JPS6029005A - Power transistor drive circuit - Google Patents

Abstract

PURPOSE:To prevent destruction of a power transistor (TR) and baking a base drive circuit by providing a comparator detecting a base-emitter voltage of a TR and stopping the supply of a base current when a detected voltage exceeds a prescribed value. CONSTITUTION:The base-emitter voltage of a TR2 is detected by comparing it with a reference voltage 5 by a comparator 6. When the detected voltage exceeds a prescribed value, it is regarded that the TR2 is in overcurrent state and the supply of the base current is stopped. Thus, the destruction of the TR2 due to overcurrent and the baking of the base drive circuit are prevented surely so as to protect them.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field to which the invention pertains] The present invention relates to a drive circuit for a power transistor widely used for electric power. Generally, in such a drive circuit, it is desirable to promptly detect an overcurrent state and quickly shut off the power transistor to protect it.

[Prior art and its problems]

This type of power transistor usually has the inherent characteristics of large power loss and increased internal temperature rise in the high current region, and especially in overcurrent conditions, a lack of pace current causes a large collector current. It is known that a high collector-to-emitter voltage is applied and enters the active region, increasing power loss (represented by the product of collector current and collector-to-emitter voltage). Furthermore, if the power transistor is destroyed and, for example, its pace and emitter are short-circuited, there is a risk that the pace drive circuit may burn out. Therefore, in order to prevent the power transistor or the base drive circuit from being destroyed or burnt out due to overcurrent, it is necessary to release the overcurrent state within the shortest possible time.

As one of the methods, a method as shown in FIG. 1 is known. Note that FIG. 1 is a configuration diagram showing a conventional system. In the figure, 1 is a pace drive circuit, 2 is a power transistor, 3 is a current transformer (current detector), and 4 is an amplifier. The base drive circuit 1 further includes AND gates 111 and 112.
, a photocoupler 121122, a resistor 131i32, transistors 141, 142, power supplies 151, 152, an inverter 16, and the like.

In other words, when turning on power trunk 3 and 2,
By supplying a signal of level No. to the terminal T1, the gate 111
By opening, etc., a base current is supplied to the transistor 141 via the photocoupler 121. by this,
Since the transistor 141 is conductive, the current (7t9151
, a forward bias current is supplied to the base of the power transistor 20 through the resistor 151 and the transistor 14t, and the transistor 2 is turned on. At this time,
Since the gate 112 is supplied with a low level signal, the gate 112 is closed. In addition, the photo coupler 12. is also used to increase noise tolerance.
Each resistor 151 is provided to limit the current. On the other hand, the current flowing through the power transistor 2 is detected by a current transformer 3 and converted into a voltage signal by an amplifier 4. However, when the detected current value is at a normal current level, it becomes a high level signal, and at an overcurrent level, it becomes a low level signal. The signals are converted into level signals and given to the game terminals 111 and 112, respectively. Further, since the signal from the amplifier 4 is inverted by the inverter 16 and applied to the gate 112, the gate 111 is opened while the gate 112 is closed at a normal current level. Therefore, when the power transistor 2 enters an overcurrent state, the gate 112 is opened by the high level signal applied via the terminal T2 and the high level signal applied via the inverter 16, so that the photocoupler 122. ) transistor 142
operates, transistor 2 has power supply 152 and resistor 1
32 and transistor 142 to be reverse biased "off", thereby causing power transistor 2
is released.

However, according to this method, it is not possible to detect an overcurrent condition at the same time as it occurs due to delays in detection circuits such as current transformers and amplifiers. As a result, the current flows by about 2 to 3 times as much, and there is a risk that the current will concentrate inside the element of the power transistor, particularly in the emitter electrode, and a so-called hot spot will occur, resulting in thermal breakdown. Furthermore, since the detection circuit may malfunction, it is complicated to take countermeasures against this. Furthermore, there is a drawback that it is impossible to prevent the base drive circuit from being burnt out due to the short circuit between the base and emitter of the power transistor as described above.

[Purpose of the invention]

The present invention has been made in view of the above, and an object of the present invention is to reliably prevent and protect the power transistor from being destroyed by overcurrent and the pace drive circuit from being burnt out.

[Key points of the invention]

By keeping the base current supplied to the power transistor almost constant, detecting the voltage between the base and the power transistor, and stopping the supply of base current when this detected value exceeds a certain ζpel. This is designed to protect the power transistor and its drive circuit.

[Embodiments of the invention]

FIG. 2 is a block diagram showing an embodiment of the present invention, and FIG. 3 is a graph showing the relationship between base-emitter voltage and collector current of a power transistor.

As is clear from FIG. 2, the pace drive circuit 1' is characterized in that it is provided with a constant current circuit 17 consisting of a transistor 171 and a resistor 172, and that it is provided with a comparator 6.

The pace drive circuit 1' is shown with a dashed signal in the sense that it differs slightly from that in FIG.
Since a Darlington connection type transistor is used here, ' is given a dash to mean that it is slightly different from the transistor 141. Note that 5 indicates a comparison reference power source. Further, in FIG. 2, the details of the pace drive circuit 1' are omitted and only the main parts thereof are shown, but the contents are the same as in FIG. 1.

In the device shown in FIG. 2, instead of detecting the current flowing through the power transistor 2 as in the conventional example, the base
It is also possible to detect the emitter voltage by comparing it with the reference voltage 5 using the comparator 6. In other words, the base-emitter voltage VBK of the power transistor 2 is expressed as a function of its base current IB and collector current IO, as shown in the following equation: By using a constant current circuit 17 to keep the voltage VBx substantially constant, it is possible to grasp the voltage VBx as a function of the collector current Ia. 5. Considering the pace current IB as a constant, the relationship between voltage VBK and IO is expressed as shown in FIG. It can be considered to be in a current state. Note that such detection is performed by the comparator 6, and when a predetermined output is obtained from the comparator 6, the power transistor 2 can be shut off in the same manner as in FIG. Note that instead of doing this, a switch may be provided between the power transistor 2 and the base drive circuit 1' to separate them, or a switch may be provided between the base and emitter to disconnect them. It is also possible to short-circuit between the two.

〔Effect of the invention〕

As described above, according to the present invention, the voltage between the base and emitter of a power transistor increases rapidly when a collector current of a predetermined value or more flows. 5 to detect from the emitter voltage value, it is possible to detect and cut off at high speed. Therefore, it is possible to release the power transistor from the overcurrent state and prevent its destruction. As a result, the pace drive circuit It also has the advantage of being able to be protected.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing a conventional power transistor driving method, Fig. 2 is a block diagram showing an embodiment of the present invention, and Fig. 3 is a graph showing the relationship between the voltage between the base and emitter of the power transistor and the collector current. It is. Symbol explanation 1.1'...Base drive circuit, 11t, 11
2-...and gate, 121, 122...
...Photocoupler, 131°132.172...
・Resistance, 141,142,171...Transistor, 141'-...Darlington transistor, 151,152...Power supply, 16...
Inverter, 17... Constant current circuit, 2...
・・・ノ(war transistor, 3...current transformer(
current detector), 4...amplifier, 5...
Reference voltage, 6... Konno (Later's agent, patent attorney Akio Namiki, agent, patent attorney Kiyoshi Matsuzaki Figure 1 Figure 2 Figure 3 Collector Electric Asa Ic (A)

Claims (1)

[Scope of Claims] A power transistor drive circuit that supplies current to a power transistor pace to drive it, including a constant current circuit that keeps the pace current substantially constant; 1. A power transistor drive circuit comprising a detection means for detecting an emitter voltage, and stopping supply of pace current when the detected voltage exceeds a predetermined value.