JPH0526903Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an overcurrent protection device for a power transistor that prevents damage to the power transistor due to overcurrent.

[Conventional technology]

A conventional overcurrent protection device is applied to a pulse width control inverter device, etc., as shown in FIG.
It was constructed by providing a fuse 6 in the power supply line 5. and power transistor 2
If the power transistors 2 and 4 or 3 and 5 are turned on at the same time due to a failure or noise in the control device (not shown) that controls the motors 5 to 5, or if the motor 1, which is the load, is short-circuited, the fuse 6 will open. It was blown out by the overcurrent, and no more current was prevented from flowing through the power transistors 2 to 5.

However, in the above-mentioned conventional configuration, the response time of the fuse 6 is on the order of milliseconds, and the power transistors 2 to 5 may be damaged before the fuse 6 blows, resulting in poor reliability as an overcurrent protection device. Missing. Furthermore, once the fuse 6 blows due to an overcurrent flow, the fuse 6 must be replaced in order to operate the motor 1 again, which has disadvantages such as poor maintainability.

Therefore, as an example of improving responsiveness and maintainability, an overcurrent protection device was previously proposed that detects when a current exceeding a certain level flows through a power transistor and stops the operation of the power transistor using a semiconductor switch. There is.

By the way, power transistors are generally
It has anti-surge characteristics as shown by curve A in the figure, and if the energization time is short, it is possible to flow a large current through the collector. That is, even if the current value is large, as long as it is a short pulse, the power transistor can remain in operation without any problem.

[The problem that the idea attempts to solve]

However, when the above-mentioned overcurrent protection device detects a current equal to or higher than a preset current value J _D0 , it immediately stops the operation of the power transistor, regardless of the energization time.

For this reason, as shown by the shaded area B in the figure, the operating range of the power transistor becomes narrow, resulting in the disadvantage that its performance cannot be fully demonstrated.

The present invention was devised in consideration of the above-mentioned conventional problems, and is capable of reliably protecting power transistors from overcurrent, fully demonstrating the performance of power transistors, and improving maintainability. It is an object of the present invention to provide an overcurrent protection device for a power transistor that can also achieve the following.

[Means to solve the problem]

In order to solve the above-mentioned problems, the overcurrent protection device for a power transistor according to the present invention includes a current detection means that converts the current flowing through the power transistor into a voltage and detects it, and has a low-pass filter that inputs the voltage. , a current determining means for determining whether the current detected by the current detecting section is at least a predetermined value, using the voltage, for example, by a comparator; and a current determining means for determining whether the current is at least the predetermined value by the current determining means. and operation stopping means for stopping the operation of the power transistor, and the characteristics of the low-pass filter are set so that the operation characteristics of the current determining means correspond to the surge resistance characteristics of the power transistor. It is characterized by the presence of

[Effect]

In the above configuration, the current flowing through the power transistor is detected as a voltage by the current detection means, but since this voltage is input to the low-pass filter, it gradually rises at a time corresponding to its magnitude. Subsequently, the current determining means determines whether the current flowing through the power transistor is equal to or greater than a predetermined value, for example, based on whether the voltage is equal to or greater than a comparison voltage of a comparator. If the current determining means determines that the current is equal to or greater than a predetermined value, the operation stopping means stops the operation of the power transistor.

In the above operation, the characteristics of the low-pass filter are set so that the operating characteristics of the current determining means correspond to the surge resistance characteristics of the power transistor, so the overcurrent flowing through the power transistor is relatively large. However, if the current flows only for a very short time, the operation stopping means will not stop the operation of the power transistor as determined by the current determining means.

Therefore, according to the above configuration, the power transistor is reliably protected from overcurrent, the operating range of the power transistor is expanded to fully demonstrate its performance, and furthermore, after the overcurrent is cut off, the power transistor is automatically Maintainability can also be improved by returning it to the original state.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 to 5.

As shown in FIG. 1, the pulse width control inverter device 11 includes a motor drive circuit 12 and a protection circuit 13 which is an overcurrent protection device.

The motor drive circuit 12 mainly consists of photocouplers 14-17 and power transistors 18-21.

As the details of the photocoupler 14 are shown in FIG. 2, the photocoupler 14 is made up of an optical isolation amplifier 14a, a driver 14b, and output transistors 14c, 14d connected to a control circuit 22 in this order. It has a similar configuration.
Further, independent power supplies 23 and 24 are always connected to the photo couplers 14 and 15, respectively, and a common power supply 25 is connected to the photo couplers 16 and 17 via a control transistor 26 of the protection circuit 13. .

The outputs of photocouplers 14 to 17 are, respectively,
It is connected to the bases of the power transistors 18 to 21, and the collectors of the power transistors 18 and 19 are both connected to a motor drive power source 28. Further, the emitters of power transistors 18 and 19 are connected to power transistors 20 and 2, respectively.
1 and two different terminals of the motor 27. The emitters of the power transistors 20 and 21 are both connected to an overcurrent detection resistor 29 of the protection circuit 13.

On the other hand, the protection circuit 13 includes an overcurrent detection resistor 29,
An overcurrent detection unit 31 as a current detection means including a low pass filter 30 consisting of a resistor 30a and a capacitor 30b, an amplifier 32, a comparator 33 as a current determination means, and a state holding capacitor 34a and a discharge resistor 34b connected in parallel. A monostable multivibrator (hereinafter referred to as monomulti) 35 having a state holding section 34 and a control transistor 26 are connected in this order, and a stabilizing circuit 36 is provided in the power supply path. Further, the circuit consisting of the monomulti 35 and the control transistor 26 constitutes an operation stopping means that stops the operation of the power transistors 18 to 21 when the output of the comparator 33 becomes high level. moreover,
This embodiment includes a status output section 37 that transmits to the control circuit 22 whether or not the protection circuit 13 is in a protection operation state.

In the above configuration, the operations of the protection circuit 13 and the motor drive circuit 12 will be explained below.

First, in the protection circuit 13, the motor drive circuit 12
A collector current (hereinafter referred to as collector current) flowing through the power transistors 18 to 21 flows through the overcurrent detection resistor 29 and generates a voltage across the overcurrent detection resistor 29. This voltage is input to an amplifier 32 via a low-pass filter 30, amplified there, and then input to a comparator 33. Then, it is compared with a comparison voltage set in advance in the comparator 33.

However, the input voltage of the comparator 33 changes as shown in FIGS.
It changes over time depending on the magnitude of the collector current. Therefore, the input voltage of the comparator 33 is, for example, if the magnitude of the collector current is J _D1 .
If the magnitude of the collector current is J _D2 , the comparison voltage is reached in _the energization time of T ₁ .
The relationship between the magnitude of the collector current and the energization time when the input voltage of the comparator 33 reaches the comparison voltage is represented by a curve C shown in FIG. 4. and,
This curve C also shows the operating characteristics of the comparator 33, and by appropriately setting the characteristics of the low-pass filter 30, as shown in the figure,
It can be made to follow the anti-surge characteristic curve D of the power transistors 18 to 21.

Therefore, if the collector current has a value in the range below curve C, as shown by the shaded area E in the figure, the input voltage of comparator 33 will not exceed the comparison voltage, and the comparator 33 will not exceed the comparison voltage. The output of 33 is in a high impedance state. Therefore, the state holding capacitor 34a is connected to the discharge resistor 34.
Since it is discharged through b, it is not charged. Therefore, the monomulti 35 is turned on, and the control transistor 26 is also turned on.

However, if the collector current is in a range above curve C in FIG. As a result, the output of the comparator 33 becomes high level. Therefore, the state holding capacitor 34a is charged, the monomulti 35 is turned off, the control transistor 26 is also turned off, and the protection circuit 13 enters the protective operation state.

Once the protective operation state is reached, the collector current is cut off and even if the output of the comparator 33 returns to high impedance, the charge stored in the state holding capacitor 34a is discharged through the discharge resistor 34b, and the input of the monomulti 35 is The protective operation state is maintained until the voltage drops below the threshold voltage. Then, when the input to the monomulti 35 falls below the threshold voltage, the protection state is canceled and the original state is restored.

Furthermore, the protection circuit 13 of this embodiment includes a status output unit 37, and by transmitting the status of the protection operation from the status output unit 37 to the control circuit 22, the control circuit 22 handles the abnormality. It is now possible to perform operations such as

On the other hand, in the motor drive circuit 12, when the protection circuit 13 is not in the protective operation state in the normal operating state, the control transistor 26 of the protection circuit 13 is turned on as described above, and the photocoupler 1 is turned on.
In addition to photocouplers 4 and 15, a power source is also connected to photocouplers 16 and 17. This photo coupler 14~
17, based on the signal from the control circuit 22, the fifth
A power transistor 18, as shown in Figures a-d.
~21 ON signal is output. As a result, power transistors 18 and 21 are turned on at the same time, and power transistors 19 and 20 are turned on at the same time.
The ON state is repeated alternately.

In the former state, current flows through the path of the power transistor 18 , motor 27 , power transistor 21 , and overcurrent detection resistor 29 . On the other hand, in the latter state, the power transistor 19, the motor 27,
A current flows through the path between the power transistor 20 and the overcurrent detection resistor 29. Then, these two states are alternately repeated, and the motor 27 is driven by alternately passing current in the opposite direction to the motor 27.

Therefore, if the power transistors 18 and 20 or 19 and 21 are turned on at the same time due to a failure in the control circuit 22 or noise, etc., or if the motor 2
In the event of abnormal operation, such as when the output to 7 is short-circuited, an overcurrent flows through the ON state of the power transistors 18 to 21. Protection circuit 13
However, as mentioned above, when the above overcurrent is detected,
It enters the protective operation state, and the control transistor 26
It turns off. As a result, photocoupler 16,
The power supply path to the power transistor 17 is cut off, and regardless of the output signal of the control circuit 22, the power transistor 2
0 and 21 are turned OFF, and the overcurrent is cut off.
Therefore, power transistors 18-21 stop operating and are protected from damage due to overcurrent.

[Effect of idea]

As described above, the overcurrent protection device for a power transistor according to the present invention includes a current detecting means having a low-pass filter that converts the current flowing through the power transistor into a voltage and detects the voltage, and inputs the voltage, and a current detecting means for detecting the current flowing through the power transistor. current determining means for determining whether the current detected by the unit is equal to or greater than a predetermined value; and when the current determining means determines that the current is equal to or greater than the predetermined value, the operation of the power transistor is stopped. and an operation stop means, and the characteristics of the low-pass filter are set so that the operation characteristics of the current determining means correspond to the anti-surge characteristics of the power transistor.

This has the effect that the power transistor can be reliably protected from overcurrent, the performance of the power transistor can be fully exhibited, and maintainability can also be improved.

[Brief explanation of drawings]

1 to 5 show an embodiment of the present invention, in which FIG. 1 is a circuit diagram showing the overall configuration of a pulse width control inverter device, and FIG. 2 is a circuit diagram showing details of a photocoupler. Figure 3a is a graph showing the relationship between energization time and collector current, Figure 3b is a graph showing the relationship between energization time and collector current.
is a graph showing the relationship between the conduction time for each collector current and the comparator input voltage, FIG. 4 is a graph showing the operating range of the power transistor, and FIGS. 5 a to d are time charts showing the state of the ON signal of the power transistor. FIG. 6 is a circuit diagram showing a conventional example, and FIG. 7 is a graph showing the operating range of a power transistor in another conventional example. 18 to 21 are power transistors, 26 is a control transistor, 30 is a low-pass filter, 31 is an overcurrent detection section (current detection means), 33 is a comparator (current determination means), and 35 is a monomulti.

Claims (1)

[Claims for Utility Model Registration] A current detection means having a low-pass filter that converts and detects the current flowing through the power transistor into a voltage and inputs the voltage, and the current detected by the current detection section is greater than or equal to a predetermined value. current determining means for determining whether or not the current is greater than or equal to a predetermined value; and operation stopping means for stopping the operation of the power transistor when the current determining means determines that the current is equal to or higher than a predetermined value. An overcurrent protection device for a power transistor, characterized in that characteristics of the low-pass filter are set so that operating characteristics of the current determining means correspond to anti-surge characteristics of the power transistor.