JPH0669136B2 - Switch device with overcurrent protection function - Google Patents

Description

The present invention relates to a switch device having an overcurrent protection function that uses a conductive modulation type MOSFET as a switch element.

[Conventional device] A conduction modulation type MOSFET has a MOS gate input and is a FET that operates in a bipolar mode. It has the advantages of high switching speed and low on-voltage. Therefore, high-power high-frequency control, which was impossible with conventional bipolar transistors and MOSFETs, becomes possible.
Miniaturization and cost reduction of various devices can be achieved.

The conductive modulation type MOSFET is called BIFET (Bipolar mode FET), but recently, IGBT (Insulated Gate Bipolar)
Transistor), henceforth abbreviated as IGBT.

Figure 3 shows the basic chopper circuit of an IGBT. In the figure, reference numeral 1 is an IGBT, and power is supplied from a DC power supply 2 to a load 3 by turning on / off the IGBT 1.

The gate signal generator that controls the on / off of IGBT1 is IGBT
It is composed of a gate power supply 4 for supplying a positive voltage to the gate of T1, a gate power supply 5 for supplying a negative voltage, and bipolar transistors 6 to 9 for amplifying the control signal input to the control signal input terminal 10. When a positive signal is applied to the control signal input terminal 10, the transistors 6 and 7 are turned on, a positive voltage is supplied from the gate power source 4 to the gate of the IGBT1 via the output terminal 11, and the IGBT1 is turned on. When a negative signal is input to the control signal input terminal 10, the transistors 8 and 9 are turned on and a negative voltage from the gate power source 5 is output via the output terminal 11.
It is supplied to the gate of IGBT1 and IGBT1 turns off.

FIG. 4 is a characteristic example showing the relationship between the drain voltage V _D and the drain current I _D with the gate voltage of the IGBT as a parameter.
When the gate voltage V _G is driven to be high as shown in the figure, the on-voltage is lowered and the power loss of the IGBT can be reduced. However, when a short circuit occurs in the load 3 in the circuit of FIG. 3, the drain-source voltage V _D of the IGBT1 rises to the voltage of the DC power supply 2. As a result, the power loss in the IGBT1 becomes excessive and the IGBT1 is destroyed. When the gate voltage is driven low in consideration of such a load accident, there is a problem that the ON voltage of the IGBT1 becomes high as shown in FIG. 4 and the power loss becomes large.

In order to solve such a problem, it may be possible to provide an overcurrent protection circuit as shown in FIG.

As shown in the figure, resistors 12 and 13 are connected in series between the drain and source of the IGBT 1, and the drain-source voltage is detected at both ends of the resistor 13. A resistor 41 and a transistor 42 are connected in series between the gate and source of the IGBT 1, and the gate of the transistor 42 is connected to the high potential side of the resistor 13 via a Zener diode 43. The gate of the IGBT 1 is connected to the output terminal 11 of the gate signal generation circuit via the resistor 44.

The operation when such a protection circuit is provided is as follows. When a short circuit occurs in the load 3 and an overcurrent flows in the IGBT1, the ON voltage of the IGBT1 rises. This voltage is the resistance 12,
When the voltage across the resistor 13 is divided by 13 and exceeds the Zener voltage of the Zener diode 43, a current flows through the base of the transistor 42. This turns on the transistor 42, and the voltage of the gate power supply 4 is divided by the resistors 41 and 44 to decrease. For example, the voltage of the gate power supply 4 is 15V, the resistance 41
When both 44 and 44 are set to 50Ω, IG is set during normal operation.
The gate voltage of BT1 is 15V, and when a short-circuit accident occurs in the load 3, the gate voltage drops to 7.5V, and the current flowing through the IGBT1 can be reduced. On the other hand, when the load 3 is normal and the IGBT 1 is turned on, there is a delay time of several tens of nanoseconds in the initial stage. Therefore, from the time when the positive gate voltage is applied to the IGBT1 for several tens of nanoseconds, the IGBT is
The voltage of the DC power supply 2 is applied to 1. During this period, a current flows through the base of the transistor 42, so the gate voltage of the IGBT1 has a low value. But over time IG
The on-voltage of BT1 drops and finally drops to several volts. At this time, when the voltage generated in the resistor 13 becomes lower than the Zener voltage value, the transistor 42 is turned off and the IG
The gate voltage of BT1 rises to the voltage of the gate power supply 4,
It can be driven until the on-voltage of T1 is sufficiently low.

Here, FIG. 6 shows the relationship between the drain current I _D (max) and the drain-source voltage V _D when an overcurrent flows through the IGBT and the IGBT is destroyed. As is clear from the figure, I _D (max) is V _D
It is inversely proportional to, and it is important to minimize the overcurrent especially when the IGBT is used in a high voltage circuit. For that purpose, it is necessary to turn off the current by setting the gate voltage to Vth (minimum gate voltage for turning on the IGBT) or less, or to set Vth + 3V or less to sufficiently reduce the current substantially flowing.

[Problems to be Solved by the Invention] However, in the protection circuit of FIG. 5, when the resistors 41 and 44 are set so that the gate voltage becomes Vth or lower when an overcurrent flows through the IGBT, Such problems arise.

As shown in FIG. 1, as described above, the voltage of the DC power supply 2 is applied between the drain and source of the IGBT1 at the initial stage of turning on the IGBT1 and the transistor 42 is in the ON state. At this time, the gate voltage of the IGBT1 is about Vth. Or it will be less than that. As a result, IGBT1 will not turn on or the turn-on time will be extremely long.

Secondly, when the load 3 has an accident and the protection circuit operates, the overcurrent flowing in the IGBT1 decreases sharply.
The voltage applied to the IGBT 1 oscillates due to the stray inductance component of the circuit, and the voltage temporarily generated in the resistor 13 becomes lower than the Zener voltage value of the Zener diode 43. At this time, the transistor 42 is turned off, a high gate voltage is applied to the IGBT1 again, and an overcurrent starts to flow. This circuit causes an oscillation phenomenon in this circuit.

The present invention has been made to solve the above-mentioned problems, and its purpose is to eliminate the decrease in the gate voltage at the initial stage of turn-on, and when the IGBT is turned off and an oscillating voltage is generated, the IGBT immediately An object of the present invention is to provide a switching device having a highly reliable overcurrent protection function that does not turn on.

[Means for Solving the Problems] In order to achieve the above object, the present invention is a conductivity modulation type in which the conductivity between a drain and a source changes according to a gate voltage applied between the gate and the source to turn on and off. In a switch device using a MOSFET as a switch element (1),
The gate voltage control includes a voltage detection circuit (12, 13) for detecting a drain-source voltage of the switch element, and a gate voltage control circuit controlled by an output of the voltage detection circuit to lower the gate voltage. In the circuit, the drain is connected to the gate of the switch element through the diode (15), and the gate is the resistor (16) and the diode (17).
The MOSFET (14) connected to the output of the voltage detection circuit via the parallel circuit of the switch element and the output of the gate signal generation circuit of the switch element to control the voltage for the off period of the switch element and the predetermined period of initial turn-on of the voltage. And a circuit that short-circuits the output terminal of the detection circuit.

[Operation] The predetermined period in the initial stage of turn-on is set to be longer than the turn-on time of the switch element, and during this period, the output terminal of the voltage detection circuit is short-circuited to the MOSFET (14)
The gate voltage of the switch element is fixed to zero, and the gate voltage of the switch element does not drop at the initial stage of turn-on. Also,
MO when overcurrent occurs and the output of the voltage detection circuit increases
The SFET (14) is turned on, the gate voltage of the switch element is reduced to zero, and the overcurrent decreases rapidly. At this time, even if the oscillating voltage is generated and the output of the voltage detection circuit is reduced, the MOSFET (14) is maintained in the ON state for a predetermined period by the gate input capacitance of the MOSFET (14) and the discharge time constant of the resistor (16). This prevents the switch element from turning on again.

[Embodiment] FIG. 1 shows a circuit configuration of an embodiment of the present invention. Portions corresponding to the basic circuit shown in FIG. 3 are denoted by the same reference numerals as those in FIG. 3 and detailed description thereof will be omitted.

As a voltage detection circuit that detects the drain-source voltage of the IGBT1, resistors 12 and 13 are connected in series between the drain and source of the IGBT1. This is the same as in FIG. IGBT1
As a circuit that lowers the gate-source voltage when an overcurrent flows through the
A series circuit of 15 and MOSFET 14 is provided. The gate of the MOSFET 14 is connected to the high potential side terminal of the resistor 13 which is the output terminal of the voltage detection circuit via the parallel circuit of the resistor 16 and the diode 17. The resistor 16 and the diode 17 constitute a predetermined time constant circuit together with the gate input capacitance of the MOSFET 14. That is, when the detection voltage of the resistor 13 increases, the gate voltage of the MOSFET 14 is immediately increased via the diode 17 and the resistance 13
The diode 17 is reverse-biased when the detection voltage of (3) decreases, and the gate voltage of the MOSFET 14 is reduced with a predetermined time constant. The high-potential terminal of the resistor 13, which is the output terminal of the voltage detection circuit, has a MOS circuit composed of the MOSFET 20 and the resistor 23 as a circuit for selectively short-circuiting this.
Inverter and MOS controlled by this MOS inverter
FET18 is provided. The input terminal of the MOS inverter is connected to the output terminal 11 of the gate signal generation circuit via the resistor 21. The resistor 21 constitutes a delay circuit together with the gate input capacitance of the MOSFET 20. A Zener diode 19 for preventing overvoltage is provided between the drain and source of the MOSFET 18.

The time constant of the delay circuit formed by the resistor 21 and the gate input capacitance of the MOSFET 20 is set so that the MOSFET 14 does not turn on before the IGBT 1 turns on. For example, to prevent the MOSFET 18 from turning off until the drain-source voltage of the IGBT1 drops below 10% when the on-gate signal is input,
The time constant is set.

In the switch device configured as above, when a positive signal is applied to the control signal input terminal 10 of the gate signal generation circuit, the on-gate signal is output from the output terminal 11 via the resistor 22.
It is supplied to the gate of IGBT1 and IGBT1 turns on. At this time, an on-gate signal is also applied to the gate of the MOSFET 20 via the resistor 21, but the time at which the MOSFET 20 is turned on is delayed from the time at which the IGBT 1 is turned on by a delay circuit formed by the resistor 21 and the gate input capacitance. When the MOSFET 20 turns on, the gate voltage of the MOSFET 18 becomes zero and the MOSFET 18 connected in parallel with the resistor 13 turns off. That is, the MOSFET 18 is short-circuited at both ends of the resistor 13 for a predetermined period at the initial stage of turn-on of the IGBT.
The gate voltage is kept from dropping.

When a negative control signal is applied to the control signal input terminal 10 of the gate signal generation circuit, a negative off gate signal is output from the output terminal 11 and the IGBT is turned off. At this time, an off-gate signal is also given to the gate of the MOSFET 20, and the MOSFET 20 is turned off. Therefore, the MOSFET 18 is turned on, and the resistance 13 for the voltage detection capacitor is short-circuited.

As described above, in this switch device, the MOSFET 18 is kept in the ON state during the OFF period of the IGBT 1 and the predetermined period in the initial stage of turn-on, and the high potential side terminal of the resistor 13 which is the output terminal of the voltage detection circuit is kept in the ground state.

In such a circuit configuration, if an accident occurs in the load 3,
The operation when an overcurrent flows through the IGBT1 will be described. When an overcurrent flows through the IGBT1, its on-voltage rises. At this time, the MOSFET 18 provided in parallel with the resistor 13 of the voltage detection circuit is in the OFF state, and the resistor 13 can obtain a voltage corresponding to the ON voltage of the IGBT 1. This voltage is transferred to the MOSFET via diode 17.
The MOSFET 14 is turned on when the gate voltage of the gate 14 exceeds the threshold voltage. As a result, the gate-source voltage of IGBT1 is
It drops to the sum of 15 forward voltages. This sum is set to a value that is sufficiently lower than the threshold of IGBT1.
The overcurrent of BT1 is cut off.

When the overcurrent is cut off in this way, the IG
The drain-source voltage of BT1 oscillates. In the circuit of this embodiment, the IGBT 1 is not turned on again and the overcurrent does not flow out again even if such vibration occurs. The reason is as follows. In the gate of MOSFET14 to short-circuit between the gate and source of IGBT1, a resistor 17 is connected in parallel with diode 17
Is provided.

When the drain-source voltage of the IGBT 1 oscillates and the terminal voltage of the resistor 13 drops, the charge stored in the gate input capacitance of the MOSFET 14 is discharged via the resistor 16. However, in this embodiment, the time constant of this discharge is increased so that the discharge time until the gate voltage of the MOSFET 14 reaches the threshold value Vth is I
It is set to be longer than the oscillation cycle of the GBT1 drain-source voltage. As a result, even if the drain-source voltage of the IGBT1 becomes zero for a short period of time, the MOSFET 14 is not turned off, which prevents the IGBT1 from turning on again and causing an overcurrent to flow out.

Further, in the circuit of the present embodiment, when the circuit is operating normally, the MOSFET 18 is kept in the ON state for a certain period of time after the ON gate signal is applied to the IGBT 1 as described above. Therefore, during the turn-on delay time of the IGBT1, the terminal of the resistor 13 which is the output terminal of the voltage detection circuit and the short-circuit are short-circuited, the MOSFET 14 does not turn on, and failure or delay of turn-on of the IGBT1 can be prevented.

FIG. 2 shows an embodiment in which, in the circuit of FIG. 1, when an overcurrent flows through the IGBT1 and its gate voltage drops, it is detected and the operation of the gate signal generating circuit is controlled. In the figure, reference numeral 24 is a photocoupler, the light emitting element of which is inserted in series with the diode 15 and the resistor 25 is connected to the light receiving element side to detect that the MOSFET 14 is turned on. The terminal voltage of this resistor 25 is the waveform shaping circuit 26,
It is input to one input terminal of the AND gate 29 via the flip-flop 27. The control signals of "1" and "0" are input to the other input terminal 30 of the AND gate 29. 3
1 is a level conversion circuit that converts the output of the AND gate 29 into positive and negative signals, the output terminal of which is connected to the control input terminal 10 of the gate signal generation circuit.

The normal operation of the circuit thus configured will be described. A signal is applied to the reset terminal 28 of the flip-flop 27 so that the output of the flip-flop 27 is always "1". On the other hand, the IGBT 1 is connected to the control input terminal 30 of the AND gate 29.
A control signal of "1" or "0" is input to turn on or off. At this time, the output of the AND gate 29 becomes the same as the signal given to the control input terminal 30, and this is converted into a positive or negative signal by the level conversion circuit 31 and supplied to the control signal input terminal 10 of the gate signal generation circuit. .

Next, the operation when an overcurrent flows in the IGBT 1 will be described. IGB
When an overcurrent flows through T1, MOSFET 14 is turned on as described above, and the gate voltage of IGBT1 drops. At this time, MOSFET 14
A current flows to the light emitting element side of the photocoupler 24 connected in series with and the terminal voltage of the resistor 25 rises. This voltage is converted into a predetermined logic level signal by the waveform shaping circuit 26, the output of the flip-flop 27 is inverted and becomes "0", and as a result, the output of the AND gate 29 also becomes "0", and the control input of the gate signal generation circuit. Even if a negative voltage is applied to the terminal 10 and the ON signal “1” continues to be input to the control input terminal 30 of the AND gate 29, the IGBT1
The supply of the gate signal to is stopped.

Thus, in the circuit of this embodiment, not only the overcurrent of the IGBT 1 but also automatic control for stopping the gate signal generation circuit when an overcurrent flows is performed.

[Effects of the Invention] In the switch device according to the present invention, the MOSFET is used as a control element for short-circuiting the gate and the source when an overcurrent flows through the switch element. The gate of this MOSFET is provided with a time constant circuit composed of a resistor and its own gate input capacitance. Therefore, when an overcurrent flows through the switch element and this MOSFET works to reduce the gate-source voltage of the switch element, even if an oscillating voltage occurs between the drain and source due to a sudden decrease in overcurrent, The MOSFET can be kept in the ON state by the time constant circuit, and it is possible to prevent a situation in which an overcurrent again flows through the switch element. In addition, the output terminal of the voltage detection circuit is provided with a circuit that short-circuits the switch element containing the ON-gate signal for a predetermined period at the initial turn-on time, and the overcurrent protection circuit works at the initial turn-on time of the switch element to prevent the switch element from turning on. Also, the situation where the turn-on is delayed is prevented. Therefore, according to the present invention, a switch device having a highly reliable overcurrent protection function can be realized.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of a switch device having an overcurrent protection function of the present invention, FIG. 2 is a block diagram of another embodiment of the present invention, and FIG. 3 is a switch device using an IGBT. Basic configuration diagram, FIG. 4 is a characteristic diagram showing the voltage-current relationship of the IGBT, FIG. 5 is a configuration diagram of a switch device having an overcurrent protection function according to the conventional concept, and FIG. 6 is a dangerous operation area of the IGBT. FIG. 1 ... Conduction modulation type MOSFET (IGBT) 2 ... DC power supply, 3 ... Load 4,5 ... Gate power supply, 6-9 ... Transistor 10 ... Control signal input terminal, 11 ... Gate signal output terminal 12 , 13 ...... Resistance (voltage detection circuit) 14,18,20 ...... MOSFET, 15,17 …… Diode 16,21 to 23 …… Resistance, 19 …… Zener diode

Claims (3)

[Claims] 1. A switch device using as a switch element (1) a conductivity modulation type MOSFET whose conductivity between drain and source changes according to a gate voltage applied between the gate and source to turn on and off. Voltage detection circuit (12,13) that detects the drain-source voltage of the
And a gate voltage control circuit that is controlled by the output of the voltage detection circuit to reduce the gate voltage. The gate voltage control circuit has a drain connected to the gate of the switch element via a diode (15). A switch whose gate is controlled by a MOSFET (14) whose gate is connected to the output of the voltage detection circuit through a parallel circuit of a resistor (16) and a diode (17) and the output of a gate signal generation circuit of the switch element. And a circuit that short-circuits the output terminal of the voltage detection circuit for a predetermined period during the off period of the device and the initial stage of turn-on, and a predetermined discharge time constant is formed by the gate input capacitance of the MOSFET and the resistor. Switch device with current protection function. 2. The circuit for short-circuiting the output terminal of the voltage detection circuit is driven by the output of the gate signal generation circuit,
The input section includes a MOS inverter (20, 21) including a delay element longer than the turn-on time of the switch element, and a MOSFET (18) driven by the MOS inverter to short-circuit the output terminal of the voltage detection circuit. A switch device having an overcurrent protection function according to claim 1. 3. Means for detecting that the MOSFET (14) is turned on to lower the gate voltage, and means for keeping the input signal of the gate signal generating circuit off by the detection signal. A switch device having an overcurrent protection function according to claim 1.