JP2861169B2 - Snubber circuit of power converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a semiconductor device for protecting a semiconductor device from a surge voltage when a power conversion device including a self-extinguishing power semiconductor device operates. It relates to a snubber circuit.

(Prior Art) As a self-extinguishing type power semiconductor element, for example, two power transistors are connected in series, this series circuit is connected between the positive and negative electrodes of a DC power supply, and one power transistor and the other power transistor are connected to each other. It is known that power conversion can be performed by alternately turning on / off. Therefore, by connecting three sets of such power transistor series circuits in parallel with each other and connecting a DC power supply thereto,
The prior art will be described below using a three-phase inverter that converts DC power from the DC power supply into three-phase AC power as an example.

In a voltage source three-phase inverter constituted by power transistors, freewheeling diodes are connected in anti-parallel to the respective power transistors. Here, when the power transistor is turned off (or at the time of reverse recovery of the freewheeling diode) and a surge voltage generated by the stray inductance of the circuit is applied to the power transistor, the power transistor may be destroyed. Therefore, in order to avoid occurrence of such inconvenience, a snubber circuit is attached to the three-phase inverter.

FIG. 9 is a circuit diagram showing a first conventional example of a snubber circuit attached to a power converter.

In FIG. 9, 1 to 6 are six power transistors, and 7 to 12 are six freewheeling diodes. A three-phase invar is formed by bridge-connecting three sets of anti-parallel freewheeling diodes to each of these six power transistors. Here, 34 is a DC power supply and 44 is a stray inductance of the circuit. Reference numeral 31 denotes a diode, 32 denotes a resistor, and 33 denotes a capacitor. These constitute a snubber circuit. Tenth
The figure shows a power transistor for one phase, for example, 1, 4 and a snubber circuit thereof in the first conventional example.

As described above, in the first conventional example shown in FIG. 9, a snubber circuit (commonly called an R-C-D snubber) including the resistor 32, the capacitor 33, and the diode 31 is connected between the positive electrode and the negative electrode of the DC power supply 34. When the power transistor is turned off, the energy stored in the stray inductance 44 of the circuit is absorbed by the capacitor 33,
Surge voltage is suppressed. Here, if the resistor 32 and the diode 31 constituting the snubber circuit are removed, the surge voltage suppression effect increases.

Next, FIG. 11 is a circuit diagram showing a second conventional example of the snubber circuit attached to the power converter.

In FIG. 11, six power transistors 1 to
The six freewheeling diodes 7 to 12, the DC power supply 34 and the stray inductance 44 have the same functions as in FIG.

A snubber circuit composed of a capacitor and a resistor connected in series is connected in parallel to each of the power transistors constituting the three-phase inverter. In FIG. 11, 13 to 18 are six capacitors constituting the snubber circuit, and 38 to 43 are resistors constituting the snubber circuit,
FIG. 12 shows a power transistor for one phase of the second conventional example,
For example, 1, 4 and its snubber circuit are shown.

Such a snubber circuit composed of a resistor and a capacitor is commonly called an RC snubber, and a capacitor constituting the RC snubber turns off a power transistor to which the RC snubber is connected. It acts so as to suppress the surge voltage that occurs when this occurs.

(Problems to be Solved by the Invention) In recent years, semiconductor devices for power conversion (especially IGBTs (insulated gate bipolar transistors), MOS-FETs, etc.) have become capable of extremely high-speed switching operation, Since the decay rate (-di / dt) of the main current that is interrupted at that time is large, the surge voltage generated by the stray inductance of each part wiring is very large.

In a snubber circuit connected between a positive electrode and a negative electrode of a DC power supply 34 as shown in FIG. 10 as a first conventional example, there is an effect of a stray inductance 45 existing in the snubber circuit itself.
For this reason, it is not possible to sufficiently prevent the generation of surge voltage, especially when the stray inductance is large or when the collector current to be interrupted is large, the generated surge voltage exceeds the breakdown voltage of the semiconductor element, or the collector current and the collector-emitter voltage
There is a problem that the operation locus with V _CE deviates from the reverse bias safe operation area (hereinafter referred to as RBSOA) of the semiconductor element, and the power transistor is destroyed. This will be described below with reference to FIGS. 3 and 4.

The solid lines in FIGS. 3 and 4 are waveform diagrams showing the operation for one phase shown in FIG. 10, and FIG. 3 shows changes in the collector currents I _C and V _CE of the power transistor 1 with respect to time. FIG. 4 shows the operation locus of I _C and V _CE shown in FIG. 3 together with the RBSOA of the transistor when I _C and V _CE are plotted on the vertical axis and the horizontal axis, respectively.

That is, when the power transistor 1 in FIG. 10 is turned off and V _CE reaches the DC power supply voltage, the diode 31 of the snubber circuit conducts, and the energy stored in the floating inductance 44 is absorbed by the capacitor 33. Due to the effects of the stray inductance 45 and the transient forward voltage drop of the diode 31 existing on this RCD snubber circuit,
The surge voltage generated by the energy of the stray inductance 44 is not sufficiently suppressed. As a result, a very high surge voltage is generated as shown by the solid line in FIGS. 3 and 4, which deviates from the RBSOA of the power transistor 1 and the power transistor 1 is destroyed.

In addition, as shown in FIG.
In an RC snubber circuit individually connected to the six power transistors constituting the phase inverter, for example,
When 12 view of V _CE of power transistor 1 starts to increase, the energy stored in the stray inductance 44 is absorbed in the capacitor 13 through the resistor 38, the surge voltage generated during switching by increasing the capacity of the capacitor 13 reduces And the operation trajectory of the power transistor
Can be stored inside. An example of the operation waveform and operation locus of the power transistor 1 at the time of this turn-off is shown as a dashed line in FIGS.

However, in this conventional example, it is necessary to consume the energy of the stray inductance 44 stored in the capacitor 13 by some method. In this case, the energy is discharged through the resistor 38 (the resistors 38 to 43 as a whole). Consumes energy.

Therefore, when the frequency of the switching of the power transistor is increased, the energy loss generated in the snubber circuit increases, and there is a problem that the components of the snubber circuit increase in size.

SUMMARY OF THE INVENTION The present invention has been proposed to solve the above-described problems. An object of the present invention is to suppress a surge voltage generated at the time of switching of a power semiconductor element to protect the semiconductor element and to generate a voltage in a snubber circuit. It is an object of the present invention to provide a snubber circuit of a power conversion device in which a semiconductor device for power conversion can be used at a high frequency by reducing energy loss to be performed.

(Means for Solving the Problems) In order to achieve the above object, a snubber circuit according to the present invention is configured such that a series circuit of a self-extinguishing power semiconductor element to which a freewheeling diode is connected in antiparallel is connected to a power supply. A power converter in which one semiconductor element and the other semiconductor element constituting the series circuit alternately turn on / off to perform power conversion, wherein a first constant voltage diode and a reverse direction with respect to the constant voltage diode are provided. A first series circuit including a first diode connected thereto, and a second series circuit including a second constant voltage diode and a second diode connected in a reverse direction to the constant voltage diode,
The first and second constant voltage diodes are connected in parallel so that they are opposite to each other, and a series circuit of this parallel connection circuit and a capacitor is connected in parallel to each semiconductor element.

(Operation) According to the present invention, the first and second series circuits in which a constant voltage diode, which is a device having a breakdown voltage in the forward and reverse bidirectional directions, and a diode are connected in the reverse direction so that each constant voltage diode is in the opposite direction. By connecting a series circuit of the parallel connection circuit and the capacitor in parallel with the semiconductor element to form a snubber circuit, a surge voltage at the time of switching of the semiconductor element is suppressed, and the element is prevented from being destroyed. be able to. Further, the size of the snubber circuit component is reduced by reducing the loss generated in the snubber circuit, and the frequency at which the semiconductor element is switched is increased.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a main circuit diagram of a power converter when a snubber circuit which is a basis of an embodiment of the present invention is configured.

In FIG. 1, the functions of six power transistors 1 to 6 as self-extinguishing type power semiconductor elements, six freewheeling diodes 7 to 12 and a DC power supply 34 are shown in FIG. 9 and FIG. Since they are the same as those already described in FIG. 11, their description is omitted to avoid duplication. In FIG. 1, a diode 3 is connected between the positive and negative electrodes of the DC power supply 34.
1, an RCD snubber circuit including a resistor 32 and a capacitor 33 is connected.

Further, in FIG. 1, two constant voltage diodes are connected in series in the reverse direction as voltage drop asymmetrical elements having a breakdown voltage in both forward and reverse directions, and six capacitors are connected in series to this as a snubber circuit. Power transistor 1
To 6 are connected in parallel. That is, the first
In the figure, reference numerals 19 to 30 denote constant voltage diodes constituting the snubber circuit, and reference numerals 13 to 18 denote capacitors.

FIG. 2 is a circuit diagram showing a power transistor for one phase of the power converter shown in FIG. 1, for example, 1, 4 and a snubber circuit thereof.

In FIG. 2, a series circuit of a series body of constant voltage diodes 19 and 20 as a voltage drop asymmetric element and a capacitor 13 is connected in parallel to the power transistor 1. At this time, one of the constant voltage diodes 19 The Zener voltage is set slightly higher than the voltage of the DC power supply 34.

Here, for example, when the power transistor 1 is turned off, the collector-emitter voltage of the power transistor 1
When V _CE reaches the voltage value of the DC power supply 34 (at this time, the capacitor 13 has already been charged to the power supply voltage value), the energy stored in the floating inductance 44 on the main circuit becomes the R-C-D snubber. It is absorbed by the capacitor 33 through the diode 31 of the circuit. At this time, a surge voltage generated by the energy of the floating inductance 44 is not sufficiently suppressed due to the influence of the transient forward voltage drop of the floating inductance 45 and the diode 31 existing on the RCD snubber circuit.

However, when V _CE reaches the zener voltage of the constant voltage diode 19, the constant voltage diode 19 conducts, and the floating inductance 44 passes through the constant voltage diode 20.
Will be absorbed by the capacitor 13.

FIGS. 3 and 4 are waveform diagrams showing the operation of one phase shown in FIG. 2. FIG. 3 shows changes in I _C and V _CE of the power transistor 1 with respect to time as described above. Yes, fourth
The figure shows the operation trajectories of I _C and V _CE shown in FIG. 3 when I _C and V _CE are plotted on the vertical axis and the horizontal axis, respectively.

That is, the power transistor 1 is turned off, and V _CE
When There rises to the Zener voltage V _Z of the Zener diode 19 is set to a value slightly higher than the supply voltage, the point Z after the third diagram, the current flowing to the stray inductance 44 zener diode 19 → the constant-voltage diode 20 3 and 4, the surge voltage during the collector current decreasing period can be reduced as shown by the broken line in FIGS. 3 and 4, and the operation trajectory of the power transistor 1 can be contained in the RBSOA. it can.

Thereafter, when the charging of the capacitor 13 is completed, the resonance by the stray inductance 44 and the capacitor 13 starts, but this energy is absorbed by the constant voltage diode 20.

Next, when the power transistor 1 is turned on,
Charge stored in capacitor 13 is a constant voltage diode
It is discharged through 20, 19 and the power transistor 1. At this time, if the zener voltage of the constant voltage diode 20 is set higher than the voltage between both ends of the capacitor 13, a disadvantage occurs in that the capacitor 13 does not discharge when the power transistor 1 is turned on. It is desirable to make the value as low as possible within a range in which can be suppressed.

When a snubber circuit including the constant voltage diodes 19 and 20 having a predetermined Zener voltage and the capacitor 13 is configured in this manner, the V _{CE of the} power transistor 1 becomes constant.
It does not operate until the Zener voltage reaches 9, and suppresses only a large surge voltage during the period during which the collector current falls, thereby preventing destruction of the semiconductor element.

The amount of charge stored in the capacitor 13 is much smaller than that of the RC snubber circuit shown in FIGS. 11 and 12 shown as the second conventional example, so that the capacitance of the capacitor 13 can be reduced. Energy loss generated in the snubber circuit can be made very small as compared with the conventional snubber circuit, and the snubber circuit can be reduced in size by reducing its capacity.

Further, by reducing the loss of the snubber circuit, the frequency at which the power transistor is switched can be increased.

FIG. 5 shows an example in which an NPN-type bipolar transistor is used in place of the constant voltage diode.
It is the same as the figure. The base terminal of the transistor 35 is open, and a series circuit of the transistor 35 and the capacitor 13 is connected to the power transistor 1 in parallel. Of course, the snubber circuit having the above configuration is also connected in parallel to the other five power transistors.

Here, when the base terminal is opened between the collector and the emitter of a general bipolar transistor, the bipolar transistor has a breakdown voltage in both forward and reverse directions as shown in FIG. That is, the forward breakdown voltage (point A in FIG. 6) of the transistor 35 is set slightly higher than the voltage value of the DC power supply 34, and the reverse breakdown voltage (point B in FIG. 6) is set as shown in FIGS. By setting the described value, the same effect as in FIGS. 2 to 4 can be obtained.

Note that FIG. 7 is obtained by replacing the example of FIG. 5 with a PNP-type bipolar transistor 46.

Now, an embodiment of the present invention will be described with reference to FIG.
In FIG. 8, reference numerals 19 and 20 denote constant voltage diodes as described above. For convenience, reference numeral 19 denotes a first constant voltage diode, and reference numeral 20 denotes a second constant voltage diode. First constant voltage diode
A first diode 36 is connected in the reverse direction to 19 to form a first series circuit, and a second constant voltage diode 20 is connected to a second diode.
Are connected in the reverse direction to form a second series circuit. These first and second series circuits correspond to the first
And the second constant voltage diodes 19 and 20 are connected in parallel so as to be opposite to each other. Further, a series circuit of the parallel connection circuit and the capacitor 13 is connected in parallel with the power transistor 1, that is, connected between the collector and the emitter of the power transistor 1, thereby forming a snubber circuit. This snubber circuit is connected, for example, to each of the power transistors 1 to 6 of the power converter shown in FIG. 7 is a freewheeling diode, and 44 is a stray inductance of the main circuit.

Next, the operation will be described.
When the collector-emitter voltage V _CE reaches the Zener voltage by the constant voltage diode 19 at the turn-off of
This constant voltage diode 19 conducts and the stray inductance
The energy stored in 44 is absorbed by the capacitor 13 through the diode 36. At this time, as shown in FIGS. 3 and 4, the surge voltage during the period in which the collector current is reduced is reduced, and the operation trajectory of the power transistor 1 is changed by RB.
It can fit in SOA. Thereafter, when the charging of the capacitor 13 is completed, resonance by the stray inductance 44 and the capacitor 13 starts, but this energy is absorbed by the constant voltage diode 20.

When the power transistor 1 is turned on, the electric charge stored in the capacitor 13 is transferred to the constant voltage diode 20,
It is discharged through the diode 37 and the power transistor 1.

As described in the description of FIGS. 2 to 4,
The Zener voltage of one constant voltage diode 19 is
The zener voltage of the other constant voltage diode 20 is set to a value as low as possible within a range in which resonance at the time of turn-off can be suppressed in order to promote discharge at the time of turn-on of the power transistor 1.

In this manner, the snubber circuit including the constant voltage diodes 19 and 20, the diodes 36 and 37, and the capacitor 13 having a predetermined zener voltage operates until the V _{CE of the} power transistor 1 reaches the zener voltage of the constant voltage diode 19. Instead, only a large surge voltage during the collector current falling period is suppressed to prevent the semiconductor element from being destroyed.

The amount of charge stored in the capacitor 13 is shown in FIG.
It is very small compared to the capacitor of the RC snubber circuit shown in FIG. 12, so that the capacitance of the capacitor 13 can be reduced and the snubber circuit can be downsized by reducing the capacitance. Further, the switching frequency of the power transistor can be increased by reducing the loss of the snubber circuit.

Particularly, in this embodiment, the diodes 36 and 37 are connected so that the current flows only in the zener direction to each of the constant voltage diodes 19 and 20 and the forward current does not flow. The loss caused by the directional current can be reduced. Therefore, FIG. 1, FIG. 2, FIG.
7, the size and capacity of the constant voltage diodes 19 and 20 can be reduced.

In the above-described embodiment of the present invention, a power transistor is described as a power semiconductor element. However, instead of a power transistor, another self-extinguishing type power semiconductor element such as an IGBT or a field effect transistor is used. It is needless to say that the present invention can be applied to the case where is used.

(Effect of the Invention) As described above, according to the present invention, in series with a capacitor,
Forming a snubber circuit by connecting a first and a second series circuit comprising a constant voltage diode and a diode in parallel with each other so that the respective constant voltage diodes are opposite to each other;
Since this snubber circuit is connected in parallel with each of the self-extinguishing type semiconductor elements constituting the power conversion device, it is possible to suppress a surge voltage at the time of switching of the semiconductor element and prevent the semiconductor element from being destroyed. Further, the loss generated in the snubber circuit can be reduced to downsize the components of the snubber circuit, and the switching frequency of the semiconductor element can be increased to be applied to a high-frequency inverter and the like.

[Brief description of the drawings]

FIG. 1 is a main circuit diagram of a power converter using a snubber circuit which is a basic embodiment of the present invention, FIG. 2 is a configuration diagram of a snubber circuit which is a basic embodiment of the present invention, FIG. 4 is an operation waveform diagram of a power transistor for one phase, FIG. 5 is a circuit diagram showing a modification of FIG. 2, FIG. 6 is a characteristic diagram of a bipolar transistor, and FIG. 7 is another diagram of FIG. FIG. 8 is a circuit diagram showing a modification, FIG. 9 is a circuit diagram showing an embodiment of the present invention, FIG. 9 is a main circuit diagram of a power conversion device to which the first conventional example is applied, and FIG.
FIG. 11 is a circuit diagram of a first conventional example, FIG. 11 is a main circuit diagram of a power conversion device to which the second conventional example is applied, and FIG. 12 is a circuit diagram of a second conventional example. 1-6 Power transistor 7-12,31,36,37 Diode 13-18,33 Capacitor 19-30 Constant voltage diode 32 Resistance, 34 DC power supply 35,46 Bipolar transistor 44,45 Floating Inductance

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-55-58733 (JP, A) JP-A-52-120352 (JP, A) JP-A-58-93039 (JP, U) JP-A-63 5431 (JP, Y2) J. 45-41333 (JP, Y2) (58) Fields investigated (Int. Cl. ⁶ , DB name) H02H 7/12 H02M 1/00-1/06 H03K 17/08- 17/16

Claims (1)

(57) [Claims] 1. A series circuit of self-extinguishing type power semiconductor elements having a freewheeling diode connected in anti-parallel is connected to a power supply, and one semiconductor element and the other semiconductor element forming the series circuit are connected to each other. A power converter that alternately turns on and off to perform power conversion, comprising: a first series circuit including a first constant voltage diode and a first diode connected in a reverse direction to the constant voltage diode; And a second series circuit including a second constant voltage diode and a second diode connected in reverse direction to the constant voltage diode so that the first and second constant voltage diodes are in opposite directions to each other. And a series circuit of the parallel connection circuit and the capacitor is connected in parallel to each semiconductor element.