JP2011229215A - Dc-ac conversion apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a DC-AC conversion apparatus with a small power loss.SOLUTION: A DC-AC conversion apparatus 1 has: a bridge circuit 10 having four switching elements S1-S4, and freewheel diodes D1-D4 connected in antiparallel to the respective switching elements; and an output filter 20 in which a coil L and a capacitor C are connected in series. A DC voltage source E is connected to input terminals A and B of the DC-AC conversion apparatus 1, and the output filter 20 is connected to output terminals of the bridge circuit 10. In addition, a switching circuit 30 for short-circuit between the output terminals of the bridge circuit 10 is provided between the bridge circuit 10 and the output filter 20. The switching circuit 30 is a series circuit in which two FETs S5 and S6 are connected in series in an opposite direction to each other.

Description

  The present invention relates to a DC-AC converter including a bridge circuit including a plurality of switching elements, a free wheel diode connected in antiparallel with each element, and an output filter including a coil.

  A DC-AC converter (DC / AC converter, inverter) electrically generates AC power having a desired frequency such as a commercial frequency (50 Hz or 60 Hz) from a DC power source such as a solar cell or a storage battery (battery), for example. (See, for example, Patent Document 1).

  FIG. 2A is a circuit diagram showing a circuit configuration of a conventional DC-AC converter. Here, in order to simplify the description, a DC-AC converter that converts a DC voltage into a single-phase AC voltage will be described as an example. A conventional DC-AC converter 100 shown in FIG. 2A includes a bridge circuit 10 in which four switching elements S1 to S4 are connected in a full bridge, an output filter 20 in which a coil L and a capacitor C are connected in series, . A DC voltage source E is connected to input terminals A and B of the DC-AC converter 100, and an output filter 20 is connected to an output terminal of the bridge circuit 10.

  The bridge circuit 10 is a basic circuit that converts the DC voltage of the DC voltage source E into an AC voltage. In the bridge circuit 10, a series circuit of switching elements S1 and S2 and a series circuit of switching elements S3 and S4 are connected in parallel, and free wheel diodes D1 to D4 are connected in reverse parallel to the switching elements S1 to S4. Yes. The switching elements S1 to S4 are semiconductor elements such as transistors.

  The principle and operation of converting DC voltage to AC voltage by the DC-AC converter 100 will be described. A pair of switching elements S1 and S4 of the bridge circuit 10 and a pair of switching elements S2 and S3 opposed to the switching elements S1 and S4 are turned on and off alternately (see FIG. 2B). When switching element S1 / S4 is ON and switching element S2 / S3 is OFF, it passes through output filter 20 (coil L and capacitor C) through switching element S1 as shown by the thin line arrow in FIG. A current path passing through the switching element S4 is formed. At this time, a positive voltage having the same magnitude as that of the DC voltage source E is applied between the output terminals CD of the DC-AC converter 100. Similarly, when switching element S1 / S4 is OFF and switching element S2 / S3 is ON, a current path is formed through switching element S3, through output filter 20 (capacitor C and coil L), and through switching element S2. Is done. At this time, a negative voltage having the same magnitude as that of the DC voltage source E is applied between the output terminals CD of the DC-AC converter 100. Hereinafter, alternating voltage is output from the output terminals C and D by repeating the operation of alternately turning on and off the switching elements S1 to S4.

  The time from when switching element S1 / S4 is turned off to when switching element S2 / S3 is turned on (similarly, the time from when switching element S2 / S3 is turned off until switching element S1 / S4 is turned on) ) T is appropriately set according to a desired frequency of the AC voltage (see FIG. 2B). For example, when the AC frequency is set to the commercial frequency, the time T during which all the switching elements S1 to S4 are turned off is about 1 to 10 msec.

  The output filter 20 is a series circuit of a coil L and a capacitor C, and has a function of removing a high frequency component of the current Ia (see FIG. 2A) output from the bridge circuit 10 and generating a sine wave.

JP 2008-54383 A

  However, in the conventional DC-AC converter 100, power loss occurs due to current flowing through the freewheel diodes D1 to D4.

  Now, after switching element S1 / S4 is turned on and current Ia flows through output filter 20, when switching element S1 / S4 is turned off, current Ia flows through coil L inserted in output filter 20. A counter electromotive force is generated in the coil L. As a result, as indicated by a broken line arrow in FIG. 2A, an induced current Ib flows through the freewheel diode D2, the coil L, and the freewheel diode D3. For this reason, the forward voltage drop Vf (about 0.3 to 2 V) is caused by the induction current Ib flowing through the freewheel diodes D2 and D3, so that a power loss of Vf × Ib occurs. Similarly, when the switching element S2 / S3 is turned on and the current Ia flows through the output filter 20 and then the switching element S2 / S3 is turned off, the freewheel is caused by the back electromotive force generated in the coil L. An induction current Ib flows through the diode D4, the coil L, and the freewheel diode D1. For this reason, the forward voltage drop Vf (about 0.3 to 2 V) is caused by the induction current Ib flowing through the freewheel diodes D1 and D4, and thus a power loss of Vf × Ib occurs. That is, after the switching elements S1 to S4 are turned on, the induced current Ib caused by the counter electromotive force of the coil L flows through the freewheel diodes D1 to D4 at a time T when all the switching elements S1 to S4 are turned off. As a result, power loss occurs.

  The present invention has been made in view of the above circumstances, and one of its purposes is to provide a DC-AC converter with low power loss.

  The DC-AC converter of the present invention includes a bridge circuit including a plurality of switching elements, a freewheel diode connected in antiparallel with each element, and an output filter including a coil. An output filter is connected to the output terminal of the bridge circuit. In addition, a switching circuit for short-circuiting the output terminals of the bridge circuit is provided between the bridge circuit and the output filter.

  The switching circuit short-circuits between the output terminals of the bridge circuit when in the ON state, and opens between the output terminals of the bridge circuit when in the OFF state. According to this configuration, when the switching circuit is ON, an electrical loop circuit can be formed by the switching circuit and the output filter. Therefore, a part of the induced current caused by the back electromotive force of the coil inserted in the output filter can be prevented from flowing to the free wheel diode of the bridge circuit, realizing a DC-AC converter with low power loss. it can.

  As one form of the DC-AC converter of the present invention, the switching circuit includes a series circuit in which at least two FETs are connected in series, and the FETs at both ends of the series circuit are connected in opposite directions. It is mentioned.

  According to this configuration, the switching circuit can be realized with a simple configuration. Examples of FET (Field Effect Transistor) include unipolar transistors such as MOSFET (Metal Oxide Semiconductor Field Effect Transistor, Metal-Oxide-Semiconductor FET) and JFET (Junction Field Effect Transistor, Junction FET). In the present invention, a bipolar transistor such as an IGBT (Insulated Gate Bipolar Transistor) may be used as the switching element used in the switching circuit. However, the FET can reduce the forward voltage drop due to the current flow, and can realize a DC-AC converter with less power loss than the bipolar transistor.

  Further, among the series circuit of FETs constituting the switching circuit, the FETs at both ends are connected in opposite directions, so that currents in both directions can flow through the switching circuit (series circuit). That is, when the FET is ON, not only a direct current but also an alternating current can be applied.

  One form of the DC-AC converter of the present invention is that the switching circuit is turned on in a time zone when all the switching elements of the bridge circuit are turned off.

  According to this configuration, when all the switching elements of the bridge circuit are turned off, the switching circuit (the FET of the series circuit constituting the switching circuit) is turned on, so that the induced current flows through the freewheel diode. Can be prevented.

  It is preferable that at least one of the FETs in the switching circuit is a SiC-MOSFET or a JFET.

  As described above, it is preferable to use a unipolar transistor such as a MOSFET or JFET having a small forward voltage drop as the FET. In particular, by using a SiC-MOSFET having a small ON resistance and a small switching loss, a DC-AC converter with a smaller power loss can be realized.

  The DC-AC converter according to the present invention includes a switching circuit that short-circuits the output terminals of the bridge circuit between the bridge circuit and the output filter, thereby reducing power loss.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining the principle and operation | movement of the DC-AC converter which concerns on Embodiment 1 of this invention, (A) is a circuit diagram which shows the circuit structure of an apparatus, (B) is operation | movement of a switching element. Indicates. It is a figure explaining the principle and operation | movement of the conventional DC-AC converter, (A) is a circuit diagram which shows the circuit structure of an apparatus, (B) shows operation | movement of a switching element.

  Embodiments of the present invention will be described with reference to the drawings. In the drawings, the same reference numerals indicate the same or corresponding parts.

(Embodiment 1)
A DC-AC converter 1 according to Embodiment 1 of the present invention shown in FIG. 1A includes four switching elements S1 to S4, and a free wheel diode D1 connected in antiparallel to each element S1 to S4. To D4, and an output filter 20 in which a coil L and a capacitor C are connected in series. A DC voltage source E is connected to input terminals A and B of the DC-AC converter 1, and an output filter 20 is connected to an output terminal of the bridge circuit 10. The most characteristic feature of the DC-AC converter 1 is that a switching circuit 30 for short-circuiting the output terminals of the bridge circuit 10 is provided between the bridge circuit 10 and the output filter 20. Note that the bridge circuit 10 and the output filter 20 have the same configuration as the conventional DC-AC converter 100 described in the above-mentioned “Background Art”, and thus description thereof is omitted.

  The switching circuit 30 is a series circuit in which two FETs S5 and S6 are connected in series in opposite directions. The FETs S5 and S6 are N-channel JFETs. The forward voltage drop of the FETs S5 and S6 is smaller than the free wheel diodes D1 to D4 of the bridge circuit 10. Further, the switching circuit 30 is in the ON state, that is, the FET S5 / S6 is turned on (see FIG. 1B) at the time T when all the switching elements S1 to S4 of the bridge circuit 10 are turned off. Short-circuit between the output terminals.

  As described in the above section “Problem to be Solved by the Invention”, the back electromotive force is applied to the coil L inserted in the output filter 20 at the time T when all the switching elements S1 to S4 of the bridge circuit 10 are OFF. Will occur. At this time, in the DC-AC converter 1 shown in FIG. 1, the FET S5 / S6 is turned ON, and the switching circuit 30 is short-circuited between the output terminals of the bridge circuit 10, whereby the switching circuit 30 and the output filter 20 Thus, an electric loop circuit is formed. Therefore, the induced current Ib resulting from the counter electromotive force of the coil L flows through the loop circuit passing through the switching circuit 30 as shown by the broken line in FIG. It can be prevented from flowing to the wheel diodes D1 to D4. Therefore, power loss can be reduced while the FETs S5 / S6 are ON. 1A indicates the direction of the induced current Ib flowing through the loop circuit when the switching elements S1 / S4 are turned off from ON. When the switching elements S2 / S3 are turned from ON to OFF, the direction of the induced current Ib flowing through the loop circuit is the opposite direction (in this case, counterclockwise) to the broken line arrow in FIG.

  In the conventional DC-AC converter (see FIG. 2), for example, the switching elements S1 / S4 are turned off from ON and all the switching elements S1 to S4 are turned off. When Ib flows, a reverse voltage is generated (similarly, when S2 / S3 is turned OFF from ON, induced current Ib flows through D1 and D4). For this reason, in the time zone in which all the switching elements S1 to S4 are OFF, the current value that increases according to the time zone (pulse width) in which the switching elements S1 / S4 (S2 / S3) are ON decreases. Therefore, especially in the case of PWM (Pulse Width Modulation) control, the ON-OFF operation of the switching elements S1 to S4 is frequently repeated, so that the output current ripple increases. Such a problem can also occur in a system in which a current is circulated by turning on a switching element opposite to a switching element that has been turned off from ON instead of circulating a current through the free wheel diode when the switching element is off.

  In contrast, the DC-AC converter (see FIG. 1) according to the present invention includes the switching circuit 30 so that an electrical loop circuit can be formed by the switching circuit 30 and the output filter 20. Therefore, the switching circuit 30 (FET S5 / S6) can be turned on and the current can be recirculated to the loop circuit during the time when all the switching elements S1 to S4 are turned off. Therefore, since the current value hardly decreases in the time zone in which the current is returned to the loop circuit, the ripple of the output current can be reduced.

  Here, the time t from the time when all the switching elements S1 to S4 are turned OFF to the time when the FET S5 / S6 is turned ON (see FIG. 1B) is a power loss according to the switching performance of the FETs S5 and S6. It is sufficient to set as appropriate so as to reduce the number. This time t is preferably 500 nsec or less, and more preferably 100 nsec or less.

  In the DC-AC converter 1 shown in FIG. 1, the source terminals of the FETs S5 and S6 of the switching circuit 30 are connected to each other. However, the present invention is not limited to this, and the drain terminals may be connected to each other.

  Note that the present invention is not limited to the above-described embodiment, and can be modified as appropriate without departing from the gist of the present invention. For example, the number and type of FETs used in the switching circuit may be changed as appropriate.

  The DC-AC converter of the present invention can be suitably used for, for example, a solar cell inverter.

1, 100 DC-AC converter (inverter)
10 Bridge circuit
S1-S4 switching element
D1-D4 freewheeling diode
20 Output filter L Coil C Capacitor
30 Switching circuit
S5, S6 FET (switching element)
E DC voltage source (DC power supply)

Claims (4)

A DC-AC converter comprising a plurality of switching elements, a bridge circuit comprising a free wheel diode connected in antiparallel with each element, and an output filter comprising a coil,
The output filter is connected to an output terminal of the bridge circuit;
A DC-AC converter comprising a switching circuit for short-circuiting between output terminals of the bridge circuit between the bridge circuit and the output filter. The switching circuit includes a series circuit in which at least two FETs are connected in series,
2. The DC-AC converter according to claim 1, wherein FETs at both ends of the series circuit are connected in opposite directions to each other.   3. The DC-AC converter according to claim 1, wherein the switching circuit is turned on during a time period when all the switching elements of the bridge circuit are turned off.   The DC-AC converter according to claim 2, wherein at least one of the FETs of the switching circuit is a SiC-MOSFET or a JFET.

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