JPS599464Y2 - Protection circuit in switching circuit - Google Patents

Description

[Detailed Description of the Invention] The present invention provides a switching circuit equipped with a switching transistor connected in series to a load and having a control signal applied to its control electrode. , when the switching saturation voltage of the switching transistor becomes abnormal, such as when a series short occurs when the power is turned on (i.e., when both upper and lower transistors turn on in a SEPP circuit, etc.), the switching transistor This invention relates to a protection circuit that switches the switching transistor to an off state, thereby protecting the switching transistor from excessive current and excessive voltage.

This type of switching circuit, especially a power switching circuit, can handle large amounts of power with little loss, but if a conventional current-, voltage-, or power-operated type protection circuit is used,
The operating range during normal operation may be restricted, or
Protection may be insufficient during incomplete switching operations due to abnormalities in the preceding stage circuit.

The present invention was devised to correct the above-mentioned defects, and is a switching transistor connected in series to a load such as a motor and having a control signal supplied to its base.
, a control transistor whose collector is connected to the base of the switching transistor, and a capacitor whose one end is connected between the base and emitter of the control transistor, and a voltage between the collector and emitter of the switching transistor. The control transistor is configured to supply a divided voltage to the control transistor and its base, and when a control signal that turns off the switching transistor is applied, the base potential of the control transistor becomes higher than its collector potential. The divided voltage is selected so that the charge stored in the capacitor is discharged through the base and collector of the control transistor.

With this structure, it is possible to extremely effectively protect the load from abnormalities, prevent damage to switching transistors, etc. due to incomplete switching in the case of an internal abnormality in the switching circuit, and reduce power consumption in the event of an abnormality. This ensures that losses do not increase at all.

Next, an embodiment of the present invention will be described with reference to the drawings. In FIG. 1, 11 is a positive power supply terminal, 12 is an input terminal, 13 is a ground terminal, and 22 is an output terminal.

A square wave input signal supplied from the input terminal 12 is connected to the circuit 24.
is applied to the base of the switching transistor 1.

The output of the circuit 24 is connected to the circuit 2 from a junction 41 between the collector of the switching transistor 1 and the resistor 15 and a junction 42 between the emitter of the switching transistor 1 and the resistor 9.
3a and the input side of circuit 23b, respectively.

The two outputs of the circuit 24 are the circuits 23 a and 23
The voltage is applied to the bases of switching transistors 3a and 3b through electrolytic capacitors 17a and 17b and resistors 18a and 18b connected in series thereto, respectively.

The emitters of the transistors 3a and 3b and the resistor 20
The outputs taken out from the junctions 43 a and 43 b with the switching transistors 43 a and 20 b, respectively, are connected to the switching transistors 4 a and 20 b.
and 4b, respectively.

Transistors 4a and 4b are SEPP (Singl
The output is sent to the output terminal 22 via a junction 44 between the emitter of the transistor 4a and the collector of the transistor 4b.
taken from.

The circuit 10a and the circuit 10b are each a protection circuit,
The voltage between the collector and emitter of transistors 4a and 4b is divided by resistors 6a, 7a and 6b, 7b, respectively.

One of the divided voltages is applied to the base of the switching transistor 2a from the junction 45a between the resistors 6a and 7a, and the other is applied from the junction 45b between the resistors 6b and 7b.
to the base of switching transistor 2b.

Capacitors 8a and 8b are connected to transistors 2a and 2.
This makes it possible to turn transistors 4a and 4b from an off state to an on state, as will be described later.

The respective collectors of transistors 2a and 2b are connected to the bases of transistors 3a and 3b, respectively, and the emitters of transistors 2a and 2b are connected to the output terminal 22 and the ground terminal via diodes 5a and 5b to prevent reverse transistor operation. 13, respectively.

Diodes 21a and 2lb connected in parallel between the respective collectors and emitters of transistors 4a and 4b prevent back-up caused by some induction of a motor (not shown) connected as a load to output terminal 22. It's stopping the rush.

Further, diodes 19a and 19b connected in parallel between the respective collectors of transistors 2a and 2b and the respective cathodes of diodes 5a and 5b prevent backlash in the same manner as the diodes 21a and 2lb. ing.

A square wave input signal supplied from the input terminal 12 is connected to the circuit 24.
is applied to the base of transistor 1.

The circuit 24 has a phase separation function, and outputs an output in phase opposite to the input signal to the circuit 23a from the junction 41 between the collector of the transistor 1 and the resistor 15, and outputs an output having the opposite phase to the input signal from the junction 41 between the collector of the transistor 1 and the resistor 9. The circuits 23b and 23b respectively supply outputs that are in phase with the input signal.

These phase relationships are shown in FIGS. 2A to 2D, where FIG. 2A shows the input signal waveform and FIG. 2B shows the input signal waveform at the junction point 4.
1 to the circuit 23a, and FIG. 2C shows the signal waveform supplied from the junction 42 to the circuit 23b.

At time A, the transistor 1 enters the on state, and this state is maintained until time B in accordance with the input signal, at which time it is switched to the off state.

Transistor 1, which is turned off at time B, remains off until time C in response to the input signal.

As described above, the transistor 1 can repeatedly turn on and off depending on the input signal.

The DC components of the outputs shown in FIGS. 2B and 2C of the circuits 24, which are supplied to the circuits 23a and 23b and are 180 degrees out of phase with each other, are blocked by the electrolytic capacitors 17a and 17b, respectively, and only these AC components are passed through the resistor 18. a and 1
8b to the bases of transistors 3a and 3b, respectively.

Between times A and B, the transistor 3a to which the signal shown in FIG. 2B is applied to its base is in an OFF state,
Further, the transistor 3b to which the signal shown in FIG. 2C is applied to its base is turned on.

Further, between times B and C, transistor 3a is on and transistor 3b is off.

As mentioned above, the outputs of the transistors 3a and 3b, which repeat on and off operations in response to the signals shown in FIGS. 2B and 2C, are output from the junction points 43a and 43b between their respective emitters and the resistors 20a and 20b, respectively. It is taken out and applied to the bases of transistors 4 a and 4 b, respectively.

These outputs are taken out from the emitter sides of transistors 3a and 3b, so these transistors 3a,
This waveform is in phase with the waveform applied to the base of 3b.

The transistors 4 a and 4 b are driven by signals applied to their bases that are 180° out of phase with each other as follows.

That is, between times A and B, the transistor 4a
is turned off, and transistor 4b is turned on.

Further, between times B and C, transistor 4a is in an on state, and transistor 4b is in an off state.

The output of the SEPP circuit consisting of transistors 4a and 4b is the emitter of transistor 4a and transistor 4b.
Its phase relationship with respect to the input side signal shown in FIG. 2A is shown in FIG. 2D.

The output of the SEPP circuit is connected via a terminal 22 to a motor (not shown) as a load.

The circuit operation in the normal operating state has been described above, but it will be explained in detail below that the protection circuits 10a and 10b protect against abnormalities in the load and prevent destruction due to abnormalities inside the circuit.

Note that since the protection circuits 10a and 10b operate in substantially the same way, only the protection circuit 10a will be described in detail below.

The voltage between the collector and emitter of switching transistor 4a is divided by resistors 6a and 7a, and the divided voltage is applied to the base of transistor 2a from junction 45a between resistors 5a and 7a.

That is, the voltage between the collector and emitter of the transistor 4a is detected by the resistors 5a and 7a and applied to the base of the transistor 2a.

A capacitor 8a, one end of which is connected to a junction 45a between the resistors 5a, 7a and the base of the transistor 2a, is charged with a time constant determined by the capacitor 8a and the resistors 5a, 7a.

A voltage obtained by dividing the collector-emitter voltage of the transistor 4a is applied to the base of the transistor 2a by resistors 5a and 7a.

Then. When a voltage that is slightly higher or higher than the saturation voltage of the transistor 4a in the normal operating state is applied to the resistors 5a and 7a, the transistor 2a is turned on, and the saturation voltage of the transistor 4a in the normal operating state is applied to the resistor 5a. , 7a, the transistor 2a is set to be turned off.

That is, in a normal operating state, when transistor 4a is on, transistor 2a is turned off, and when transistor 4a is off, transistor 2a is turned on.

In normal operating conditions, between times A and B in FIG. 2B, transistors 3a and 4a are in the off state, so the collector-emitter voltage of transistor 4a is greater than its saturation voltage, and therefore transistor 2a is on.

In this state, if there is no capacitor 8a, even if a signal that turns on transistors 3a and 4a is applied, since transistor 2a is on, this signal will be short-circuited through transistor 2a, and transistor 3a will turn on. a, 4a maintains the off state regardless of the signal.

However, by connecting the capacitor 8a to the base of the transistor 2a as shown in FIG. 1, it becomes possible to release the off state of the transistors 3a and 4a.

That is, the reason why transistors 3a and 4a are in the off state is that in a normal operating state, the bases of transistors 3a and 4a are driven to the negative side relative to their emitter potentials by a negative input signal.

Further, in this case, since the input signal is on the negative side, the collector voltage of transistor 2a becomes lower than its base voltage.

Therefore, the charge of the capacitor 8a can flow between the base and collector of the transistor 2a which is in the on state.

Therefore, the charge of the capacitor 8a is transferred between the base and collector of the transistor 2a, the resistor 18a, and the electrolytic capacitor 17.
a, the resistor 15, and the electrolytic capacitor 16, respectively, and are lost in a closed loop that returns to the capacitor 8a.

Therefore, the base voltage of transistor 2a drops and transistor 2a turns off.

Therefore, while the capacitor 8a is being recharged with a time constant determined by the resistors 5a, 7a and the capacitor 8a, a positive signal enters the base of the transistor 3a, turning on the transistors 3a, 4a.

The recharging time is set to be longer than the switching time of the transistor 4a and within a safe operation range (for example, about several tens of microseconds).

The diode 5a is for preventing the transistor 2a from operating as a reverse transistor when the capacitor 8a is discharged.

Note that when the transistor 4a is in an on state and the saturation voltage between its collector and emitter in a normal operating state is reached, the transistor 2a is in an off state.

In the present invention, a problem to be considered is when the transistor 4a or 4b is in the on state and the load is short-circuited, the drive pulse waveform is abnormal, or a series short-circuit occurs when the power is turned on.

In any of the above states, the saturation voltage of the transistor 4a increases abnormally, so that the capacitor 8a described above
Immediately after the recharging time has elapsed, the protection circuit 10a operates as described above.

That is, by detecting this abnormal or normal voltage with the resistors 5a and 7a and turning on the transistor 2a, the base voltage of the transistor 3a is forcibly pulled to the negative side with respect to its emitter voltage, and the transistors 3a, 4 A is pulled back from on to off.

Furthermore, since the transistors 4a and 2a constitute a bistable circuit, power destruction due to incomplete switching is prevented even in the event of any abnormality in the ambient conditions.

Furthermore, since this protective operation is reset every cycle, the normal state is automatically restored as soon as the abnormal state disappears.

The above is about the operation of the protection circuit 10a.

Further, the protection circuit 10b operates in exactly the same way as the protection circuit 10a.

As described above, the present invention can extremely effectively protect against load abnormalities, and the power loss does not increase at all in the event of an abnormality, so it is possible to apply a protection circuit that is most suitable for application to a power switching circuit.

Furthermore, since the switching transistor and the control transistor constitute a bistable circuit, it is possible to prevent power destruction due to incomplete switching in response to various abnormalities in the ambient conditions.

In addition, even in the event of an abnormality, the control transistor returns to its normal operating state once the capacitor discharge that starts after the switching operation of the control transistor is completed, so that it automatically returns to its normal operating state as soon as the abnormal state disappears. It has the advantage of recovering.

Furthermore, since it can basically consist of a switching transistor, a control transistor, a capacitor, and a voltage dividing means, its circuit structure is simple and its operation is reliable.

[Brief explanation of drawings]

The drawings show an example in which the present invention is applied to a power switching circuit for driving a motor with a rectangular wave. FIG. 1 is a wiring diagram, and FIG. 2 is a waveform diagram of each part. In addition, in the symbols used in the drawings, 1a, 1b
, 2a, 2b, 3a, and 3b are X switching transistors, 12 is an input terminal, and 22 is an output terminal.

Claims (1)

[Scope of utility model registration request] A switching transistor connected in series to a load and having its base supplied with a control signal, a control transistor whose collector is connected to the base of this switching transistor, and a capacitor connected between the base and emitter of this control transistor. are configured to supply a voltage obtained by dividing the voltage between the collector and emitter of the switching transistor to the base of the control transistor, and a control signal for turning off the switching transistor is applied. The divided voltage is selected such that the base potential of the control transistor is higher than the collector potential of the control transistor, so that the accumulated charge of the capacitor is discharged through the base-collector of the control transistor. Protection circuit in switching circuit.