JP3056188B1 - Power circuit overcurrent protection circuit - Google Patents

Abstract

To provide an overcurrent protection circuit that can mix an output that does not turn off a switch with an open contact on the input side of a power supply circuit and an output that turns off when an overcurrent occurs. SOLUTION: When an overcurrent is detected by an overcurrent detection element 6 of an output 1, a voltage drop detection circuit 10 detects that an output voltage VOUT1 has become equal to or lower than a predetermined level under the control of a PWM control circuit 4, The open-contact drive circuit 2 operates to drive the switch 1 with open contacts. On the other hand, when the overcurrent is detected by the current detection element 12 of the output 2, the operation of the open contact drive circuit 2 is invalidated by the output of the current-voltage conversion circuit 15, so that the switch 1 with the open contact does not operate.
Thus, an output that turns off the switch with an open contact on the input side during the overcurrent protection operation and an output that does not turn off can be mixed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent protection circuit for a multi-output power supply circuit for obtaining two or more power supply outputs from one transformer.

[0002]

2. Description of the Related Art An example of this type of conventional overcurrent protection circuit will be described with reference to FIGS. FIG. 8 is a block diagram of a power supply circuit to which a conventional overcurrent protection circuit is applied, and FIG. 9 is a circuit diagram showing details of the open contact drive circuit 2 and the voltage drop detection circuit 10 of FIG. 8 and 9, reference numeral 1 denotes a switch with an open contact (a circuit breaker or the like) that disconnects an input during an overcurrent operation. An open contact drive circuit 2 controls the switch 1 with open contacts. 3 is a switching transistor, and 4 is a PWM (Pulse Width Modulation) which controls on / off of the transistor 3.
It is a control circuit. Reference numeral 5 denotes a transformer composed of one primary winding 5a and two secondary windings 5b and 5c. Reference numeral 6 denotes a current detecting element for detecting a current flowing through the secondary winding 5b of the transformer 5.

[0003] Reference numeral 7 denotes a current-voltage conversion circuit for converting a detection current value of the current detection element 6 into a voltage, and an output thereof is supplied to a PWM control circuit 4 via a diode 11. Reference numeral 8 denotes a rectifying / smoothing circuit that rectifies and smoothes the output from the secondary winding 5b of the transformer 5, and 9 denotes a PW that outputs the output of the rectifying / smoothing circuit 8.
The feedback control circuit feeds back to the M control circuit 4. Reference numeral 10 denotes a voltage drop detection circuit for detecting a drop in the output voltage VOUT1 of the output 1, and its output is the open contact drive circuit 2
Supplied to The output voltage of the secondary winding 5c of the transformer 5 is supplied to the rectifying / smoothing circuit 13 via the current detecting element 12, where it is rectified and smoothed, stabilized in the stabilizing circuit 14, and output as the output voltage VOUT2. Is output. 9, 17, 25, 26, and 30 are resistors, 1
8 is a capacitor. Reference numeral 19 denotes a comparator for comparing the reference voltage V20 with the voltage of the non-inverting input terminal. 28 is a photocoupler and 29 is a Zener diode.

Next, the operation of the above-described circuit will be described. In FIG. 8, when the overcurrent protection circuit is not operating, the Zener diode 29 of the voltage drop detection circuit 10 is in the ON state because the output voltage VOUT1 of the output 1 is a constant level voltage. The phototransistor is on, and both ends of the capacitor 18 are short-circuited. Therefore, the non-inverting input terminal of the comparator 19 is at a low level, so that the output of the comparator 19 is at a low level, and the transistor 21 is in a cutoff state.

When the output current IOUT1 of the output 1 increases and the output voltage of the current-voltage conversion circuit 7 increases and exceeds the overcurrent protection operating point corresponding to the overcurrent detection voltage of the PWM control circuit 4, PWM The ON ratio of the transistor 3 is reduced by the control of the control circuit 4. As a result, the output voltage VOUT1
Begins to drop. Further, the output current IOUT1 increases, and accordingly, the output voltage VOUT1 decreases, and the voltage drop detection circuit 10
Is lower than the predetermined output voltage, the Zener diode 29 of the voltage drop detection circuit 10 is turned off, whereby the phototransistor of the photocoupler 28 is turned off. As a result, the voltage V18 at the non-inverting input terminal of the comparator 19 (FIG. 9) rises at a time constant determined by the resistor 17 and the capacitor 18, and when the voltage exceeds the reference voltage V20, the output of the comparator 19 becomes high level, 21 base is biased on. As a result, a current flows through the transistor 21, the switch 1 with the open contact is driven, and the open contact is cut off. As a result, both output 1 and output 2 are turned off.

Next, the output current IOUT2 of the output 2 increases,
When the current exceeds the overcurrent protection operation point corresponding to the overcurrent detection voltage of the PWM control circuit 4, the operation of the PWM control circuit 4 causes the output voltage VOUT2 to start decreasing. Further, the output voltage VOUT1 of the output 1 starts to decrease at the same time when the output voltage VOUT2 decreases. The output current IOUT2 further increases, and the PWM control circuit 4
When the output voltage VOUT1 drops and the detection voltage of the voltage drop detection circuit 10 falls below a predetermined output voltage, the Zener diode 29 of the voltage drop detection circuit 10 turns off, thereby turning off the phototransistor of the photocoupler 28. Turns off. As a result, the voltage at the non-inverting input terminal of the comparator 19 rises with a time constant determined by the resistor 17 and the capacitor 18, and when the voltage V18 exceeds the reference voltage V20, the output of the comparator 19 becomes high level, Bias the base on. Thereby, the transistor 2
1, a current flows, and the switch 1 with the open contact is driven.
The open contact is disconnected. Thereby, output 1 and output 2
Are both dismissed.

As described above, in the circuit shown in FIG.
Even when the output 2 does not want to turn off the switch 1 with the open contact on the input side even when the overcurrent occurs,
There is a problem that the switch 1 with the open contact is disconnected. That is, in the conventional overcurrent protection circuit, when an output that turns off the switch 1 during an overcurrent and an output that does not turn off are requested by the load side, 1
A power supply circuit that obtains two or more outputs from one transformer cannot meet the demand.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and has been made in consideration of the above problem. To provide an overcurrent protection circuit that can mix an output that does not turn off a switch with an open contact on the input side of a power supply circuit and an output that turns off a switch with an open contact on the input side during overcurrent protection operation. With the goal.

[0009]

In order to achieve the above object, an invention according to claim 1 is a primary circuit of a transformer.
And a switch for turning on / off the power supply and the primary side circuit periodically.
Switching means for turning on / off;
A first and a second output circuit provided on the next side;
First and second currents respectively provided in the second output circuit
Detecting means and an output of the first or second current detecting means
Is the first or second preset corresponding to each
Is greater than a predetermined value, the first or second current detection is performed.
When the switching means is turned on in accordance with the output of the output means
Control means for controlling the distance between the first output and the first output.
When the output voltage of the power circuit falls below a certain value,
Switch control means for turning off, and the second current detection means
When the output of the switch exceeds a certain value, the
The control element that turns off the switch is controlled not to operate
And a control circuit for performing the control .

[0010] The invention according to claim 2 is the invention according to claim 1.
The overcurrent protection circuit of the power supply circuit, wherein the control circuit
Compares the output of the second current detecting means with a constant voltage.
Driven by the output of the comparing means
Control for turning off the switch in the switch control means.
And a transistor for controlling the control element so as not to operate.
It is characterized by having .

[0011]

[0012]

FIG. 1 is a block diagram showing an embodiment of the present invention.
This will be described with reference to FIG. FIG. 1 is a block diagram of a power supply circuit to which the overcurrent protection circuit according to the embodiment is applied, and FIG. 2 is a circuit diagram showing details of the open contact drive circuit 2 and the voltage drop detection circuit 10 of FIG. 1 and 2, the same parts as those in FIGS. 8 and 9 are denoted by the same reference numerals, and the description thereof will be omitted. The circuit shown in FIGS. 1 and 2 is different from the circuits shown in FIGS. 8 and 9 in that the transistor 21 is cut off in the open contact drive circuit 2 shown in FIG. Is provided with a holding circuit 50 (FIG. 2).

The holding circuit 50 includes a comparator 23 having a reference voltage V24 and an output VC2 of the current-voltage conversion circuit 15 as inputs, and a transistor 22 having an output of the comparator 23 connected to the base of the comparator 23 via a resistor 27. And the collector and the emitter of the transistor 22 are connected to the base and the emitter of the transistor 21, respectively.

Next, the operation of the embodiment of the present invention will be described.
This will be described with reference to FIGS. FIG. 3 is a time chart showing the operation when the output 1 (VOUT1) shown in FIG. 1 is overcurrent. FIG. 4 is a time chart showing the operation when the output 2 shown in FIG. 1 is overcurrent. First, when the overcurrent protection circuit is not operating, the Zener diode 29 of the voltage drop detection circuit 10 is in an ON state, and both ends of the capacitor 18 (FIG. 2) are short-circuited by the phototransistor of the photocoupler 28. . Therefore, the output of the comparator 19 is at a low level.

Here, as the output current IOUT1 of the output 1 increases, the detection current of the current detection element 6 detecting the current increases, and the output voltage VC1 of the current-voltage conversion circuit 7 increases. (FIG. 3B). Further, the output current IOUT1
When the current exceeds the overcurrent protection operating point I1 corresponding to the overcurrent detection voltage VTH of the PWM control circuit 4, the ON ratio of the transistor 3 is reduced by the control of the PWM control circuit 4, and the output voltage VOUT1 starts to decrease. (FIG. 3 (a)). Also,
The other output voltage VOUT2 starts to decrease from the time when the control voltage of the stabilizing circuit 14 starts to be insufficient after the output voltage VOUT1 starts to decrease. This means that the output voltage VOUT2 is
This is because they are obtained from the same transformer.

When the output current IOUT1 further increases and the output voltage VOUT1 drops under the control of the PWM control circuit 4 and the detection voltage of the voltage drop detection circuit 10 falls below the output voltage corresponding to I2, the voltage drop detection circuit 10 Is turned off. Thus, when the phototransistor of the photocoupler 28 is turned off, the output current IOUT1 becomes IOUT1.
After time t0 when the voltage becomes 2, the voltage at the non-inverting input terminal of the comparator 19 rises with a time constant determined by the resistor 17 and the capacitor 18, and when the voltage V18 exceeds the reference voltage V20 at time t1 (see FIG. )), The output of the comparator 19 becomes high level,
The base voltage V25 of the transistor 21 is biased to a high level (FIG. 3D), and the transistor 21 is turned on. As a result, the switch 1 with the open contact is driven (FIG. 3).
(e)), the open contact of the switch 1 with the open contact is disconnected.

Next, a case where the output 2 is in an overcurrent state will be described. As the output current IOUT2 of the output 2 increases, the detection current of the current detection element 12 increases accordingly.
The output voltage VC2 of the current-voltage detection circuit 15 increases (FIG. 4 (f)). When the output current IOUT2 further increases and exceeds the overcurrent protection operation point I3 of the output 2 corresponding to the overcurrent detection voltage VTH of the PWM control circuit 4, the ON ratio of the transistor 3 is reduced by the control of the PWM control circuit 4, Output voltage V
OUT2 starts to decrease (FIG. 4 (a)). Further, the output voltage VOUT1 of the output 1 starts to decrease at the same time when the output voltage VOUT2 decreases. This is because the output voltage VOUT1 is obtained from the same transformer as the output voltage VOUT2.

When the output current IOUT2 becomes I3, the current-
If the reference voltage V24 is set in advance so that the output voltage VC2 of the voltage conversion circuit 15 exceeds the reference voltage V24 of the comparator 23 in FIG.
Rises to the high level (FIG. 4 (g)), and drives the transistor 22 to the ON state to fix the base of the transistor 21 to the low level (FIG. 4 (d)). Further, the output current IOUT2 increases, the ON ratio of the transistor 3 decreases under the control of the PWM control circuit 4, the output voltages VOUT2 and VOUT1 decrease, and the detection voltage of the output 1 voltage drop detection circuit 10 becomes the output current I4. When the output voltage falls below the corresponding output voltage, the Zener diode 29 of the voltage detection circuit 10 is turned off, whereby the phototransistor of the photocoupler 28 is turned off.

As a result, after time t0 when the output current IOUT2 becomes I4, the voltage V18 at the non-inverting input terminal of the comparator 19 is obtained.
Rises at a time constant determined by the resistor 17 and the capacitor 18, and when the voltage V18 exceeds the reference voltage V20 at time t1 (FIG. 4C), the output of the comparator 19 becomes high level, and the base of the transistor 21 Try to bias on. However, since the base of the transistor 21 is fixed to the low level by the operation of the transistor 22 (FIG. 4D), the transistor 21 is not turned on (FIG. 4E).
The switch 1 with the open contact is not driven, and the open contact of the switch 1 with the open contact is not disconnected.

As described above, in a power supply circuit that obtains two or more power supply outputs from one transformer, an output that does not disconnect a switch with an open contact on the input side of the power supply circuit only during overcurrent protection operation during overcurrent protection operation. In addition, the output that turns off the switch with the open contact on the input side during the overcurrent protection operation can be mixed.

Next, another embodiment of the present invention will be described. FIG. 5 shows the present invention applied to a three-output power supply circuit. As shown in this figure, the outputs of the current-voltage conversion circuits 7, 15, 37 corresponding to the three outputs are respectively connected to the open-contact drive circuit 2 and the PWM control circuit via diodes 11, 16, 31, 32, 33. 4, the number of outputs can be increased to three or more.

FIG. 6 is a circuit diagram showing another example of the configuration of the holding circuit 50 shown in FIG. 2. The holding circuit 51 shown in FIG. 6 replaces the comparator 23, the resistor 27, and the transistor 22 shown in FIG. A comparator 24 having an open collector type output is used. FIG. 7 is a circuit diagram showing still another configuration example of the holding circuit 50 of FIG. 2. The circuit shown in FIG.
The output VC2 of the current-voltage conversion circuit 15 is applied to the base of the transistor 40 via the zener diode 39, and the collector of the transistor 40 holds the base potential of the transistor 21 at a low level.
In this circuit, when the output VC2 of the current-voltage conversion circuit 15 exceeds the Zener voltage of the Zener diode 39, the transistor 40 is turned on.
Is held at a low level.

[0023]

As described above, according to the present invention, in a power supply circuit that obtains two or more power supply outputs from one transformer, the switch with an open contact on the input side of the power supply circuit is turned off during overcurrent protection operation. The effect of being able to mix the output that does not fail and the output that does not matter is obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a power supply circuit to which an overcurrent protection circuit according to an embodiment of the present invention is applied.

FIG. 2 is a circuit diagram showing details of an open contact drive circuit 2 and a voltage drop detection circuit 10 of the overcurrent protection circuit in the same embodiment.

FIG. 3 is a time chart showing the operation of the overcurrent protection circuit when the output 1 shown in FIG. 1 is overcurrent.

4 is a time chart showing the operation of the overcurrent protection circuit when the output 2 shown in FIG. 1 is overcurrent.

FIG. 5 is a block diagram showing a configuration of another embodiment of the present invention.

FIG. 6 is a circuit diagram showing another configuration example of the holding circuit 50 in FIG. 2;

FIG. 7 is a circuit diagram showing still another configuration example of the holding circuit 50 in FIG. 2;

FIG. 8 is a block diagram showing a configuration of a power supply circuit using a conventional overcurrent protection circuit.

9 is a circuit diagram showing details of an open contact drive circuit 2 and a voltage drop detection circuit 10 of the overcurrent protection circuit shown in FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Switch with open contact 2 ... Open contact drive circuit 3, 21, 22, 40 ... Transistor 4: PWM
Control circuit 5 Transformer 6, 12, 34 Current detecting element 7, 15, 37 Current-voltage conversion circuit 8, 13,
35 rectifier / smoothing circuit 9 feedback control circuit 10 voltage drop detection circuit 11, 16, 31, 32, 33 diode 1
4, 36: stabilizing circuit 17, 25, 26, 27, 30, 38: resistor 1
8. Capacitors 19, 23, 24 Comparators V20, V24 Reference voltage 28 Photocoupler 29, 39 Zener diodes 50, 51, 52 Holding circuit

Claims (2)

(57) [Claims] 1. A primary circuit of a transformer is turned on / off.
A switch and switching for periodically turning on / off the primary side circuit
Means, and first and second output circuits provided on the secondary side of the transformer.
And first and second circuits respectively provided in the first and second output circuits.
The output of the second current detecting means and the output of the first or second current detecting means is
More than the first or second fixed value set corresponding to each
The output of the first or second current detecting means
The on-time of the switching means is shortened corresponding to
Control means for controlling the output voltage of the first output circuit when the output voltage falls below a certain value.
When the output of the switch control means for turning off the switch and the output of the second current detecting means exceeds a certain value,
Control element for turning off the switch in the switch control means
Overcurrent protection circuit for a power supply circuit, comprising: a control circuit that controls so as not to operate . 2. The control circuit according to claim 2, wherein the control circuit detects the second current.
Comparing means for comparing the output of the means with a constant voltage;
Driven by the output of the comparing means,
Control element to turn off the switch
A control transistor.
An overcurrent protection circuit for a power supply circuit according to claim 1 .