JP2005033611A - Transistor protection circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a protection circuit which is simple in configuration and is capable of protecting an output control transistor from the overvoltage (surge voltage) of a power source to be applied to a collector. <P>SOLUTION: The protection circuit protects, from the overvoltage of the power source, a base voltage drive output control transistor 15 the collector of which is connected with a load (control target) 50 connected with the power source and in which voltage for controlling the load is inputted to a base. The protection circuit is provided with an overvoltage protection circuit 20 including: a first current detection means 22 for detecting the collector current of the output control transistor 15; a comparison means 25 for comparing voltage variation by the detected collector current with preset first reference voltage Vref1; and base voltage cutoff means 30, 35 for making base voltage zero when the voltage variation detected on the basis of the overvoltage is higher than the first reference voltage. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a protection circuit for a transistor, and more particularly to a protection circuit for protecting a gate voltage driven output control transistor in a control IC from an overvoltage (surge voltage) of a power supply.

  A surge voltage may be generated when the current flowing through the circuit including the inductance is turned on and off by the switch element. For example, a primary coil of a vehicle ignition device has one end connected to a power source (battery) and the other end connected to a collector of an output control transistor (eg, IGBT) of an igniter. During operation of the igniter, an overvoltage such as a load ramp surge is likely to be applied to the collector, which may cause destruction of the IGBT.

  In order to prevent the destruction of the IGBT, in the conventional example shown in FIG. 3 (see Patent Document 1), a diode 104 and a thyristor 106 are interposed between the gate G and the intermediate point between the drain D of the MOSFET 100 and the reactor 102 connected thereto. The protection circuit 110 including these is arranged. The cathode of the diode 104 is connected to the drain of the MOSFET 100 and detects an overvoltage surge. The thyristor 106 is turned on by receiving the detection voltage at the gate, and outputs power supplied from the power supply 114 via the resistor 112 to the gate of the MOSFET 100.

When an overvoltage surge occurs between the drain and source and the magnitude exceeds the breakover voltage of the diode 104, the thyristor 106 is turned on, the MOSFET 100 is turned on by the drive voltage from the power supply 114, and the surge current is discharged to GND.
Japanese Patent Laid-Open No. 5-299990

  However, the conventional example uses the voltage of the power source 114 as the power source of the control IC including the MOSFET 100. The voltage of the power supply 114 varies due to various causes. Therefore, in this case, another protection circuit is required to protect the MOSFET 100 from the surge voltage of the power supply 114, and as a result, the configuration of the number of protections becomes complicated.

  The present invention has been made in view of the above circumstances, and provides a protection circuit capable of effectively protecting an output control transistor of a control IC from an overvoltage (surge voltage) of a power supply applied to a collector while having a simple configuration. The purpose is to do.

  The inventor of the present application drives the protection circuit with a control voltage from a control circuit (control CPU) with little fluctuation, and when the power supply surge voltage is generated, the gate voltage of the output control transistor is cut off (the transistor is turned off). The present invention has been completed.

  In the transistor protection circuit according to the present invention, the collector is connected to the load connected to the power supply, and the output control transistor in which the voltage for controlling the load is input to the gate is used as the overvoltage of the power supply. A first current detection means for detecting a collector current of the output control transistor; a comparison means for comparing a voltage based on the detected collector current with a first reference voltage set in advance; And an overvoltage protection circuit including a gate voltage cut-off means for setting the gate voltage of the output control transistor to zero when the voltage detected by the overvoltage is larger than the comparison voltage.

  In this protection circuit, when the comparison means determines that the voltage (the amount of change) due to the current detected by the first current detection means due to the surge voltage is greater than the reference voltage, the gate voltage cutoff means is connected to the gate of the output control transistor. Zero the voltage. As a result, the output control transistor is deactivated.

  According to a second aspect of the present invention, there is provided the protection circuit according to the first aspect, wherein the difference between the second current detection means for detecting the collector current and the voltage based on the detected collector current and a preset second reference voltage is less than a predetermined value. There is provided an overcurrent protection circuit including an operation amplifier for maintaining and a gate voltage reduction means for reducing the gate voltage when a voltage difference exceeds a predetermined value due to overcurrent. According to a third aspect of the protection circuit of the present invention, in the second aspect, the gate voltage cutoff means is not subjected to the voltage reduction action of the gate voltage reduction means.

  According to a protection circuit of a fourth aspect of the present invention, in the third aspect, the first current detection means and the second current knowledge means use a current mirror of the collector current of the output control transistor. The protection circuit of claim 5 is the protection circuit of claim 3, wherein the first reference voltage is higher than the second reference voltage. According to a sixth aspect of the present invention, the overvoltage protection circuit and the overcurrent protection circuit are driven by the voltage of the control ECU.

  According to the transistor protection circuit of the present invention, when the voltage detected due to the surge voltage is higher than the reference voltage, the gate voltage cutoff means zeroes the gate voltage of the output control transistor. As a result, the output control transistor and the control IC including the output control transistor are deactivated. Even if a surge voltage is applied in a non-operating state, the output control transistor does not have to be destroyed.

  According to the protection circuit of the second aspect, since the current protection circuit including the gate voltage reducing means is provided, the output control transistor is reliably protected from the overcurrent. According to the protection circuits of claims 3 and 5, the gate voltage cutoff means outputs the surge voltage when it is generated without being affected by the reduction of the gate voltage by the gate voltage reduction means caused by the overcurrent. The gate voltage of the control transistor is cut off.

  According to the protection circuit of the fourth aspect, the configuration of the first current detection means and the second current detection means is simplified. According to the protection circuit of the sixth aspect, it is not necessary to consider the destruction of the output control transistor due to the surge voltage of the power source that drives the protection circuit.

The control IC arranged between the control ECU and the load driven by the power supply is based on at least an output control transistor for controlling the operation timing of the load by a signal from the control ECU and a surge voltage of the power supply applied to the collector. And an overvoltage protection circuit for protecting the output control transistor. An overcurrent protection circuit for protecting the output control transistor from an overcurrent applied to the collector can also be provided.
(1) Overvoltage protection circuit The overvoltage protection circuit includes at least a first current detection transistor, a comparator, and a gate voltage cutoff transistor. Furthermore, a flip-flop (FF) can be included.

A current mirror of the collector current of the output control transistor can be used for the first current detection means and the second current detection means described below. That is, the collector and gate of the output control transistor are directly connected, and the gate of the output control transistor, the gate of the first current detection transistor, and the gate of the second current detection transistor are connected to a common wiring. When a predetermined reference current (collector current) of the output control transistor flows, a predetermined voltage V _BE is generated between the gate and the emitter. This voltage V _BE is similarly applied between the gate and emitter of the first current detection transistor and the second current detection transistor, and the same current as the reference current flows through these two transistors.

  When the surge voltage is applied, the current mirror that flows through the first current detection transistor rises, and this is used to detect the surge voltage. Similarly, overcurrent control is performed using a current mirror that flows through a second current detection transistor that rises when an overcurrent flows.

The comparator has two input terminals and one output terminal, and compares the amount of voltage change caused by the detected current input to the first input terminal with a reference voltage preset in the second input terminal. The output of the comparator is input to the first input terminal of the FF, and the control signal is input to the second input terminal. The gate voltage cut-off means for making the gate voltage zero when the surge voltage is generated comprises a transistor.
(2) Overcurrent protection circuit The overcurrent protection circuit includes second current detection means, an operational amplifier, and gate current reduction means. The operational amplifier (operational amplifier) keeps the difference between the amount of change in current due to the detected current input to the first input terminal and the reference voltage set at the second input terminal below a predetermined value. The gate voltage reducing means for reducing the gate voltage to a predetermined value comprises a transistor.
(3) Relationship between the two etc. It is desirable that the gate voltage cut-off means is not subjected to the voltage reduction action by the gate voltage reduction means. Specifically, first, the gate of the voltage cut-off transistor constituting the gate voltage cut-off means is closer to the input terminal side of the control IC (from the gate of the output control transistor than the gate of the voltage reduction transistor constituting the gate voltage reduction means). It is connected to the far side.

  Second, the output part of the gate voltage cutoff transistor driven by the comparator that receives the voltage change amount from the first current detection transistor is the gate that is driven by the operational amplifier that receives the voltage change amount from the second current detection transistor. It is desirable to be closer to the input terminal side than the output of the reduction transistor. The first reference voltage of the comparator is preferably higher than the second reference voltage of the operational amplifier.

  Further, it is desirable that the overvoltage protection circuit and the overcurrent protection circuit are driven by the electric power of the control ECU.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
(Constitution)
In this embodiment, the present invention is applied to an igniter of a vehicle. In FIG. 1, a control IC (igniter) 10 is arranged between a primary coil 60 of an ignition coil (not shown) and a control CPU 65. The other end of the primary coil 60 is connected to a battery (not shown). The control signal of the control CPU 65 is input to the input terminal 11 of the igniter 10, and the control signal of the igniter 10 is output from the output terminal 12 to the primary coil 60.

  The igniter 10 includes an output control transistor 15 that controls the operation timing of the primary coil 60. The collector of the output control transistor 15 is connected to the output terminal 12, the emitter is connected to the ground terminal 13, and the gate is connected to the input terminal 11 by the high-level wiring 66.

  The igniter 10 further includes an overvoltage protection circuit 20 that protects the output control transistor 15 from an overvoltage (surge voltage) applied to the collector, and an overcurrent protection circuit 40 that protects the output control transistor 15 from an excessive collector current.

  The overvoltage protection circuit 20 includes a first current detection transistor 22, a comparator (comparator) 25, a flip-flop circuit 30, and a gate voltage cutoff transistor 35. The collector of the first current detection transistor 22 is connected to the collector of the output control transistor 15 at the connection point 67, the emitter is connected to the ground terminal 13 via the resistor 21, and the gate is connected to the high-level wiring 66 via the first resistor 23. The connection points 68 are connected respectively.

  The output of the emitter is input to the first input terminal of the comparator 25. The second input terminal reference current Vref1 is input. The output terminal is connected to the set terminal S of the FF 30, and the reset terminal R is connected to the high-level wiring 66 at the connection point 69. The collector of the gate voltage cutoff transistor 35 having the output terminal of the FF 30 connected to its gate is connected to the high level wiring 66 at the connection point 68 and the emitter is connected to the low level wiring 71. A resistor 70 is disposed between the connection points 68 and 69 in the high-level wiring 66.

  The overcurrent protection circuit 40 includes a second current detection transistor 42, an operational amplifier 45, and a gate current reduction transistor 50. The collector of the second current detection transistor 42 is connected to the collector of the output control transistor 15 at the connection point 73. The emitter is connected to the ground terminal 13 via the first resistor 41, and the gate is connected to the high-level wiring 66 at a connection point 74 between the gate of the first current transistor 22 and the connection point 68.

The output of the emitter is input to the first input terminal of the operational amplifier 45. The reference voltage Vref2 is input to the second input terminal. The collector of the gate voltage reducing transistor 50 whose output terminal is connected to its gate is connected to the high-level wiring 66 at a connection point 76 farther from the input terminal 11 than the connection point 68 (closer to the gate of the output control transistor 15). Yes. The emitter is connected to the lower wiring 71 at a connection point 77 between the emitter of the gate voltage cutoff transistor 35 and the ground terminal 13. A resistor 78 is disposed between the connection points 68 and 76 in the high-level wiring 66.
(Function)
The operation of the embodiment will be described with reference to FIG. The left half shows a normal state (a state where no surge voltage is applied) from the power source, and the right half shows a state where a surge voltage is applied.
(1) Normal State As shown in FIG. 2B, the voltage B of the power source connected to the primary coil 60 is maintained at a constant value (for example, 12V). As shown in FIG. 2A, the ignition signal A is applied from the control CPU 65 to the input terminal 11.

  As indicated by c1 in FIG. 2 (c), the current C flowing from the collector to the emitter of the output control transistor 15 increases with the RC time constant. Then, the current mirror of the collector current of the output control transistor 15 detected by the second current detection transistor 42 rises, and the current and resistance flowing through the second current detection transistor 42 as indicated by e1 in FIG. The product with 41 gradually increases. The gate voltage F of the output control transistor 15 increases as indicated by f1 in FIG. 2 (f), and the output voltage of the primary coil 60 decreases from the output terminal 13 as indicated by d1 in FIG. 2 (d).

When the difference between the product of the current flowing through the second current detection transistor 42 and the resistor 41 and the second reference voltage Vref2 set in the operational amplifier 45 becomes larger than a predetermined value, the process proceeds to overcurrent control. That is, the operational amplifier 45 turns on the voltage reduction transistor 50 and discharges (squeezes) the gate current of the output control transistor 15. As a result, the main current is maintained at a constant value as indicated by c2 in FIG. 2 (c), the gate voltage F decreases as indicated by f2 in FIG. 2 (f), and indicated by d2 in FIG. 2 (d). Thus, the output voltage D rises.
(2) When a surge voltage is generated When the ignition signal A shown in FIG. 2 (a) is turned on, if a surge voltage is generated in the power source as shown in FIG. 2 (b) b, as indicated by c3 in FIG. 2 (c). In addition, the main current that has been increasing in the same manner as during overcurrent control increases rapidly. Along with this, the current mirror of the collector current of the output control transistor 15 detected by the first transistor 22 also suddenly rises.

As a result, the power that has been increased in the same way as during overcurrent control and that can be accumulated by the current flowing through the first current detection transistor 22 and the resistor 21 suddenly increases as indicated by e3 in FIG. 2 (e). To rise. When the voltage exceeds the reference voltage Vref1 set in the comparator 25, the voltage drops instantaneously as indicated by e4, and the flip-flop 30 holds the state. As indicated by f4 in FIG. 2 (f), the voltage cutoff transistor 35 makes the gate voltage of the output control transistor zero. Thus, as indicated by c4 in FIG. 2 (c), the main current stops rising further and is held at a constant value.
(effect)
According to the present embodiment, the following effects can be obtained. First, since the overvoltage protection circuit 20 is provided in addition to the overcurrent protection circuit 40, the output control transistor 15 is protected from the overcurrent of the power supply and from the surge voltage. Moreover, protection from surge voltage is reliable for the following reasons.

  First, the connection point 68 between the gate voltage cutoff transistor 35 controlled by the comparator 25 and the wiring 66 is closer to the input terminal 11 side than the connection line 76 between the gate voltage reducing transistor 50 controlled by the operational amplifier 45 and the wiring 66. In this case, the output control transistor 15 is located away from the gate.

  Therefore, a relatively high voltage that is not subjected to feedback (voltage reduction action) by the operational amplifier 45 is applied to the gate of the first current detection transistor 22, and the comparator 25 operates the gate current cutoff transistor 35 accordingly. Moreover, the first reference voltage Vref1 of the comparator 25 is set higher than the second reference voltage Vref2 of the operational amplifier 45. Thus, the gate voltage is turned off immediately before the surge voltage is applied to the output control transistor 15.

Secondly, the igniter 10 including the overvoltage protection circuit 20 and the overcurrent protection circuit 40 is driven not by the voltage of the power source to which the primary coil 60 is connected but by the voltage from the control CPU 65. Since the power supply voltage of the control CPU 65 varies little, there is no fear that the output control transistor 15 is destroyed by the surge voltage.
<Comparative example>
As a comparative example, it is also conceivable to use a control signal from the control CPU as a power source for the control IC. However, the control IC has no information regarding the voltage of the power supply. Therefore, the transistor cannot be protected from an overvoltage applied from the power supply during the operation of the control IC.

It is circuit explanatory drawing which shows the Example of this invention. (A) (b) (c) (d) (e) and (f) are time charts showing the operation of the embodiment. It is circuit explanatory drawing which shows a prior art example.

Explanation of symbols

10: igniter (control IC) 15: output control transistor 20: overvoltage protection circuit 22: first current detection transistor 25: comparator 30: flip-flop 35: gate voltage cutoff transistor 40: overcurrent protection circuit 42: second current detection transistor 45: Operational amplifier 50: Gate voltage reduction transistor

Claims (6)

A protection circuit that protects an output control transistor in which a collector is connected to a load connected to a power supply and a voltage for controlling the load is input to a gate from an overvoltage of the power supply,
First current detection means for detecting a collector current of the output control transistor;
A comparing means for comparing a voltage due to the detected collector current with a preset first reference voltage;
And an overvoltage protection circuit including gate voltage cut-off means for making the gate voltage of the output control transistor zero when the detected voltage is higher than the first reference voltage due to an overvoltage of the power supply. Transistor protection circuit.   Further, the second current detecting means for detecting the collector current, an operational amplifier for keeping the difference between the detected collector current voltage and the preset second reference voltage below a predetermined value, and the voltage difference due to overcurrent 2. The protection circuit according to claim 1, further comprising an overcurrent protection circuit including gate voltage reduction means for reducing the gate voltage when the voltage exceeds a predetermined value.   The protection circuit according to claim 2, wherein the gate voltage cutoff unit is not subjected to a voltage reduction action by the gate voltage reduction unit.   4. The protection circuit according to claim 3, wherein the first current detection unit and the second current detection unit use a current mirror of a collector current of the output control transistor.   The protection circuit according to claim 3, wherein the first reference voltage is higher than the second reference voltage.   The protection circuit according to claim 3, wherein the overvoltage protection circuit and the overcurrent protection circuit are driven by a voltage of a control ECU.

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