JPH0622322B2 - Switching transistor protection circuit - Google Patents

Description

The present invention relates to a protection circuit for a switching transistor such as an output transistor of a motor drive circuit.

FIG. 1 shows a bridge type motor drive circuit. This motor drive circuit is composed of output transistors 2, 4, 6, 8 as switching transistors and drive transistors 10, 12 and has external terminals 14, 16
A motor 18 that rotates normally or reversely is connected between the two. A driving power source (not shown) is connected between the power source terminal 20 and the common line, and Vcc indicates the driving voltage.

A rotation control circuit 22 is connected to the bases of the transistors 4, 8, 10 and 12, and a switching control signal is added based on a forward or reverse rotation command signal applied to the input terminal 24. That is, in this case, the transistors 10, 2, and 8 are conducting during the forward rotation, and the transistor 1 is rotating during the reverse rotation.
2, 6 and 4 are conducting and the current flowing in the motor 18 is switched to give the direction of rotation.

In such a motor drive circuit, the transistor 2 or 6
Is switched from the conducting state to the non-conducting state, and the transistor 4 or 8 is shifted from the non-conducting state to the conducting state, the transistors 2 and 4 or the transistors 6 and 8 are simultaneously brought into the conducting state, and the power supply terminal 20 -There may be a timing time when a short circuit occurs between common lines. That is, this is because the transition of the transistor 2 or 6 to the non-conduction state is delayed when the transition of the transistor 2 or 6 from the conduction state to the conduction state of the transistor 4 or 8.

When such a situation occurs, the output transistors 2, 4
Alternatively, a large current may flow through the transistors 6 and 8 and destroy them.

Therefore, conventionally, from the rotation control circuit 22 to the transistor 4,
The generation timing of the switching control signal applied at the time of rotation switching of 8, 10 and 12 is limited so that the transistors 2 and 4 or the transistors 6 and 8 do not become conductive at the same time, or the switching control signal is externally attached. A method such as delaying with a capacitor is used.

However, limiting the switching control signal causes inconveniences such as limiting the usage conditions when the drive circuit is configured by a semiconductor integrated circuit, and the external capacitor is installed by using external terminals (pins) or There are drawbacks such as an increase in product price due to an increase in the number of parts.

Therefore, the present invention prevents the switching transistors that are alternately switched from being turned on at the same time,
Switching transistor It is an object of the present invention to provide a protection circuit for a switching transistor, which is prevented from being damaged by a large current.

That is, the protection circuit for the switching transistor of the present invention is connected in series between the power supply terminal 20 to which the power supply should be connected and the one terminal 14 side of the load 18, and when conducting, supplies a drive current from the power supply side to the load 18. First transistor 2 to be poured
And a driving current in a direction opposite to the driving current is supplied from the power supply terminal 20 side to the load 18 when connected in series between the power supply terminal 20 and the other terminal 16 side of the load 18 and conducting. 2 is connected in series between the second transistor 6 and the one terminal 14 side of the load 18 and the ground point, and when conducting, a drive current flowing to the load 18 through the second transistor 6 is transferred to the ground point side. When the third transistor 4 that flows and the other terminal 16 side of the load 18 and the ground point are connected in series and are conductive, the drive current that flows to the load 18 through the first transistor 2 is connected to the ground point. The fourth transistor 8 flowing to the side and power is supplied from the power supply terminal 20 side, and the base is connected to the one terminal 14 of the load 18 to detect the conduction state of the first transistor 2. Conducting fifth P
A sixth P that is supplied with power from the power supply terminal 20 side and has its base connected to the other terminal 16 of the load 18 to detect the conduction state of the second transistor 6 and conduct electricity.
It is connected between the NP-type transistor 28B and the base of the third transistor 4 and the ground point, and receives the conduction current of the fifth transistor 28A at its base to conduct, and when the first transistor 2 conducts. A seventh NPN transistor 30A that controls the third transistor 4 to be in a non-conducting state, and a conduction current of the sixth transistor 28B that is connected between the base of the fourth transistor 8 and a ground point. An eighth NPN transistor 30B which receives the base and conducts, and controls the fourth transistor 8 to be in a non-conducting state when the second transistor 6 is conducting.

FIG. 2 shows an embodiment of a protection circuit for a switching transistor of the present invention, and the same parts as those of the motor drive circuit shown in FIG. 1 are designated by the same reference numerals.

In FIG. 2, in this motor drive circuit, the switching delay from the conducting state to the non-conducting state of the driving transistor 10 is detected, and the output transistors 2 and 4 as the first and third transistors are simultaneously brought into the conducting state. A switching suppression circuit 26A that suppresses the occurrence of the switching delay, and a switching suppression that detects the same switching delay of the driving transistor 12 and suppresses the output transistors 6 and 8 serving as the second and fourth transistors from being simultaneously turned on. The circuit 26B is installed.

The switching suppression circuit 26A includes a PNP transistor 28A that constitutes a fifth transistor, an NPN transistor 30A that constitutes a seventh transistor, and a resistor 3.
It is composed of 2A. The transistor 28A detects the conductive state of the transistor 10 and becomes conductive. Resistance 3
2A sets the current value when the transistor 28A is in the conductive state. Further, the transistor 30A short-circuits the base side of the transistor 4 to the ground side based on the conduction of the transistor 28A, and suppresses the switching signal to the base of the transistor 4.

In this embodiment, transistor 30A conducts when its base is at a high level, thus forcing the base of transistor 4 to remain at a low level where it is non-conductive.

The switching suppression circuit 26B includes a PNP transistor 28B forming a sixth transistor, an NPN transistor 30B forming an eighth transistor, and a resistor 3.
It is composed of 2B. The transistor 28B detects the conduction state of the transistor 12 and becomes the conduction state. Resistance 3
2B sets the current value when the transistor 28B is in the conductive state. Further, the transistor 30B short-circuits the base side of the transistor 8 to the ground side based on the conduction of the transistor 28B, and suppresses the switching signal to the base of the transistor 8.

In this embodiment, transistor 30B conducts when its base is at a high level, thus holding the base of transistor 8 at a low level, which forces it to be non-conductive.

With the above-described configuration, when the transistor 10 is in the conductive state, the transistors 28A and 30A of the switching suppression circuit 26A are in the conductive state, so that the transistor 4 is in the non-conductive state. It is necessary that the transistor 10 is always non-conductive. That is, even if the base of the transistor 10 shifts to a high level, the transistor 4 does not turn on while the transistors 10 and 2 do not turn off. Therefore, it is possible to prevent the transistor 2 and the transistor 4 from becoming conductive at the same time and to prevent the short-circuit current from flowing.
In addition, the breakdown of the transistor 4 can be prevented and the reliability of operation can be improved.

When the transistor 12 is in the conductive state, the transistors 28B and 30B of the switching suppression circuit 26B are in the conductive state, so that the transistor 8 is in the non-conductive state. Therefore, in order for the transistor 8 to be in the conductive state, the transistor 12 is always in the non-conductive state. It is necessary to be. That is, even if the base of the transistor 12 shifts to a high level, the transistor 8 does not turn on unless the transistors 12 and 6 shift to the non-conducting state. Therefore, it is possible to prevent the transistor 6 and the transistor 8 from becoming conductive at the same time and to prevent the short-circuit current from flowing, so that the transistor 6 and the transistor 8 can be prevented from being broken and the reliability of the operation can be improved.

Particularly, in the motor drive circuit, there is an advantage that the braking delay due to the limitation of the control signal can be prevented and the rotation direction can be switched quickly.

According to the experiment, it was confirmed that the short-circuit current, which was flowing for several amperes when the switching suppression circuit was not installed, could be reduced to about several milliamperes.

In addition, since the switching control signal is limited as in the past, there are no inconveniences such as the setting of usage conditions, and since the delay time setting capacitor can be removed, the pin for that is also unnecessary, and the circuit configuration Can be simplified and the product price can be reduced.

Although the motor drive circuit has been described in the embodiment,
The present invention can be expected to have the same effect even when implemented in a switching element that drives an addition other than the motor.

As described above, according to the present invention, the first and third transistors forming the series circuit or the third transistor or the third transistor forming the series circuit can be formed without limiting or delaying the switching control signal for selectively turning on the switching transistor. It is possible to prevent the second and fourth transistors from conducting simultaneously and prevent the power supply from being short-circuited via both of them, and to prevent the first and third transistors or the second and fourth transistors from being destroyed by a large current at the time of short circuit. Can be protected.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a conventional motor drive circuit, and FIG. 2 is a circuit diagram showing an embodiment of a switching transistor protection circuit of the present invention. 2 ... Output transistor (first transistor) 4 ... Output transistor (third transistor) 6 ... Output transistor (second transistor) 8 ... Output transistor (fourth transistor) 18 ... Motor (load) ) 20 ... Power supply terminals 26A, 26B ... Switching prevention circuit 28A ... Fifth transistor 28B ... Sixth transistor 30A ... Seventh transistor 30B ... Eighth transistor

Claims (1)

[Claims] 1. A power supply terminal 20 to which a power supply is connected and a load 18
Of the first transistor 2 which is connected in series with the one terminal 14 side and flows a drive current from the power source side to the load 18 when conducting, and the power source terminal 20 and the other terminal 16 side of the load 18. A second transistor 6 that is connected in series between the power source terminal 20 side and a load 18 in which a drive current in a direction opposite to the drive current flows into the load 18, and is connected to the one terminal 14 side of the load 18. A third transistor 4 which is connected in series with a point and conducts a drive current flowing to the load 18 through the second transistor 6 to the ground point side when conducting, and the other terminal 16 side of the load 18 And a ground point, and a fourth transistor 8 which is connected in series between the first transistor 2 and the load 18 to flow a drive current flowing through the load 18 to the ground point side, and is fed from the power supply terminal 20 side. At the same time, a base is connected to the one terminal 14 of the load 18 to detect the conduction state of the first transistor 2 and conduct a fifth P
A sixth P that is supplied with power from the power supply terminal 20 side and has its base connected to the other terminal 16 of the load 18 to detect the conduction state of the second transistor 6 and conduct electricity.
It is connected between the NP-type transistor 28B and the base of the third transistor 4 and the ground point, and receives the conduction current of the fifth transistor 28A at its base to conduct, and when the first transistor 2 conducts. A seventh NPN transistor 30A that controls the third transistor 4 to be in a non-conducting state, and a conduction current of the sixth transistor 28B that is connected between the base of the fourth transistor 8 and a ground point. An eighth NPN transistor 30B which receives the base and conducts, and controls the fourth transistor 8 to be in a non-conducting state when the second transistor 6 is conducting; circuit.