JPH05111145A - Overcurrent protective circuit for transistor - Google Patents

Abstract

PURPOSE:To provide a safe and secure overcurrent protective circuit which has solved complexity of the protective circuit due to low overcurrent withstand capacity, when using an insulated gate bipolar transistor(IGBT) for the power switching element of a motor driving inverter. CONSTITUTION:This circuit is equipped with a means, which detects an overcurrent by a shunt resistor 5 and an overcurrent detecting circuit 7, a means which detects the rate of change of a value of an overcurrent, a means which measures the duration of overcurrent after its detection by a timer circuit 8, and a means which breaks the gate input of an IGBT by a trip circuit 9. And this is an overcurrent protective circuit for an IGBT, which breaks the gate input of the IGBT at a point of time when the overcurrent exceeds the specified duration, practically, 10mus, and besides when the rate of change of a value of the overcurrent is lower than the specified value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transistor overcurrent protection circuit for protecting an insulated gate bipolar transistor (hereinafter abbreviated as IGBT) used in an inverter or the like for driving a motor from an overcurrent such as a short circuit accident. ..

[0002]

2. Description of the Related Art In recent years, demand for inverter devices has been increasing with the development of power electronics technology. In some cases, a large current may momentarily flow through a power switching element in a motor drive inverter, and in such a case, various protections are performed to prevent destruction of the element due to over-resistance of the element. Among them, there is a short circuit protection function for preventing the power switching element from being destroyed by a large current due to a short circuit when the motor causes an accident such as a layer short circuit. The conventional short circuit protection will be described below. FIG. 2 shows the configuration of a conventional protection circuit.
Is a three-phase or single-phase power source, 2 is a bridge diode, 3
Is a capacitor, 4 is an inverse converter (hereinafter referred to as an inverter), 5 is a shunt resistor, and 6 is a motor. here,
The power switching element that constitutes the inverter unit 4 is
Transistor, MOSFET (Metal Oxide Semiconduct
or Field Effect Transistor), IGBT (InsulatedG)
ateBipolar Transistor) etc.

Now, when the motor is short-circuited, the energy stored in the capacitor 3 flows into the inverter unit 4, and a voltage proportional to the large current flowing into the inverter unit 4 is generated in the shunt resistor 5. Reference numeral 7 denotes an overcurrent detection circuit, which is a circuit for determining whether or not the current value exceeds the withstand voltage of the power switching element by the voltage generated by the shunt resistor 5, that is, a means for detecting the overcurrent, and the detected signal is a timer circuit 8 And to the trip memory circuit 9. The timer circuit 8 is a means for measuring the duration time after the overcurrent is detected, and the timer is set by the signal from the overcurrent detection circuit 7, and the duration time of the overcurrent exceeds the preset predetermined timer time. Trip memory circuit 9
Send a signal to. The trip memory circuit 9 normally transmits a gate signal from the control circuit 10 (a signal for turning on / off the power switching element of the inverter unit 4) to the gate drive circuit 11, but when an overcurrent is detected, an overcurrent detection circuit is provided. 7 is a circuit for blocking the transmission of the gate signal based on the signals from 7 and the timer circuit 8, that is, a means for cutting off the gate input of the power switching element. That is, when the overcurrent flows through the shunt resistor 5, the gate inputs of all the power switching elements are cut off after a predetermined timer time of the timer circuit 8. This timer time is the reverse recovery current of the diode inserted in parallel with the power switching element, and the short-circuit current for a short time that does not lead to the destruction of the element such as when it malfunctions due to noise or surge voltage. Is set to about several tens of μs so as not to stop, and the protective operation is not performed during that period.

Next, a case where an IGBT is also used for the power switching element of the inverter section 4 will be described. IGB
T has undergone rapid development and its characteristics are known to be as follows. First, the so-called conductivity modulation phenomenon occurs in the IGBT due to the injection of minority carriers, and a large current can flow in the conductive state. Transistor, MOSFET, I
Table 1 shows the comparison value of the peak current value (when the capacitor voltage is DC300V) and the chip area ratio with respect to the rated current of each element of the GBT.

[0005]

[Table 1]

Secondly, as shown in (Table 1), the IGBT has a small chip area. Also, the speed to turn off the third is very fast. Finally, the cost is also cheaper than MOSFET. In the circuit of FIG. 2, the IGBT is turned off at a low speed when an overcurrent occurs due to a short circuit accident. That is, the overcurrent protection when the IGBT is short-circuited is cut off within a short time without element destruction, and a soft cutoff method is adopted in order to keep the operation locus at this time within RBSOA (reverse bias safe operation area).

[0007]

As described above, when the load short-circuit protection IGBT is softly cut off, as shown in (Table 1), the ratio of the rated current of the device to the chip area and the peak current value at the time of short circuit Has the highest IGBT, and if the rated current is the same, the IGBT has a problem in that it easily breaks due to energy resistance, and the gate drive circuit is complicated and costly.

An object of the present invention is to solve the above-mentioned conventional problems and to provide a safe and simple transistor overcurrent protection circuit which prevents load short-circuit breakdown, surge voltage dV / dt breakdown and latch-up breakdown. And

[0009]

In order to achieve the above object, an overcurrent protection circuit for a transistor according to the present invention is an IGBT.
And a means for detecting the rate of change of the detected current, and the gate input of the IGBT is cut off after the rate of change of the overcurrent reaches zero or a value close to zero.

[0010]

With this structure, the latch-up is prevented when the overcurrent of the IGBT is cut off, and the thermal destruction due to the junction over of the IGBT is also prevented.

[0011]

EXAMPLES Examples of the present invention will be described. First IG
The mechanism of current flow when the BT is short-circuited will be described with reference to FIG. From the equivalent circuit of the IGBT shown in FIG.
The collector current IC of the GBT is the current I1 flowing through the MOSFET Q1 and the collector current I of the PNP transistor Q2.
It can be separated into two, and the ratio of the current values of I1 and I2 depends on the physical properties of the device such as the life time killer that influences the characteristics of the IGBT.
And I2, that is, I2 / I1 is in the range of 0.5 to 2. This ratio means the hFE (current amplification factor) of the PNP transistor. Generally, hFE rapidly decreases when the junction temperature exceeds 150 ° C, but in the case of IGBT, hFE is considerably low at 0.5 to 2 even at room temperature, and when it reaches a high temperature of 150 ° C, so-called conductivity modulation phenomenon is expected. It becomes impossible, and the collector current I2 of the PNP transistor Q2 becomes almost zero.

FIG. 4 is a diagram showing this on the time axis, and the collector current IC of the IGBT is the sum of I1 and I2. In FIG. 4, t1 is the time point of occurrence of overcurrent, and time t
The collector current IC increases up to 2. At time t2, the collector current reaches its peak, and the junction temperature of the IGBT exceeds 150 ° C before and after this, and after that the MOSFET
The current I1 of Q1 gradually decreases due to the increase of the ON resistance value thereof, but the current I2 of the PNP transistor Q2 on one side sharply decreases from time t2 and becomes almost zero at time t3.

At this point, most of the current flowing through the IGBT becomes the current I1 flowing through the MOSFET Q1 shown in FIG.
Since I2 is almost zero, NPN transistor Q3
No voltage is generated in the resistance between the base and the emitter of the IGBT, so that the latch-up of the IGBT does not occur. If the current is cut off at the time when the current value at time t2 reaches its peak, the IGBT is likely to wrap up because the current is cut off at the maximum increase rate (dV / dt) of the collector voltage. As described in section. Further, during the period from the time t3 to the time t4, the short circuit current flows around the current I1 flowing through the MOSFET Q1 and if the time t4 is too long, IG
The junction temperature of BT further rises and causes thermal destruction. The time immediately after t3 in FIG. 4 is optimal for surely protecting the IGBT from an overcurrent by the above mechanism.
It can be seen that the rate of change of the short-circuit current in the period from time t3 to just after t4 when the IGBT is destroyed is almost zero as compared with the large increase rate in the periods t1 to t2 and the large decrease rate in the periods t2 to t3.

The present invention utilizes the fact that the rate of change of the overcurrent is small to provide the most safe shutoff protection of the IGBT. The details will be described with reference to the embodiment shown in FIG. Of the circuit configuration in FIG. 1, 1 to 11 are the same as in FIG. The difference between the circuit configurations of FIGS. 2 and 1 is that the current change rate detection circuit 12 is added. This current change rate detection circuit 12
It is composed of a differential circuit and a circuit that detects the level of its output with a window comparator, and the change of the short-circuit current is predetermined by the level detection circuit with the differential circuit and the window comparator of the input voltage proportional to the overcurrent generated in the shunt resistor 5. When the timing becomes smaller than a predetermined value, that is, when the time t3 in FIG. 4 is reached, a signal is transmitted to the trip storage circuit, and the trip storage circuit 9 outputs the signal of the current change rate detection circuit 12 and the signal of the overcurrent detection circuit 7. , AND of the signal of the timer circuit 8 indicating that the duration of the overcurrent exceeds the predetermined timer time, the existence of the overcurrent and the time t3 at which the rate of change thereof becomes small are recognized, and I
The gate drive circuit 11 is instructed to shut off the gate input of the GBT.

In addition to the analog circuit, the current change rate detection circuit 12 may be constructed by a method such as an A / D converter circuit and a microcomputer or DSP (digital signal processor). In the case of the present invention, the timer time of the timer circuit 8 is about 10 μs in the section t1 to t3 of FIG.
Therefore, a value close to 10 μs is appropriate.

[0016]

As described above, according to the overcurrent protection circuit of the present invention, it is possible to cover the greatest drawback of the IGBT having a small overcurrent withstand capability and improve the overcurrent protection performance.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an overcurrent protection circuit of the present invention.

FIG. 2 is a block diagram showing a configuration of a conventional overcurrent protection circuit.

FIG. 3 is an equivalent circuit diagram of the IGBT.

FIG. 4 is a current change diagram for explaining the mechanism of the short circuit current of the IGBT.

[Explanation of symbols]

4 Inverter unit (IGBT) 5 Shunt resistance (means for detecting overcurrent) 7 Overcurrent detection circuit (means for detecting overcurrent) 8 Timer circuit (means for measuring the duration time after overcurrent is detected) 9 Trip Memory circuit (means for cutting off the gate input of the IGBT) 12 Current change rate detection circuit (means for detecting the rate of change in the value of overcurrent)

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[Procedure amendment]

[Submission date] September 24, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Full text

[Correction method] Change

[Correction content]

[Document name] Statement

Patent application title: Transistor overcurrent protection circuit

[Claims]

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transistor overcurrent protection circuit for protecting an insulated gate bipolar transistor (hereinafter abbreviated as IGBT) used in an inverter or the like for driving a motor from an overcurrent such as a short circuit accident. ..

[0002]

2. Description of the Related Art In recent years, demand for inverter devices has been increasing with the development of power electronics technology. In some cases, a large current may momentarily flow through a power switching element in a motor drive inverter, and in such a case, various protections are performed to prevent destruction of the element due to over-resistance of the element. Among them, there is a short circuit protection function for preventing the power switching element from being destroyed by a large current due to a short circuit when the motor causes an accident such as a layer short circuit. The conventional short circuit protection will be described below. FIG. 2 shows the configuration of a conventional protection circuit.
Is a three-phase or single-phase power source, 2 is a bridge diode, 3
Is a capacitor, 4 is an inverse converter (hereinafter referred to as an inverter), 5 is a shunt resistor, and 6 is a motor. here,
The power switching element that constitutes the inverter unit 4 is
Transistor, MOSFET (Metal Oxide Semiconduct
or Field Effect Transistor), IGBT (InsulatedG)
ateBipolar Transistor) etc.

Now, when the motor is short-circuited, the energy stored in the capacitor 3 flows into the inverter unit 4, and a voltage proportional to the large current flowing into the inverter unit 4 is generated in the shunt resistor 5. Reference numeral 7 denotes an overcurrent detection circuit, which is a circuit for determining whether or not the current value exceeds the withstand voltage of the power switching element by the voltage generated by the shunt resistor 5, that is, a means for detecting the overcurrent, and the detected signal is a timer circuit 8 And to the trip memory circuit 9. The timer circuit 8 is a means for measuring the duration time after the overcurrent is detected, and the timer is set by the signal from the overcurrent detection circuit 7, and the duration time of the overcurrent exceeds the preset predetermined timer time. Trip memory circuit 9
Send a signal to. The trip memory circuit 9 normally transmits a gate signal from the control circuit 10 (a signal for turning on / off the power switching element of the inverter unit 4) to the gate drive circuit 11, but when an overcurrent is detected, an overcurrent detection circuit is provided. 7 is a circuit for blocking the transmission of the gate signal based on the signals from 7 and the timer circuit 8, that is, a means for cutting off the gate input of the power switching element. That is, when the overcurrent flows through the shunt resistor 5, the gate inputs of all the power switching elements are cut off after a predetermined timer time of the timer circuit 8. This timer time is the reverse recovery current of the diode inserted in parallel with the power switching element, and the short-circuit current for a short time that does not lead to the destruction of the element such as when it malfunctions due to noise or surge voltage. Is set to about several tens of μs so as not to stop, and the protective operation is not performed during that period.

Next, a case where an IGBT is also used for the power switching element of the inverter section 4 will be described. IGB
T has undergone rapid development and its characteristics are known to be as follows. First, the so-called conductivity modulation phenomenon occurs in the IGBT due to the injection of minority carriers, and a large current can flow in the conductive state. Transistor, MOSFET, I
Table 1 shows the comparison value of the peak current value (when the capacitor voltage is DC300V) and the chip area ratio with respect to the rated current of each element of the GBT.

[0005]

[Table 1]

Secondly, as shown in (Table 1), the IGBT has a small chip area. Also, the speed to turn off the third is very fast. Finally, the cost is also cheaper than MOSFET.

Utilizing the above characteristics, the IGBT has been used in recent years.
The gate drive circuit has a soft shutoff for short-circuit current protection.
The method to do is.

A conventional example of these contents will be described.
It FIG. 3 shows an equivalent circuit of the IGBT. Where Q1 is
MOSFET, Q2 is PNP transistor, Q3 is NP
N transistor, R1 is the base-emitter resistance of Q3
Anti-C is the depletion layer between the collector and base of Q2 and Q3.
Capacity.

In operation, the gate G has a threshold.
When the voltage is exceeded, I1 and I2 flow, the sum of which is I
It is a collector current Ic of GBT.

When a load short circuit occurs, the IGBT
The short-circuit current of the
It will always be a large value. When Ic becomes large, R
A voltage of 1 × I2 is generated between the base and emitter of Q3
Then, when Q3 turns on, the gate voltage is thresholded.
The IGBT will not turn off even if the voltage is below the threshold voltage.
Therefore, latch-up occurs.

Besides the latch-up, there is the following reason.
Be understood. Here, when a large current is suddenly cut off,
Increase rate of collector-emitter voltage due to ΔVce / Δt
Becomes a large value, and C1 and ΔV of the depletion layer of Q2 and Q3
current I3 due to ce / Δt = C1 × ΔVce / Δt is R
1 and the sum current of I2 flows to R1 and the base of Q3
A voltage of R1 × (I2 + I3) is generated between the emitters,
As a result, Q3 turns on and the IGBT latches.
Up.

That is, in order to prevent latch-up, a short
At the same time as the magnitude of the leakage current,
Size is also an important factor. Therefore, load short circuit
When it happens, conventionally, the IGBT is slowly shut off and the
How to reduce current I3 and prevent latch-up
ing.

A conventional example will be described below. IGBT is
Since it takes less time to destroy than a transistor,
Need to be cut off. However, especially for high current devices, short
When the overcurrent at the time of the fault is cut off at high speed by the IGBT,
The surge voltage applied between the emitter and emitter is RBSOA (reverse voltage
There is a risk that the device may be damaged by leaving the safe operating area).
This avoidance turns off the IGBT at a low speed only when a short circuit occurs.
Is effective.

Example of driving circuit for executing this operation
Will be described. This circuit is
When a short circuit occurs, both are turned off at the same time, and the IGBT
Due to the relatively high resistance connected between the gate and collector, I
It employs a method of turning off the GBT at a low speed. That is, the overcurrent protection when the IGBT is short-circuited is cut off within a short time without element destruction, and a soft cutoff method is adopted in order to keep the operation locus at this time within RBSO A.

[0015]

As described above, when the load short-circuit protection IGBT is softly cut off, as shown in (Table 1), the ratio of the rated current of the device to the chip area and the peak current value at the time of short circuit Has the highest IGBT, and if the rated current is the same, the IGBT has a problem in that it easily breaks due to energy resistance, and the gate drive circuit is complicated and costly.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above conventional problems and to provide a safe and simple transistor overcurrent protection circuit which prevents load short-circuit breakdown, surge voltage dV / dt breakdown and latch-up breakdown. And

[0017]

In order to achieve the above object, an overcurrent protection circuit for a transistor according to the present invention is an IGBT.
And a means for detecting the rate of change of the detected current, and the gate input of the IGBT is cut off after the rate of change of the overcurrent reaches zero or a value close to zero.

[0018]

With this structure, the latch-up is prevented when the overcurrent of the IGBT is cut off, and the thermal destruction due to the junction over of the IGBT is also prevented.

[0019]

EXAMPLES Examples of the present invention will be described. First IG
The mechanism of current flow when the BT is short-circuited will be described with reference to FIG. From the equivalent circuit of the IGBT shown in FIG.
The collector current Ic of GBT is the current I1 flowing through the MOSFET Q1 and the collector current I of the PNP transistor Q2.
It can be separated into two, and the ratio of the current values of I1 and I2 depends on the physical properties of the device such as the life time killer that influences the characteristics of the IGBT.
And I2, that is, I2 / I1 is in the range of 0.5 to 2. This ratio means the hFE (current amplification factor) of the PNP transistor. Generally, hFE rapidly decreases when the junction temperature exceeds 150 ° C, but in the case of IGBT, hFE is considerably low at 0.5 to 2 even at room temperature, and when it reaches a high temperature of 150 ° C, so-called conductivity modulation phenomenon is expected. It becomes impossible, and the collector current I2 of the PNP transistor Q2 becomes almost zero.

FIG. 4 shows this on the time axis, and the collector current Ic of the IGBT is the sum of I1 and I2. In FIG. 4, t1 is the time point of occurrence of overcurrent, and time t
The collector current Ic increases up to 2. At time t2, the collector current reaches its peak, and the junction temperature of the IGBT exceeds 150 ° C before and after this, and after that the MOSFET
The current I1 of Q1 gradually decreases due to the increase of the ON resistance value thereof, but the current I2 of the PNP transistor Q2 on one side sharply decreases from time t2 and becomes almost zero at time t3.

At this point, most of the current flowing through the IGBT becomes the current I1 flowing through the MOSFET Q1 shown in FIG.
Since I2 is almost zero, NPN transistor Q3
No voltage is generated in the resistance between the base and the emitter of the IGBT, so that the latch-up of the IGBT does not occur. Note that if current interruption when the current value of the time t2 reaches a peak, that there is a risk that IGBT is latch-up to cut off at the maximum rate of rise of the collector voltage (dV / dt) associated therewith, prior art As described in section. Further, during the period from the time t3 to the time t4, the short circuit current flows around the current I1 flowing through the MOSFET Q1 and if the time t4 is too long, IG
The junction temperature of BT further rises and causes thermal destruction. The time immediately after t3 in FIG. 4 is optimal for surely protecting the IGBT from an overcurrent by the above mechanism.
It can be seen that the rate of change of the short-circuit current in the period from time t3 to just after t4 when the IGBT is destroyed is almost zero as compared with the large increase rate in the periods t1 to t2 and the large decrease rate in the periods t2 to t3.

The present invention utilizes the fact that the rate of change of the overcurrent is small to provide the safest interruption protection of the IGBT, and the details thereof will be described with reference to the embodiment shown in FIG. Of the circuit configuration in FIG. 1, 1 to 11 are the same as in FIG. The difference between the circuit configurations of FIGS. 2 and 1 is that the current change rate detection circuit 12 is added. This current change rate detection circuit 12
It is composed of a differential circuit and a circuit that detects the level of its output with a window comparator, and the change of the short-circuit current is predetermined by the level detection circuit with the differential circuit and the window comparator of the input voltage proportional to the overcurrent generated in the shunt resistor 5. When the timing becomes smaller than a predetermined value, that is, when the time t3 in FIG. 4 is reached, a signal is transmitted to the trip storage circuit, and the trip storage circuit 9 outputs the signal of the current change rate detection circuit 12 and the signal of the overcurrent detection circuit 7. , AND of the signal of the timer circuit 8 indicating that the duration of the overcurrent exceeds the predetermined timer time, the existence of the overcurrent and the time t3 at which the rate of change thereof becomes small are recognized, and I
The gate drive circuit 11 is instructed to shut off the gate input of the GBT.

The current change rate detection circuit 12 may be configured by a method such as an A / D converter circuit and a microcomputer or DSP (digital signal processor) in addition to the above analog circuit. In the case of the present invention, the timer time of the timer circuit 8 is about 10 μs in the section t1 to t3 of FIG.
Therefore, a value close to 10 μs is appropriate.

[0024]

As described above, according to the overcurrent protection circuit of the present invention, it is possible to cover the greatest drawback of the IGBT having a small overcurrent withstand capability and improve the overcurrent protection performance.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an overcurrent protection circuit of the present invention.

FIG. 2 is a block diagram showing a configuration of a conventional overcurrent protection circuit.

FIG. 3 is an equivalent circuit diagram of the IGBT.

FIG. 4 is a current change diagram for explaining the mechanism of the short circuit current of the IGBT.

[Explanation of Codes] 4 Inverter Unit (IGBT) 5 Shunt Resistor (Means for Detecting Overcurrent) 7 Overcurrent Detection Circuit (Means for Detecting Overcurrent) 8 Timer Circuit (Time after Detection of Overcurrent) Means for measuring) 9 Trip memory circuit (means for shutting off gate input of IGBT) 12 Current change rate detection circuit (means for detecting rate of change of overcurrent value)

Claims (1)

[Claims] 1. A means for detecting an overcurrent, a means for detecting a rate of change of the value of the overcurrent, a means for measuring a duration time after the overcurrent is detected, and a gate input of an insulated gate bipolar transistor being cut off. An overcurrent protection circuit for a transistor, which cuts off the gate input of the transistor after a predetermined duration of the overcurrent and when the rate of change of the value of the overcurrent is smaller than a predetermined value.