JPS6243210A - Overcurrent detecting circuit for transistor - Google Patents

Abstract

PURPOSE:To attain a simple constitution as well as the reduction of cost without using an extra overcurrent detecting circuit, by using an overcurrent protecting circuit of a primary transistor (TR) also to a device which uses said primary TR. CONSTITUTION:The voltage energing at an output terminal A of an ON/OFF signal circuit 1 is also applied to the base of a control TR 5 via a resistance 9 and a constant voltage diode 8 and with an extremely slight time delay. The currents flowing through the resistance 9 all flow to the collector of a primary TR 4 via a diode 6 in a normal mode. Thus the TR 5 is kept OFF. Here if the collector current of the TR 4 exceeds a prescribed level, the TR 5 is turned on. The light emitting diode part of a photocoupler 12 is connected in series to the TR 5. Then the base-emitter current of the TR 4 is commutated to the TR 5 to turn off the TR 4. At the same time, an insulated overcurrent detecting signal is extracted from the phototransistor side of the photocoupler 12 and can be used for various types of control signals.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to an overcurrent detection circuit for a transistor used as a switching element.

[Prior art and its problems]

Comparing a transistor and a thyristor, which have the same current capacity, the overcurrent withstand capacity of the transistor is smaller than that of the thyristor, so it is difficult to protect the transistor from overcurrent with a fuse. In addition, by connecting transistors in a bridge to operate on/off,
In an inverter that converts DC power to AC power, if there is a period when the transistors that make up the upper and lower arms of the same phase are turned on at the same time due to some kind of failure, a so-called arm short circuit, the short-circuit current at this time can be extremely high. Since the rise occurs with a large temporal change (di/dt), the usual overcurrent protection method is to detect the overcurrent of the transistor using a current transformer, etc., and use this detection signal to shut off the base signal circuit of the transistor. Since it cannot be protected by
This can even damage other healthy transistors.

Therefore, by utilizing the characteristic that when an overcurrent flows through a transistor, the voltage between the collector and emitter of this transistor increases, the overcurrent of the transistor is suppressed by using a control transistor as a control switching element connected in parallel between the base and emitter of the transistor. A transistor overcurrent protection circuit designed to cut off the current is disclosed in Japanese Patent Application Laid-Open No. 59-103567.
It is introduced in the issue.

FIG. 3 is a circuit diagram showing the transistor overcurrent protection circuit described in this publication, and the outline of its operation is as follows. That is, in the aga diagram, when an on signal is output from the output terminal A of the on/off signal circuit 1,
A forward bias current flows from the base to the emitter of the main transistor 4 via the resistor 2, turning the main transistor 4 on. At this time, the voltage between the collector and emitter of the main transistor 4 becomes a sufficiently small value of 0. In addition, due to the delay circuit composed of the resistor 10 and the capacitor 11, the voltage across the capacitor 11 becomes the voltage 1 that appears at the output terminal A of the on/off signal circuit 1 after a very short delay time (for example, several microseconds). Near value already charged. At this time, the forward voltage of the diode 6, the constant voltage diode 8
If the relationship between the Zener voltage of the control transistor 5, the base-emitter voltage of the control transistor 5, and the collector-emitter voltage when the main transistor 4 is on is properly selected, all of the current flowing through the resistor 9 flows through the diode 6. The current flows into the collector of the main transistor 4 through the current, and since no current flows into the base of the control transistor 5, the control transistor 5 is in an off state.

When the collector current of the main transistor 4 becomes an overcurrent exceeding the specified value and the collector-emitter voltage becomes large, the control transistor 5 becomes conductive, and the forward bias current flowing from the base of the main transistor 4 to the emitter Since the current is commutated to the control transistor 5, the main transistor 4 can be turned off with almost no time delay. Note that the protection diode 3 is connected to the main transistor 4.
The resistor 7 is used to compensate for the Zener voltage of the constant voltage diode 8. Also, the output terminal B of the on-off signal circuit 1 outputs a reverse bias current for turning off the main transistor 4.

As mentioned above, the conventional circuit shown in FIG. 3 can provide fast and reliable protection against overcurrent in the main transistor 4 despite its simple circuit configuration. At times, the protective operation is automatically performed, but on the other hand, the protective operation is automatically released when the overcurrent is eliminated.

Therefore, when composing an inverter device or chopper device by combining transistors equipped with such an overcurrent protection circuit, a separate overcurrent determination circuit is prepared as a protection system for the entire device. It is necessary to stop the overcurrent protection circuit and issue an alarm, and the overcurrent protection circuit is required to be equipped with double overcurrent protection circuits, which has the disadvantage of complicating the circuit and increasing cost.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a transistor overcurrent image 11 circuit which can also be used in an overcurrent protection circuit of a device using a main transistor.

[Key points of the invention]

This invention provides a bridging control transistor between the base and emitter of the main transistor, and when the main transistor is in a forward bias state, if an overcurrent flows through it, the voltage between the collector and emitter of the main transistor increases. Therefore, by detecting this voltage rise and activating the control transistor, the overcurrent of the main transistor is cut off in a very short time, the operating state of this control transistor is detected, and this detection signal is sent to the overcurrent of the raw transistor. The idea is to treat this as a current detection signal and use it for various control signals.

[Embodiments of the invention]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention. In FIG. 1, reference numeral 1 denotes an on/off signal circuit, in which forward bias current flows from the output terminal A to turn on the main transistor 4, and from output terminal B turns the main transistor 4 to the OFF state. The voltage that appears at the output terminal A of the zero on/off signal circuit 1, which outputs a reverse bias current, is controlled by the resistor 9 and the constant voltage diode 8 by a delay circuit composed of a resistor 10 and a capacitor 11. The forward voltage v6 of the diode 6, the zener voltage v1 of the voltage regulator diode 8, and the zener voltage v1 of the control transistor 5 are also applied to the base of the control transistor 5 with a very small time delay. If the relationship between the base-emitter voltage V and the collector-emitter voltage v4 of the main transistor 4 is selected appropriately, that is, so as to satisfy the relationship in equation (1) below, the current flowing through the resistor 9 All of flows through the diode 6 to the collector of the raw transistor 4, so the control transistor 5 maintains the off state.OV4+ Va < Vs + VW ...-...
・・・・・・・・・ Song... Beat... (1) Here, when the collector current of the main transistor 4 becomes an overcurrent exceeding the specified value, the collector-emitter voltage v4 of the main transistor 4 increases. Since the value increases to v41, the relationship between each voltage shown in equation (1) changes to the relationship shown in equation (2).

Va1 +V・＞V, +Vs・・・・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・・・・
・(2) Therefore, the control transistor 5 changes from OFF to ON state. In the present invention, since the light emitting diode part of the photocoupler 12 is connected in series to this control transistor 5, the main transistor 4 is commutated to the control transistor 5 to turn off the main transistor 4, and an isolated overcurrent detection signal can be extracted from the phototransistor side of the photocoupler 12 to an external base. . Therefore, when an inverter device is configured by combining a plurality of main transistors 4, each transistor is automatically cut off due to overcurrent, and the inverter device also handles the overcurrent of the device based on the overcurrent detection signal from the net coupler 12. It can be carried out.

Note that the resistor 2 in FIG. 1 is for limiting the base current of the main transistor 4, the protection diode 3 is for protecting the main transistor 4, and the resistor 7 is for compensating the Zener voltage of the constant voltage diode 8. , is the same as the case of the conventional circuit shown in FIG.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention. In FIG. 2, parts having the same functions as the parts shown in FIG. WJ1 are given the same numbers, so their explanation will be omitted.

In the second embodiment shown in FIG. 2, an overcurrent detection transistor 14 is provided as a second control switching element that operates exactly the same as the control transistor 5 that cuts off the current of the main transistor 4 in the event of an overcurrent. When the main transistor 4 becomes overcurrent, the control transistor 5 and the overcurrent detection transistor 14 are turned on, and current flows from the DC power supply 16 to the overcurrent detection transistor 14 via the protective resistor 15 and the photocoupler 12. Since an isolated overcurrent detection signal is provided on the output side of 12, this signal can be used to activate the overcurrent protection device.

〔Effect of the invention〕

According to this invention, when the main transistor becomes overcurrent, when the control transistor which quickly commutates the base current of this main transistor to cut off the main circuit current is turned on, or when the control transistor is exactly the same as this control transistor, When the overcurrent detection transistor, which operates on and off, turns on, the overcurrent detection circuit is configured to insulate and detect that state, so a power conversion device consisting of a combination of these main transistors (For example, an inverter, etc.) can also use the above-mentioned overcurrent detection signal to activate the overcurrent protection system of the power converter. Therefore, there is no need to provide a separate overcurrent detection circuit, which has the effect of realizing a simple and inexpensive overcurrent protection device.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a first embodiment of the present invention, Fig. 2 is a circuit diagram showing a second embodiment of the invention, and Fig. 3 is a conventional example of a transistor overcurrent protection circuit. 0 1 ON/OFF signal circuit, 2... Resistor, 3... Protection diode, 4... Main transistor, 5... Control transistor as a control switching element, 6... Diode, 7,9.10... Resistor, 8... Constant voltage diode, 11. Capacitor, 12 Photocoupler, 14
- Overcurrent detection transistor as a second control switching element, 15... protection resistor, 16... DC power supply. lh++1

Claims (1)

[Claims] 1) Interposed between the base and emitter of a main transistor, which is a switching element, and an on/off signal circuit that controls this main transistor, and capable of cutting off the forward bias current of the main transistor. The base of the main transistor
It has a control switching element that directly bridges the emitters or bridges between the base and emitter of the auxiliary transistor that leads the base forward bias current to the main transistor, and the base current of the main transistor is forward biased. In a transistor drive circuit configured to cause the control switching element to conduct when the collector-emitter voltage of the main transistor exceeds a predetermined limit value at a certain time, the transistor drive circuit includes means for detecting an operation of the control switching element. A transistor overcurrent detection circuit characterized in that: 2) In the overcurrent detection circuit according to claim 1, the operation detection means for the control switching element includes a second control switching element that operates in the same manner as the control switching element, and a second control switching element that operates in the same manner as the control switching element. 1. A transistor overcurrent detection circuit comprising means for detecting operation of an element.