JP5424307B2 - Isolated DC-DC converter - Google Patents

Description

  The present invention relates to an insulation type DC-DC converter that converts DC power from a DC power source such as a fuel cell or a solar cell into DC power having a boosted voltage.

  2. Description of the Related Art Conventionally, a power conversion device is known in which DC power of a DC power source such as a fuel cell or a solar cell is boosted by a converter unit and then converted into commercial AC power by an inverter unit.

  As an example of this, a DC-DC converter that converts DC power from a DC power source into voltage DC power boosted by switching of a switching element via a smoothing capacitor, and this DC power is converted to AC power by switching of the switching element. Examples include a grid-connected inverter having an inverter and DC voltage optimization means for controlling the inverter so as to minimize power conversion loss (for example, Patent Document 1).

JP 2006-101581 A

  By the way, in order to reduce the cost of a DC power supply, for example, a fuel cell can reduce the number of cells. Therefore, it is required to use a DC power supply of a DC power having a lower voltage and a larger current. In the case of voltage and large current, when the overlap between the voltage and current when the switching element is turned off and turned on is large, there is a problem that the switching loss increases and it is difficult to improve the power conversion efficiency of the converter unit.

  Generally, in a converter unit using a switching element, power conversion efficiency decreases when used at light load (dashed line in FIG. 6). However, when a power conversion device is used for home use, the power conversion efficiency is reduced. Since usage increases, there is also a demand for improvement in power conversion efficiency at this light load.

  In this case, as a method for reducing the switching loss, it is conceivable to shift to the burst oscillation in which the switching frequency of the switching element is intermittent at a predetermined interval at light load.

  However, in this case, there is a problem that the ripple current of the DC power supply increases due to burst oscillation of the switching element, which affects the life of the DC power supply such as a fuel cell.

  On the other hand, in the converter unit, there are problems that the life of the switching element and peripheral parts is shortened due to heat generation due to an excessive load of power, and the power conversion efficiency is lowered. For this reason, in order to reduce heat generation due to an excessive load of electric power in the converter unit, a plurality of converter units may be arranged in parallel so as to alleviate the electric power load to each converter unit.

  The present invention realizes high power conversion efficiency at light load even when the DC power supply is a low voltage and large current DC power, and can reduce the ripple current of the DC power supply and extend the life of the DC power supply. It aims at providing an insulation type DC-DC converter.

In order to achieve the above object, an insulated DC-DC converter according to the present invention includes a converter unit and control means, and boosts low-voltage and large-current DC power in a DC power supply by converter control by the control means. The converter unit converts a plurality of the converter units in parallel, and each converter unit includes a step-up transformer having a primary winding and a secondary winding for boost output, A full-bridge composite resonance circuit that performs resonance and voltage resonance, and a voltage doubler circuit that doubles the secondary voltage of the step-up transformer to the predetermined voltage.
In the full bridge composite resonance circuit, a voltage resonance capacitor is connected in parallel to each switching element coupled to the full bridge, and a current resonance capacitor is connected in series between the circuit and the primary winding of the step-up transformer. The control means performs control to shift the switching frequency of the full bridge composite resonance circuit to burst oscillation intermittently at a predetermined interval at light load, and for each converter unit, sets the phase of burst oscillation to each other. Control to shift is performed.

  According to this configuration, when the DC power source is a low voltage / large current DC power, the switching loss of the switching element is reduced by the current resonance and voltage resonance of the full bridge composite resonance circuit in the converter unit, Combined with the voltage doubler of the voltage doubler circuit, the power conversion efficiency of the converter unit is improved. Together, these can increase the power conversion efficiency of the converter unit and burst the full-bridge composite resonance circuit at light loads. Since the transition to oscillation is possible, it is possible to reduce useless power and further reduce the switching loss of the switching element at the time of light load, so that high power conversion efficiency can be realized even at light load. In addition, since the plurality of converter units arranged in parallel are controlled so that the phases of burst oscillation are shifted from each other, the ripple current of the DC power supply can be reduced and the life of the DC power supply can be extended.

  Preferably, two converter units are arranged in parallel, and the control unit performs control to shift the phase of burst oscillation by 180 ° for each converter unit. Therefore, the ripple current of the DC power supply can be further reduced.

It is a circuit block diagram which shows the power converter device provided with the insulation type DC-DC converter which concerns on one Embodiment of this invention. It is a characteristic view which shows the power conversion efficiency at the time of using a voltage doubler circuit for the converter part of the insulation type DC-DC converter of FIG. It is a characteristic view which shows the burst oscillation operation | movement of the converter part of FIG. (A)-(C) is a figure which shows the time chart which shows the ripple current in each burst oscillation timing. (A) to (C) are time charts showing input ripple currents at points A to C when the phase of burst oscillation is shifted by 180 °. It is a characteristic view which shows the power conversion efficiency of the converter part of FIG.

  FIG. 1 is a circuit configuration diagram of a power conversion device including an insulated DC-DC converter according to an embodiment of the present invention. In this power conversion device, for example, a DC power source 8 of a fuel cell converts DC power having a low voltage and a large current into AC power and is connected to an AC power system 1. The AC power system 1 and the DC power source 8 In addition to the above, the converter unit 3 that converts the low-voltage / high-current DC power into a predetermined voltage DC power that has been boosted, the inverter unit 2 that converts the boosted DC power from the converter 3 into predetermined AC power, and a device Control means 10 for controlling the whole is provided. The inverter section 2 has a full bridge configuration (not shown) of switching elements such as four transistors and diodes.

  The converter unit 3 includes a step-up transformer 6 having a primary winding Np and a secondary winding Ns for step-up output, a full-bridge composite resonance circuit 5 that performs current resonance and voltage resonance, and a secondary of the step-up transformer 6. It consists of an insulation type DC-DC converter provided with the voltage doubler circuit 7 which doubles a voltage to the said predetermined voltage. That is, the converter unit 3 has a configuration in which the full bridge composite resonance circuit 5 is disposed on the primary side of the step-up transformer 6 and the voltage doubler circuit 7 is disposed on the secondary side.

  A plurality of the converter units 3 are arranged in parallel, in this example, two A blocks 3A and B blocks 3B. Hereinafter, the converter units 3A and 3B may be collectively referred to simply as “converter 3”. By arranging a plurality of converter units 3 in parallel and reducing the power burden to each of the converter units 3A and 3B, the heat generation of the full bridge composite resonance circuit 5 can be reduced. In addition, the life of peripheral parts can be extended. Moreover, power conversion efficiency can be improved.

The full bridge composite resonance circuit 5 has switching elements Q (Q1 to Q4) in four switching units, respectively, and a smoothing capacitor Cs such as an electrolytic capacitor connected in parallel to both ends of the DC power supply 1, A series circuit of the switching elements Q1 and Q3 and a series circuit of the switching elements Q2 and Q4 are connected in parallel. A connection point between the switching elements Q1 and Q3 and a connection point between the switching elements Q2 and Q4 are connected to the step-up transformer 6. Is connected to the primary winding Np to form a full bridge. The voltage resonance capacitors Cv1 to Cv4 are connected in parallel to the four switching elements Q1 to Q4 coupled to the full bridge, respectively, and between the constituent circuit of the full bridge and the primary winding Np of the step-up transformer 6. A current resonance capacitor Ci is connected in series. As the switching element Q, for example, a MOS-FET, an NPN type transistor, or the like is used.

When the switching element Q is on, current resonance of the full bridge composite resonance circuit 5 is performed by a predetermined resonance frequency fo by the current resonance capacitor Ci, the leakage inductance Ll of the step-up transformer 6, and a voltage doubler capacitor Cd described later. Is called. The resonance frequency fo is expressed by the following equation when the leakage inductance value L1, the primary and secondary self-inductance values Lp and Ls, the capacitance value Ci of the current resonance capacitor, and the capacitance value Cd of the voltage doubler capacitor are expressed as follows.
fo = (1 / 2π) √ ((1 / CiLl) + (1 / CdLl) (Lp / Ls))
The voltage doubler capacitor Cd can be used for adjusting the resonance frequency by changing the set capacitance, and the resonance frequency of current resonance can be optimally adjusted.

  When the circuit 5 performs a flyback operation by adjusting the turns ratio of a primary winding Np and a secondary winding Ns, which will be described later, the current resonance is caused by the self-inductance (leakage inductance and excitation inductance of the step-up transformer 6). ) And the current resonance capacitor Ci.

  The voltage resonance of the full bridge composite resonance circuit 5 is caused by the voltage resonance capacitor Cv, the self-inductance (leakage inductance and excitation inductance) of the step-up transformer, and the current resonance capacitor Ci when the switching element Q is switched from on to off and from off to on. This is performed at a predetermined resonance frequency. Furthermore, the resonant frequency can be adjusted by setting the gap of the step-up transformer 6 to a predetermined length and changing its inductance.

  The full bridge composite resonance circuit 5 eliminates the overlap of voltage and current when the switching element Q is turned off and turned on by current resonance and voltage resonance even when the DC power supply 8 is DC power of low voltage and large current. In addition, although there is a slight switching loss when turned off, the switching loss is reduced in the entire circuit, and the power conversion efficiency of the converter unit 3 can be improved. In this example, the current resonance has a greater effect of reducing the switching loss than the voltage resonance.

  As described above, in the step-up transformer 6, the primary winding Np is connected to the full bridge composite resonance circuit 5, and the secondary winding Ns is connected to the voltage doubler circuit 7. Since the direct current power of the direct current power supply 8 is low voltage and large current, direct current of the best voltage can be obtained by boosting the step-up transformer 6 and voltage doubler of the voltage doubler circuit 7 to increase the inverter efficiency of the inverter unit 2 in the next stage. The turn ratio of the primary winding Np and the secondary winding Ns is set to a predetermined turn ratio so as to output electric power.

  The turn ratio of the primary winding Np and the secondary winding Ns of the step-up transformer 6 is set slightly higher than the predetermined turn ratio so that the core loss is reduced due to the current resonance. When the turn ratio is slightly increased with a low voltage / high current input, the secondary voltage tends to rise, but the control means 10 controls the output voltage to be a constant voltage. By narrowing the resonance interval between the current resonance capacitor and the current resonance capacitor, a so-called flyback operation interval in which power is stored in the step-up transformer 6 while the switching element Q is on is shortened. As a result of the experiment, the core loss of the step-up transformer 6 in the converter unit 3 can be further reduced by this operation. The resonance frequency can be adjusted by changing the setting of the turn ratio of the primary winding Np and the secondary winding Ns.

  The voltage doubler circuit 7 includes, for example, a voltage doubler capacitor Cd and a second diode D2 connected in series with one end of the secondary winding Ns of the step-up transformer 6, and between the voltage doubler capacitor Cd and the second diode D2. It has the 1st diode D1 connected in parallel with the both ends of the secondary winding Ns, and the direct current | flow output capacitor | condenser Co connected in parallel with the circuit output terminal. In one half cycle of the alternating voltage excited by the secondary winding Ns, the first diode D1 is turned on to charge the voltage doubler capacitor Cd, and in the other half cycle, the second diode D2 is turned on to turn on the voltage doubler. The capacitor Cd is discharged to generate a voltage charged in the voltage doubler capacitor Cd and an alternating voltage excited in the secondary winding Ns in the same direction and at the same voltage level. In this example, the voltage doubler circuit 7 doubles the secondary voltage of the step-up transformer 6, but it may be doubled or more. The voltage doubler capacitor Cd can be used as a capacitor for adjusting the resonance frequency, and the full bridge composite resonance circuit 5 performs current resonance including the voltage doubler capacitor Cd.

  FIG. 2 is a characteristic diagram comparing the power conversion efficiency of the converter unit 3 when the voltage doubler circuit 7 is connected to the secondary side of the step-up transformer 6 and when a full-wave rectifier circuit is connected. It is a thing. As shown in this figure, the power conversion efficiency of the converter unit 3 is higher when the voltage doubler circuit 7 is connected than when the full-wave rectifier circuit is connected to the secondary side of the step-up transformer 6. The combination of the step-up transformer 6 step-up and the voltage doubler circuit 7 step-up circuit allows the low-voltage and large-current direct-current power to be optimally boosted to direct-current power and has a slightly higher turn ratio. This is due to the fact that the flyback-like operation interval by setting is shortened and the core loss can be further reduced. Thus, the combination of the boosting of the step-up transformer 6 and the voltage doubler of the voltage doubler circuit 7 makes it possible to convert the DC power of low voltage and large current into the DC power of the voltage boosted optimally. Conversion efficiency can be improved. The same effect can be obtained even when the DC power source 8 is a relatively high voltage and high input solar cell, for example, a fuel cell.

  The control means 10 converts the DC power of the low-voltage / high-current DC power supply 8 into a predetermined voltage DC power boosted by converter control, and then converts the boosted DC power into predetermined AC power by inverter control. Convert. Based on the detection of the AC output of the inverter unit 2, the control unit 10 performs switching control of the switching element Q of the full bridge composite resonance circuit 5 and the switching element of the inverter unit 2 so that the value becomes a predetermined voltage value.

  The control means 10 includes a burst oscillation control unit 12 that performs control to shift to the burst oscillation in which the switching frequency of the full bridge composite resonance circuit 5 is intermittent at an arbitrary predetermined interval at light load in order to reduce the switching loss. The burst oscillation control unit 12 generates a trigger signal according to the input voltage and power range from the DC power of the DC power supply 8 and performs burst oscillation control that is arbitrarily intermittent. FIG. 3 shows a waveform of burst (intermittent) oscillation that repeats oscillation and stop for ID (drain current). In this example, burst oscillation repeats oscillation of a specified wave number based on the input of a trigger signal, but may be auto burst oscillation that repeats oscillation and stop at a specified wave number.

  By reducing the unnecessary turn-off and turn-on of each switching element Q by this burst oscillation, it is possible to reduce the switching loss, conduction loss, iron loss and copper loss in the transformer at the time of light load. . In general, in the converter, when the input voltage from the DC power source is relatively high within a low voltage range and the load is light, it is difficult to control the output voltage of the converter. The output voltage can be easily controlled by suppressing the increase in the output voltage.

  Furthermore, the control means 10 has an oscillation timing deviation control unit 14 that performs control for shifting the phase of burst oscillation from each other for each of the converter units 3A and 3B. In this example, two converter units 3 are arranged in parallel, and the oscillation timing shift control unit 14 performs control for shifting the phase of burst oscillation by 180 ° for each converter unit 3A, 3B.

  4A to 4C are time charts showing the ripple current of the DC power supply 8 at each burst oscillation timing of the A block and the B block of the converter unit 3. The phases of burst oscillation in the A block and the B block almost overlap in FIG. 4A, slightly overlap in FIG. 4B, and are shifted from each other by 180 ° in FIG. 4C. 5 (A) to 5 (C) show a case where the phase of the burst oscillation of FIG. 4 (C) is shifted by 180 ° between the DC power supply 8 of FIG. 1 and the smoothing capacitors Cs of the converter units 3A and 3B. It is a time chart which shows each input ripple current in A-C point. 5A is a point A on the + side of the DC power supply 8 in the A block, FIG. 5B is a point B on the + side of the DC power supply 8 in the B block, and FIG. 5C is a + side of the DC power supply 8. The input ripple current at the point C closest to the power source (left side of the points A and B in FIG. 1). As shown in FIGS. 5A and 5B, the input ripple currents at the points A and B are the same as the burst oscillation frequency and their phases are shifted from each other by 180 °.

  As shown in FIG. 5C, when viewed from the DC power supply 8 side, the input ripple current at the burst oscillation frequency at point A in FIG. 5A and the input ripple at the burst oscillation frequency at point B in FIG. The currents overlap each other, and the frequency of the input ripple current at the point C is apparently twice the burst oscillation frequency, and the burst oscillation intermittency is relaxed, and the ripple current of the DC power supply 8 is reduced. In FIG. 4A, the ripple current rate is large at 11.8%, for example, and when the phase of burst oscillation is shifted in FIG. 4B, the ripple current rate is as small as 7.5%. When the phase of burst oscillation is shifted by 180 °, the ripple current ratio becomes the smallest at 4.1%. As described above, the ripple current of the DC power supply 8 can be reduced by shifting the burst oscillation phases of the A block and the B block arranged in parallel in the two converter units 3 by 180 °. By reducing the ripple current, the burden on the DC power source 8 such as a fuel cell is reduced, and the life can be extended.

(Example)
The direct current power of the direct current power source 8 of the fuel cell in FIG. 1 is, for example, direct current power having a voltage of 15 V and a current of 60 A, which is a lower voltage and a larger current than the previous voltage of 35 V and a current of 30 A. The rated load per unit of the DC-DC converters (converter units) 3 arranged in parallel with each other is 400 W, for example, and the two units are 800 W. The switching element is driven by current resonance and voltage resonance of the full bridge composite resonance circuit 5. The switching loss of Q can be reduced, and the power conversion efficiency of the converter unit 3 can be improved by the combination of the boosting of the step-up transformer 6 and the voltage doubler of the voltage doubler circuit 7. Since the full bridge composite resonance circuit 5 is shifted to burst oscillation at light load, unnecessary switching loss, conduction loss, iron loss at the transformer, and copper loss at the switching element Q at, for example, 100 to 200 W when light load is applied. Can be reduced. In addition, since the converters 3A and 3B are controlled so as to shift the phase of the burst oscillation from each other, the ripple current of the DC power supply 8 can be reduced and the life of the DC power supply 8 can be extended. In the DC-DC converter 3, for example, direct-current power having a direct-current voltage of 380 V is supplied to the inverter unit 2, converted into alternating-current power by the inverter unit 2, and linked to the commercial alternating-current power system 1.

  FIG. 6 shows the power conversion efficiency per one converter unit 3. In contrast to the general power conversion efficiency characteristics (broken line) of the converter unit using the conventional switching element, as shown by the solid line, the power conversion efficiency of the converter unit 3 is the whole from light load to rated load. It has a characteristic that the power conversion efficiency further increases at a light load, that is, a characteristic that the power conversion efficiency shows a peak at a light load (100 to 200 W).

  In this case, as described above, the above characteristics can be obtained by the combination of the full-bridge composite resonance circuit 5 and the step-up transformer 6 and the voltage doubler circuit 7 included therein. By adjusting the long and voltage doubler capacitor Cd, it is possible to further reduce the core loss and obtain the optimum power conversion efficiency.

  Thus, the present invention reduces the switching loss of the switching element Q by the current resonance and voltage resonance of the full bridge composite resonance circuit 5 in the converter unit 3 when the DC power supply 8 is a low voltage / large current DC power, Since the power conversion efficiency of the converter unit 3 is improved by the combination of the boosting of the step-up transformer 6 and the voltage doubler of the voltage doubler circuit 7, these together can increase the power conversion efficiency of the converter unit 3 as a whole. Since the full bridge composite resonance circuit 5 is shifted to burst oscillation at light load, unnecessary power is saved and switching loss and conduction loss of the switching element Q at light load and iron loss and copper loss at the transformer are reduced. Therefore, even with low-voltage and large-current DC power, the power conversion efficiency of the converter unit 3 as a whole is high, and even during light loads It is possible to realize a conversion efficiency. Moreover, since the burst oscillation phases are controlled to be shifted from each other for each of the converter units 3A and 3B arranged in parallel, the ripple current of the DC power supply 8 can be reduced and the life of the DC power supply 8 can be extended. .

  In this embodiment, the control means 10 controls the inverter unit 2 and the converter unit 3 based on the detection of the AC output of the inverter unit 2, but the DC output of the converter unit 3 (DC voltage of the voltage doubler circuit 7). The converter unit 3 may be controlled based on the detection of the output.

  In this embodiment, two converter units 3 are arranged in parallel, but three or more converter units 3 may be arranged in parallel.

  In this embodiment, the DC power source 8 is a fuel cell, but it may be applied to a solar cell, wind power generation, or the like.

1: AC power system 2: Inverter unit 3 (3A, 3B): Converter unit (insulated DC-DC converter)
5: full bridge composite resonance circuit 6: step-up transformer 7: voltage doubler circuit 8: DC power supply 10: control means 12: burst oscillation control unit 14: oscillation timing shift control unit Ci: current resonance capacitors Cv1 to Cv4: voltage resonance capacitor Cs : Smoothing capacitor Cd: Double voltage capacitor Q (Q1 to Q4): Switching element

Claims (2)

An insulation type DC-DC converter comprising a converter unit and a control means, and converting the DC power of a low voltage and a large current in a DC power source into a predetermined voltage DC power boosted by converter control by the control means,
A plurality of the converter units are arranged in parallel, and each converter unit includes a step-up transformer having a primary winding and a secondary winding for boost output, and a full-bridge composite resonance circuit that performs current resonance and voltage resonance; A voltage doubler circuit that doubles the secondary voltage of the step-up transformer to the predetermined voltage,
In the full bridge composite resonance circuit, a voltage resonance capacitor is connected in parallel to each switching element coupled to the full bridge, and a current is connected in series between the full bridge component circuit and the primary winding of the step-up transformer. A resonant capacitor is connected,
The control means performs control to shift to a burst oscillation in which the switching frequency of the full-bridge composite resonance circuit is intermittent at a predetermined interval at light load,
For each converter unit, control to shift the phase of burst oscillation from each other,
Isolated DC-DC converter. In claim 1,
Two converter units are arranged in parallel,
The said control means is an insulation type DC-DC converter which performs control which mutually shifts the phase of burst oscillation 180 degrees for every converter part.

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