JP2008289232A - Switching circuit - Google Patents

Abstract

A switching circuit capable of reducing the overall loss without causing an increase in size is obtained.
An external diode (5) is connected in parallel to a series circuit in which main and secondary MOSFETs (1a, 3a) are connected in series so that their body diodes (2a, 4a) are in reverse series. The main and sub MOSFETs 1a and 3a are connected to each other between the source terminals s and the gate terminals g, the external diode 5 is connected to the same polarity as the body diode 2a, and the main and sub MOSFETs 1a and 3a are simultaneously switched by the drive circuit 6a. Be controlled. Since the current circulating in the main body diode 2a is cut off by the slave body diode 4a, both the circulating current and the recovery current flow to the external diode 5, but a high-performance external diode 5 can be freely selected. , The overall loss can be reduced.
[Selection] Figure 1

Description

  The present invention relates to a switching circuit configured by combining main switching elements having a body diode.

  In a conventional switching circuit, a plurality of MOSFETs are connected in series, and a current regenerated from a load is circulated by a body diode that is structurally parasitic on each MOSFET (see, for example, Patent Document 1).

  Further, in the conventional switching circuit, a reverse voltage application circuit is separately provided in the circulating diode, and the diode is recovered at a low voltage to reduce the loss (for example, see Patent Document 2).

JP 2007-014059 (paragraph numbers 0012, 0013, 0033 and FIG. 1) Japanese Patent Laying-Open No. 2006-141167 (paragraph number 0015 and FIG. 1)

  In such a switching circuit, a series circuit including a plurality of MOSFETs is connected between a positive line and a negative line, and a regenerative current from a load circulates through the body diodes of these MOSFETs. Normally, when one of the MOSFETs is turned off, a current flows through the body diode of the other MOSFET, and when the other MOSFET is turned on again during the period when the other body diode is conducted by the circulating current, the other body diode is turned on. Causes recovery loss and recovery noise due to the flow of recovery current.

  If a MOSFET with a fast recovery speed of its body diode is used, the above problem can be solved, but the body diode is a parasitic diode that occurs in the configuration of the MOSFET. Both costs are difficult.

  Also, if the other MOSFET is turned on while the other body diode is non-conductive, the recovery current does not flow and no recovery loss or recovery noise occurs. However, a current sensor or MOSFET for detecting the circulating current does not occur. Since a control circuit is required, the number of parts is increased, the reliability of the switching circuit is lowered, the switching circuit is increased in size, and the cost is increased. In addition, since the on-time control is restricted, the efficiency of the switching circuit is lowered and the drive control of the load is hindered.

  Furthermore, if the reverse applied voltage at the time of body diode recovery is small, the recovery loss will be low.However, in order to apply the reverse voltage immediately before the body diode recovery, a power supply circuit with a large current and a fast response is required for each switching circuit. Necessary and costly.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to obtain a switching circuit that can reduce loss without causing an increase in size.

  In the switching circuit according to the present invention, first and second main switching elements each having a main circuit switching terminal to be opened and closed, a control signal terminal, and a body diode are connected in series so that the body diodes are in reverse series. A series circuit connected to the external circuit, an external diode connected in parallel to the series circuit in the same conduction direction as the body diode of the first main switching element, and the first and second main switching circuits And a control means for supplying a control signal for controlling opening / closing of the element to the control signal terminal at substantially the same timing.

  In the switching circuit according to the present invention, first and second main switching elements each having a main circuit switching terminal to be opened and closed, a control signal terminal, and a body diode are connected in series so that the body diodes are in reverse series. A series circuit connected to the external circuit, an external diode connected in parallel to the series circuit in the same conduction direction as the body diode of the first main switching element, and the first and second main switching circuits And a control means for supplying a control signal for controlling the opening and closing of the element to the control signal terminal at substantially the same timing, so that the current circulating in the body diode of the first main switching element is supplied to the second main switching element. Can be cut off by the body diode, the reflux current flows to the external diode, and the recovery current is also external It flows through the diode, free to select a high-performance external diode than the body diode, thereby reducing the overall loss without causing a large reduction by the use of high-performance external diode.

Embodiment 1 FIG.
1 and 2 show Embodiment 1 for carrying out the present invention. FIG. 1 is a block diagram showing the configuration of a three-phase inverter circuit using the switching circuit of the present invention. FIG. It is a principal part extraction diagram for demonstrating. In FIG. 1, the three-phase inverter circuit is divided into six switching circuits 8 according to the present invention (hereinafter referred to as 8a to 8f when they need to be individually distinguished in the description and the drawings, and when they are expressed in common. The same applies to the components of the switching circuit, which is simply referred to as switching circuit 8, and is configured as a three-phase bridge. A main MOSFET 1 that is a first main switching element includes a drain terminal d and a source terminal s that are main circuit open / close terminals, a gate terminal g that is a control signal terminal, and a main body diode 2 that is parasitic on the main MOSFET 1. The slave MOSFET 3 that is the second main switching element includes a drain terminal d and a source terminal s that are main circuit open / close terminals, a gate terminal g, and a slave body diode 4 that is parasitic on the slave MOSFET 3.

In the switching circuit 8, the main MOSFET 1 and the sub MOSFET 3 are connected in series so that the body diode 3 and the body diode 4 have opposite polarities, and the source terminals s and the gate terminals g are connected. Yes. Further, it has an external diode 5 connected in parallel to the series circuit of the main and slave MOSFETs 1 and 3 connected in series. The external diode 5 is connected such that the cathode k is connected to the drain terminal d of the main MOSFET 1 and the anode a is connected to the drain terminal d of the sub-MOSFET 3 so as to have the same polarity (same conduction direction) as the body diode 2. Yes. A drive circuit 6 as a control circuit applies an open / close signal as a control signal to each gate terminal g with reference to the potential of each source terminal s to perform open / close control. The capacitor 7 is connected between the positive line P and the negative line N and bypasses the high-frequency current generated by the switching of the main and slave MOSFETs 1 and 3.
Each switching circuit 8 constitutes three sets of half bridges connected in series between a positive line P and a negative line N, and a load motor 9 is connected to a neutral point O of each half bridge.

Next, the operation will be described. FIG. 2 shows a set of half bridges made up of the switching circuits 8a and 8d in order to simplify the description of the operation of the switching circuit 8. In FIG. In FIG. 2, the main MOSFET 1a and the sub MOSFET 3a of the switching circuit 8a are both turned off, and the main MOSFET 1d and the sub MOSFET 3d of the switching circuit 8d are both off, so that the circulating current J2 from the load is on the negative line side. It is assumed that the current flows through the external diode 5d.
At this time, since the main MOSFET 1d and the sub MOSFET 3d of the switching circuit 8d on the negative line side are off, the body diode 2d of the main MOSFET 1d does not conduct.

  Here, when both the main MOSFET 1a and the sub MOSFET 3a are turned on during the period in which the circulating current J2 from the load as the forward current flows through the external diode 5d on the negative line side, the current to the load is the circulating current. At the same time as commutation from J2 to forward current J1, recovery current J3 flows in the direction opposite to the conduction direction of external diode 5 due to the recovery characteristic of external diode 5d. The main body diode 2d is cut off by the sub MOSFET 3d and no current flows, so that no recovery occurs, and therefore the body diode recovery current J4 does not flow. Since the current circulating to the main body diode 2a which is the parasitic diode of the main MOSFET 1a is cut off by the slave body diode 4a which is the parasitic diode of the slave MOSFET 3a, the circulating current flows to the external diode 5 and the recovery current is also the external diode. Will flow to 5.

The recovery current J3 flowing through the external diode 5 causes recovery loss in the external diode 5d, but the characteristics of the diode used for the external diode 5d can be set regardless of the characteristics of the main MOSFET 1d, and the main body diode 2d It is possible to select a diode with high performance, that is, sufficiently high speed and soft recovery characteristics, and it is not necessary to use a current sensor or MOSFET control circuit for detecting the circulating current. Recovery loss and recovery noise can be suppressed without incurring any problems. When the main and sub MOSFETs 1d and 3d, which are two switching elements, are connected in series, the sub MOSFET 3d, which is one of the switching elements, can have a normal performance.
Although the switching circuit 8d has been described above, the same operation is performed in the switching circuits 8a to 8c and the switching circuits 8e and 8f.

Here, since the MOSFET parasitic diode generally recovers quickly and a good characteristic cannot be obtained, when recovery occurs in the main body diode 2d of the main MOSFET 1d, the recovery loss becomes extremely large.
The same effect can be obtained by using a low-loss diode such as a Schottky barrier diode instead of the slave MOSFET 3d, instead of the slave MOSFET 3d and the slave body diode 4d. In this case, the forward voltage of the Schottky barrier diode is obtained. The loss due to the drop occurs with the same magnitude as the on-resistance loss of the main MOSFET 3d. However, in the present embodiment, since the secondary MOSFET 3d is turned on, no current flows through the secondary body diode 4d, and the synchronous rectification operation that flows through the secondary MOSFET 3d is performed, so that the loss becomes extremely small.

Further, the drain-source voltage of the sub MOSFET 3d is hardly applied when the main MOSFET 1d is cut off. Even if there is a slight deviation in the on / off operation of the main MOSFET 1d and the sub MOSFET 3d, it becomes small because it is clamped by the forward voltage of the external diode, and even if the surge voltage due to the inductance of the circuit wiring is taken into account, the drain of the sub MOSFET 3d A source withstand voltage of 20 to 60 V is sufficient.
In the case of the low breakdown voltage product, a low on-resistance MOSFET can be procured at low cost, and in the present embodiment, the source terminals s and gate terminals g of the main MOSFET 1d and the sub MOSFET 3d are connected to each other, and the gate terminal is connected to the drive circuit 6d. Since the open / close signal is given, the main MOSFET 1d and the sub MOSFET 3d can be driven by the same drive circuit 6d, so that the cost increase by adding the sub MOSFET 3d is slight.

  Also, when it is necessary to use a main MOSFET 1d having a drain-source withstand voltage of about 600 V, the withstand voltage difference between the main MOSFET 1d and the sub MOSFET 3d is 10 times or more, and the respective on-resistance differences are 20 times or more. It is possible to set. That is, by making the on-resistance of the MOSFET 3d sufficiently lower than the on-resistance of the MOSFET 1d, it is possible to slightly suppress an increase in the total on-loss including the MOSFETs 1d and 3d even when compared to the case of the MOSFET 1d alone. it can. Therefore, the increase in on-resistance loss due to the addition of the secondary MOSFET 3d can be suppressed to about 5%, and the loss can be extremely reduced as compared with the case where a Schottky barrier diode is used instead of the secondary MOSFET 3d.

As described above, by providing the external diode 5d and providing the slave MOSFET 3d that cuts off the current to the main body diode 2d of the main MOSFET 1d, recovery loss and recovery noise due to the main body diode 2d having slow recovery and poor characteristics can be avoided. .
Further, by making the sub MOSFET 3d a MOSFET having a lower withstand voltage than the main MOSFET 1d, that is, having a low on-resistance, loss and cost increase due to the addition of the sub MOSFET 3d can be effectively suppressed. That is, even if there is a loss due to the addition of the sub MOSFET 3d or a loss of the external diode 5d, the overall loss can be reduced by eliminating the recovery loss of the main body diode 2d of the main MOSFET 1d. In addition, recovery noise can be reduced by using a high-performance external diode 5d.
Further, by connecting the source terminals of the main MOSFET 1d and the sub MOSFET 3d and driving the main MOSFET 1d and the sub MOSFET 3d with the same voltage with the common reference potential, there is no need to add a new driving circuit, so that the switching circuit can be realized. Smaller and cheaper.

In addition, although each source terminal of each of the main MOSFET 1 and the sub-MOSFET 3 and each gate terminal are connected to each other and the drive circuit 6 drives each gate terminal based on each source terminal, the drive circuit 6 Each of the main MOSFET 1 and the sub MOSFET 3 may be provided individually, and there may be some deviation in the timing of each drive control, and the same effect is obtained.
Further, the example in which the circulating current J2 flows through the external diode 5d by turning off the main MOSFET 1d and the secondary MOSFET 3d is shown. However, the primary MOSFET 1a and the secondary MOSFET 3a are turned on when the circulating current J2 flows through the external diode 5d. When the conduction time of the recirculation current J2 is long, the recirculation current J2 flows to the external diode 5d if the main MOSFET 1d and the sub MOSFET 3d are turned on until just before the main MOSFET 1a and the sub MOSFET 3a are turned on. The resulting loss can be reduced.

Further, in this embodiment, the operation is shown by taking one arm of a three-phase inverter using six switching circuits according to the present invention as an example. The present invention may be applied only to a MOSFET in which recovery by a body diode occurs.
In addition to the three-phase inverter, for example, a switching circuit according to the present invention such as a buck converter or a boost converter is connected in series, or the switching circuit and a separate MOSFET, etc. Even when the switching elements are connected in series, the same effect can be obtained.
In addition, if a switching element has not only MOSFET but the body diode which is a parasitic diode, a recovery loss and recovery noise can be reduced similarly.

It is a block diagram of the three-phase inverter circuit using the switching circuit which is Embodiment 1 of this invention. The principal part extraction figure for demonstrating operation | movement of a switching circuit is shown.

Explanation of symbols

1a to 1f main MOSFET, 2a to 2f main body diode,
3a-3f secondary MOSFET, 4a-4f secondary body diode,
5a to 5f external diode, 6a to 6f drive circuit,
8, 8a-8f Switching circuit.

Claims (4)

A series circuit in which first and second main switching elements each having a main circuit open / close terminal to be opened / closed, a control signal terminal, and a body diode are connected in series so that the body diodes are in reverse series; Open / close control of the external diode connected in parallel with the series circuit in the same conduction direction as the body diode of the first main switching element and the first and second main switching elements are performed at substantially the same timing. A switching circuit comprising control means for supplying a control signal to the control signal terminal. The control signal terminals of the first and second main switching elements are connected in common, and the control means supplies the control signal to the control signal terminals connected in common. The switching circuit according to claim 1. 2. The switching circuit according to claim 1, wherein a withstand voltage between main switching terminals of the second main switching element is lower than a withstand voltage between main switching terminals of the first main switching element. The control means provides the control signal so as to close both the first and second switching elements during a period in which a forward current flows through the external diode. Item 4. The switching circuit according to Item 1.

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