JP2012029436A - Current resonance type converter and control method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low cost current resonance type converter with a relatively simple circuit configuration.SOLUTION: A converter includes a main circuit 10 and a control circuit 20 for inputting a control signal S20 to the main circuit 10. The main circuit 10 converts a DC input voltage Vin into an AC voltage and converts the AC voltage into a DC output voltage Vo by making a switching element, which performs on/off operation by the control signal S20 having a switching frequency fs, perform a soft switching operation by use of an LC resonance operation. The control circuit 20 includes: output voltage detection means 31 for detecting the startup output voltage Vo and obtaining a detection result; frequency determining means 32 for determining the frequency fs for commencement of a soft start based on a characteristic of the output voltage of the converter vs. the switching frequency and the detection result; and frequency control means 22 for controlling a startup of the switching element by the control signal S20 that gradually decreases the frequency fs after commencing the soft start at the determined frequency fs.

Description

  The present invention relates to a current resonance type converter that performs a soft switching operation using the resonance operation of an inductor and a capacitor (this is also referred to as “LLC converter”), and a control method for the current resonance type converter that controls the soft start thereof. It is about.

  2. Description of the Related Art Conventionally, a current resonance type converter has a switching element that is turned on / off by a control signal having a switching frequency, for example, as described in Patent Document 1 below, and direct current (hereinafter referred to as “DC”) by the switching element. A switching circuit that converts an input voltage of AC to AC (hereinafter referred to as “AC”), a resonance inductor, and a resonance capacitor. The AC voltage output from the switching circuit is input and a predetermined voltage is input. A resonance circuit that resonates at a resonance frequency, a primary winding that inputs a resonance signal output from the resonance circuit, and a transformer having a secondary winding (hereinafter referred to as “transformer”), and the secondary winding. A rectifier circuit that rectifies the AC voltage output from the line into a DC voltage, and an output voltage obtained by smoothing the DC voltage output from the rectifier circuit from the output side. And an output capacitor to force.

  In such a current resonance converter control method, the output voltage is controlled by controlling the switching frequency of a control signal for turning on / off the switching element. The switching frequency is designed to operate at a frequency higher than the resonance frequency of the resonance circuit, for example. When the switching frequency is increased (that is, away from the resonance frequency), the output power is reduced, and the switching frequency is reduced. When it is lowered (ie, closer to the resonance frequency), the output power increases.

  This type of current resonance type converter uses a resonance phenomenon caused by a resonance circuit when switching the switching element, and uses a soft switching technology that performs switching in a state in which the voltage or current becomes zero, thereby achieving high efficiency of the power source without switching loss. And low noise can be achieved. In soft switching, switching performed in a state where the voltage is zero is generally referred to as zero voltage switching (hereinafter referred to as “ZVS”), and switching performed in a state where the current is zero is referred to as zero current switching (hereinafter referred to as “ZCS”). ing.

JP 2006-204048 A

  However, the conventional current resonance type converter and its control method have the following problems.

  The current resonance type converter performs frequency control of a control signal for controlling the on / off operation of the switching element, and the switching frequency is uniquely determined by the output voltage and the output current. For this reason, when starting from a state where the output voltage is 0 V, the switching frequency is gradually decreased from a high frequency (that is, soft start), thereby preventing an overcurrent due to the charge current of the output capacitor. . At this time, since a current for charging the output capacitor flows through the switching element that performs switching, ZVS can be performed using this current.

  However, when the voltage remains in the output capacitor, or a plurality of current resonant converters are connected in parallel on the output side, and the current resonant converter in the output stopped state is output by the current resonant converter in operation. When the current resonant converter is started from a high switching frequency, power cannot be transmitted to the output side due to the output voltage versus switching frequency characteristics of the current resonant converter. As a result, since no current flows through the switching element, the ZVS operation cannot be performed, and a surge voltage is applied to the switching element.

  In addition to reducing the switching loss of the switching element, the soft switching current resonance type converter has an advantage (merit) in that a switching element with a low breakdown voltage can be selected by suppressing a surge breakdown voltage of the switching element at the time of turn-off. . However, if the above phenomenon occurs, the switching element must have a high breakdown voltage. Therefore, the converter efficiency is increased due to an increase in the cost of the current resonance converter and deterioration of the characteristics of the switching element (such as on-resistance in the switching element). There is a problem of lowering.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a low-cost current resonant converter and a control method thereof, which can use a switching element with a low withstand voltage and have a relatively simple circuit configuration. To do.

  A control method for a current resonance type converter according to a first aspect of the present invention is a method in which a DC input voltage is input and an on / off operation is performed by a control signal having a switching frequency by using a resonance operation of an inductor and a capacitor. In a control method of a current resonance type converter that performs a soft switching operation on a switching element, converts the input voltage into an AC voltage, converts the AC voltage into a DC output voltage, and outputs the output voltage from the output side. It has a processing.

  That is, in the control method of the current resonance type converter according to the first aspect of the present invention, an output voltage detection process for detecting the output voltage at start-up and obtaining the detection result, and a predetermined output voltage at the switching frequency with respect to the output voltage. Based on the switching frequency characteristics and the detection result, a frequency determination process for determining the soft start start switching frequency capable of supplying power to the output side, and starting at the determined switching frequency. And a frequency control process for controlling the activation of the current resonance converter by turning on / off the switching element by the control signal for gradually decreasing the switching frequency.

  A current resonance converter according to a second aspect of the present invention has a switching element that is turned on / off by a control signal having a switching frequency, and converts the DC input voltage to AC by the switching element to output a first AC voltage. A switching circuit, a resonance inductor and a capacitor, a resonance circuit that inputs the first AC voltage, resonates at a predetermined resonance frequency, and outputs a resonance signal; and 1 that inputs the resonance signal A transformer having a secondary winding and a secondary winding insulated from the primary winding and a second AC voltage output from the secondary winding are converted to DC to generate an output voltage. A rectifier circuit that outputs the output voltage from an output side, and a control circuit that detects the output voltage and generates the control signal to turn on / off the switching element. In the current resonant converter having a predetermined output voltage versus switching frequency characteristics of the switching frequency for the output voltage, the control circuit has the following means.

  That is, the control circuit detects the output voltage at start-up and obtains the detection result, and based on the output voltage versus switching frequency characteristic and the detection result, the control circuit supplies power to the output side. The switching element is turned on / off by frequency determining means for determining the switching frequency at which soft-start can be supplied, and the control signal for gradually decreasing the switching frequency after starting activation at the determined switching frequency. Frequency control means for operating and controlling activation of the current resonance type converter.

  According to the control method of the current resonance type converter in the first aspect of the invention, the ZVS operation of the switching element can always be performed at the time of soft start by the output voltage detection process at startup, the frequency determination process, and the frequency control process. A switching element with a low breakdown voltage can be selected.

  According to the current resonance type converter in the second invention, for example, when the control circuit is configured to be executed by program control using a processor, it is not necessary to add a new circuit and Since it can be processed only by changes, it does not cost. Further, when the control circuit is configured by an actual circuit, the frequency control at the time of startup can be performed simply by adding a circuit configuration at the time of startup to the circuit configuration at the time of steady operation. Therefore, a low-cost control circuit can be realized with a relatively simple circuit configuration.

FIG. 1 is a configuration diagram showing an outline of a current resonance type converter in Embodiment 1 of the present invention. FIG. 2 is a circuit diagram showing a configuration example of the converter main circuit 10 in FIG. FIG. 3A is a diagram illustrating voltage and current waveforms in the FETs 12-1 to 12-4 in FIG. 3-2 is a diagram showing voltage and current waveforms on the primary winding 14a side in FIG. FIG. 3C is a diagram illustrating voltage and current waveforms on the secondary winding 14b side in FIG. FIG. 4 is a graph showing output voltage versus switching frequency characteristics in the current resonance type converter of FIG. FIG. 5 is a flowchart showing processing at the start-up in the control circuit 20 in FIG. FIG. 6 is a schematic circuit diagram showing a configuration example of the current resonance type converter according to the second embodiment of the present invention.

  Modes for carrying out the present invention will become apparent from the following description of the preferred embodiments when read in light of the accompanying drawings. However, the drawings are only for explanation and do not limit the scope of the present invention.

(Configuration of Current Resonant Type Converter of Example 1)
FIG. 1 is a configuration diagram illustrating an outline of a current resonance type converter according to a first embodiment of the present invention.

  This current resonance type converter is a DC / DC converter used in a switching power supply device with high efficiency and low noise, and inputs a DC input voltage Vin and a DC input current Iin supplied from a DC power source such as a solar cell. An input terminal 1 is provided. An output terminal 17 that outputs a DC output voltage Vo and a DC output current Io is connected to the input terminal 1 via the converter main circuit 10.

  The converter main circuit 10 includes a switching circuit having a plurality of switching elements (for example, field effect transistors, hereinafter referred to as “FETs”) that are turned on / off by a control signal S20 having a switching frequency fs, and a resonance composed of an inductor and a capacitor. The circuit includes a resonance circuit having a frequency fr, a transformer that insulates between input and output terminals, and a rectifier circuit that rectifies the output voltage of the transformer into a DC voltage. In this converter main circuit 10, a DC input voltage Vin and a DC input current Iin are input from the input terminal 1, and a soft switching operation is performed on an FET that is turned on / off by a control signal S 20 using the resonance operation of the resonance circuit. Thus, the DC input voltage Vin and the DC input current Iin are converted into AC, and the AC voltage is transmitted through a transformer. The AC voltage and the AC current are converted by the rectifier circuit or the like into the DC output voltage Vo and the DC output current Io. To output from the output terminal 17.

  A control circuit 20 for controlling the frequency of the converter main circuit 10 is connected to the output terminal 17 side. The control circuit 20 is a circuit that generates a control signal S20 on the basis of the DC output voltage Vo and the DC output current Io to turn on / off the FET in the converter main circuit 10, and outputs the steady state output state control means 21, In addition to having the frequency control means 22, a control means 30 at the time of activation is added. The startup control means 30 includes an output voltage detection means 31 and a soft start start frequency determination means 22.

  The constant output state control means 21 has a function of determining an optimum switching frequency fs for performing constant voltage control, constant current control, and the like based on the DC output voltage Io, the DC output current Io, and the like. On the side, the frequency control means 22 is connected. The frequency control means 22 has a function of performing on / off control of the FET in the converter main circuit 10 by a control signal S20 having the determined switching frequency fs.

  The output voltage detection means 31 constituting the start-up control means 30 has a function of detecting the DC output voltage Vo at start-up and obtaining the detection result. On the output side, the frequency determination means 32 for starting the soft start. Is connected. The soft-start start frequency determining means 32 determines the soft-start start switching frequency fs that can supply power to the output terminal 17 based on the output voltage versus switching frequency characteristics of the current resonance type converter and the detection result. The frequency control means 22 is connected to this output side. In addition to the above functions, the frequency control means 22 starts the operation at the switching frequency fs determined by the frequency determination means 32 and then turns the FET on / off by a control signal S20 that gradually decreases the switching frequency fs. And has a function of controlling the activation of the current resonance type converter.

  The control circuit 20 includes a processor such as a digital signal processor (hereinafter referred to as “DSP”) that is a microprocessor specialized for digital signal processing, or an actual circuit.

When the control circuit 20 is configured by a processor, for example, the following may be performed.
The processor is controlled by, for example, a program memory storing a control program at the time of normal operation and startup, a central processing unit (hereinafter referred to as “CPU”) that performs calculation and control processing according to the control program, and program processing of the CPU. An input unit and an output unit. Therefore, the function of the output voltage detection means 31 is executed by the input section, the function of the output state control means 21 in the steady state is executed by the program processing of the input section and the CPU, and the soft start start is executed by the program processing of the CPU. The functions of the frequency determination unit 32 and the frequency control unit 22 may be executed, and the output unit may execute the function of outputting the control signal S20.

FIG. 2 is a circuit diagram showing a configuration example of converter main circuit 10 in FIG.
The converter main circuit 10 includes a smoothing input capacitor 11 and a full bridge type switching circuit 12 connected to a positive side input terminal 1a and a ground GND side input terminal 1b constituting the input terminal 1. The full bridge type switching circuit 12 has four FETs 12-1 to 12-4 as switching elements, and an FET 12-1, a node N2, and an FET 12-2 are connected in series between the input terminal 1a and the input terminal 1b. The FET 12-3, the node N1, and the FET 12-4 are connected in series. A series resonant circuit 13 and a transformer 14 are connected to the nodes N1 and N2.

  The FETs 12-1 and 12-2 are complementarily turned on / off by the control signal S20, and the FETs 12-3 and 12-4 are complementarily turned on / off by the control signal S20. Parasitic capacitances 12-1a, 12-2a, 12-3a, 12-4a, etc. exist between the drains and sources of the FETs 12-1, 12-2, 12-3, 12-4, respectively.

  For example, when the FETs 12-1 and 12-4 are turned off and the FETs 12-2 and 12-3 are turned on by the control signal S20, the DC input current Iin input from the input terminal 1a is turned on. → Node N1 → Primary side of series resonance circuit 13 and transformer 14 → Node N2 → ON-state FET 12-2 → Input terminal 1b. At this time, a current Isw flows between the drains and sources of the FETs 12-2 and 12-3, and a drain-source voltage Vsw is generated in the FET 12-2.

  The series resonance circuit 13 connected to the node N1 includes a resonance capacitor 13a having a capacitance Cr and a resonance inductor 13b having an inductance Lr, which are connected in series. The series resonance circuit 13 has a specific resonance frequency fr determined by the capacitance Cr of the resonance capacitor 13a and the inductance Lr of the resonance inductor 13b. For example, a resonance current Ir that is a resonance signal flows through the capacitor 13a and the inductor 13b. Then, a resonance voltage Vr is generated between both end electrodes of the capacitor 13a and the inductor 13b. A transformer 14 is connected to the electrode side of one end of the inductor 13b.

  The transformer 14 has a primary winding 14a connected between an electrode at one end of the inductor 13b and the node N2, and a secondary winding 14b insulated from the primary winding 14a. . The turn ratio N between the primary winding 14a and the secondary winding 14b is N1: N2. In the primary winding 14a, an inductor 14c, which is an exciting inductance Lm of the transformer 14, exists in parallel with the primary winding 14a. For example, when the resonance current Ir flows through the capacitor 13a and the inductor 13b, the resonance current Ir is divided into an excitation current Im flowing through the inductor 14c and a primary current IT flowing through the primary winding 14a (that is, Ir = Im + IT). At this time, the excitation voltage Vm generated between the both end electrodes of the inductor 14c is equal to the primary voltage VT1 generated between the both end electrodes of the primary winding 14a. When the primary current IT flows through the primary winding 14a, a secondary voltage VT2, which is a second AC voltage, is generated between the both end electrodes of the secondary winding 14b. A rectifier circuit 15 is connected to both end electrodes of the secondary winding 14b.

  The rectifier circuit 15 is a circuit for full-wave rectifying the secondary voltage VT2 generated in the secondary winding 14b into a DC current ID, and is constituted by a diode bridge circuit including, for example, four diodes 15-1 to 15-4. ing. A positive output terminal 17a and a ground GND side output terminal 17b constituting an output terminal 17 are connected to the output side of the rectifier circuit 15 via a smoothing output capacitor 16. The output capacitor 16 has a capacitance Co. When the DC output current Io is output from the output terminal 17a, the DC output voltage Vo appears between the output terminals 17a and 17b.

(Control Method of Current Resonant Type Converter of Example 1)
As a control method of the current resonance type converter according to the first embodiment, (1) a control method of the switching frequency fs during steady state and (2) a control method of the switching frequency fs during start-up will be described.

(1) Control Method of Constant Switching Frequency fs In the constant operation, the control signal S20 having the switching frequency fs is generated by the steady output state control means 21 and the frequency control means 22 in the control circuit 20 of FIG. The FETs 12-1 to 12-4 in the switching circuit 12 of FIG. 2 are on / off controlled.

  For example, when the FETs 12-1 and 12-4 are turned off and the FETs 12-2 and 12-3 are turned on by the control signal S20, the on-state is turned on by the DC input current Iin input to the input terminal 1a in FIG. A current Isw flows between the drain and source of the FET 12-3. When the current Isw flows to the node N1, the resonance current Ir flows in the series resonance circuit 13. The resonance current Ir is shunted to the inductor 14c as the exciting current Im and is shunted to the primary winding 14a of the transformer 14 as the primary current IT. The divided excitation current Im and the primary current IT are merged and flow to the node N2 in the switching circuit 12. The current Isw flowing to the node N2 flows out to the input terminal 1b through the drain and source of the FET 12-2 in the on state.

  When the primary current IT flows through the primary winding 14a of the transformer 14, a primary voltage VT1 is generated between the electrodes at both ends of the primary winding 14a. Then, a secondary current (= IT * (1 / N) = IT * (N2 / N1)) is induced in the secondary winding 14b of the transformer 14, and a secondary voltage is generated between the electrodes at both ends of the secondary winding 14b. VT2 (= VT1 * (1 / N) = VT1 * (N2 / N1)) is generated. The secondary current is full-wave rectified by the rectifier circuit 15 and converted into a DC current ID. The DC current ID is smoothed by the output capacitor 16, and the smoothed DC output current Io and output voltage Vo are output terminals. 17a.

  Here, the voltage and current waveforms of each part in the converter main circuit 10 of FIG. 2 will be described.

  3A is a diagram illustrating voltage and current waveforms in the FETs 12-1 to 12-4 in FIG. 2, FIG. 3B is a diagram illustrating voltage and current waveforms on the primary winding 14a side in FIG. 3-3 is a diagram showing voltage and current waveforms on the secondary winding 14b side in FIG. 3 to 3, the horizontal axis represents time t, and the vertical axis represents the value of voltage V or current I.

  At time t1 in FIG. 3A, for example, the FETs 12-1 and 12-4 in the switching circuit 12 are turned off by the control signal S20 of the control circuit 20 with the DC input voltage Vin being input to the input terminals 1a and 1b. When the FETs 12-2 and 12-3 are turned on, the drain-source voltage Vsw of the FET 12-2 becomes 0V. When time t2 elapses from time t1 to time t3, for example, the FET 12-1 and 12-4 are turned on and the FETs 12-2 and 12-3 are turned off by the control signal S20, and the drain-source voltage of the FET 12-2 is turned on. Vsw rises to the input voltage Vin. A period Tsw between times t1 and t3 is a switching cycle of the FETs 12-1 to 12-4. Similarly to the above, when time t4 elapses after time t3 and time t5 is reached, the FETs 12-1 and 12-4 are turned off and the FETs 12-2 and 12-3 are turned on, and the drain and source of the FET 12-2 are turned on. The inter-voltage Vsw becomes 0V.

  At time t1, due to the parasitic capacitances 12-2a and 12-3a of the FETs 12-2 and 12-3, a negative current Isw flows between the drains and sources of the FETs 12-2 and 12-3, and immediately thereafter. The current Isw increases in the + direction. When the current Isw increases in the + direction, the maximum current value is reached at a certain point in time, and then a certain current value (time) due to the operation of the series resonant circuit 13, the transformer 14 and the rectifier circuit 15 having the resonant frequency fr. It decreases until t2).

During the period Tr between times t1 and t2, the diodes 15-1 to 15-4 of the rectifier circuit 15 are conductive,
Vin-Vr> (N1 / N2) * Vo
In this state, the output voltage Vo is clamped. A period from time t2 to time t3 (Tsw−Tr) is a period during which the resonance voltage Vr of the series resonance circuit 13 is increased using the excitation current Im (that is, a period during which the current Isw is increased). When the input voltage Vin is low, the switching frequency fs of the control signal S20 is reduced by the control of the output state control means 21 and the frequency control means 22 in the steady state in the control circuit 20 to switch the FETs 12-1 to 12-4. By increasing the period Tsw, the resonance voltage Vr can be boosted.

  In FIG. 3-2, the excitation voltage Vm of the inductor 14c and the primary voltage VT1 of the primary winding 14a rise to voltage (N / N2) * Vo at time t1, and fall at time t2. At time t3, the voltage drops to-(N / N2) * Vo. The excitation voltage Vm and the primary voltage VT1 rise at time t4, and rise to voltage (N / N2) * Vo at time t5.

The primary current IT flowing through the primary winding 14a increases from the current value 0A to the current value (N2 / N1) * Ip at time t1, and then decreases to the current value 0A at time t2. The current value Ip is the maximum value in the output current ID of the rectifier circuit 15.
IP = (π / 2) * (Tsw / Tr) * Io
However, dead time is excluded.

  Further, the primary current IT has a current value of 0A between time t2 and time t3, decreases to a current value− (N2 / N1) * Ip after time t3, and then increases to a current value of 0A. Maintain until time t5.

  The excitation current Im flowing through the inductor 14c has a minimum current value of 0 A or less before the time t1, and then increases and reaches a maximum value between time t2 and t3 (≈Vm * Tsw / Lm). / 2). The excitation current Im then decreases, and the current value becomes the minimum value of 0 A or less between times t4 and t5.

  The resonance current Ir flowing through the series resonance circuit 13 has a current waveform obtained by adding the primary current IT flowing through the primary winding 14a and the exciting current Im flowing through the inductor 14c.

  3-3, the secondary voltage VT2 of the secondary winding 14b is different only in the maximum value and the minimum value from the primary voltage VT1 of the primary winding 14a. That is, the secondary voltage VT2 increases to the output voltage Vo at time t1, decreases at time t2, and decreases to voltage -Vo at time t3. Further, the secondary voltage VT2 increases at time t4, and increases to the output voltage Vo at time t5.

  The output current ID of the rectifier circuit 15 increases from the current value 0A to the maximum value Ip at time t1, and then decreases to the current value 0A at time t2. The output current ID has a current value of 0A between times t2 and t3, and thereafter, from time t3 to t5, is the same as the current waveform from time t1 to t3. The output current ID is smoothed by the output capacitor 16, and a flat output current Io is output from the output terminal 17a.

  As described above, in the steady-state control method according to the first embodiment, for example, when the input voltage Vin decreases, the control is performed by the control of the steady-state output state control unit 21 and the frequency control unit 22 in the control circuit 20. By reducing the switching frequency fs of the signal S20 and increasing the switching period Tsw of the FETs 12-1 to 12-4, the resonance voltage Vr can be boosted and the output current Io can be held at a constant value.

  Further, the output state control means 21 in the steady state monitors the fluctuation of the output voltage Vo and the output current Io (or the resonance current Ir) based on the fluctuation of the load connected to the output terminals 17a and 17b. Based on the steady state output state control means 21 and frequency control means 22, the switching frequency fs of the control signal S20 is controlled to change the switching period Tsw of the FETs 12-1 to 12-4. This makes it possible to perform constant voltage control that outputs a constant output voltage Vo, constant current control that outputs a constant output current Io, and the like.

(2) Control Method of Switching Frequency fs at Start-up FIG. 4 is a diagram showing output voltage versus switching frequency characteristics in the current resonance type converter of FIG.

  In FIG. 4, the horizontal axis represents the output voltage Vo, and the vertical axis represents the switching frequency fs. A thick broken straight line 40 is a characteristic indicating the switching frequency f1 at the start of the conventional soft start. A thick solid curve 41 is a switching frequency characteristic capable of supplying power to the output side. A thin solid curve 42 is a characteristic showing the switching frequency f2 at the start of soft start in the first embodiment. Further, a thick solid line 43 is a characteristic indicating the soft-start start switching frequency f3 in the actual operation of the first embodiment.

  The current resonance type converter controls the switching frequency fs in the control signal S20, and the switching frequency fs is uniquely determined by the output voltage Vo and the output current Io. Therefore, in the conventional current resonance type converter, when the output voltage Vo is started from a state of 0 V, the switching frequency fs is gradually decreased from a high frequency f1 (for example, 300 kHz) (ie, soft start). The overcurrent due to the charging current of the output capacitor 16 is prevented. At this time, since a current for charging the output capacitor 16 flows through the FETs 12-1 to 12-4 that perform switching, the ZVS operation can be performed using this current.

  However, when the voltage remains in the output capacitor 16 or when the output voltage Vo is applied by parallel connection on the output side in a plurality of current resonance converters, a high frequency (for example, 300 kHz) at the switching frequency f1. When the current resonance type converter is started from the above, power cannot be transmitted to the output side due to the output voltage versus switching frequency characteristic of the current resonance type converter. As a result, since no current flows through the FETs 12-1 to 12-4, the ZVS operation cannot be performed, and a surge voltage is applied to the FETs 12-1 to 12-4.

  The reason why the FETs 12-1 to 12-4 cannot perform the ZVS operation when the output voltage Vo is applied at the time of starting the current resonance type converter is that the output voltage vs. switching frequency characteristics of the resonance type converter are on the output side. This is because starting is started from the switching frequency f1 where power cannot be transmitted.

  Therefore, in the control method of the first embodiment, the output voltage Vo at the time of startup is detected by the output voltage detection means 31 in FIG. 1, and the current resonance is determined based on the detection result by the frequency determination means 32 at the start of soft start. Referring to the output voltage vs. switching frequency characteristics of the type converter (that is, the switching frequency characteristics 41 capable of supplying power to the output side shown in FIG. 4), start-up is started from the frequency f2 at which power can be transmitted to the output side. By causing the current Isw to flow through the FETs 12-1 to 12-4, the FETs 12-1 to 12-4 can always perform the ZVS operation.

  Here, since the output voltage vs. switching frequency characteristic 41 of the current resonance type converter also depends on the input voltage Vin, it is possible to perform a better ZVS operation by detecting the input voltage Vin. However, since the input voltage Vin fluctuates due to external factors, when the input voltage Vin changes instantaneously after startup, the switching frequency f2 at which startup starts is lower than the frequency at which power can be supplied to the output side. Since the power is transmitted to the output side more than necessary, the soft start operation cannot be performed and the overcurrent cannot be prevented. Therefore, in actual operation, the switching frequency f3 of the output voltage versus switching frequency characteristic 43 when the input voltage Vin is the highest (the input voltage Vin having the highest switching frequency f2) is used.

  Note that the output voltage vs. switching frequency characteristic 41 of the current resonance type converter is very difficult to calculate, and in particular, the no-load frequency characteristic (frequency characteristic at which power can be supplied to the output side) is influenced by the parasitic inductance of the winding. However, it is difficult to calculate accurately. For this reason, the characteristic 43 of the switching frequency f3 at the start of soft start with a certain margin from a value actually measured by an actual apparatus is used.

  The output voltage versus switching frequency characteristic 43 shown in FIG. 4 connects, for example, the switching frequency f3 (= 300 kHz) when the output voltage Vo is 0 V and the switching frequency f3 (= 200 kHz) when the output voltage Vo is 380 V. It is represented by a straight line.

  The characteristic equation of output voltage versus switching frequency when there is no load (when power cannot be supplied to the output side) in the current resonance type converter can be expressed by the following equation (1), for example.

Where Vin: Input voltage
Vo: Output voltage
Lm: excitation inductance (μH), resonance inductance (μH)
Cr: Resonance capacitance (μF)
N: Turn ratio of transformer (N1 / N2)
fs: switching frequency (Hz)

Hereinafter, a control method at the time of activation will be described with reference to FIG.
FIG. 5 is a flowchart showing processing at the time of startup in the control circuit 20 in FIG.

  In FIG. 5, when the control means 30 at the time of activation shown in FIG. 1 starts processing, in step ST1, the output voltage detection means 31 determines whether or not the current resonance type converter is activated, and is not activated. If it is determined (No), the process is terminated, and if it is determined that it is activated (Yes), the process proceeds to the staple ST2 of the output voltage detection process. In step ST2, the output voltage detection means 31 detects the output voltage Vo, sends this detection result to the frequency determination means 32 for starting the soft start, and proceeds to step ST3 of the frequency determination process.

In step ST3, the soft-start start frequency determining means 32 refers to the characteristic 43 of the soft-start start switching frequency f3 shown in FIG. The process proceeds to step ST4 of the frequency control process. In step ST4, the frequency control means 22 performs a soft start process based on the calculation result, starts starting at the calculated switching frequency fs, and then outputs a control signal S20 that gradually decreases the switching frequency fs. Thus, the on / off operation of the FETs 12-1 to 12-4 is controlled. Thereby, a soft start is performed at the time of start-up, and the process ends.
Thereafter, steady state frequency control is performed by the steady state output state control means 21.

(Effect of Example 1)
According to the first embodiment, there are the following effects (a) and (b).

  (A) Since the ZVS operation of the FETs 12-1 to 12-4 can always be performed at the time of soft start by the control means 30 and the frequency control means 22 at the time of starting, the FETs 12-1 to 12-4 having a low withstand voltage are selected. it can.

  (B) When the control circuit 20 is constituted by a processor, it is not necessary to add a new circuit, and the processing can be performed only by changing the control program for executing the functions of the control means 30 and the frequency control means 22 at the time of activation. Therefore, it does not cost.

(Configuration of Current Resonant Type Converter of Example 2)
FIG. 6 is a schematic circuit diagram showing a configuration example of the current resonance type converter according to the second embodiment of the present invention. Elements common to those in FIG. 1 showing the first embodiment are denoted by common reference numerals. Yes.

  In the current resonance type converter according to the second embodiment, the control circuit 20 in FIG. 1 is configured by an actual circuit. The control circuit 20 in FIG. 1 has a function of performing constant voltage control for making the output voltage Vo constant, constant current control for making the output current Io constant, etc. In the second embodiment, for example, A configuration example in the case of performing constant voltage control will be described.

  In the control circuit 20 of the second embodiment, the steady state output state control means 21 in FIG. 1 includes a reference voltage source 21 a that generates a reference voltage Vth, an operational amplifier (hereinafter referred to as “op-amp”) 21 b, and this. The limiter circuit 21c is connected to the output side of the operational amplifier 21b.

  The operational amplifier 21b is a circuit that compares the output voltage Vo output from the output terminals 17a and 17b with the reference voltage Vth and outputs the comparison result to the limiter circuit 21c. The limiter circuit 21c has a maximum limiter value LVmax and a minimum limiter value LVmin. When the input comparison result is within the set range (= LVmax to LVmin range), an output signal corresponding to the comparison result (for example, proportional) When the input comparison result exceeds the set range, the output signal is output with the excess being suppressed within the set value. The frequency control means 22 in FIG. 1 is connected to the output side of the limiter circuit 21c.

  The frequency control means 22 in FIG. 1 includes a frequency control integrated circuit (hereinafter referred to as “frequency control IC”) 22b having a frequency control terminal 22a for inputting an output signal of the limiter circuit 21c, and an output side of the frequency control IC 22b. The drive circuit 22c is connected.

  The frequency control IC 22b is a circuit that generates a signal having a switching frequency fr corresponding to the output signal of the limiter circuit 21c input from the frequency control terminal 22a, and outputs this signal to the drive circuit 22c. The drive circuit 22c is a circuit that drives the output signal of the frequency control IC 22b to generate a control signal S20, and turns on / off the FETs 12-1 to 12-4 in the converter main circuit 10 by the control signal S20.

  The start-up control means 30 in FIG. 1 includes two voltage dividing resistors 31a and 31b constituting the output voltage detection means 31 and an operational amplifier 32a constituting the frequency determination means 32 for starting the soft start.

The two voltage dividing resistors 31a and 31b are connected in series between the output terminal 17a and the ground GND, and are detected in proportion to the output voltage Vo from a node N3 that is a connection point between the two voltage dividing resistors 31a and 31b. The voltage is output to the operational amplifier 32a. The operational amplifier 32a compares the input detection voltage with a reference voltage (for example, 0V of the ground GND), and based on the detection result, the maximum limiter value LVmax and the minimum limiter value LVmin of the limiter circuit 21c are set as follows, for example: The circuit to be adjusted.
In FIG. 4, when the output voltage Vo is 0V:
Adjusting the maximum limiter value LVmax to 300 kHz When the output voltage Vo is 380 V in FIG.
Adjust the minimum limiter value LVmin to 200kHz

(Control Method of Current Resonant Type Converter of Example 2)
As a control method of the current resonance type converter according to the second embodiment, (1) a control method of the switching frequency fs during steady state and (2) a control method of the switching frequency fs during startup will be described.

(1) Control Method of Constant Switching Frequency fs In the constant operation, the operational amplifier 21b compares the output voltage Vo output from the output terminals 17a and 17b with the reference voltage Vth, and the comparison result is sent to the limiter circuit 21c. Is output. When the input comparison result is within the set range (= LVmax to LVmin), the limiter circuit 21c outputs an output signal proportional to the comparison result to the frequency control terminal 22a, and the input comparison result has the set range. When it exceeds, an output signal in which the excess is suppressed within the set value is output to the frequency control terminal 22a.

  In the frequency control IC 22b, the output signal of the limiter circuit 21c is input from the frequency control terminal 22a, and a signal having a switching frequency fr corresponding to the output signal is generated. This signal is driven by the drive circuit 22c to generate a control signal s20 having a desired switching frequency fr. The control signal S20 causes the FETs 12-1 to 12-4 in the converter main circuit 10 to be turned on / off. To do. As a result, as in the first embodiment, a constant output voltage Vo can be output even when there is a change in the load connected to the output terminals 17a and 17b.

(2) Control method of switching frequency fs at startup At startup, the output voltage Vo output from the output terminal 17a is divided by the voltage dividing resistors 31a and 31b, and a detection voltage proportional to the output voltage Vo is obtained. , And output from the node N3 to the operational amplifier 32a. The operational amplifier 32a compares the input detection voltage with the reference voltage (= 0V), and adjusts the maximum limiter value LVmax and the minimum limiter value LVmin of the limiter circuit 21c as follows based on the detection result. For example, when the output voltage Vo is 0V, the maximum limiter value LVmax is adjusted to 300 kHz, and when the output voltage Vo is 380V, the minimum limiter value LVmin is adjusted to 200 kHz.

  The limiter circuit 21c determines the switching frequency fs at the start of activation in accordance with the characteristic 43 of the switching frequency f3 at the start of soft start shown in FIG. 4, and sends this determination result to the frequency control IC 22b via the frequency control terminal 22a. The frequency control IC 22b performs a soft start process, starts activation at the determined switching frequency fs, and then outputs an output signal in which the switching frequency fs is gradually reduced. This output signal is driven by the drive circuit 22c to generate a control signal S20, and the FETs 12-1 to 12-4 in the converter main circuit 10 are turned on / off by the control signal S20. Thereby, soft start is performed at the time of starting. Thereafter, the operational frequency is controlled by the operational amplifier 21b, the limiter circuit 21c, the frequency control IC 22b, and the drive circuit 22c.

  In the second embodiment, the limiter circuit 21c determines the switching frequency fs at the start of activation. However, the switching frequency fr may be determined by the frequency control IC 22b.

(Effect of Example 2)
According to the second embodiment, there are the same effects as the effects (a) of the first embodiment, and there are also the following effects (c).

  (C) A new voltage dividing resistor 31a, 31b and operational amplifier 32a for configuring the control means 30 at the start-up are newly added to the circuit constituting the output state control means 21 at the constant time. Frequency control becomes possible. Therefore, the low-cost control circuit 20 can be realized with a relatively simple circuit configuration.

(Modification)
The present invention is not limited to the first and second embodiments, and various usage forms and modifications are possible. For example, there are the following forms (i) and (ii) as usage forms and modifications.

  (I) In the second embodiment, the configuration example in the case where the constant voltage control is performed has been described. However, the constant current control or the like can be performed by changing the circuit configuration.

  (Ii) The configuration using the processor of the first embodiment for realizing the control circuit 20 in FIG. 1 or the configuration using the actual circuit of the second embodiment may be changed to a configuration other than that illustrated. For example, the FETs 12-1 to 12-4 constituting the switching circuit 12 may be composed of switching elements such as other transistors.

10 Converter main circuit 12 Switching circuit 12-1 to 12-4 FET
DESCRIPTION OF SYMBOLS 13 Series resonance circuit 14 Transformer 15 Rectification circuit 20 Control circuit 21 Constant output state control means 22 Frequency control means 30 Control means at the time of start 31 Output voltage detection means 32 Frequency determination means of soft start start

Claims (4)

A DC input voltage is input, and a soft switching operation is performed on a switching element that is turned on / off by a control signal having a switching frequency by using a resonance operation of an inductor and a capacitor, and the input voltage is converted into an AC voltage. In the control method of the current resonance type converter that converts the AC voltage into a DC output voltage and outputs it from the output side,
An output voltage detection process for detecting the output voltage at startup and obtaining the detection result;
Based on a predetermined output voltage versus switching frequency characteristic at the switching frequency with respect to the output voltage and the detection result, a frequency determination process for determining the switching frequency at the start of soft start that can supply power to the output side;
A frequency control process for controlling the activation of the current resonance type converter by turning on / off the switching element by the control signal for gradually decreasing the switching frequency after starting the activation at the determined switching frequency;
A method for controlling a current resonance type converter. A switching circuit that has a switching element that is turned on / off by a control signal having a switching frequency, and that converts a DC input voltage into AC by the switching element and outputs a first AC voltage;
A resonance circuit having a resonance inductor and a capacitor, inputting the first AC voltage, resonating at a predetermined resonance frequency, and outputting a resonance signal;
A transformer having a primary winding for inputting the resonance signal, and a secondary winding insulated from the primary winding;
A rectifier circuit that converts the second alternating voltage output from the secondary winding into a direct current to generate an output voltage, and outputs the output voltage from the output side;
A control circuit that detects the output voltage and generates the control signal to turn on / off the switching element;
In a current resonant converter having a predetermined output voltage versus switching frequency characteristic at the switching frequency with respect to the output voltage,
The control circuit includes:
Output voltage detection means for detecting the output voltage at the time of startup and obtaining the detection result;
Based on the output voltage vs. switching frequency characteristics and the detection result, a frequency determining means for determining the switching frequency at the start of soft start capable of supplying power to the output side;
Frequency control means for controlling the activation of the current resonant converter by turning on / off the switching element by the control signal for gradually decreasing the switching frequency after starting the activation at the determined switching frequency;
A current resonance type converter. The output voltage detection means, the frequency determination means, and the frequency control means are:
3. The current resonance type converter according to claim 2, wherein the current resonance type converter is configured to be executed by the control program of a processor that performs predetermined calculation and control according to the control program. The output voltage detection means, the frequency determination means, and the frequency control means are:
3. The current resonance type converter according to claim 2, wherein the current resonance type converter is constituted by an actual circuit.

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