JPH051652B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an overcurrent protection circuit for a conduction modulation type MOSFET.

[Technical background of the invention and its problems]

A conduction modulation type MOSFET is a FET that has a MOS gate input and operates in bipolar mode.
It has the advantage of fast switching speed and low on-state voltage. This makes it possible to control high power and high frequency, which was impossible with conventional bipolar transistors and MOSFETs.
Various devices can be made smaller and lower in cost. In this specification, this conductivity modulation type MOSFET will be described below.
Abbreviated as BIFET (Bipolar mode FET).

Figure 4 shows the basic chopper circuit of BIFET. In the figure, 1 is the BIFET, and this BIFET
Power is supplied from the DC power supply 2 to the load 3 by turning on and off the power supply 1. The gate signal generation circuit that controls BIFET1 on and off is
It is composed of a gate power supply 4 that supplies a positive voltage to the gate of the BIFET 1, a gate power supply 5 that supplies a negative voltage, and bipolar transistors 6 to 9 that amplify a control signal input to a control signal input terminal 10. When a positive signal is applied to the control signal input terminal 10 of this gate signal generation circuit, the transistors 6 and 7 are turned on, and a positive voltage is applied from the gate power supply 4 to the output terminal 11.
is supplied to the gate of BIFET1 through
1 turns on. When a negative signal is input to the control signal input terminal 10, the transistors 8 and 9 are turned on, and the negative voltage from the gate power supply 5 is applied to the output terminal 1.
1 to the gate of BIFET1,
BIFET1 turns off.

Figure 5 shows the gate voltage V _G as a parameter.
An example of the drain voltage V _D -drain current I _D characteristic of BIFET is shown. When driving with a high gate voltage V _G as shown in the figure, the on-voltage becomes low and the BIFET
power loss can be reduced. However, if a short circuit occurs in load 3 in the circuit shown in Figure 4,
The voltage between the drain and source of BIFET 1 rises to the voltage of DC power supply 2. As a result, the power loss in BIFET1 becomes excessive, leading to destruction of BIFET1. If the gate voltage is lowered and driven in consideration of such a load accident, as can be seen from FIG. 5, the on-voltage of BIFET 1 becomes high, and there is a problem that the power loss in the on-state of BIFET 1 becomes large.

In order to solve this problem, an overcurrent protection circuit as shown in FIG. 6, for example, can be considered. As shown in the figure, resistors 12 and 13 are connected in series between the drain and source of BIFET 1, and the voltage between the drain and source is detected at both ends of resistor 13. Further, a resistor 41 and a transistor 42 are connected in series between the gate and source of the BIFET 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 BIFET 1 is connected to the output terminal 11 of the gate signal generation circuit via a resistor 44.

The operation when such a protection circuit is provided is as follows. When an accident occurs in load 3 and overcurrent flows through BIFET 1, the on-voltage of BIFET 1 increases. This voltage is divided by resistors 12 and 13, and the voltage across resistor 13 is connected to Zener diode 4.
When the Zener voltage value of 3 is exceeded, the transistor 42
A current flows through the base of. This turns on the transistor 42, and the gate control voltage supplied from the gate power supply 4 is divided by the resistors 41 and 44 and lowered. For example, the voltage of gate power supply 4
When 15V and resistors 41 and 44 are both 50Ω,
During normal operation, the gate voltage of BIFET1 is
15V, and if a short-circuit accident occurs in load 3, the gate voltage will drop to 7.5V, and the current flowing through BIFET 1 can be reduced. On the other hand, when the load 3 is normal and the BIFET 1 is turned on, there is an initial delay time of several tens of nanoseconds. Therefore, the voltage of the DC power supply 2 is applied to the BIFET 1 for several tens of nanoseconds after the positive gate voltage is applied to the BIFET 1. During this period, current flows through the base of transistor 42, so
The gate voltage of BIFET1 becomes a low value. However, as time passes, the on-voltage of BIFET 1 drops, eventually dropping to several volts. At this time, when the voltage generated across the resistor 13 becomes lower than the Zener voltage value, the transistor 42 is turned off.
The gate voltage of BIFET 1 rises to the voltage of gate power supply 4, and the BIFET 1 can be driven until the on-voltage of BIFET 1 becomes sufficiently low.

FIG. 7 shows the relationship between the BIFET current ID (max) and the drain-source voltage V _D when an overcurrent flows through the BIFET and the BIFET _is destroyed. In the figure, the shaded area is the area where the BIFET is destroyed. As is clear from the figure, I _D (max) is inversely proportional to V _D , and it is especially important to minimize overcurrent when using BIFET in a high voltage circuit. To do so, set the gate voltage to Vth
(minimum gate voltage to turn on BIFET) or below, or turn off the current, or Vth + 3V
It is necessary to make the current that actually flows sufficiently small.

However, in the conventional protection circuit shown in Figure 6,
When overcurrent flows through BIFET1, the gate voltage
Resistors 41 and 44 so that it is Vth or lower
When setting, the following problems occur. 1st
As mentioned above, at the initial stage when BIFET 1 turns on, the voltage of DC power supply 2 is applied between the drain and source of BIFET 1, and transistor 4
BIFET 2 is in the on state, and at this time the gate voltage of BIFET 1 becomes approximately Vth or lower. As a result, BIFET1 either does not turn on or the turn-on time becomes extremely long.
Second, if load 3 causes an accident and the protection circuit is activated, the overcurrent flowing through BIFET 1 will suddenly decrease, and the voltage applied to BIFET 1 will oscillate due to the stray inductance component of the circuit, causing a temporary The voltage 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, and the transistor 42 is turned off again.
A high gate voltage is applied to BIFET1 and overcurrent begins to flow. This repetition causes an oscillation phenomenon in this circuit.

[Purpose of the invention]

The present invention solves the above problems and is highly reliable.
The purpose is to provide an overcurrent protection circuit for BIFET.

The BIFET overcurrent protection circuit according to the present invention is
A thyristor is installed between the gate and source of the BIFET.
A series circuit of MOSFETs is connected and the BIFET
By detecting that the voltage between the drain and source of the BIFET exceeds a predetermined voltage and applying a trigger signal to the gate of the thyristor, the gate of the BIFET,
Must be configured to reduce source-to-source voltage.

Further, a gate control signal is input to the BIFET and the MOSFET individually through resistors, but at this time, the charge time constant between the capacitance of the gate of the MOSFET and the corresponding resistor
This configuration prevents the MOSFET from turning on during the turn-on period with the BIFET.

〔Effect of the invention〕

In the protection circuit according to the present invention, a thyristor is used as the main switch element that shorts the gate and source of the BIFET when an overcurrent flows through the BIFET. Once a thyristor is turned on, it remains on unless a reverse voltage is applied between the anode and cathode, so a BIFET that is once turned off will turn on again due to the influence of stray inductance.
Oscillation phenomena in the BIFET circuit are prevented. In addition, a MOSFET is inserted in series with the thyristor.
The floating capacitor of that gate and the
A delay circuit is configured by a resistor provided between the output terminals of the BIFET gate signal generation circuit. Therefore, the BIFET with the on-gate signal
There is a certain delay time before this MOSFET turns on at the beginning of turn-on. Therefore, a situation in which the overcurrent protection circuit operates at the initial stage of turn-on of the BIFET and the BIFET does not turn on or the turn-on is delayed is also prevented. Therefore, according to the present invention, a highly reliable BIFET overcurrent protection circuit can be realized.

[Embodiments of the invention]

Examples of the present invention will be described below.

FIG. 1 shows the circuit configuration of one embodiment. Portions corresponding to the basic circuit shown in FIG. 4 are given the same reference numerals as in FIG. 4, and detailed description thereof will be omitted. As a voltage detection circuit that detects the voltage between the drain and source of BIFET1, there is a resistor 1 between the drain and source of BIFET1.
2 and 13 are connected in series. This is the same as the conventional figure 6. A thyristor 14 is installed between the gate and source of BIFET1 as a circuit to reduce the voltage between the gate and source when an overcurrent flows through BIFET1.
A series circuit of MOSFET 15 and MOSFET 15 is provided. The gate of the thyristor 14 is connected to the high potential side terminal of the resistor 13, which is the output terminal of the voltage detection circuit, via a Zener diode 16 as a trigger diode. A resistor 18 is provided between the gate of BIFET 1 and the output terminal 11 of the gate signal generating circuit, and a resistor 19 is also provided between the gate of MOSFET 15 and the output terminal 11 of the gate signal generating circuit. The resistor 19 and the floating capacitor at the gate of the MOSFET 15 constitute a delay circuit.
A Zener diode 17 for overvoltage prevention is connected between the drain and source of the MOSFET 15.

The time constant of the delay circuit formed by resistor 19 and the floating capacitor at the gate of MOSFET 15 is set so that MOSFET 15 does not turn on before BIFET 1 turns on. Specifically, for example, until the on-gate signal is input and the voltage between the drain and source of BIFET1 becomes 10% or less,
The time constant is set so that MOSFET 15 will not turn on.

In the protection circuit configured as described above, a case will be considered in which a positive signal is applied to the control signal input terminal 10 of the gate signal generation circuit, the BIFET 1 is in an on state, and a short circuit accident occurs in the load 3. At this time, an overcurrent flows through the BIFET 1, the on-voltage of the BIFET 1 increases, and this voltage is divided by the resistors 12 and 13 and detected. At this time, the MOSFET 15 is in the on state due to the on gate signal from the gate signal generation circuit. When the voltage generated across the resistor 13 exceeds the Zener voltage of the Zener diode 16, a gate current flows through the thyristor 14 and the thyristor 14 is turned on. When the thyristor 14 turns on, the gate-source voltage of BIFET1 is equal to the on-state voltage of the thyristor 14 and MOSFET15.
is the sum of the on-voltage. This value can easily be set to 2V or less. Since BIFET1Vth is approximately 5V, when an overcurrent flows through BIFET1, the voltage between the gate and source can be set to Vth or less, and the overcurrent can be completely cut off. and thyristor 1
Once BIFET 4 is turned on, it remains on as long as the anode is at a positive potential, so the overcurrent of BIFET 1 decreases rapidly, the voltage oscillates, and thyristor 1
Even if the gate voltage of BIFET 4 decreases, the gate-source voltage of BIFET1 is kept below Vth, and the BIFET
1, no overcurrent will flow again.

Next, the operation at the initial turn-on of BIFET1 will be explained. When a positive control signal is applied to the control input terminal 10 of the gate signal generation circuit, a positive on-gate signal is transmitted from the output terminal 11 to the BIFET via the resistor 18.
1 gate. At the same time, the on-gate signal is also applied to the gate of MOSFET 15 via resistor 19. At this time, the gate voltage of MOSFET 15 rises due to the charging time constant of resistor 19 and the gate floating capacitor, and when this reaches Vth, MOSFET 15 is turned on. Here, in this embodiment, the time until this MOSFET 15 turns on is set to be longer than the turn-on delay time of BIFET 1, and the thyristor 14 is kept in the OFF state while the on-voltage is high at the initial turn-on of BIFET 1. . Therefore, a high on-gate signal is supplied to the gate of BIFET1. As time passes, MOSFET15 turns on, but at this time
The on-voltage of BIFET 1 is sufficiently low, and thyristor 14 will not turn on. Therefore, in the overcurrent protection circuit of this embodiment, a high gate voltage can be supplied to BIFET 1 except when an overcurrent flows through BIFET 1, and it is possible to prevent turn-on failure or turn-on delay of BIFET 1.

FIG. 2 shows a circuit configuration of an improved embodiment of the circuit of the embodiment shown in FIG. In the previous example, BIFET1
It takes some time for the overcurrent to be cut off after the overcurrent flows. This time is determined by the time it takes for the thyristor 14 to turn on, and is usually 2 to 3 μs.
It is. During this period, overcurrent flows through BIFET1,
This may cause BIFET1 to be destroyed. In this embodiment, this point has been improved. In other words, in addition to the protection circuit shown in Figure 1, the bipolar transistor 20 and MOSFET between the gate and source of BIFET1 are
21 series circuits are provided. A series circuit of resistors 24 and 25 is newly provided between the drain and source of BIFET 1 as a voltage detection circuit. The base of the transistor 20 is connected to the high potential side terminal of a resistor 25 via a Zener diode 22. The gate of MOSFET21 is
It is connected to the output terminal 11 via a resistor 19 in common with the gate of the MOSFET 15. Also
A Zener diode 23 for overvoltage prevention is connected between the drain and source of the MOSFET 21.

When a short-circuit accident occurs in the load 3 in the circuit configured as described above, an overcurrent flows through the BIFET 1 as described above, and its on-voltage increases. Then,
When the terminal voltages of the resistors 13 and 25 of the voltage detection circuit increase and these voltages exceed the Zener voltage values of the Zener diodes 16 and 22, respectively,
Current flows through the gate of thyristor 14 and the base of transistor 20. At this time, thyristor 14
As mentioned above, there is a tone-on time of 2 to 3 μs, during which the transistor 20 is in the on state. In other words, the gate-source voltage of BIFET1 decreases to the sum of the on-voltage of transistor 20 and the on-voltage of MOSFET21, and as a result, BIFET1
The overcurrent is cut off. When the overcurrent is cut off, the voltage between the drain and source of BIFET1 may oscillate as described above, but 2 to 3μs after the overcurrent starts flowing, the thyristor 14 is completely turned on, so the voltage between the BIFET1 The gate voltage is kept below Vth to prevent overcurrent from flowing out again.

In this way, the circuit of this embodiment can protect the BIFET 1 from overcurrent more effectively than the circuit of the previous embodiment.

FIG. 3 shows an embodiment in which, in the circuit of FIG. 1, when an overcurrent flows through BIFET 1 and its gate voltage drops, this is detected and the operation of the gate signal generation circuit is controlled. In the figure, 31 is a photocoupler, and its light emitting element is connected to the thyristor 14.
A resistor 32 is connected to the light receiving element side to detect that the thyristor 14 and the MOSFET 15 are turned on. The terminal voltage of this resistor 32 is inputted to one input terminal of an AND gate 36 via a waveform shaping circuit 33 and a flip-flop 34. Control signals of "1" and "0" are input to the other input terminal 37 of the AND gate. 38 is a level conversion circuit that converts the output of the AND gate 36 into positive and negative signals, and its output terminal is connected to the control input terminal 10 of the gate signal generation circuit.

The normal operation of the circuit configured in this way will be explained. A signal is applied to the reset terminal 35 of the flip-flop 34 so that the output of the flip-flop 34 is always "1". On the other hand, AND
A "1" or "0" signal is input to the control input terminal 37 of the gate 36 in order to turn the BIFET 1 on or off. At this time, the output of the AND gate 36 becomes the same as the signal applied to the control input terminal 37, which is converted into a positive or negative signal by the level conversion circuit 38 and supplied to the control signal input terminal 10 of the gate signal generation circuit. . As a result, a positive on-gate signal or a negative off-gate signal is supplied from the output terminal 11 to the gate of BIFET1.

Next, the operation when an overcurrent flows through BIFET1 will be explained. When an overcurrent flows through BIFET1, thyristor 14 is turned on, and the gate voltage of BIFET1 decreases. At this time, a current flows to the light emitting element side of the photocoupler 31 connected in series with the thyristor 14, and the terminal voltage of the resistor 32 increases. This voltage is converted into a predetermined logic level signal by a waveform shaping circuit 33 and input to a flip-flop 34. As a result, the output of the flip-flop 34 is inverted and becomes "0", and as a result, the output of the AND gate 36 also becomes "0", a negative voltage is applied to the control input terminal 10 of the gate signal generation circuit, and the gate signal to BIFET1 is applied. The signal supply is stopped.

In this way, the circuit of this embodiment not only protects the BIFET from overcurrent, but also performs automatic control to stop the gate signal generation circuit when an overcurrent flows.

The circuit for automatically controlling the gate signal generation circuit shown in FIG. 3 can be similarly applied to the circuit of the embodiment shown in FIG.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the BIFET circuit configuration of one embodiment of the present invention, FIG. 2 is a diagram showing the BIFET circuit configuration of another embodiment, and FIG. 3 is a diagram showing the BIFET circuit configuration of yet another embodiment.
Figure 4 shows the basic configuration of the BIFET circuit; Figure 5 shows an example of BIFET voltage-current characteristics; Figure 6 shows the circuit configuration with a conventional overcurrent protection circuit. 7 are diagrams showing the dangerous operation area of BIFET. 1...BIFET, 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... Resistor (voltage detection circuit), 14... Thyristor, 15... …MOSFET,
16, 17... Zener diode, 18, 19
……resistance.

Claims (1)

[Claims] 1. The anode is connected to the gate of the conductivity modulation type MOSFET 1, the cathode is connected to the source of the conductivity modulation type MOSFET 1 via the MOSFET 15, and the voltage between the drain and source of the conductivity modulation type MOSFET 1 is a predetermined voltage. A thyristor 14 is provided which is triggered when the conductivity modulation type
A gate control signal is input to the gates of MOSFET 1 and the MOSFET 15 through input resistors 18 and 19, respectively, and the MOSFET 15 is turned on within the turn-on period of the conductivity modulation type MOSFET 1 by the gate capacitance of the MOSFET 15 and the charging time constant by the input resistor 19. This is an overcurrent protection circuit for a conductive modulation type MOSFET, which is characterized by preventing the current from turning on. 2 The anode is connected to the gate of the conductivity modulation type MOSFET 1, the cathode is connected to the source of the conductivity modulation type MOSFET 1 via the MOSFET 15, and it is triggered when the voltage between the drain and source of the conductivity modulation type MOSFET 1 exceeds a predetermined voltage. a thyristor 14 having one end connected to the conductive modulation type thyristor 14;
Connected to the gate of MOSFET1, the other end is
The conductivity modulation type via MOSFET21
A transistor 20 connected to the source of the MOSFET 1 and turned on when the voltage between the drain and source of the conductivity modulating MOSFET 1 exceeds the predetermined voltage.
A gate control signal is input to the gates of the conductivity modulation type MOSFET 1 and the MOSFETs 15 and 21 via input resistors 18 and 19, respectively, and a charging time constant due to the capacitance of the gates of the MOSFETs 15 and 21 and the input resistor 19 is provided. Therefore, during the turn-on period of the conductivity modulation type MOSFET 1, the
An overcurrent protection circuit for a conductivity modulation type MOSFET characterized by preventing MOSFETs 15 and 21 from turning on.