CN115694203B - DC isolated converter capable of bidirectional conversion and control method thereof - Google Patents

Abstract

The application relates to a direct current isolation type converter capable of bidirectional conversion and a control method thereof, wherein the direct current isolation type converter comprises an energy storage follow current unit, a high-frequency conversion unit, a high-frequency isolation and transformation unit and a rectifying and filtering unit; the energy storage freewheel unit comprises a second capacitor and a freewheel inductor, and the high-frequency conversion unit comprises a first switching tube, a fifth switching tube and an absorption filter capacitor; the energy storage follow current unit is connected with the first direct current source, the input end of the high-frequency conversion unit is connected with the energy storage follow current unit, the input end of the high-frequency isolation and transformation unit is connected with the output end of the high-frequency conversion unit, the output end of the high-frequency isolation and transformation unit is connected with the input end of the rectifying and filtering unit, and the output end of the rectifying and filtering unit is connected with the second direct current source. The invention can realize high-efficiency conversion of the soft switch, can relatively simply and meet the bidirectional conversion of a wide range of voltage, so as to meet the wide range of a direct current end in a practical use scene, and is simple and efficient to realize.

Description

DC isolated converter capable of bidirectional conversion and control method thereof

Technical Field

The application relates to the technical field of power electronics and the field of battery equipment, in particular to a direct current isolation type converter capable of bidirectional conversion and a control method thereof.

Background

With the current demands of renewable energy source utilization and 'carbon peak', 'carbon neutralization', and the like, the rapid development of energy storage products and battery equipment related fields, many application scenarios related to battery energy storage, such as charging piles, household energy storage, commercial energy storage, and the like, are occurring, and the demands on power supply products capable of bidirectional conversion are also increasing. Many devices gradually use batteries, which need to be charged or discharged, and because of the nature of the wide voltage range of the batteries, and the compatibility of different products, the corresponding voltage range is also wider and wider, so that conventional converters that use two sets of circuits, i.e., a charging circuit and a discharging circuit, to realize bidirectional conversion have no cost advantage, and the conventional single-stage circuit has defects in terms of efficiency and meeting the requirement of charging or discharging in a wide voltage range.

As shown in fig. 1, the current conversion circuit for making the low-voltage battery pack is usually implemented by two stages, namely, a one-stage boosting or step-down scheme and then a one-stage DC/DC voltage stabilizing conversion, and the two stages have high cost, and meanwhile, the efficiency is reduced due to the two stages of conversion. The principle is simple and direct, but the change of high turn ratio can cause higher switch tube stress, and meanwhile, the inductance and leakage inductance parameters of the original main transformer can be changed, new current loop interference is introduced, and the mutation of voltage can bring about another series of parameter change in control, and the problems of easy oscillation and the like are caused by the step duty ratio adjustment. Furthermore, the realisation of these two converters in soft switching synergies is relatively poor; the whole converter is complex and difficult to popularize and apply because a conversion circuit and a transformer are additionally added. In the prior art, a traditional one-stage voltage reduction method is also adopted, and the defect is that a circuit is difficult to realize soft switching and has lower efficiency because of wide duty ratio range caused by wide-range voltage regulation requirements.

Disclosure of Invention

The invention aims to provide a direct current isolation type converter capable of realizing high-efficiency conversion of a soft switch, and the control method thereof, which can realize the bidirectional conversion of a voltage with a relatively simple and wide range so as to meet the wider range of a direct current end in a practical use scene, and is simple and efficient to realize; the method solves the technical problems that the prior art cannot meet the requirement of direct current wide range or needs two-stage converter to perform multiple buck-boost conversion, so that the loss is large, the conversion is complex, and the method is not suitable for being applied to places with limited volumes or relatively high cost requirements.

The invention adopts a technical scheme that: a DC isolation type converter capable of bidirectional conversion is used between two DC power supplies and comprises an energy storage follow current unit, a high-frequency conversion unit, a high-frequency isolation and transformation unit and a rectification filter unit; the energy storage follow current unit comprises a second capacitor and a follow current inductor, and the high-frequency conversion unit comprises a first switching tube, a second switching tube, a third switching tube, a fourth switching tube, a fifth switching tube and an absorption filter capacitor; two ends of the second capacitor are connected with a first direct current source, one end of the follow current inductor is connected with the first direct current source, and the other end of the follow current inductor is connected with the input end of the high-frequency conversion unit; the drain electrode of the first switching tube is connected with the source electrode of the fifth switching tube, the source electrode is connected with the drain electrode of the third switching tube, the drain electrode of the second switching tube is connected with the drain electrode of the fifth switching tube, the source electrode of the fourth switching tube is connected with the source electrode of the third switching tube, the drain electrode of the first switching tube and the source electrode of the third switching tube form two input ends of the high-frequency conversion unit, and the two input ends of the high-frequency conversion unit are connected with two ends of a first direct-current source in a bridging mode through a follow current inductor; the source electrode of the first switching tube and the source electrode of the second switching tube form two output ends of the high-frequency conversion unit, one end of the absorption filter capacitor is connected with the drain electrode of the second switching tube, and the other end of the absorption filter capacitor is connected with the source electrode of the fourth switching tube; the input end of the high-frequency isolation and transformation unit is connected with the output end of the high-frequency conversion unit, the output end of the high-frequency isolation and transformation unit is connected with the input end of the rectifying and filtering unit, and the output end of the rectifying and filtering unit is connected with the second direct current source.

Further, the high-frequency isolation and transformation unit is a high-frequency isolation transformer directly connected with the output end of the high-frequency conversion unit or a high-frequency isolation transformer with a high-frequency isolation capacitor connected in series with the primary side or a high-frequency isolation transformer with a resonance inductor and a resonance capacitor connected in series with the primary side, and the secondary side of the high-frequency isolation transformer is a single winding or multiple windings.

The rectification filter unit comprises a high-frequency rectification circuit and a direct current filter capacitor, wherein the high-frequency rectification circuit is a full-bridge rectification circuit, a full-wave rectification circuit or a voltage doubling rectification circuit, and the input end of the high-frequency rectification circuit is directly connected with the secondary side of the high-frequency isolation transformer or is connected with the secondary side of the high-frequency isolation transformer after being connected with the high-frequency isolation capacitor or the resonant inductor and the resonant capacitor in series; the output end of the high-frequency rectifying circuit is connected with a direct-current filter capacitor, and the direct-current filter capacitor is also connected with a second direct-current source.

Further, the high-frequency conversion unit has a rectification mode and an inversion mode; in the rectification mode, the first switching tube and the third switching tube are used as a high-frequency conversion switching tube and a boosting tube, the two switching tubes form a composite functional bridge arm, the fifth switching tube is used as a boosting flywheel diode and also used as a reverse buck conduction connecting wire, and the first switching tube to the fifth switching tube are used for making different pulse conduction combinations according to drive control to realize boosting and high-frequency conversion; in the inversion mode, the first switching tube to the fourth switching tube play a role in high-frequency rectification, are equivalent to high-frequency rectification diodes, the fifth switching tube is used as a step-down switch, pulse conduction or direct connection can be realized, and the first switching tube and the third switching tube are matched with the fifth switching tube to be used as a follow current diode, so that direct-current reverse step-down output or direct connection output is realized; the third switching tube and the fourth switching tube are commonly used as a boost tube, and are conducted by pulse according to drive control, so that reverse boost energy storage output of direct current is realized.

Further, the follow current inductor is two follow current inductors in series connection or an equivalent inductor obtained by equivalent of the two follow current inductors, and the inductance value of the equivalent inductor is the sum of the inductance values of the two follow current inductors; when the free-wheeling inductors are two free-wheeling inductors in series connection, one end of the first free-wheeling inductor is connected with the positive output end of the first direct-current source, the other end of the first free-wheeling inductor is connected with the drain electrode of the first switching tube and the source electrode of the fifth switching tube, one end of the second free-wheeling inductor is connected with the negative output end of the first direct-current source, and the other end of the second free-wheeling inductor is connected with the source electrodes of the third switching tube and the fourth switching tube; when the free-wheeling inductor is an equivalent inductor obtained by equivalent of the two free-wheeling inductors, one end of the equivalent inductor is connected with one output end of the first direct current source, the other end of the equivalent inductor is connected with one input end of the high-frequency conversion unit, and the other input end of the high-frequency conversion unit is connected with the other output end of the first direct current source through a wire.

Further, the first to fifth switching tubes are high-frequency switching tubes provided with anti-parallel diodes, or equivalently high-frequency switching tubes with the same function; the anti-parallel diode is an integrated diode, a parasitic diode or an external diode; the absorption filter capacitor is a high-frequency nonpolar capacitor or a high-frequency polar capacitor; when the absorption filter capacitor is a high-frequency capacitor with polarity, the positive electrode of the absorption filter capacitor is connected with the drain electrode of the fifth switching tube, and the negative electrode of the absorption filter capacitor is connected with the source electrode of the fourth switching tube.

The invention adopts another technical scheme that: a control method of a bidirectional-conversion direct-current isolation type converter, which is used for controlling the bidirectional-conversion direct-current isolation type converter according to the technical scheme, and comprises the following steps:

s100: selecting a working mode, wherein the working mode comprises a rectification mode and an inversion mode, the rectification mode is input from a first direct current source end, the second direct current source end is output, the inversion mode is input from the second direct current source end, and the first direct current source end is output; in the rectification mode, the first switching tube and the third switching tube of the high-frequency conversion unit are used as a boost tube in addition to the high-frequency conversion switching tube, and the two switching tubes form a composite functional bridge arm;

s200: according to the selected working mode, no driving signal or only synchronous rectification driving signal is applied to a switching tube which works as a diode or a continuous tube in the direct current isolation type converter, a high-level direct-connection driving signal is applied to the switching tube which needs to work in a direct-connection mode, and no driving signal or a low-level non-conduction signal is applied to the switching tube which does not need to be conducted;

s300: if the working mode is a rectification mode, judging whether the direct current isolation type converter needs to be subjected to voltage reduction or voltage boosting according to the voltage instantaneous value voltage of the current first direct current source, output voltage setting and conversion ratio conversion to the primary side of the high frequency isolation and transformation unit, determining whether to carry out voltage boosting PWM driving control on a switching tube serving as a voltage boosting tube in a composite function bridge arm of the high frequency conversion unit, and determining whether to carry out PWM driving conduction on other switching tubes; if the voltage needs to be boosted, common driving is needed to be applied to the first switching tube and the third switching tube to form the boosted voltage, and the second switching tube and the fourth switching tube are matched with the corresponding bridge type pair tube to perform high-frequency conversion; if the voltage needs to be reduced or the voltage does not need to be increased, normal bridge driving is applied to the first switching tube to the fourth switching tube to perform high-frequency conversion; the fifth switching tube is closed when the direct current isolation type converter is boosted or closed in a non-conduction interval when the direct current isolation type converter is reduced, and is opened when the bridge type high-frequency conversion is conducted;

S400: if the working mode is an inversion mode, converting the voltage instantaneous value voltage and the transformation ratio of the current second direct current source to a first direct current source end, comparing the output voltage setting to judge whether the direct current isolated converter needs to be subjected to voltage reduction or voltage increase, and determining whether a fifth switching tube and a composite function bridge arm of the high frequency conversion unit are subjected to voltage reduction PWM driving control; meanwhile, determining whether PWM driving conduction is carried out on a switching tube of a high-frequency rectifying circuit in the rectifying filter unit; if the voltage needs to be boosted, common driving is needed to be applied to the third switching tube and the fourth switching tube to form boosting energy storage; if the voltage is required to be reduced or the voltage is not required to be increased, normal bridge driving is applied to a switching tube of a high-frequency rectifying circuit in the rectifying and filtering unit to carry out high-frequency conversion; the fifth switching tube is closed in a non-conduction interval of the freewheeling or the buck-rectifying of the composite functional bridge arm, and is conducted in a bridge rectifying conduction interval or a boost energy storage interval.

Further, in steps S300 to S400, applying a driving signal for shorting the coil of the high-frequency isolation transformer to the switching tube of the high-frequency rectification circuit in the rectification filter unit in the rectification mode can bring the dc isolation converter into a boosting energy storage state; in the inversion mode, the direct-current isolated converter can realize the step-down regulation of the output voltage by applying driving signals with variable duty ratios to the high-frequency rectifying circuit and the fifth switching tube.

Further, in steps S300 to S400, when the fifth switching tube is in the PWM operating state, the PWM switching frequency of the fifth switching tube is identical to or double the PWM switching frequency of the switching tube of the high-frequency conversion unit or the high-frequency rectification circuit.

Further, in steps S300 to S400, in the buck rectification mode, PWM driving signals with central symmetry are applied to the first switching tube and the fourth switching tube, and the second switching tube and the third switching tube, respectively, which are used as the pair tubes in the high-frequency conversion unit, and PWM driving signals which are integrated corresponding to the conduction intervals of the diagonal pair tubes formed by the first switching tube and the fourth switching tube are applied to the fifth switching tube; in the buck inversion mode, central symmetrical PWM driving signals are respectively applied to bridge type geminate transistors or single transistors which are inverted in the high-frequency rectification circuit, and comprehensive PWM driving signals corresponding to the conduction intervals of oblique geminate transistors formed by the first switching tube and the fourth switching tube are applied to a fifth switching tube in the high-frequency conversion unit.

The invention has the beneficial effects that:

(1) The complexity of the traditional multi-stage circuit conversion is overcome in terms of structure and performance, so that the loss of a power device of the direct-current converter at the rear end is reduced, the limitation is reduced, and the design margin is larger;

(2) In terms of control, the voltage control mode of the traditional series resonance transformation which needs wide frequency modulation is changed, and the voltage control mode is realized by adjusting the duty ratio of each switching tube, so that the control mode is simpler;

(3) The invention can realize direct current isolated bidirectional conversion, can reduce voltage and boost voltage, and has simpler circuit and wider adaptive voltage range compared with the traditional bidirectional conversion circuit;

(4) The invention avoids the combination switching of a plurality of converters or transformer coils due to the normalization control on the structure, so that the performance is more stable, and the comprehensive cost performance is high.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings that are needed in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present application, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.

Fig. 1 is a schematic structural diagram of a conventional dc bidirectional isolated converter;

FIG. 2 is a schematic diagram of an embodiment of the present invention;

FIG. 3 is a schematic circuit diagram of embodiment 1;

FIG. 4 is a schematic diagram illustrating the use state of embodiment 1 in the rectifying mode;

FIG. 5 is an equivalent schematic diagram of embodiment 1 in buck rectification mode;

FIG. 6 is an equivalent schematic diagram of embodiment 1 in boost rectifying mode;

FIG. 7 is a schematic diagram of the driving of embodiment 1 in the boost rectifying mode;

fig. 8 is a schematic diagram of the usage state of embodiment 1 in the inversion mode;

fig. 9 is an equivalent schematic diagram of embodiment 1 in boost inversion mode;

fig. 10 is an equivalent schematic diagram of embodiment 1 in buck-boost inversion mode;

fig. 11 is a schematic diagram of driving in the buck-boost inversion mode according to embodiment 1.

Detailed Description

In order that the above-recited objects, features and advantages of the present invention will be more clearly understood, a more particular description of the invention will be rendered by reference to the appended drawings and appended detailed description. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, however, the present invention may be practiced in other ways than those described herein, and therefore the present invention is not limited to the specific embodiments disclosed below.

Unless defined otherwise, technical or scientific terms used herein should be given the ordinary meaning as understood by one of ordinary skill in the art to which this application pertains. The terms "first," "second," and the like in the description and in the claims, are not used for any order, quantity, or importance, but are used for distinguishing between different elements. Likewise, the terms "a" or "an" and the like do not denote a limitation of quantity, but rather denote the presence of at least one. The terms "connected" or "connected," and the like, are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "upper", "lower", "left", "right", etc. are used merely to indicate a relative positional relationship, which changes accordingly when the absolute position of the object to be described changes.

As shown in fig. 2, a bi-directional switchable dc isolated converter is used between two dc power sources, and comprises an energy storage freewheel unit, a high-frequency conversion unit, a high-frequency isolation and transformation unit and a rectifying and filtering unit; the energy storage freewheel unit comprises a second capacitor C2 and a freewheel inductor, and the high-frequency conversion unit comprises a first switching tube Q101, a second switching tube Q102, a third switching tube Q103, a fourth switching tube Q104, a fifth switching tube Q105 and an absorption filter capacitor Cs1; two ends of the second capacitor C2 are connected with a first direct current source, one end of the follow current inductor is connected with the first direct current source, and the other end of the follow current inductor is connected with the input end of the high-frequency conversion unit; the drain electrode of the first switching tube Q101 is connected with the source electrode of the fifth switching tube Q105, the source electrode is connected with the drain electrode of the third switching tube Q103, the drain electrode of the second switching tube Q102 is connected with the drain electrode of the fifth switching tube Q105, the source electrode is connected with the drain electrode of the fourth switching tube Q104, the source electrode of the fourth switching tube Q104 is connected with the source electrode of the third switching tube Q103, the drain electrode of the first switching tube Q101 and the source electrode of the third switching tube Q103 form two input ends of a high-frequency conversion unit, and the two input ends of the high-frequency conversion unit are connected with two ends of a first direct current source in a bridging mode through a follow current inductor; the source electrode of the first switching tube Q101 and the source electrode of the second switching tube Q102 form two output ends of the high-frequency conversion unit, one end of the absorption filter capacitor Cs1 is connected with the drain electrode of the second switching tube Q102, and the other end is connected with the source electrode of the fourth switching tube Q104; the input end of the high-frequency isolation and transformation unit is connected with the output end of the high-frequency conversion unit, the output end of the high-frequency isolation and transformation unit is connected with the input end of the rectifying and filtering unit, and the output end of the rectifying and filtering unit is connected with the second direct current source.

In the embodiment of the invention, the high-frequency isolation and transformation unit is a high-frequency isolation transformer Tra directly connected with the output end of the high-frequency conversion unit or a high-frequency isolation transformer Tra with a high-frequency isolation capacitor or a resonant inductor and a resonant capacitor connected in series on the original side, and when the high-frequency isolation capacitor or the resonant inductor and the resonant capacitor are connected in series, the resonance soft switching transformation of the original side can be obtained; the secondary side of the high-frequency isolation transformer Tra is single-winding or multi-winding.

The rectification filter unit comprises a high-frequency rectification circuit and a direct-current filter capacitor C1, wherein the high-frequency rectification circuit is a full-bridge rectification circuit, a full-wave rectification circuit or a voltage doubling rectification circuit and has a bidirectional conversion function; the input end of the high-frequency rectifying circuit is directly connected with the secondary side of the high-frequency isolation transformer Tra, or is connected with the secondary side of the high-frequency isolation transformer after being connected in series with the high-frequency isolation capacitor or the resonant inductor and the resonant capacitor, so that a resonant soft switch in the process of reverse conversion can be obtained; the output end of the high-frequency rectifying circuit is connected with a direct current filter capacitor C1, and the direct current filter capacitor C1 is also connected with a second direct current source.

The high-frequency conversion unit is provided with a rectification mode and an inversion mode; in the rectification mode, the first switching tube Q101 and the third switching tube Q103 are used as a high-frequency conversion switching tube and a boosting tube, the two switching tubes form a composite functional bridge arm, the fifth switching tube Q105 is used as a boosting freewheeling diode and also is used as a reverse buck conduction connecting wire, and the first switching tube Q101 to the fifth switching tube Q105 are used for making different pulse conduction combinations according to drive control so as to realize boosting and high-frequency conversion; in the inversion mode, the first to fourth switching tubes Q101 to Q104 play a role in high-frequency rectification, and can be equivalently used as high-frequency rectification diodes, the fifth switching tube Q105 is used as a step-down switch, and can be conducted in a pulse or directly connected mode, and the first switching tube Q101 and the third switching tube Q103 are matched with the fifth switching tube Q105 to be used as a follow current diode, so that direct-current reverse step-down output or direct-connection output is realized; the third switching tube Q103 and the fourth switching tube Q104 are commonly used as boost tubes, and are turned on according to driving control to realize reverse boost energy storage output of direct current.

The free-wheeling inductor is two free-wheeling inductors in series connection or an equivalent inductor obtained by equivalent of the two free-wheeling inductors, and the inductance value of the equivalent inductor is the sum of the inductance values of the two free-wheeling inductors; when the free-wheeling inductors are two free-wheeling inductors in a series connection, one end of the first free-wheeling inductor L1 is connected with a positive output end DC1+ of a first direct current source, the other end of the first free-wheeling inductor L1 is connected with a drain electrode of the first switching tube Q101 and a source electrode of the fifth switching tube Q102, one end of the second free-wheeling inductor L2 is connected with a negative output end DC 1-of the first direct current source, and the other end of the second free-wheeling inductor L2 is connected with source electrodes of the third switching tube Q103 and the fourth switching tube Q105; when the free-wheeling inductor is an equivalent inductor obtained by equivalent of the two free-wheeling inductors, one end of the equivalent inductor is connected with one output end of the first direct current source, the other end of the equivalent inductor is connected with one input end of the high-frequency conversion unit, and the other input end of the high-frequency conversion unit is connected with the other output end of the first direct current source through a wire.

The first to fifth switching tubes Q101 to Q105 are high-frequency switching tubes provided with anti-parallel diodes or high-frequency switching tubes equivalent to the same function; the anti-parallel diode is an integrated diode, a parasitic diode or an external diode; the absorption filter capacitor Cs1 is a low-capacity high-frequency nonpolar capacitor or a high-frequency polar capacitor, the absorption filter capacitor Cs1 can absorb the bus peak voltage of the switching tube in the high-frequency conversion unit, if the high-frequency isolation and transformation unit at the rear end is provided with a resonant circuit, the absorption filter capacitor Cs1 can also absorb and release the difference part between the inductance current and the resonance current of the first follow current inductance L1 and the second follow current inductance L2 so as to assist in realizing the resonance soft switching conversion; when the absorption filter capacitor Cs1 is a high-frequency capacitor having polarity, the positive electrode of the absorption filter capacitor Cs1 is connected to the drain electrode of the fifth switching tube Q105, and the negative electrode is connected to the source electrode of the fourth switching tube Q104.

The working principle of the invention is described below with reference to specific examples:

example 1

As shown in fig. 3, in embodiment 1, the freewheeling inductance of the energy storage freewheeling unit is two freewheeling inductances in a series connection, one end of the first freewheeling inductance L1 is connected with the positive output terminal DC1+ of the first DC source, the other end is connected with the drain electrode of the first switching tube Q101 and the source electrode of the fifth switching tube Q102, one end of the second freewheeling inductance L2 is connected with the negative output terminal DC 1-of the first DC source, and the other end is connected with the source electrodes of the third switching tube Q103 and the fourth switching tube Q105. The high-frequency isolation and transformation unit is a high-frequency isolation transformer Tra with a resonant inductor Lr and a resonant capacitor Cr connected in series on the original side, and a single winding is arranged on the secondary side of the high-frequency isolation transformer Tra. The high-frequency rectifying circuit is a full-bridge rectifying circuit, and the full-bridge rectifying circuit comprises a seventh switching tube Q107, an eighth switching tube Q108, a ninth switching tube Q109 and a tenth switching tube Q110; the drain electrode of the seventh switching tube Q107 is connected with the drain electrode of the eighth switching tube Q108 to form a positive output end of the full-bridge rectifying circuit, and the source electrode of the ninth switching tube Q109 is connected with the source electrode of the tenth switching tube Q110 to form a negative output end of the full-bridge rectifying circuit; the source electrode of the seventh switching tube Q107 is connected with the drain electrode of the ninth switching tube Q108 to form one input end of the full-bridge rectifier circuit, the source electrode of the eighth switching tube Q108 is connected with the drain electrode of the tenth switching tube Q110 to form the other input end of the full-bridge rectifier circuit, and the two input ends are connected with the secondary side of the high-frequency isolation transformer Tra. The positive output end and the negative output end of the full-bridge rectifying circuit are connected with a direct current filter capacitor C1, and the direct current filter capacitor C1 is also connected with the positive output end DC < 2+ > of the second direct current source and the negative output end DC < 2 > -of the second direct current source.

The control method of embodiment 1 includes the steps of:

s100: and selecting a working mode, wherein the working mode comprises a rectification mode and an inversion mode, the rectification mode is input from a first direct current source end, the second direct current source end is output, the inversion mode is input from the second direct current source end, and the first direct current source end is output. In the rectification mode, the first switching tube Q101 and the third switching tube Q103 of the high-frequency conversion unit are used as boost tubes in addition to high-frequency conversion switching tubes, and the two switching tubes form a composite functional bridge arm.

S200: according to the selected working mode, no driving signal or only synchronous rectification driving signal is applied to a switching tube which works as a diode or a continuous tube in the direct current isolation type converter, a high-level direct-connection driving signal is applied to the switching tube which needs to work in a direct current mode, and no driving signal or a low-level non-conduction signal is applied to the switching tube which does not need to conduct.

S300: if the working mode is a rectification mode, judging whether the direct current isolation type converter needs to be subjected to voltage reduction or voltage boosting according to the voltage instantaneous value voltage of the current first direct current source, output voltage setting and conversion ratio conversion to the primary side of the high frequency isolation and transformation unit, determining whether to carry out voltage boosting PWM driving control on a switching tube serving as a voltage boosting tube in a composite function bridge arm of the high frequency conversion unit, and determining whether to carry out PWM driving conduction on other switching tubes; if the voltage needs to be boosted, common driving needs to be applied to the first switching tube Q101 and the third switching tube Q103 to form the boosted voltage, and the second switching tube Q102 and the fourth switching tube Q104 are matched with corresponding bridge type pair tube high-frequency conversion; if the voltage needs to be reduced or the voltage does not need to be increased, normal bridge driving is applied to the first switch tube to the fourth switch Q101-Q104 tubes to carry out high-frequency conversion; the fifth switching transistor Q105 is turned off during the step-up period or the step-down period of the dc-isolated converter, and is turned on during the conduction period of the bridge high-frequency conversion.

S400: if the working mode is the inversion mode, converting the voltage instantaneous value voltage and the transformation ratio of the current second direct current source to the first direct current source end, comparing the output voltage setting to judge whether the direct current isolated converter needs to be subjected to voltage reduction or voltage increase, and determining whether to carry out voltage reduction PWM driving control on a fifth switching tube Q105 and a composite function bridge arm of the high frequency conversion unit; meanwhile, determining whether PWM driving conduction is carried out on a switching tube of a high-frequency rectifying circuit in the rectifying filter unit; if the voltage needs to be boosted, common driving is needed to be applied to the third switching tube Q103 and the fourth switching tube Q104 to form boosting energy storage; if the voltage is required to be reduced or the voltage is not required to be increased, normal bridge driving is applied to a switching tube of a high-frequency rectifying circuit in the rectifying and filtering unit to carry out high-frequency conversion; the fifth switching tube Q105 is closed in the non-conducting interval of the freewheeling or buck-rectifying of the composite functional bridge arm, and is conducted in the conducting interval of the bridge rectifying or the boosting energy storage.

In steps S300 to S400, applying a driving signal for shorting the coil of the high-frequency isolation transformer to the switching tube of the high-frequency rectification circuit in the rectification filter unit in the rectification mode can make the dc isolation converter enter a boosting energy storage state; in the inversion mode, the application of a driving signal with a variable duty ratio to the high-frequency rectifying circuit and the fifth switching tube Q105 enables the dc-isolated converter to realize the step-down regulation of the output voltage. When the fifth switching tube Q105 is in the PWM operating state, the PWM switching frequency of the fifth switching tube Q105 is identical to or double the PWM switching frequency of the switching tube of the high-frequency conversion unit or the high-frequency rectification circuit. In a buck rectification mode, a central symmetrical PWM driving signal is respectively applied to a first switching tube Q101 and a fourth switching tube Q104 which are paired tubes in a high-frequency conversion unit, a second switching tube Q102 and a third switching tube Q103, and a PWM driving signal which is synthesized corresponding to a conduction interval of an oblique paired tube formed by the first switching tube Q101 and the fourth switching tube Q104 is applied to a fifth switching tube Q105; in the buck inversion mode, central symmetrical PWM driving signals are respectively applied to bridge type geminate transistors or single transistors which are inverted in the high-frequency rectification circuit, and PWM driving signals which are integrated corresponding to the conduction interval of the diagonal geminate transistors formed by the first switching tube Q101 and the fourth switching tube Q104 are applied to the fifth switching tube Q105 in the high-frequency conversion unit.

The working principle of example 1 is as follows:

in the rectification mode, the first direct current source is connected with an input power supply, and a load or a circuit equivalent to the load can be connected between the positive output end and the negative output end of the second direct current source. According to the basic principle of circuit voltage reduction, voltage reduction is formed when the output voltage is set to be reduced to the voltage Ve at the first direct current source side according to the transformation ratio and is lower than the input voltage; when Ve is set to be larger than the input voltage, the output operation state of embodiment 1 is the boost state. When the input voltage is a stepped rectified power supply or the voltage of the output load varies widely, the operating state of embodiment 1 may be both boosted and dropped.

(1) Determining a step-down state according to the input/output voltage requirement

At this time, the seventh to tenth switching transistors Q107 to Q110 are not required to be turned on by PWM driving, and the seventh to tenth switching transistors Q107 to Q110 may be equivalently diodes, and may be naturally rectified or turned on by freewheeling, and the circuit diagram of fig. 3 is equivalent to the state shown in fig. 4, and if the seventh to tenth switching transistors Q107 to Q110 are high-frequency switching transistors provided with anti-parallel diodes, PWM driving may be applied to perform synchronous rectification when the anti-parallel or equivalent diodes are turned on, and meanwhile, the fifth switching transistor Q105 may be equivalently a wire, and fig. 4 may be further equivalent to fig. 5.

In the high-frequency conversion unit, when the fifth switching tube Q105 is applied with a through driving signal, which is equivalent to a wire, the output ports of the absorption filter capacitor Cs1, the first freewheeling inductor L1 and the second freewheeling inductor L2 are directly connected in parallel, and the first switching tube Q101 to the fourth switching tube Q104 operate according to a full-bridge operation mode, and the duty cycle of each switching tube can be regarded as approximately 50% when the dead zone is ignored, as shown in fig. 7 a. Therefore, the portion of the inductance current of the first freewheeling inductor L1 and the second freewheeling inductor L2 exceeding the resonance current is absorbed by the absorption filter capacitor Cs1, and the portion of the inductance current of the first freewheeling inductor L1 and the second freewheeling inductor L2 not exceeding the resonance current is released and supplemented by the absorption filter capacitor Cs1 along with the progress of the high-frequency conversion. If the filter capacitor Cs1 is not absorbed, the resonant current of the full-bridge resonant transformation is clamped and interfered with the input inductor current. If the operating frequency of the converter is higher or lower than the natural resonant frequency of the full-bridge resonant conversion Where Lr is the inductance value of the resonant inductor Lr, cr is the capacitance value of the resonant capacitor Cr, and the high-frequency conversion unit may generate buck or boost due to the resonant conversion characteristic. When the first to fourth switching tubes Q101 to Q104 operate according to the full-bridge conversion operation mode, the seventh to tenth switching tubes Q107 to Q110 may be regarded as typical full-bridge rectifiers, and the current is output to the dc filter capacitor C1 and the dc output load. The related working principle is the prior art and is not accumulated here 。

Besides the above frequency modulation can realize output voltage regulation, inductance is connected between the secondary side of the high-frequency isolation transformer Tra and the high-frequency rectifying circuit, and conventional duty ratio regulation can be applied to the high-frequency converting circuit to regulate output voltage, and the PWM driving square wave of each pair of tubes can be a conventional square wave signal with half a switching period interval or a direct duty ratio reduction based on fig. 7 (a), and can also be a square wave signal as shown in fig. 11 (b), and PWM driving signals are sent to the other pair of tubes immediately after the PWM driving square wave of the upper pair of tubes is closed, that is, central symmetrical PWM driving signals are respectively applied to the first switching tube Q101 and the fourth switching tube Q104 and the second switching tube Q102 and the third switching tube Q103 in the high-frequency converting unit. In this condition, the fifth switching tube Q105 cannot be applied with a fully-on signal, or the fifth switching tube Q105 cannot be considered as a wire, but PWM driving corresponding to the on interval of the pair of tubes is applied to the fifth switching tube Q105, and when the pair of tubes is turned off, the fifth switching tube Q105 is also turned off. Therefore, the PWM switching frequency of the fifth switching tube Q105 should be double the PWM switching frequency of the switching tube of the high frequency conversion unit or coincide with the PWM switching frequency of the switching tube of the high frequency conversion unit.

(2) Determining a boost state according to the input/output voltage requirement

Unlike the buck state, in the boost state, the fifth switching tube Q105 cannot be directly turned on, and the fifth switching tube Q105 is equivalent to a boost freewheeling diode, and the boost conversion must be performed by matching with the composite functional bridge arm formed by the first switching tube Q101 and the third switching tube Q103, and fig. 4 can be equivalent to fig. 6. Because the boost is needed, the short-circuit channel is formed in the alternating current input loop, so that the first freewheeling inductor L1 and the second freewheeling inductor L2 can store energy and boost, and meanwhile, the DC/DC conversion at the rear end cannot be influenced, and therefore, a composite functional bridge arm consisting of the fifth switching tube Q105, the first switching tube Q101 and the third switching tube Q103 must be reasonably utilized to perform boost conversion under a specific driving time sequence. When the third switching tube Q103 and the second switching tube Q102 are combined to perform high-frequency conversion, if the first switching tube Q101 is not already turned on, the inductor current of the first freewheeling inductor L1 and the second freewheeling inductor L2 and the absorption filter capacitor Cs1 can both supply power to the channel of the third switching tube Q103 and the second switching tube Q102, if the first switching tube Q101 is turned on during the fast completion or conduction period of the third switching tube Q103 and the second switching tube Q102, i.e. the driving of the first switching tube Q101 is turned on in advance on the basis of fig. 7 (a), and the driving signal at this time is as shown in fig. 7 (b), the first switching tube Q101 and the third switching tube Q103 are simultaneously applied with driving conduction, so that the direct connection short circuit of the first freewheeling inductor L1 and the second freewheeling inductor L2 can be realized and energy storage can be performed, and at this time, the fifth switching tube Q105 is not applied with PWM driving, and can be regarded as a diode, so that the capacitance function of the absorption filter capacitor Cs1 is isolated, and the power supply function of the absorption filter capacitor Cs1 is not affected by continuing high-frequency conversion. When the first switching tube Q101 is turned off, the current cannot be reversed due to the existence of the first freewheeling inductor L1 and the second freewheeling inductor L2, and the current continues to keep in the original direction, the reverse energy release freewheeling of the inductance electromotive force occurs, and the reverse energy release freewheeling is connected in series with the input voltage, so as to supply power to the high-frequency conversion unit together with the first dc source, and at this time, the fifth switching tube is turned on, and PWM driving can be applied to perform synchronous rectification. When the working of the pair of tube paths consisting of the third switch tube Q103 and the second switch tube Q102 is completed, the pair of tube paths consisting of the first switch tube Q101 and the fourth switch tube Q104 are switched to work, and at the moment, the power supply path of the alternating-current end can directly supply power to the first switch tube Q101 and the fourth switch tube Q104, meanwhile, in order to avoid the influence of inductance current and resonance transformation current difference value parts, the fifth switch tube Q105 needs to be opened. Regarding the moment and the end moment of the boost-on of the first switching tube Q101, when the first freewheeling inductor L1 and the second freewheeling inductor L2 start to release energy and boost power, the boost-on is selected, and the high-frequency conversion is performed by switching the diagonal pair of the first switching tube Q101 and the fourth switching tube Q104, because the current of the pair of the third switching tube Q103 and the second switching tube Q102 is in the descending interval before, the influence caused by the superposition of the inductive energy storage currents of the first freewheeling inductor L1 and the second freewheeling inductor L2 can be relatively reduced, meanwhile, the first switching tube Q101 does not need to be turned off, the natural turn-off of the third switching tube Q103 can end the energy storage boost, and soft turn-off can be realized. At this time, if the first switching tube Q101 is continuously turned on, it can be used as an inductance freewheeling channel and a next-stage switching channel for high-frequency conversion, so that the loss of turning off in the middle and turning on again at one time can be reduced. Therefore, the boosting sequence of the first switching tube Q101 is preferably set to be a section for performing the boosting composite operation of the first switching tube Q101 before the operation of the pipe path composed of the third switching tube Q103 and the second switching tube Q102 is completed, that is, before the completion time.

As is clear from the above-described operation principle, the boosting operation is mainly performed in the common mode between the pair of tube sections formed by the third switching tube Q103 and the second switching tube Q102 and the first switching tube Q101, and the driving method shown in fig. 7 (c) or fig. 7 (d), that is, the driving method shown in fig. 7 (a), may be used, and the first switching tube Q101 and the third switching tube Q103 are commonly boosted in the adjacent conduction sections between the pair of tubes formed by the third switching tube Q103 and the second switching tube Q102 and between the pair of tubes formed by the first switching tube Q101 and the fourth switching tube Q104, and the fifth switching tube Q105 must be turned off at this time. Therefore, the boost driving method shown in fig. 7 (a) or fig. 7 (b) can realize boost expansion, but has a disadvantage that the dc conversion is interrupted when the boost is performed, so that the PWM switching frequency of the fifth switching transistor Q105 still matches the PWM switching frequency of the switching transistor of the high frequency conversion unit or is double the PWM switching frequency of the switching transistor of the high frequency conversion unit, which is required to be selected as appropriate in use.

In addition, a driving signal for shorting the winding of the high-frequency isolation transformer Tra is applied to the switching tube of the high-frequency rectification circuit in the rectifying and filtering unit, for example, a shorting conduction signal is applied to the seventh switching tube Q107 and the eighth switching tube Q108 (or the ninth switching tube Q109 and the tenth switching tube Q110) when resonance is about to cross zero, so that the secondary side of the high-frequency isolation transformer Tra is shorted, at this time, boost energy storage is performed on the leakage inductance of the high-frequency isolation transformer Tra and the resonance inductance Lr connected in series with the winding of the high-frequency isolation transformer Tra, then the PWM driving signal is turned off, the secondary side short-circuit condition of the high-frequency isolation transformer Tra is interrupted, the leakage inductance of the resonant inductance Lr or the high-frequency isolation transformer Tra is equivalently connected in series with the output rectifying circuit, and the inductance electromotive force is reversed, and the output rectification is connected in series in the same direction at this time, so that boost is formed.

From the above analysis, it can be seen that in the boost state of embodiment 1, unnecessary boosting or reducing and intermediate capacitor energy storage processes in the conventional two-stage converter shown in fig. 1 are avoided, multiplexing of the boost switching tube is achieved, loss of the conventional boost switching tube is reduced, and system efficiency is improved.

In the inversion mode, the second dc source end is connected to the dc power supply and is connected in parallel with the dc filter capacitor C1, and the first dc source end is connected to an equivalent load, which may be a power supply or a load that absorbs energy. According to the basic principle of dc voltage reduction, voltage reduction is formed when the voltage Ve at the second dc source side is converted to the voltage Ve at the first dc source side to be higher than the output voltage of the first dc source side, otherwise voltage boosting may be required. Therefore, as shown in fig. 8, in the case where embodiment 1 is operated in the inversion state, the first to fourth switching transistors Q101 to Q104 perform the rectifying function, and thus can be equivalently used as diodes.

(1) Assume that the voltage boosting state is determined according to the input/output voltage conversion requirement

The method comprises the steps that firstly, a fifth switching tube Q105 is fully conducted, meanwhile, a full-bridge rectifying circuit in a rectifying and filtering unit on the secondary side is subjected to full-bridge inversion operation at the moment, a seventh switching tube to tenth switching tubes Q107-Q110 form a typical H bridge, when the seventh switching tube Q107 and the tenth switching tube Q110 are simultaneously applied with PWM driving as shown in the (a) of the fig. 11, or the eighth switching tube Q108 and the ninth switching tube Q109 are simultaneously applied with the pair of tubes, voltage on the direct current side is directly transmitted to the primary side through coupling of a high-frequency transformer Tra, then full-bridge automatic rectification is carried out by a first switching tube to fourth switching tube Q101-Q104 to form direct current voltage, an absorption filter capacitor Cs1 is charged, the voltage can be regarded as an equivalent direct current source, meanwhile, the positive electrode of the absorption filter capacitor Cs1 and the drain electrode of the second switching tube are connected with a first free-wheel inductor L1, the negative electrode of the absorption filter capacitor Cs1 and the source of the fourth switching tube Q104 are connected with a second free-wheel inductor L2, and the high-frequency converting unit is equivalent to the fourth free-wheel inductor is connected with the first free-wheel inductor L2 and the second free-wheel inductor L2; meanwhile, when PWM driving is applied to the fifth switching tube Q105 of the energy storage freewheel unit, fig. 8 can be equivalent to fig. 9, and since the fifth switching tube Q105 is always in the on state and can be regarded as a wire, embodiment 1 can be changed into a soft-switching full-bridge dc converter with secondary side series resonance. The resonant inductor Lr and the resonant capacitor Cr are connected in series in the circuit, so that the function of boosting or reducing voltage caused by frequency change of the high-frequency isolation and transformation unit can be fully utilized in use, and under the same load condition, the higher the switching frequency is, the more the boosting is, and the specific working principle is the prior art, so that tiredness is not performed.

In addition, a driving signal for shorting the winding of the high-frequency isolation transformer Tra is applied to the switching tube of the high-frequency rectifying part in the rectifying and filtering unit, and the driving signal can enter a boost energy storage mode, for example, a short-circuit conducting signal is applied to the third switching tube Q103 and the fourth switching tube Q104 immediately before the rectifying current is zero or before the next rectifying starts, so that the output winding of the high-frequency isolation transformer Tra is shorted, at this time, boost energy storage is performed on the leakage inductance of the high-frequency isolation transformer Tra and the resonance inductance Lr connected in series with the winding of the high-frequency isolation transformer Tra, then the PWM driving signal is turned off, the secondary side short-circuit working condition of the high-frequency isolation transformer Tra is interrupted, the resonance inductance Lr or the leakage inductance of the high-frequency isolation transformer Tra in the original loop is equivalently connected in series with the output rectifying circuit, and the inductance electromotive force is reversely connected in series with the same direction, so that the boost is formed and the full-bridge from the first switching tube to the fourth switching tube Q101 to Q104.

(2) The step-down state is determined based on the voltage Ve requirement of the second DC source voltage converted to the first DC source side

At this time, fig. 8 can be equivalent to fig. 10, which is equivalent to adding a dc input voltage-reducing circuit in the rectifying output loop of the full-bridge inverter.

The equivalent dc source voltage to be applied to the first and second freewheeling inductances L1 and L2 must therefore be reduced or changed to PWM regulation mode because of the presence of the first and second freewheeling inductances L1 and L2, which voltage would be reduced in terms of duty cycle conversion if the equivalent dc source was applied in terms of duty cycle calculated in a control like a Buck circuit. If the time for connecting the equivalent direct current source formed by the full-bridge rectification to the first freewheeling inductor L1 and the second freewheeling inductor L2 is reduced, the step-down is formed, so that a typical H-bridge formed by the seventh to tenth switching transistors Q107 to Q110 is applied to the PWM driving signal with the duty ratio reduced based on the PWM driving signal with the duty ratio of 50% shown in fig. 11 (a), the PWM driving signal is applied to the fifth switching transistor Q105 to conduct, the connection of the absorption filter capacitor Cs1, the second switching transistor Q102 and the third switching transistor Q103 to the tube and the connection of the first switching transistor Q101 and the fourth switching transistor Q104 to the first freewheeling inductor L1 and the second freewheeling inductor L2 are reduced, the Buck application of the equivalent direct current source is formed, that is, after the second switching transistor Q102 and the third switching transistor Q103 form rectification, the fifth switching transistor Q105 is turned off according to the duty ratio requirement, and at this time, the first inductor L1 and the second inductor freewheeling L2 are conducted by the first switching transistor Q101 and the third switching transistor Q103, and the second switching transistor Q102 continue to charge the absorption filter capacitor Cs1 and the third switching transistor Q103.

In addition, the aforementioned equivalent dc source PWM mode is applied to the first freewheeling inductor L1 and the second freewheeling inductor L2 in another way, that is, the duty ratio of the full-bridge inversion of the rectifying and filtering unit at the second dc source side is changed from approximately 50% by 2 to the PWM driving duty ratio D required for the operation control of the converter, and the PWM conduction driving of the full-bridge inversion is that the central side is tightly connected, the non-conduction driving is on two sides, if the pair of the seventh switching tube Q107 and the tenth switching tube Q110 is denoted as a, the pair of the eighth switching tube Q108 and the ninth switching tube Q109 is denoted as B, as shown in fig. 11 (B), the primary side rectification is that the pair of the full-bridge transistors, that is, the first switching tube Q101 and the fourth switching tube Q104 and the second switching tube Q102 and the third switching tube Q103D are also rectified dc pulse voltages with central symmetrical 2D time conduction, where T is one switching period; meanwhile, PWM driving is applied to the fifth switching tube Q105 when rectification is conducted, and the voltage of the absorption filter capacitor Cs1 and the rectified voltage, namely the equivalent direct current source, are applied to the ports of the first follow current inductor L1 and the second follow current inductor L2 according to a PWM mode of 2*D, so that the purpose of reducing the voltage is achieved.

According to the above working principle, when the fifth switching tube Q105 is in the PWM working state, the PWM switching frequency of the fifth switching tube Q105 is consistent with or is twice the PWM switching frequency of the switching tube of the high-frequency rectifying circuit. In addition, in some occasions with relatively high output voltage, the switching tube and the filter capacitor can be connected in series or similar in series to improve the withstand voltage by considering the selection of the switching tube and the back-end filter voltage.

The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (10)

1. The DC isolation type converter capable of bidirectional conversion is used between two DC power supplies and is characterized by comprising an energy storage follow current unit, a high-frequency conversion unit, a high-frequency isolation and transformation unit and a rectification filter unit; the energy storage follow current unit comprises a second capacitor and a follow current inductor, and the high-frequency conversion unit comprises a first switching tube, a second switching tube, a third switching tube, a fourth switching tube, a fifth switching tube and an absorption filter capacitor; two ends of the second capacitor are connected with a first direct current source, one end of the follow current inductor is connected with the first direct current source, and the other end of the follow current inductor is connected with the input end of the high-frequency conversion unit; the drain electrode of the first switching tube is connected with the source electrode of the fifth switching tube, the source electrode is connected with the drain electrode of the third switching tube, the drain electrode of the second switching tube is connected with the drain electrode of the fifth switching tube, the source electrode of the fourth switching tube is connected with the source electrode of the third switching tube, the drain electrode of the first switching tube and the source electrode of the third switching tube form two input ends of the high-frequency conversion unit, and the two input ends of the high-frequency conversion unit are connected with two ends of a first direct-current source in a bridging mode through a follow current inductor; the source electrode of the first switching tube and the source electrode of the second switching tube form two output ends of the high-frequency conversion unit, one end of the absorption filter capacitor is connected with the drain electrode of the second switching tube, and the other end of the absorption filter capacitor is connected with the source electrode of the fourth switching tube; the input end of the high-frequency isolation and transformation unit is connected with the output end of the high-frequency conversion unit, the output end of the high-frequency isolation and transformation unit is connected with the input end of the rectifying and filtering unit, and the output end of the rectifying and filtering unit is connected with the second direct current source.

2. The bidirectional-convertible direct-current isolation type converter according to claim 1, wherein the high-frequency isolation and transformation unit is a high-frequency isolation transformer directly connected with an output end of the high-frequency conversion unit or a high-frequency isolation transformer with a high-frequency isolation capacitor connected in series with an original side or a resonant inductor and a resonant capacitor connected in series with an original side, and a secondary side of the high-frequency isolation transformer is a single winding or multiple windings.

3. The bidirectional-convertible direct-current isolation type converter according to claim 2, wherein the rectifying and filtering unit comprises a high-frequency rectifying circuit and a direct-current filtering capacitor, the high-frequency rectifying circuit is a full-bridge rectifying circuit, a full-wave rectifying circuit or a voltage doubling rectifying circuit, and an input end of the high-frequency rectifying circuit is directly connected with a secondary side of the high-frequency isolation transformer or is connected with the secondary side of the high-frequency isolation transformer after being connected with the high-frequency isolation capacitor or the resonant inductor and the resonant capacitor in series; the output end of the high-frequency rectifying circuit is connected with a direct-current filter capacitor, and the direct-current filter capacitor is also connected with a second direct-current source.

4. The bi-directional switchable dc-isolated converter of claim 1, wherein said high frequency conversion unit has a rectifying mode and an inverting mode; in the rectification mode, the first switching tube and the third switching tube are used as a high-frequency conversion switching tube and a boosting tube, the two switching tubes form a composite functional bridge arm, the fifth switching tube is used as a boosting flywheel diode and also used as a reverse buck conduction connecting wire, and the first switching tube to the fifth switching tube are used for making different pulse conduction combinations according to drive control to realize boosting and high-frequency conversion; in the inversion mode, the first switching tube to the fourth switching tube play a role in high-frequency rectification, are equivalent to high-frequency rectification diodes, the fifth switching tube is used as a step-down switch, pulse conduction or direct connection can be realized, and the first switching tube and the third switching tube are matched with the fifth switching tube to be used as a follow current diode, so that direct-current reverse step-down output or direct connection output is realized; the third switching tube and the fourth switching tube are commonly used as a boost tube, and are conducted by pulse according to drive control, so that reverse boost energy storage output of direct current is realized.

5. The bidirectional-switchable direct-current isolation type converter of claim 1, wherein the freewheeling inductor is two freewheeling inductors in series connection or an equivalent inductor obtained by equivalent of the two freewheeling inductors, and the inductance value of the equivalent inductor is the sum of the inductance values of the two freewheeling inductors; when the free-wheeling inductors are two free-wheeling inductors in series connection, one end of the first free-wheeling inductor is connected with the positive output end of the first direct-current source, the other end of the first free-wheeling inductor is connected with the drain electrode of the first switching tube and the source electrode of the fifth switching tube, one end of the second free-wheeling inductor is connected with the negative output end of the first direct-current source, and the other end of the second free-wheeling inductor is connected with the source electrodes of the third switching tube and the fourth switching tube; when the free-wheeling inductor is an equivalent inductor obtained by equivalent of the two free-wheeling inductors, one end of the equivalent inductor is connected with one output end of the first direct current source, the other end of the equivalent inductor is connected with one input end of the high-frequency conversion unit, and the other input end of the high-frequency conversion unit is connected with the other output end of the first direct current source through a wire.

6. The bi-directionally switchable dc-isolated converter of claim 1, wherein the first to fifth switching transistors are high frequency switching transistors provided with anti-parallel diodes or equivalently high frequency switching transistors of the same function; the anti-parallel diode is an integrated diode, a parasitic diode or an external diode; the absorption filter capacitor is a high-frequency nonpolar capacitor or a high-frequency polar capacitor; when the absorption filter capacitor is a high-frequency capacitor with polarity, the positive electrode of the absorption filter capacitor is connected with the drain electrode of the fifth switching tube, and the negative electrode of the absorption filter capacitor is connected with the source electrode of the fourth switching tube.

7. A control method of a bi-directionally switchable dc-isolated converter, characterized in that it is used for controlling the bi-directionally switchable dc-isolated converter according to any one of claims 1 to 6, comprising the steps of:

s100: selecting a working mode, wherein the working mode comprises a rectification mode and an inversion mode, the rectification mode is input from a first direct current source end, the second direct current source end is output, the inversion mode is input from the second direct current source end, and the first direct current source end is output; in the rectification mode, the first switching tube and the third switching tube of the high-frequency conversion unit are used as a boost tube in addition to the high-frequency conversion switching tube, and the two switching tubes form a composite functional bridge arm;

s200: according to the selected working mode, no driving signal or only synchronous rectification driving signal is applied to a switching tube which works as a diode or a continuous tube in the direct current isolation type converter, a high-level direct-connection driving signal is applied to the switching tube which needs to work in a direct-connection mode, and no driving signal or a low-level non-conduction signal is applied to the switching tube which does not need to be conducted;

s300: if the working mode is a rectification mode, judging whether the direct current isolation type converter needs to be subjected to voltage reduction or voltage boosting according to the voltage instantaneous value voltage of the current first direct current source, output voltage setting and conversion ratio conversion to the primary side of the high frequency isolation and transformation unit, determining whether to carry out voltage boosting PWM driving control on a switching tube serving as a voltage boosting tube in a composite function bridge arm of the high frequency conversion unit, and determining whether to carry out PWM driving conduction on other switching tubes; if the voltage needs to be boosted, common driving is needed to be applied to the first switching tube and the third switching tube to form the boosted voltage, and the second switching tube and the fourth switching tube are matched with the corresponding bridge type pair tube to perform high-frequency conversion; if the voltage needs to be reduced or the voltage does not need to be increased, normal bridge driving is applied to the first switching tube to the fourth switching tube to perform high-frequency conversion; the fifth switching tube is closed when the direct current isolation type converter is boosted or closed in a non-conduction interval when the direct current isolation type converter is reduced, and is opened when the bridge type high-frequency conversion is conducted;

S400: if the working mode is an inversion mode, converting the voltage instantaneous value voltage and the transformation ratio of the current second direct current source to a first direct current source end, comparing the output voltage setting to judge whether the direct current isolated converter needs to be subjected to voltage reduction or voltage increase, and determining whether a fifth switching tube and a composite function bridge arm of the high frequency conversion unit are subjected to voltage reduction PWM driving control; meanwhile, determining whether PWM driving conduction is carried out on a switching tube of a high-frequency rectifying circuit in the rectifying filter unit; if the voltage needs to be boosted, common driving is needed to be applied to the third switching tube and the fourth switching tube to form boosting energy storage; if the voltage is required to be reduced or the voltage is not required to be increased, normal bridge driving is applied to a switching tube of a high-frequency rectifying circuit in the rectifying and filtering unit to carry out high-frequency conversion; the fifth switching tube is closed in a non-conduction interval of the freewheeling or the buck-rectifying of the composite functional bridge arm, and is conducted in a bridge rectifying conduction interval or a boost energy storage interval.

8. The method according to claim 7, wherein in steps S300 to S400, the step-up energy storage state of the dc-isolated converter is enabled by applying a driving signal for shorting the coil of the high-frequency isolation transformer to the switching tube of the high-frequency rectifying circuit in the rectifying filter unit in the rectifying mode; in the inversion mode, the direct-current isolated converter can realize the step-down regulation of the output voltage by applying driving signals with variable duty ratios to the high-frequency rectifying circuit and the fifth switching tube.

9. The method according to claim 7, wherein in steps S300 to S400, when the fifth switching transistor is in the PWM operating state, the PWM switching frequency of the fifth switching transistor is identical to or double the PWM switching frequency of the switching transistor of the high-frequency conversion unit or the high-frequency rectification circuit.

10. The method according to claim 7, wherein in steps S300 to S400, in the buck rectification mode, PWM driving signals are applied to the first switching tube and the fourth switching tube and the second switching tube and the third switching tube, respectively, which are symmetrical in center, in the high-frequency conversion unit, and PWM driving signals are applied to the fifth switching tube, which are integrated in correspondence with the conduction intervals of the diagonal tube formed by the first switching tube and the fourth switching tube; in the buck inversion mode, central symmetrical PWM driving signals are respectively applied to bridge type geminate transistors or single transistors which are inverted in the high-frequency rectification circuit, and comprehensive PWM driving signals corresponding to the conduction intervals of oblique geminate transistors formed by the first switching tube and the fourth switching tube are applied to a fifth switching tube in the high-frequency conversion unit.