JP2018088772A - Switching module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress a surge voltage applied to a switching element by using a Zener diode having a low yield voltage. A switching module having a high potential terminal, a low potential terminal, and a control terminal, and when the potential of the control terminal exceeds a threshold value, the high potential terminal and the low potential terminal are turned on. It has a switching element, and a diode and a Zener diode connected in series between the low potential terminal and the control terminal. The diode is connected so that its cathode faces the control terminal side and its anode faces the low potential terminal side. The Zener diode is connected so that its cathode faces the low potential terminal side and its anode faces the control terminal side. [Selection diagram] Fig. 1

Description

  The technology disclosed in this specification relates to a switching module.

  Patent Document 1 discloses a switching module having an n-type MOSFET (Metal Oxide Semiconductor Field Effect Transistor), a diode and a Zener diode connected in series between its drain and gate. The diodes are connected with their anodes facing the drain side and their cathodes facing the gate side. The Zener diode is connected such that its anode faces the gate side and its cathode faces the drain side.

  When the MOSFET is turned off, a surge voltage (a high voltage that is instantaneously applied at the turn-off timing) is applied between the drain and source of the MOSFET. The switching module of Patent Literature 1 reduces the surge voltage by turning on the MOSFET when the surge voltage is excessive. That is, when a surge voltage is applied to the MOSFET, the drain potential rises. When the potential difference between the drain and the gate exceeds a predetermined value (a value obtained by adding the forward voltage drop of the diode to the breakdown voltage of the Zener diode), the Zener diode and the diode are turned on. Then, a current flows from the drain to the gate through the Zener diode and the diode. When current flows through the gate in this way, the gate potential rises. For this reason, the resistance between the drain and the source of the MOSFET decreases, and the current easily flows between the drain and the source. As a result, the collector-source voltage (surge voltage) is reduced. Therefore, an excessive surge voltage is prevented from being applied to the MOSFET.

JP, 2006-046935, A

  In general, the potential difference between the drain and the gate during normal operation of the MOSFET (when no surge voltage is applied) is large. Since the Zener diode needs to be turned off during normal operation, the breakdown voltage of the Zener diode needs to be higher than the drain-gate voltage during normal operation. For example, if the drain-gate voltage is 700 V or higher during normal operation, the breakdown voltage of the Zener diode needs to be 700 V or higher. In order to realize such a high breakdown voltage, it is necessary to use a large Zener diode or to use a plurality of Zener diodes in series, which increases the size of the switching module. In Patent Document 1, an n-type MOSFET is used as a switching element, but the same problem occurs when another switching element (for example, IGBT) is used. Therefore, the present specification provides a technique that enables suppression of a surge voltage applied to a switching element by using a Zener diode having a low breakdown voltage.

  The switching module disclosed in this specification includes a switching element, a diode, and a Zener diode. The switching element includes a high potential terminal, a low potential terminal, and a control terminal. When the potential of the control terminal exceeds a threshold value, the high potential terminal and the low potential terminal are turned on. The diode and the Zener diode are connected in series between the low potential terminal and the control terminal. The diode is connected such that its cathode faces the control terminal and its anode faces the low potential terminal. The Zener diode is connected such that its cathode faces the low potential terminal side and its anode faces the control terminal side.

  When the switching element is turned off, a surge voltage is generated between the high potential terminal and the low potential terminal of the switching element. At this time, since the current flowing through the switching element decreases, the potential of the cathode of the Zener diode rises with respect to the potential of the control terminal due to the electromotive force of the parasitic inductance existing in the wiring between the low potential terminal and the cathode of the Zener diode. To do. Thus, when the voltage applied to the Zener diode exceeds the breakdown voltage, the Zener diode and the diode are turned on. Then, a current flows from the low potential terminal to the control terminal via the Zener diode and the diode. As a result, the potential of the control terminal increases. For this reason, the resistance between the high potential terminal and the low potential terminal is reduced (that is, the switching element is in an on state), and current easily flows between the high potential terminal and the low potential terminal. As a result, the voltage (surge voltage) between the high potential terminal and the low potential terminal is reduced. Therefore, an excessive surge voltage is prevented from being applied to the switching element. Further, the potential difference between the low potential terminal and the control terminal of the switching element during normal operation (when no surge voltage is applied) is small. Therefore, a Zener diode having a low breakdown voltage can be used as a Zener diode connecting the low potential terminal and the control terminal. As described above, according to this switching module, a surge voltage applied to the switching element can be suppressed using a Zener diode having a low breakdown voltage.

The circuit diagram of a switching module. The graph which shows each value when a switching element turns off.

  The switching module 10 shown in FIG. 1 includes an IGBT 20a and an IGBT 20b connected in series between a high potential wiring 12 and a low potential wiring 14. The collector c1 of the IGBT 20a is connected to the high potential wiring 12, the emitter e1 of the IGBT 20a is connected to the collector c2 of the IGBT 20b, and the emitter e2 of the IGBT 20b is connected to the low potential wiring 14. An intermediate wiring 16 is connected to the emitter e1 of the IGBT 20a and the collector c2 of the IGBT 20b. Although the potential of the intermediate wiring 16 varies, the average potential of the intermediate wiring 16 is higher than the potential of the low potential wiring 14 which is subtracted from the potential of the high potential wiring 12. The gate g1 of the IGBT 20a is connected to the gate drive circuit 32a through the gate resistor 30a. The gate drive circuit 32a controls the potential of the gate g1 by charging and discharging the gate g1. The gate g2 of the IGBT 20b is connected to the gate drive circuit 32b via the gate resistor 30b. The gate drive circuit 32b controls the potential of the gate g2 by charging and discharging the gate g2. The switching module 10 is used as a part of a DC-DC converter or an inverter. The IGBT 20a and the IGBT 20b are alternately turned on. Thereby, the potential of the high potential wiring 12 and the potential of the intermediate wiring 16 are controlled.

  A series circuit of a Zener diode 40a and a diode 42a is connected between the emitter e1 and the gate g1 of the IGBT 20a. The cathode of the Zener diode 40a is connected to the emitter e1 of the IGBT 20a, the anode of the Zener diode 40a is connected to the anode of the diode 42a, and the cathode of the diode 42a is connected to the gate g1 of the IGBT 20a. Further, a parasitic inductance 50a of wiring exists in the emitter e1 of the IGBT 20a. A parasitic inductance 50a illustrated in FIG. 1 represents a parasitic inductance between the emitter e1 of the IGBT 20a and the cathode of the Zener diode 40a. The parasitic inductance 50a is an inductance that a bonding wire, a lead, a connector, a bus bar, and the like have.

  A series circuit of a Zener diode 40b and a diode 42b is connected between the emitter e2 and the gate g2 of the IGBT 20b. The cathode of the Zener diode 40b is connected to the emitter e2 of the IGBT 20b, the anode of the Zener diode 40b is connected to the anode of the diode 42b, and the cathode of the diode 42b is connected to the gate g2 of the IGBT 20b. Further, a parasitic inductance 50b of wiring exists in the emitter e2 of the IGBT 20b. A parasitic inductance 50b shown in FIG. 1 represents a parasitic inductance between the emitter e2 of the IGBT 20b and the cathode of the Zener diode 40b. The parasitic inductance 50b is an inductance that a bonding wire, a lead, a connector, a bus bar, or the like has.

  In the present embodiment, the breakdown voltage of the Zener diodes 40a and 40b is about 6V, and the forward voltage drop of the diodes 42a and 42b is about 0.6V. These values are arbitrarily determined during circuit design. You can choose.

  Next, the operation of the switching module 10 when switching the IGBT will be described. Since the operation of the IGBT 20a and the operation of the IGBT 20b are the same, the operation of the IGBT 20b will be described below.

  When turning off the IGBT 20b, the gate drive circuit 32b controls the potential of the gate g2 (potential with respect to the emitter e2) to a potential equal to or lower than a threshold (in this embodiment, substantially the same potential as the potential of the emitter e2). When the IGBT 20b is turned on, the gate drive circuit 32b controls the potential of the gate g2 to a potential (for example, several V) higher than the threshold value. Thus, in normal operation (that is, in a state where no surge voltage is applied), the potential of the emitter e2 is equal to or lower than the potential of the gate g2. Although the breakdown voltage of the Zener diode 40b is not so high, the potential of the emitter e2 is about the same as or lower than the potential of the gate g2, so that the Zener diode 40b is turned off in normal operation. In normal operation, the diode 42b is off.

  Next, the operation of the switching module 10 when a high surge voltage is generated when the IGBT 20b is turned off will be described. In the period before timing t1 in FIG. 2, the gate voltage Vge of the IGBT 20b (the potential of the gate g2 with respect to the emitter e2) is maintained at a high potential by the gate drive circuit 32b. Therefore, during this period, the IGBT 20b is on, and the collector-emitter current Ice (hereinafter referred to as collector current Ice) of the IGBT 20b is high.

  At timing t1, the gate drive circuit 32b starts to lower the gate voltage Vge. When the gate voltage Vge decreases to the mirror voltage Vmrr at the timing t2, the resistance between the collector c2 and the emitter e2 of the IGBT 20b becomes high, and the collector current Ice starts to decrease. After timing t2, the gate voltage Vge is maintained at the mirror voltage Vmrr, but the gate g2 is discharged during this time. In addition, after the timing t2, the collector current Ice rapidly decreases as the resistance between the collector c2 and the emitter e2 increases. For this reason, a voltage Vce (hereinafter referred to as a collector voltage Vce) between the collector c2 and the emitter e2 of the IGBT 20b is generated by an electromotive force of a parasitic inductance (parasitic inductance of wiring other than the parasitic inductance 50b) existing in the circuit of the switching module 10. It rises rapidly. That is, a surge voltage Vs is generated. Further, since the current flowing through the parasitic inductance 50b (that is, the collector current Ice) rapidly decreases, the induced voltage VL generated at the parasitic inductance 50b increases rapidly. For this reason, the potential of the cathode of the Zener diode 40b is rapidly increased, and the voltage Vzd (potential of the cathode with respect to the anode of the cathode) applied to the Zener diode 40b is also rapidly increased. When the voltage Vzd reaches the breakdown voltage Vz at timing t3 (more specifically, the potential of the cathode of the Zener diode 40b with respect to the cathode of the diode 42b is the voltage obtained by adding the forward voltage drop of the diode 42b to the breakdown voltage Vz of the Zener diode 40b. When the value reaches the zener diode 40b and the diode 42b, the current flows from the emitter e2 to the gate g2. The current that flows to the gate g2 flows to the ground via the gate resistor 30b and the gate drive circuit 32b. Therefore, at the timing t3, the gate voltage Vge rises to a voltage Vge1 that is slightly higher than the mirror voltage Vmrr. For this reason, after timing t3, the IGBT 20b is in a state close to an on state (that is, a state in which the collector c2 and the emitter e2 have a relatively low resistance), and the reduction rate of the collector current Ice becomes small. Since current easily flows between the collector c2 and the emitter e2 of the IGBT 20b, the collector voltage Vce decreases after the timing t3. Therefore, the surge voltage Vs is prevented from further increasing. Since the induced voltage VL of the parasitic inductance 50b gradually decreases after the timing t3, the voltage Vzd applied to the Zener diode 40b also gradually decreases after the timing t3. When the voltage Vzd falls below the breakdown voltage Vz at the timing t4, the Zener diode 40b is turned off. Therefore, at the timing t4, the gate voltage Vge is reduced to the mirror voltage Vmrr, and the IGBT 20b is turned off (that is, the state between the collector c2 and the emitter e2 is high resistance). Therefore, the collector current Ice rapidly decreases after the timing t4. When the collector current Ice becomes substantially zero at timing t5, the surge voltage Vs and the induced voltage VL of the parasitic inductance 50b become substantially zero, and the collector voltage Vce is stabilized at a constant value. Thereafter, the gate voltage Vge becomes substantially 0 V, and the turn-off of the IGBT 20b is completed.

  As described above, according to the switching module 10, it is possible to prevent the surge voltage Vs from becoming excessive when the IGBT 20b is turned off. Further, since a high voltage is not applied to the Zener diode 40b connected between the gate g2 and the emitter e2 during normal operation, a Zener diode having a low breakdown voltage can be used as the Zener diode 40b. Therefore, the switching module 10 can be reduced in size.

  In the above-described embodiment, the IGBT is used as the switching element. However, a MOSFET may be used as the switching element.

  In the embodiment described above, the positions of the diode 42b and the Zener diode 40b may be interchanged. That is, the anode of the Zener diode 40b is connected to the gate g2, the cathode of the Zener diode 40b is connected to the cathode of the diode 42b, and the anode of the diode 42b is connected to the emitter e2 (ie, the parasitic inductance 50b). May be.

  The embodiments have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. The technical elements described in this specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology exemplified in this specification or the drawings achieves a plurality of objects at the same time, and has technical usefulness by achieving one of them.

10: switching module 12: high potential wiring 14: low potential wiring 16: intermediate wiring 30a, 30b: gate resistors 32a, 32b: gate drive circuits 40a, 40b: Zener diodes 42a, 42b: diodes 50a, 50b: parasitic inductances

Claims (1)

A switching module,
A switching element having a high-potential terminal, a low-potential terminal, and a control terminal, the switching element that is turned on between the high-potential terminal and the low-potential terminal when the potential of the control terminal exceeds a threshold;
A diode and a Zener diode connected in series between the low potential terminal and the control terminal;
Have
The diode is connected with the cathode facing the control terminal and the anode facing the low potential terminal;
The Zener diode is connected with the cathode facing the low potential terminal side and the anode facing the control terminal side,
Switching module.

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