JP3309039B2 - Overcurrent protection circuit for inverter control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an overcurrent protection circuit for detecting an overcurrent of a switch element of an inverter control device and preventing destruction of the switch element.

[0002]

2. Description of the Related Art A conventional overcurrent protection circuit using a power transistor as a switching element of an inverter control device will be described below with reference to FIG.

The switching control signal circuit 1 has, for example, N
This is a circuit for controlling the switching of the PN type main transistor 3. The ON signal terminal TON is connected to the base of the main transistor 3 via the resistor 2, and the common terminal TCOM is connected to the emitter of the main transistor 3. . And
A forward bias current for turning on the main transistor 3 is supplied to the base of the main transistor 3 from the ON signal end TON.

A time constant circuit composed of a resistor 8 and a capacitor 9 is connected between the ON signal terminal TON and the common terminal TCOM. The output terminal of the time constant circuit, that is, the connection point between the resistor 8 and the capacitor 9 is connected to the collector of the main transistor 3 via the resistor 7, and the other is connected to the collector of the main transistor 3 via the diode 5. It is connected to the base of the control transistor 4 via a diode 6. The control transistor 4 is, for example, an NPN transistor. The collector is connected to the base of the main transistor 3 and the emitter is connected to the emitter of the main transistor 3.

Further, the off signal end TOFF of the switching control signal circuit 1 is connected to the base of the main transistor 3, and the off signal end TOFF minus the common end TCOM is connected to the base-emitter section of the main transistor 3. Have been. Then, a base reverse bias current for shortening the turn-off time of the main transistor 3 is supplied to the base of the main transistor 3 from the off signal end TOFF.

In the above configuration, when an ON signal is output from the ON signal terminal TON of the switching control signal circuit 1, a forward bias current flows into the base of the main transistor 3, and the main transistor 3 is turned on, and the collector is turned on. A current Ic is generated. At this time, if the collector current Ic is equal to or less than the specified value, the collector-emitter voltage Vce of the main transistor 3 has a sufficiently small value. Then, in the time constant circuit constituted by the resistor 8 and the capacitor 9,
The capacitor 9 is charged, and the voltage across the capacitor 9 becomes a voltage value close to the voltage value of the ON signal end of the switching control signal circuit 1 several microseconds after the generation of the ON signal.

Here, the sum of the collector-emitter voltage Vce of the main transistor 3 and the forward voltage Vf of the diode 5 is the sum of the Zener voltage Vz of the constant voltage diode and the base-emitter voltage Vbe of the control transistor 4. , The entire current flowing through the resistor 7 flows into the collector of the main transistor 3 through the diode 5, and the control transistor 4 is turned off.

On the other hand, the collector current I of the main transistor 3
When c flows over a specified value, the collector-emitter voltage Vce of the main transistor 3 increases. Therefore, the sum of the collector-emitter voltage Vce and the forward voltage Vf of the diode 5 becomes the Zener voltage Vz of the constant voltage diode 6.
And the base-emitter voltage Vbe of the control transistor 4
Is greater than the sum of In this case, the current flowing out of the resistor 7 via the constant voltage diode 6 is supplied to the base of the control transistor, and the control transistor 4 is turned on. As a result, the current flowing from the base to the emitter when the main transistor 3 is turned on flows between the collector and the emitter of the control transistor 4, the main transistor 3 is turned off, and the collector current Ic is cut off.

FIG. 5 shows the breakdown time characteristic of the main transistor of the inverter control device and the protection time characteristic of the overcurrent protection circuit. The vertical axis represents time t, and the horizontal axis represents the collector-emitter connection of the main transistor. It takes the voltage Vce.

In the figure, a curve a represents a relationship between the collector-emitter voltage Vce of the transistor 3 and the time t until the main transistor 3 is destroyed when a current of a prescribed value or more flows through the main transistor 3. FIG. Region 1 divided by the breakdown time curve indicates a region where the main transistor 3 is destroyed. As shown by the breakdown time curve a, the time required for the transistor to be destroyed tends to be longer when the collector-emitter voltage Vce is lower and shorter when the collector-emitter voltage Vce is higher.

A straight line b in FIG. 5 shows a protection time curve in which the overcurrent protection circuit in FIG. A region 2 divided by the protection time curve and each axis indicates a region where the main transistor 3 is turned on. Therefore, as is apparent from FIG. 5, in the overcurrent protection circuit of FIG. 4, when the collector-emitter voltage Vce of the main transistor 3 exceeds a certain value, the forward bias current to the base of the main transistor 3 is uniformly increased. Cut off.

[0012]

However, in the conventional overcurrent protection circuit, when the collector-emitter voltage Vce of the main transistor 3 exceeds the fixed value as described above, the main transistor 3 is turned off. . Therefore,
As shown by a curve a in FIG.
When the emitter-to-emitter voltage Vce is low, the main transistor 3
Operates so as to turn off the main transistor 3 in an unnecessarily short time even though it has not yet been destroyed.
For this reason, there has conventionally been a problem that the main transistor 3 cannot be operated with its maximum capacity.

An object of the present invention is to provide a simple circuit configuration.
It is an object of the present invention to provide an inexpensive overcurrent protection circuit that can make the most of the breakdown strength of a main transistor.

[0014]

[0015]

The present invention has the following configuration in addition to the current detecting means and the voltage detecting means. That is, a first comparison means for comparing a current detection signal from the current detection means with a first reference voltage, and as a result of the comparison by the first comparison means, the current detection signal from the current detection means is equal to the first reference voltage. When the voltage exceeds the voltage , the voltage detection
Charged internally with a predetermined time constant by the detection voltage from the means
A time constant circuit, a second comparing means for comparing a charging voltage of the time constant circuit with a second reference voltage, and a signal supplied from the time constant circuit as a result of the comparison by the second comparing means. And a control element for turning off the switch element when the voltage becomes equal to or higher than two reference voltages.

In general, the time resistance until the switch element is destroyed corresponds to the voltage between the terminals of the switch element. Therefore, if the time until the switch element is turned off is made variable in accordance with the voltage between the terminals of the switch element as in the overcurrent protection circuit of the present invention, the overcurrent protection circuit protects in an unnecessarily short time. Will not work. For this reason, the switch element can always be operated in the maximum area of its capability.

[0017]

FIG. 1 shows an example of the configuration of an overcurrent protection circuit of an inverter control device according to an embodiment of the present invention. The same components as those in FIG. Simplify.

(Circuit Configuration) Switching Control Signal Circuit 1
Of the main transistor 3 via the resistor 2
A forward bias current for controlling on / off of the main transistor 3 is supplied to the base of the main transistor 3 from the on signal end TON. Common end T
COM is connected to the emitter of the main transistor 3 via a current detection resistor 13 for detecting a current flowing through the emitter of the main transistor 3. The current detection resistor 13 may be constituted by a resistor externally attached to the semiconductor chip on which the main transistor 3 is formed, or a separate cell is provided in the semiconductor chip to form a resistor on this cell. Any method of detecting a current proportional to the current flowing through the main transistor 3 may be used.

The collector of the main transistor 3 and the common terminal T
Between COM and COM, voltage dividing resistors 10 and 11 for detecting the collector-emitter voltage Vce of the main transistor 3 are provided. One input terminal of an operational amplifier 12 is connected to a connection point between the voltage dividing resistors 10 and 11, and
The collector-emitter voltage Vce divided by the voltage dividing resistors 10 and 11 is supplied to the operational amplifier 12. The output side of the operational amplifier 12 is connected to the control power supply 15 via the diode 5 and the resistor 22.

One input terminal of the first comparator 14 is connected to a connection point between the current detection resistor 13 and the emitter of the main transistor 3, and has a voltage value corresponding to a voltage generated at both ends of the current detection resistor 13. Is detected by the first comparator 14
Is supplied to one of the input terminals. The reference voltage V1 is supplied to the other input terminal of the first comparator 14,
The output side of the first comparator 14 is connected to the control power supply 15 via the resistor 21 and the diode 16 and the resistor 22.
Is connected to the control power supply 15 via the.

One end of the resistor 17 is connected to a connection point between the resistor 22 and the diodes 5 and 16, and the resistor 1
The other end of 7 is connected to a capacitor 18. Resistance 1
A time constant circuit is constituted by 7 and the capacitor 18, and this time constant circuit is arranged between the resistor 22 and the common terminal TCOM. One input terminal of the second comparator 19 is connected to the other end of the resistor 17, that is, the output terminal of the time constant circuit.
The output side of the second comparator 19 is connected to the control power supply 15 via the resistor 20 and to the base of the control transistor 4. The control transistor 4 has a collector connected to the base of the main transistor 3 and an emitter connected to the common terminal TCOM of the switching control signal circuit 1. The OFF signal terminal TOFF of the switching control signal circuit 1 is connected to the base of the main transistor 3 as in the conventional case, and supplies a reverse bias current to the base when the main transistor 3 is turned off.

(Circuit Operation) Next, the operation of the overcurrent protection circuit will be described with reference to FIGS. FIG. 2 is a time chart showing the operation of the circuit until the overcurrent protection circuit detects that an overcurrent has flowed through the main transistor 3 and turns off the main transistor 3 in this embodiment. .

When an ON signal is output from the ON signal terminal TON of the switching control signal circuit 1, a voltage necessary for turning on the main transistor 3 is applied to the base of the main transistor 3 via the resistor 2. As a result, a current flows from the base of the main transistor 3 toward the emitter, and the main transistor 3 is turned on. The collector-emitter voltage Vce of the main transistor 3 is equal to the resistance 1
The voltage is divided by a resistor 11 and input to an operational amplifier 12. The operational amplifier 12 is a voltage follower, and outputs the divided voltage as it is as a voltage detection signal as shown in FIG.

The first comparator 14 is supplied with a current detection signal corresponding to the voltage generated across the current detection resistor 13 as shown in FIG. When a current equal to or more than the specified value flows through the collector current Ic of the main transistor 3, the voltage of the current detection signal output from the current detection resistor 13 changes to the reference voltage V1.
2B, and the output from the first comparator 14 becomes H level as shown in FIG.

When the output of the first comparator 14 goes high, the diode 16 turns off, and the resistor 17 and the capacitor 1
As shown in FIG. 2D, a voltage corresponding to a voltage detection signal from the operational amplifier 12 is input to the time constant circuit constituted by 8 through the diode 5 in the ON state. The time constant circuit has a predetermined time constant. The time constant circuit supplies the second comparator 19 with the time constant and the input voltage to the time constant circuit, as shown in FIG. A voltage signal whose voltage value increases at a corresponding speed is output. Second comparator 19
As shown in FIG. 2 (f), the voltage signal is compared with the reference voltage V2, and when the voltage value becomes higher than the reference voltage V2, a signal which becomes H level in response thereto is output.

When the output signal from the second comparator 19 becomes H level, a current flows from the control power supply 15 to the base of the control transistor 4 via the resistor 20, and the control transistor 4 is turned on. As a result, the current flowing from the base of the main transistor 3 to the emitter flows to the collector of the control transistor 4, and the main transistor 3 is rapidly turned off as shown in FIG. Ic is turned off.

On the other hand, the collector current I of the main transistor 3
When c is within the specified value, the voltage generated across the current detection resistor 13 is lower than the reference voltage V1, and the output from the first comparator 14 is at the L level. When the output of the first comparator 14 becomes L level, the diode 16 turns on and the diode 5
Is turned off, the voltage detection signal from the operational amplifier 12 is not supplied to the time constant circuit. Accordingly, the output voltage from the time constant circuit becomes lower than the reference voltage V2, the output of the second comparator 19 becomes L level, no current is supplied to the base of the control transistor 4, and the control transistor 4 is turned off. Become. Therefore, the main transistor 3 keeps the ON state in response to receiving the ON signal from the switching control signal circuit 1 as a base.

FIG. 3 is a diagram showing the breakdown time characteristic of the main transistor and the protection time characteristic of the overcurrent protection circuit in the inverter control device according to the present embodiment. The collector-emitter voltage Vce of the transistor is taken. In FIG. 3, a curve a shows a breakdown time curve of the main transistor 3, and a curve c shows a protection time curve in the overcurrent detection circuit of the present embodiment. A region 1 defined by the curve a indicates a region where the main transistor 3 is destroyed, and a region 2 defined by the curve c and each axis indicates a region where the main transistor 3 is turned on. .

Here, the voltage dividing ratio of the voltage dividing resistors 10 and 11 in FIG. 1 is A, the time constant of the time constant circuit constituted by the resistor 17 and the capacitor 18 is τ, and the second comparator 1 is passed through the time constant circuit.
Assuming that the voltage compared by V.9 is V2, in the overcurrent protection circuit of FIG. 1, the second comparator 19 sets the H-level voltage in accordance with the collector-emitter voltage Vce of the main transistor 3 and the voltage.
The relationship with the time t until the level reaches the level is expressed by the following equation (1).

[0030]

V2 = A · Vce (1−exp (t / τ)) (1) By transforming equation (1), equation (2) is obtained.

[0031]

T = −τ · ln (1−V2 / (A · Vce)) (2) The breakdown time characteristic of the main transistor 3 has a characteristic like a curve a in FIG. Therefore, it can be simply approximated by a logarithmic function as shown in Expression (2). Therefore, by substituting the two points on the destruction time curve a into the equation (2) and calculating the respective constants τ, V2, and A in the equation (2), the overcurrent shown in FIG. Voltage dividing resistors 10 and 11 in the protection circuit,
The reference value of each constant of the resistor 17 and the capacitor 18 can be determined. An example in which the above constants τ, V2 and A are set to predetermined values so that the protection time curve is inside the curve a indicating the boundary value at which the main transistor 3 is destroyed, that is, passes through the non-destructive region, It is curve c of FIG.

As is apparent from the above, according to the configuration of the overcurrent protection circuit of the present embodiment, the collector and the
The time until the main transistor is turned off (protection time) is made variable according to the breakdown strength of the main transistor that changes based on the emitter-to-emitter voltage Vce. Therefore, the main transistor can be driven in a region where the breakdown strength is maximized.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an overcurrent protection circuit of an inverter control device according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an operation of each unit of the overcurrent protection circuit of FIG. 1;

FIG. 3 is a diagram illustrating a breakdown time characteristic of a main transistor and a protection time characteristic of an overcurrent protection circuit in the inverter control device.

FIG. 4 is a diagram showing a configuration of an overcurrent protection circuit of a conventional inverter control device.

FIG. 5 is a diagram showing a breakdown time characteristic of a main transistor and a protection time characteristic of an overcurrent protection circuit in a conventional inverter control device.

[Explanation of symbols]

1 switching control signal circuit, 2, 17, 20, 2
1,2 resistor, 3 main transistor, 4 control transistor, 10,11 voltage dividing resistor, 12 operational amplifier, 1
3 Current detection resistor, 5, 16 diode, 14 first
Comparator, 15 control power supply, 18 capacitor, 19 second comparator.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-6-351296 (JP, A) JP-A-58-224577 (JP, A) JP-A-4-1388073 (JP, A) JP-A-5-224 219759 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H02M 7/48

Claims (1)

(57) [Claims] 1. An overcurrent protection circuit of an inverter control device for preventing a switch element, which receives a minute drive signal to a control electrode and allows a large current to flow, to prevent an overcurrent from flowing and being destroyed, the current flowing through the switch element. Current detecting means for detecting a voltage between the terminals of the switch element; a first comparing means for comparing a current detection signal from the current detecting means with a first reference voltage; As a result of the comparison by the first comparing means, when the current detection signal from the current detecting means becomes equal to or higher than the first reference voltage,
A predetermined time constant is determined by the detection voltage from the voltage detection means.
A time-constant circuit for charging internally, a second comparing means for comparing a charging voltage of the time-constant circuit with a second reference voltage, and a result of the comparison by the second comparing means, which is supplied from the time-constant circuit. A control element for turning off the switch element when a signal becomes equal to or higher than the second reference voltage.