JP4393207B2 - DC-DC converter - Google Patents

Description

  The present invention relates to a DC-DC converter used in a grid interconnection system.

  Conventionally, several kW of relatively small-capacity distributed power supply equipment equipped with direct current power supplies such as solar cells and fuel cells is connected (connected) to the commercial power system via an inverter and supplied to loads such as home appliances. A grid interconnection system is used. In this grid interconnection system, a push-pull DC-DC converter is used as a circuit for boosting a voltage supplied from a DC power source and supplying the boosted DC voltage to an inverter.

  FIG. 5 shows a push-pull type voltage type DC-DC converter that has been conventionally used. The voltage from the DC power source 1 (for example, a fuel cell) is converted to AC by turning on and off the two transistors Q1 and Q2, and then the AC is boosted via the transformer 2 to the rectifying and smoothing circuit 3 The output is further converted into direct current by the rectifying and smoothing circuit 3, and a direct current voltage is output to the pair of output terminals 4 and 4.

  In the voltage type DC-DC converter as described above, when the polarity of the voltage in the secondary winding 22 of the transformer 2 is switched, the polarity of each of the rectifying diodes D1 to D4 constituting the rectifying and smoothing circuit 3 is opposite to the polarity. Current (referred to as recovery current) flows, and the energy accumulated in the floating inductance (not shown) of the transformer 2 becomes an excessive surge voltage at the time of reverse recovery of each of the rectifier diodes D1 to D4. It is applied to each of D4, resulting in a problem that efficiency is reduced due to energy loss. Further, the rectifier diodes D1 to D4 must have a withstand voltage that is about twice as large as the surge voltage, resulting in high manufacturing costs. Further, when the withstand voltages of the rectifier diodes D1 to D4 are increased, the forward voltage is increased and the energy loss is increased. Furthermore, a snubber circuit for absorbing the surge voltage is required, and energy loss in the snubber circuit is also increased.

  Conventionally, a push-pull current type DC-DC converter as shown in FIG. 6 has been used. This current type DC-DC converter basically operates in the same manner as the voltage type DC-DC converter, and an inductance element is provided between the positive side of the DC power source 1 and the intermediate tap 23 of the primary winding 21 of the transformer 2. L is inserted. When the DC-DC converter is activated, that is, when the DC voltage output from the output terminals 4 and 4 is minimum, the switching waveforms of the transistors Q1 and Q2 are as shown in FIG. 7A, and both the transistors Q1 and Q2 are connected. There is no period for turning on or off, and the transistors Q1 and Q2 are alternately turned on and off at an on-duty of 50%. Then, as shown in FIG. 7B, the DC voltage output from the output terminals 4 and 4 increases as the ON period Ton in which both the transistors Q1 and Q2 are ON increases. In this way, the transistors Q1 and Q2 are on / off controlled to increase the DC voltage until a desired voltage value is obtained.

At this time, energy is stored in the inductance element L during the on-period Ton when both the transistors Q1 and Q2 are on, and energy stored in the inductance element L is released when either one of the transistors Q1 and Q2 is turned off. As described above, the transistors Q1 and Q2 are controlled to be turned on / off.
JP 2001-112253 A (paragraphs 0003 and 0004)

  However, in the current type DC-DC converter described above, when starting, that is, when the voltage value of the DC voltage from the output terminal is minimum, the switching waveforms of the transistors Q1 and Q2 are as shown in FIG. The voltage value of the voltage does not become zero but becomes a voltage value having a certain value determined by the turn ratio of the transformer 2 and the on-duty of the transistors Q1 and Q2. For this reason, at the time of start-up, an excessive inrush current flows through the electrolytic capacitor C constituting the rectifying / smoothing circuit 3, and an excessive current flows through the two transistors Q1 and Q2, thereby reducing the efficiency due to energy loss. The problem of end up occurs. Therefore, a current limiting circuit for reducing the inrush current is required, which increases the manufacturing cost and further complicates the circuit. Therefore, if the on-duty of the transistors Q1 and Q2 is gradually increased from the 0% state (see FIG. 7C), the voltage value of the DC voltage at the time of starting becomes zero, and at the time of starting Although the problem of the inrush current flowing through the electrolytic capacitor C of the rectifying and smoothing circuit 3 is solved, when one of the two transistors Q1 and Q2 is turned on after the start-up, both the two transistors Q1 and Q2 are turned off. In addition, since the energy stored in the inductance element L is not released, an excessive voltage corresponding to this energy is applied between the collectors and emitters of the transistors Q1 and Q2, and the transistors Q1 and Q2 may be damaged. There is.

  The present invention is intended to solve such problems of the prior art, and an object of the present invention is to provide a DC-DC converter that achieves high efficiency.

This onset Ming, between a DC power supply, a transformer having an intermediate tap and comprising a primary winding and a secondary winding the primary winding, the positive electrode side of the DC power supply and the intermediate tap of the primary winding An inductance element connected to each other, first and second switching elements connected between both ends of the primary winding and the negative electrode side of the DC power supply, and a rectifying / smoothing connected to the secondary winding of the transformer And a pair of output terminals provided in the rectifying / smoothing circuit, and on-off control of the first and second switching elements to output a DC voltage higher than the voltage of the DC power supply to the output terminal. In a DC-DC converter used in a grid interconnection system in which this DC voltage is converted into a commercial AC voltage via an inverter, a diode and a third switching element are directly connected to both ends of the inductance element. Energy release means connected, and control means for sending a control signal to the energy release means, and the first and second switching elements are turned on from either one of the first and second switching elements. When both elements are turned off, the control means detects an off period in which both the first and second switching elements are off, and sends the control signal to the energy emitting means to send only the off period. By maintaining the third switching element in the ON state, the energy stored in the inductance is discharged by the energy release means.

  According to the DC-DC converter of the present invention, since the voltage value of the DC voltage from the output terminal can be increased from zero at the start-up, an excessive current does not flow through the first and second switching elements at the start-up, The problem of reduced efficiency is eliminated. Furthermore, after starting the DC-DC converter, when either the first switching element or the second switching element is turned off from the state where either the first switching element or the second switching element is turned on, it is accumulated in the inductance element. Since energy can be released, the first and second switching elements are not damaged, and a highly efficient DC-DC converter can be provided.

First, an embodiment of the DC-DC converter serving as the onset bright D C-DC converter premise (hereinafter referred to as "first embodiment") will be described with reference to FIGS.

A DC-DC converter which is a premise of the present invention is used in a grid interconnection system, and includes a DC power source 1, a primary winding 21 and a secondary winding 22, and a transformer having an intermediate tap 23 on the primary winding 21 side. 2, an inductance element L composed of a winding L <b> 1 connected between the positive side of the DC power source 1 and the intermediate tap 23 of the primary winding 21, both ends of the primary winding 21, and the negative side of the DC power source 1. A first switching element Q1 and a second switching element Q2 connected in between, a rectifying / smoothing circuit 3 connected to the secondary winding 22 of the transformer 2, and a pair of output terminals 4 provided in the rectifying / smoothing circuit 3, 4, a series circuit 5 including a diode L and a sub-winding L 2 magnetically coupled to the winding L 1 connected between the output terminals 4 and 4.

  First, each component of the DC-DC converter will be described. The DC power source 1 is composed of a fuel cell and outputs about 37V DC. Both the first switching element Q1 and the second switching element Q2 are formed of transistors, and each form a closed loop together with the DC power supply 1, the inductance element L, and the primary winding 21 of the transformer 2. The first and second switching elements Q1 and Q2 are on / off controlled by the control circuit 8, respectively. Here, the control circuit 8 is configured to send an on signal or an off signal to the bases of the first and second switching elements Q1 and Q2, respectively, and to control the duty (on duty) of both the switching elements Q1 and Q2. It is.

  The inductance element L is composed of a choke coil including a winding L1 and a sub winding L2, and the winding L1 and the sub winding L2 are magnetically coupled via an iron core. The rectifying / smoothing circuit 3 includes a diode bridge 31 composed of four rectifying diodes D1 to D4 and a smoothing electrolytic capacitor C. The rectifying / smoothing circuit 3 rectifies and smoothes the AC voltage from the transformer 2 to DC and outputs the output terminal 4. 4 to output DC voltage.

  The series circuit 5 is connected between the output terminals 4 and 4 and is configured by connecting a sub winding L2 of the inductance element L and a diode D in series. At this time, the cathode side of the diode D is connected to the positive side of the output terminals 4 and 4.

  Next, the operation of the DC-DC converter configured as described above will be described with reference to FIGS. The DC 37V output from the DC power source 1 is converted to AC by turning on and off the first and second switching elements Q1 and Q2, respectively, and this AC is output to the rectifying and smoothing circuit 3 via the transformer 2. After being converted to direct current by the diode bridge 31 in the rectifying and smoothing circuit 3, the direct current is smoothed by the smoothing capacitor C and a direct current voltage is output to the output terminals 4 and 4. This DC voltage (DC about 350 V at the maximum) is output to an inverter (not shown) connected to the output terminals 4 and 4 to be converted into a commercial AC voltage, and connected to a commercial power system (not shown). (Connecting.

  When starting the DC-DC converter, the on-duty of the switching waveforms of the first and second switching elements Q1, Q2 is gradually increased from 0% (see FIG. 2 (a)). At this time, energy is stored in the winding L1 of the inductance element L in a state where either one of the first and second switching elements Q1 and Q2 is on. When both the switching elements Q1 and Q2 are turned off, the energy accumulated in the winding L1 is transferred from the output terminals 4 and 4 through the sub-winding L2 and the diode D of the series circuit 5 to the inverter ( (Not shown).

  Then, the switching elements Q1 and Q2 are alternately turned on and off so that the on-duty of each of the switching elements Q1 and Q2 is 50% (see FIG. 2B). In order to provide an on period Ton in which both Q2 are on (see FIG. 2C), the on / off operation of both switching elements Q1 and Q2 is controlled. Energy is stored in the inductance element L when both the switching elements Q1 and Q2 are on, and when the first switching element Q1 is on and the second switching element Q2 is off (or the first switching element Q1 is off and the second switching element is off) The energy stored in the inductance element L and the energy from the DC power source 1 are superimposed and discharged to the inverter connected to the output terminals 4 and 4 when the element Q2 is on.

  At this time, the magnitude of the DC voltage generated at the output terminals 4 and 4 increases as the ON period Ton during which both the switching elements Q1 and Q2 are both turned on becomes longer. That is, by varying the on period Ton by the control circuit 8, the magnitude of the DC voltage generated at the output terminals 4 and 4 can be controlled.

  With the above-described configuration, when the DC-DC converter is started, when either one of the switching elements Q1 and Q2 is turned on and when both the switching elements Q1 and Q2 are turned off, the inductance element L Energy can be discharged to the inverter via the series circuit 5, so that an excessive voltage is not applied to both switching elements Q1 and Q2, respectively, and a reduction in efficiency can be prevented. A highly efficient DC-DC converter can be realized.

Next, the DC-DC converter of the real施例according to the present invention (hereinafter referred to as "the second embodiment") will be described with reference to FIGS. In FIG. 3, the same or corresponding parts as those shown in FIG.

  In the DC-DC converter according to the second embodiment of the present invention, the inductance element L includes only the winding L1 (that is, there is no sub-winding), the diode D ′, the resistance R, And a third switching element Q3 made of a transistor and connected in series, and a control means 7 for sending a control signal to the energy release means 6. The operation of controlling the DC voltage generated at the output terminals 4 and 4 by controlling the first and second switching elements Q1 and Q2 on and off is the same as that of the first embodiment. At the time of starting the DC-DC converter, the on-duty of both switching elements Q1 and Q2 is gradually increased from 0% as in the first embodiment. At this time, the control means 7 detects an off period Toff in which both the first and second switching elements Q1 and Q2 are off, sets the off period Toff to a high level, and controls the first and second switching elements Q1 and Q2. A control signal is output to the base of the third switching element Q3 to make the period during which either one is on low (see FIG. 4).

  The control means 7 turns on the third switching element Q3 when the control signal is at a high level, and turns it off when the control signal is at a low level. Accordingly, the third switching element Q3 is turned off when one of the first and second switching elements Q1, Q2 is on, and the first and second switching elements Q1, Q2 are turned on from the first state. When the second switching elements Q1 and Q2 are both turned off, the energy stored in the inductance element L is released by the energy release means 6 by switching the third switching element Q3 from off to on. At this time, the flow of the released energy is as indicated by a solid arrow A in FIG. Thereby, the same effect as the first embodiment can be obtained.

  Each of the above-described embodiments is merely an example of the present invention, and it is obvious that modifications and modifications as appropriate within the spirit of the present invention are included in the claims of the present application. For example, in the present embodiment, the first to third switching elements are constituted by transistors, but may be constituted by MOSFETs.

The circuit diagram for explanation of the DC-DC converter (the 1st example) used as the premise of the present invention. The figure which shows the switching waveform of the 1st, 2nd switching element by 1st Example. The circuit diagram for description of the DC-DC converter (2nd Example) of this invention. It shows the switching waveform, and control signal waveforms of the third switching element by Real施例of the present invention. An explanatory circuit diagram showing a conventional voltage type DC-DC converter. An explanatory circuit diagram showing a conventional current type DC-DC converter. The figure which shows the switching waveform by the current type DC-DC converter of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 DC power supply 2 Transformer 21 Primary winding 22 Secondary winding 23 Middle tap 3 Rectification smoothing circuit 31 Diode bridge 4 Output terminal 5 Series circuit 6 Energy discharge means 7 Control means 8 Control circuit Q1 First switching element Q2 Second switching element Q3 Third switching element C, C1, C2 Electrolytic capacitor D, D ′ Diodes D1, D2, D3, D4 Rectifier diode L Inductance element L1 Winding L2 Subwinding R Resistance

Claims (1)

A DC power source, a transformer having a primary winding and a secondary winding and having an intermediate tap on the primary winding side, and an inductance connected between the positive side of the DC power source and the intermediate tap of the primary winding Elements, first and second switching elements connected between both ends of the primary winding and the negative electrode side of the DC power source, a rectifying and smoothing circuit connected to the secondary winding of the transformer, and the rectification A pair of output terminals provided in the smoothing circuit, and on and off control of the first and second switching elements to output a DC voltage higher than the voltage of the DC power supply to the output terminal, In a DC-DC converter used in a grid interconnection system that converts commercial AC voltage through an inverter, a diode and a third switching element are connected in series at both ends of the inductance element. Energy release means, and control means for sending a control signal to the energy release means, and the first and second switching elements are both turned on from either one of the first and second switching elements. When it is turned off, the control means detects an off period in which both the first and second switching elements are off, and sends the control signal to the energy release means so that only the third period is the third time. A DC-DC converter characterized in that the energy stored in the inductance is released by the energy release means by maintaining the switching element in an ON state.

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