JP3039092B2 - Short circuit protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a short-circuit protection circuit for preventing a voltage-driven MOS element or a current-driven bipolar element as an output element from being damaged by an overcurrent flowing when a load is short-circuited.

[0002]

2. Description of the Related Art As a power switching element, for example, a voltage drivable MO such as a MOSFET or an IGBT is used.
An S element or a bipolar element that can be driven by current is known. FIG. 2 conceptually shows a circuit when an IGBT is used as a switching element. A load 23 is connected between the DC power supply 22 and the IGBT 1 having the gate connected to the drive circuit 21. In the normal operation, when the IGBT 1 is in the off state, the current is cut off, and the voltage of the power supply 22 is applied to both ends of the IGBT 1. On the other hand, in the ON state, the power supply 22
Is almost applied, and almost no voltage is applied to the IGBT1. However, a predetermined current is flowing. That is, the IGBT 1 does not have a state in which a high voltage is applied and the IGBT 1 is in the ON state. However load 23
However, there is a case where a short circuit occurs with a power supply as shown by a dotted line 24 due to some failure. This is called a load short circuit. When the semiconductor element 1 is turned on in this state, the power supply 22 is directly applied to the semiconductor element 1, and a large current flows through the IGBT 1 with a voltage applied. Therefore, very large heat is generated and the IGBT 1 is broken. In order to prevent this, a protection circuit is usually included in the drive circuit 21.

A circuit as shown in FIG. 3 is known as such a load short-circuit protection circuit. Main IGBT element 1
The discharge MOSFET 2 is connected through a reverse blocking diode 3 for preventing a gate current from flowing through the parasitic diode 20 of the discharge MOSFET 2 when the gate is negatively biased. on the other hand,
Part of the main element is an IGBT for current detection by emitter separation.
An element 11 is formed, and its emitter is MOSFET2.
And the emitter of the main element 1 via the current detection resistor 4. When an overcurrent flows through the main element 1 due to a load short circuit while the element is on, the detection element
Overcurrent also flows through 11 and a large voltage drop occurs at both ends of the current detection resistor 4. Therefore, the discharging MOSFET 2
Is turned on by the rise of the gate voltage, and the electric charge stored in the gate of the main element 1 is discharged by the MOSFET 2. On the other hand, a gate voltage for turning on the element is applied to the gate terminal 5, but the gate voltage of the main element 1 is divided by the sum of the impedance of the gate resistor 6 and the impedance of the discharging MOSFET 2 and the reverse blocking diode 3. The voltage drops to Due to the decrease in the gate voltage, the short-circuit current is reduced, the time until the element is destroyed is increased, and protection by an external circuit can be easily performed.

In this case, in the circuit of FIG. 3, since the gate voltage of the main element 1 is reduced and the gate voltage of the current detecting element 11 is also reduced, the current flowing through the detecting element 11 is also reduced.
Therefore, the voltage drop across the current detection resistor 4 decreases, and the impedance of the discharge MOSFET 2, that is, the on-resistance increases again. Since this negative feedback has a delay element due to the gate capacitance of the main element, an oscillation phenomenon is easily generated. FIG. 4 shows a measurement circuit for simulating a load short circuit.
The switching element 41 is connected to the power supply 42 without load impedance.
And a gate signal is applied between the gate and the source thereof via a gate resistor 44 by a gate drive circuit 43. 5, the solid line 51 and 53 indicates the waveform of the gate signal V _G and a collector current I _C as measured by the measurement circuit of Fig. When the gate to apply a threshold voltage greater than V _G element is turned on, but the load impedance is 0
(Actually, there is a stray inductance), so that a current determined by the characteristics of the element and the gate voltage flows. When the gate voltage is sufficiently high, this current is very large, and the element is destroyed, for example, at a destruction point 55 due to heat generation and the accompanying latch-up phenomenon. In order to prevent this, the collector current I _C when the short-circuit protection circuit of FIG. 3 is used.
The waveform and the waveform of the gate voltage V _G of the main device 1 in FIG. 5 indicated by dotted lines 52 and 54. When the gate voltage applied to the gate terminal 5 is equal to or higher than the gate threshold, the collector terminal 7
When a large current flows between the gate electrode and the emitter terminal 8, the gate voltage applied to the main element decreases as described above. Thereby, the current of the main element 1 is reduced, but at the same time, the current of the current detecting element 11 is also reduced, and the voltage drop of the current detecting resistor 4 is also reduced. Therefore, the on-resistance of the discharging MOSFET 2 increases, and the gate voltage applied to the main element 1 increases again. Gate voltage is this repeated return V _G and a collector current I _C becomes oscillation state.

FIG. 6 shows a load short-circuit protection circuit that prevents such oscillation. 6, in which the same reference numerals are given to the same parts as in FIG. 3, the current detecting resistor 4 and the discharging M
A diode 9 and a discharge resistor 10 in parallel with the diode 9 are inserted between the gates of the OSFET 2. If this resistor 10 is made sufficiently large, the charge stored in the gate of the discharging MOSFET will not be discharged immediately when an overcurrent is detected through the diode 9, and even if the voltage drop across the resistor 4 becomes small due to the operation of the short-circuit protection circuit. Since the gate voltage of the main element 1 is kept low, an oscillation phenomenon due to negative feedback does not occur.

[0006]

However, in the circuit shown in FIG. 6, a discharge MOS is required for a predetermined time until an external circuit operates.
Resistor to keep constant charge at the gate of FET2
10 needs to be big. However, even if the gate signal is input to turn on the device immediately after the external circuit is operated, the charge is retained, so that the discharging MOSFE is held.
T2 remains on. Therefore, a sufficient gate voltage is not applied to the main element 1, and the main element 1 operates in a high impedance state. Then, a loss occurs, the temperature rises, and the device is destroyed.

Such a problem similarly exists in a short-circuit protection circuit that uses a bipolar transistor as a switching element and shunts a base current using a discharging element when a load is short-circuited.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problems and to reduce the increase in drive voltage or drive current in order to prevent the main current from oscillating during the operation of the protection circuit. An object of the present invention is to provide a short-circuit protection circuit that can turn on an element again by a signal.

[0009]

In order to achieve the above object, the present invention provides a main switching device driven by a signal input from a control terminal to a control electrode, and a control electrode and a first main electrode. A control electrode of the switching element and a current detection switching element connected to the first main electrode respectively, the second main electrode is connected to the second main electrode of the main switching element via a current detection resistor, The drive signal of the main switching element is reduced by turning on the discharging switching element inserted in the branch circuit of the circuit from the control terminal to the control electrode of the main switching element when an excessive current flows through the current detection resistor, And, in the case of maintaining the reduction of the drive signal even if the overcurrent flowing to the main switching element is suppressed by the reduction of the drive signal,
After the drive signal is reduced by turning on the discharge switching element, the discharge switching element is turned off when the signal input to the drive terminal becomes zero or reverse polarity.
Control electrode and main switching element of discharge switching element
Between the control electrodes of the
Thus, a forward diode is connected .

According to the present invention, even if the drive signal is a voltage,
Is effective even if The discharge switching element is a MOSFET, and a gate electrode of the discharge MOSFET, a second main electrode of the current detection switching element, and a main switching element for maintaining a reduction in the drive signal when the overcurrent is suppressed. This is effective when means for shortening the charge time of the gate charge of the discharge MOSFET and increasing the discharge time is inserted between the second main electrode and the connection circuit of the second main electrode. Means for shortening the charging time of the gate charge of the discharging MOSFET and increasing the discharging time include a diode connected to the current detecting switching element side of the current detecting resistor and the second main electrode side of the main switching element. It is effective that the resistor is connected to the discharge resistor. Then, by connecting a diode between the gate of the discharging MOSFET and the control electrode of the main switching element, the driving signal is reduced. It is effective that the MOSFET is turned off. In that case, the branch circuit including the discharging MOSFET, the current detecting resistor and the discharging resistor are connected to the second main electrode of the main switching element via the reverse blocking diode, respectively. It is also effective that the detection resistor and the discharge resistor are connected to the second main electrode of the main switching element via a common reverse blocking diode. Further, it is effective that the above-mentioned short-circuit protection circuit has the main switching element and the current detection switching element formed on the same semiconductor substrate.

[0011]

When the signal inputted to the drive terminal by the external circuit becomes zero or reverse polarity after the short-circuit protection circuit operates, for example, the discharge switching element is a MOSFET,
If the discharge switching element is turned off in such a way that the electric charge accumulated in the gate is discharged through the diode connected to the control electrode of the main switching element,
Since the short-circuit protection circuit is in the reset state, the main switching element can be operated immediately by a signal applied to the drive terminal again.

[0012]

FIG. 1 shows a short-circuit protection circuit for an IGBT according to one embodiment of the present invention. In this circuit, a reset diode 12 is added between the gate of the discharging MOSFET 2 and the gate of the main IGBT element 1 in the circuit of FIG. FIG.
(A) shows the gate potential of the main element 1, (b) shows the collector current of the main element 1, and (c) shows the current detecting resistor 4 as shown in the figure. (D) represents the gate potential of the discharging MOSFET 2. Assuming that a load short circuit occurs at time t ₁ , the main element 1 and the current detecting element
11 collector current of increases rapidly, also the protection circuit operates at t ₂ rises sensor potential therewith. MOS
The gate potential of the FET ₂ rises almost in the same manner as the sensor potential from t ₁ to t ₂ . exceeds t _2, the protection circuit operates the main current limit (b), the although the sensor potential is lowered, the gate potential of the diode 9 and the resistor 10 MOSFET 2 is maintained. Time point t ₃
As a result, the voltage applied to the gate of the main element by the external circuit is reduced, and the gate potential of the main element is reduced. In this case, in the protection circuit shown in FIG. 6, since the electric charge at the gate of the MOSFET 2 only flows through the resistor 10, the MOSFET 2
The gate potential of No. 2 gradually decreases as shown by the dotted line 71 in FIG. However, in the protection circuit of FIG.
Since the are added between the gate of the main element 1, the gate signal becomes zero at t _3, the gate potential of the discharging MOSFET by the diode 12 is also reset to zero. That is, after t ₃ , the protection circuit remains operating in the conventional circuit shown in FIG. 6, but the voltage applied to the gate terminal 5 by the operation of the external circuit is 0 V in the circuit shown in FIG.
At the same time, the protection circuit can also be reset, so that a return operation can be performed immediately. The discharge resistor 10 is
Although it may be connected in parallel with the diode 9 as shown in FIG.
Since the potential of the current detecting resistor 4 on the element 11 side is high,
In order to prevent T2 from being reset, it is desirable to connect the resistor 4 to the emitter terminal 8 side.

To turn off the main element 1, a gate terminal 5
Often, the voltage applied to is not negatively biased but 0V. In this case, the current flows from the emitter terminal 8 to the gate terminal 5 through the diode 12 in the state of FIG. Fig. 8 shows how to prevent this.
It is an Example shown in FIG. In this protection circuit, the current detection resistor 4, discharge resistor 10 and MOSFET 2 of the circuit of FIG.
And a reverse blocking diode 13 is commonly connected in series. Thus, no current flows from the emitter to the gate even if a negative bias is applied to the gate. In this circuit, the reverse blocking diode 3 in FIG. 1 is unnecessary and omitted. However, while this is left, separate reverse blocking diodes may be connected to the emitter terminal 8 side of the current detecting resistor 4 and the discharging resistor 10, respectively.

In the above embodiment, the short-circuit protection circuit of the IGBT has been described. However, the short-circuit protection circuit of the MOSFET or the MOSFET for discharging the base current when an overcurrent flows.
A short circuit protection circuit such as a bipolar transistor that shunts through the circuit can be similarly implemented.

[0015]

According to the present invention, when an overcurrent flows in the main switching element, the operation of the protection circuit for reducing the drive signal input to the drive electrode, which is performed to suppress the current, is performed by the overcurrent. It is maintained after the suppression, but when the signal applied to the drive terminal by the external circuit becomes zero or reverse polarity, the protection circuit is reset, so that the main element can be controlled by the drive signal from that point. Since the high impedance state of the main element can be avoided, the main element can be prevented from being broken.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a short-circuit protection circuit according to one embodiment of the present invention.

FIG. 2 is a circuit diagram illustrating an example of a circuit using an IGBT;

FIG. 3 is a circuit diagram of a conventional short-circuit protection circuit.

FIG. 4 is a circuit diagram for simulating a load short circuit.

FIG. 5 is a waveform diagram of a gate signal and a collector current when a short-circuit protection circuit is not used and when the short-circuit protection circuit of FIG. 3 is used.

FIG. 6 is a circuit diagram of a short-circuit protection circuit that prevents oscillation.

FIG. 7 is a waveform chart showing the operation of the short-circuit protection circuit of FIG.

FIG. 8 is a circuit diagram of a short-circuit protection circuit according to another embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Main IGBT 2 Discharge MOSFET 4 Current detection resistor 5 Gate terminal 6 Resistance 7 Collector terminal 8 Emitter terminal 10 Discharge resistor 11 Current detection IGBT 12 Reset diode 13 Reverse blocking diode

Claims (8)

(57) [Claims] A main switching element driven by a signal input to a control electrode from a control terminal; a control electrode and a first main electrode connected to a control electrode and a first main electrode of the main switching element, respectively; The second main electrode includes a current detection switching element connected to the second main electrode of the main switching element via a current detection resistor, and when an excessive current flows through the current detection resistor, main switching is performed from the control terminal. The drive signal of the main switching element is reduced by turning on the discharge switching element inserted in the branch circuit of the circuit leading to the control electrode of the element, and the reduction of the drive signal suppresses the overcurrent flowing through the main switching element. However, if the drive signal is reduced by turning on the discharge switching element, the drive signal is input to the drive terminal. Discharge so as to turn off the discharge switching element when a signal is becomes zero or the opposite polarity
Control electrode of the switching element for
Between the control electrode and the control electrode of the main switching element
A short-circuit protection circuit characterized by connecting a diode in a forward direction . 2. The short-circuit protection circuit according to claim 1, wherein the drive signal is a voltage. 3. The short-circuit protection circuit according to claim 1, wherein the drive signal is a current. 4. The discharge switching element is a MOSFET, and a gate electrode of the discharge MOSFET, a second main electrode of the current detection switching element, and a power supply for maintaining a reduction in a drive signal when an overcurrent is suppressed. 4. The short-circuit protection circuit according to claim 1, further comprising means for shortening the charging time of the gate charge of the discharging MOSFET and increasing the discharging time between the second main electrode of the main switching element. 5. A means for shortening the charging time of the gate charge of the discharging MOSFET and lengthening the discharging time includes a diode connected to the current detecting resistor side of the current detecting resistor and a second main electrode of the main switching element. The short-circuit protection circuit according to claim 4, wherein the short-circuit protection circuit is a discharge resistor connected to the side. 6. A branch circuit including a discharging MOSFET and a current
The detection resistor and discharge resistor are reverse blocking diodes, respectively.
Connected to the second main electrode of the main switching element
The short-circuit protection circuit according to claim 4 or 5, wherein 7. A discharging MOSFET, a current detecting resistor and a current detecting resistor.
Main discharge through a common reverse blocking diode.
5. The switching device according to claim 4, wherein the switching device is connected to a second main electrode of the switching element.
7. The short-circuit protection circuit according to any one of the chairs 6. 8. A main switching element and a current detection switch.
2. The method according to claim 1, wherein the switching elements are formed on the same semiconductor substrate.
8. The short-circuit protection circuit according to any one of the chairs 7.