JP3542768B2 - Overvoltage protection circuit in regulated power supply - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an overvoltage protection circuit in a stabilized power supply.
[0002]
[Prior art]
FIG. 3 is a circuit diagram showing an example of an overvoltage protection circuit in a conventional stabilized power supply. The stabilized power supply includes a primary-side drive circuit including input terminals 1 and 2, a smoothing capacitor 3, a main switch MOSFET 4 and a primary winding of a transformer 5, a rectifier switch MOSFET 6, a freewheel switch MOSFET 7, a choke coil 8, an output capacitor. 9, a synchronous rectification type forward switching power supply comprising output terminals 10 and 11, voltage dividing resistors 12 and 13, and a secondary output circuit comprising a control circuit 14.
[0003]
The overvoltage protection circuit 15 includes a voltage dividing resistor 16, 17 connected to the output terminal 10, a comparator 18 to which a voltage divided by the voltage dividing resistors 15, 16 is input to one input terminal, and the other of the comparator 18. A reference voltage source 19 connected to the input terminal and having a first overvoltage detection reference voltage Vref1, a photocoupler 20 including a photodiode 20a and a phototransistor 20b connected to the output terminal of the comparator 18, and a photocoupler 20; The malfunction preventing capacitor 21 connected between the phototransistor 20b and the ground, the input terminal is connected to the connection point between the phototransistor 20b and the capacitor 21, and the output terminal is connected to the operation control port of the control circuit 14. And a latch circuit 22.
[0004]
In the above configuration, the main switch MOSFET 4 is PWM-controlled by the control circuit 14 so that the output voltage obtained from the output terminals 10 and 11 is stabilized.
[0005]
The protection operation of the overvoltage protection circuit 15 is performed as follows. First, when the output voltages at the output terminals 10 and 11 to which a load (not shown) is connected are within a normal range, the overvoltage protection circuit 15 does not operate. In this case, the output voltage of the latch circuit 22 connected to the operation control port of the control circuit 14, that is, the control voltage Vg is at a high level, and does not affect the PWM control operation of the control circuit 14.
[0006]
Next, when the output voltage obtained from the output terminals 10 and 11 becomes overvoltage for some reason, the divided voltage V0 by the voltage dividing resistors 16 and 17 becomes the overvoltage detection voltage of the reference voltage source 19 as shown in FIG. Exceeds Vref1. As a result, the output voltage Vc of the comparator 18 changes from the high level to the low level, a current flows from the + Vcc1 power supply to the photodiode 20a of the photocoupler 20, and the phototransistor 20b is turned on. When the phototransistor 20b is turned on, the input side of the latch circuit 22 changes from low level to high level.
[0007]
As a result, the output voltage of the latch circuit 22, that is, the control circuit stop signal Vg is latched from the low level to the high level after the delay time t, and stops the PWM control of the control circuit 14 so that the overvoltage at the output terminals 10 and 11 is reduced. Will be resolved. The delay time t is a delay caused by the photocoupler 20, the malfunction preventing capacitor 21, and the latch circuit 2.
[0008]
[Problems to be solved by the invention]
The protection operation of the voltage protection circuit 15 includes detecting an overvoltage in order to prevent an excessive voltage from being applied to loads (not shown) connected to the output terminals 10 and 11 when the output voltage becomes overvoltage. It is necessary to perform the protection operation as soon as possible after the stop. That is, the delay time t must be as small as possible.
[0009]
However, the quicker the stop operation, the higher the possibility of causing a malfunction due to not only true overvoltage but also other external factors (for example, noise, lightning surge, droop release, etc.). There is a problem that the above reliability is lowered.
[0010]
Therefore, an object of the present invention is to provide an overvoltage protection circuit in a highly reliable stabilized power supply that does not malfunction due to external factors.
[0011]
[Means for Solving the Problems]
In view of the above-mentioned object, an overvoltage protection circuit in a stabilized power supply according to the present invention includes a first protection unit that clamps an output voltage of the stabilized power supply to a predetermined voltage equal to or lower than an overvoltage set voltage when an overvoltage is detected, A second protection means for latching the operation of the stabilized power supply when the clamp operation by the protection means has continued for a predetermined time or more, wherein the first protection means determines that the output voltage has exceeded the overvoltage set voltage. A comparator for detecting, and a transistor which is turned on / off by an output of the comparator connected between the output terminals and clamps an output voltage, wherein the second protection means includes a comparator for the first protection means. based on the output value, the clamping operation by the first protection means detects that it has continued for a predetermined time or more, to latch stop the switching operation of the stabilized power supply It is way.
[0012]
Thus, highly reliable overvoltage protection that operates only at the time of a true overvoltage and does not operate under other external factors (for example, noise, lightning surge, droop release, etc.) is performed.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0014]
FIG. 1 is a circuit diagram showing an embodiment of an overvoltage protection circuit in a stabilized power supply according to the present invention. 1, the same components as those of the conventional circuit diagram of FIG. 3 will be described with the same reference numerals, but the stabilized power source is a synchronous rectification type forward switching power source having the same configuration as the conventional circuit diagram of FIG. .
[0015]
The overvoltage protection circuit 15 includes voltage dividing resistors 16 and 17 connected to the output terminal 10, a comparator 18 to which a divided voltage V0 by the voltage dividing resistors 15 and 16 is input to a positive input terminal, and a negative input terminal of the comparator 18. , A reference voltage source 19 having a first overvoltage detection reference voltage Vref1, a photodiode 20a of a photocoupler 20 connected between the output terminal and the output terminal 11 of the comparator 18, and a negative input terminal of the comparator 18. And a resistor 23 connected between the output terminal and the output terminal of the comparator 18, a drain connected to the output terminal 10, a source connected to the output terminal 11, and a first protection. It has means 15A.
[0016]
The first protection means 15A clamps the output voltage to a clamp level equal to or lower than the overvoltage set voltage when an overvoltage is detected, as described later.
[0017]
The overvoltage protection circuit 15 further includes a phototransistor 20b of the photocoupler 20 having one end connected to the + Vcc power supply, a series connection of a resistor 25 and a resistor 26 connected between the phototransistor 20b and the ground, a resistor 25 and a resistor. 26, a capacitor 27 connected between the series connection point and the ground, a plus input terminal connected to the series connection point of the resistors 25 and 26, and a comparator 28 connected between the minus input terminal of the comparator 28 and the ground. A reference voltage source 29 having a second overvoltage detection reference voltage Vref2, and a latch circuit 22 having an input terminal connected to the output terminal of the comparator 28 and an output terminal connected to the operation control port of the control circuit 14. It has the second protection means 15B. As will be described later, the second protection unit 15B stops the operation of the stabilized power supply when the clamp operation by the first protection unit 15A continues for a certain period of time or more.
[0018]
Hereinafter, the protection operation of the overvoltage protection circuit 15 having the above-described configuration will be described with reference to the signal timing chart of each unit shown in FIG. First, when the output voltage Vout at the output terminals 10 and 11 to which a load (not shown) is connected is within a certain range from the stable voltage Vcst by the PWM control of the control circuit 14, the overvoltage protection circuit 15 does not operate. In this case, the voltage divided by the voltage dividing resistors 16 and 17 does not exceed the first overvoltage detection reference voltage Vref1 of the reference voltage source 10, so that the output voltage Vc1 of the comparator 18 is low level, The photodiode 20a and the MOSFET 24 are both off.
[0019]
Further, since the phototransistor 20b is also turned off by turning off the photodiode 20a of the photocoupler 20, the voltage Vc2 at the series connection point of the resistors 25 and 26 is at a low level, and the second voltage of the reference voltage source 29 is Is lower than or equal to the overvoltage detection reference voltage Vref2. Therefore, the output of the comparator 28 is at a low level, and the output voltage of the latch circuit 22 is also at a low level. Therefore, the control circuit stop signal Vg applied to the operation control port of the control circuit 14 is at a low level and does not affect the PWM control operation.
[0020]
Next, when the output voltage Vout obtained from the output terminals 10 and 11 rises from the stable voltage Vcst for some reason and becomes an overvoltage state, the divided voltage Vo by the voltage dividing resistors 16 and 17 in the first protection means 15A is changed. , Exceeds the first overvoltage detection reference voltage Vref1 of the reference voltage source 19. As a result, the output voltage Vc1 of the comparator 18 changes from the low level to the high level, and the MOSFET 24 is turned on. The high-level output voltage Vc1 is fed back to the minus input terminal of the comparator 18.
[0021]
As a result, the output voltage Vout of the output terminals 10 and 11 is clamped at the clamp level Vm slightly higher than the stable voltage Vcst. This clamp level is a voltage level equal to or lower than an overvoltage setting voltage (that is, a voltage set to be an overvoltage for the load).
[0022]
The above-described clamping operation by the first protection unit 15A is continued as long as the overvoltage state at the output terminals 10 and 11 continues. However, when the output terminals 10 and 11 are no longer in an overvoltage state, the output of the comparator 18 returns to a low level, and the MOSFET 24 is turned off. Therefore, the stabilized power supply automatically recovers to the normal operation, and the output voltage Vout of the output terminals 10 and 11 returns to the stable voltage Vcst.
[0023]
On the other hand, during the clamping operation by the first protection means 15A, the output voltage Vc1 of the comparator 18 is at a high level, so that a current flows through the photodiode 20a of the photocoupler 20 to emit light, and the phototransistor 20b is turned on. It becomes. When the phototransistor 20b is turned on, a current flows from the + Vcc power supply to the resistors 25 and 26, and the voltage Vc2 at the series connection point of the resistors 25 and 26 increases according to the charging time constant of the resistor 25 and the capacitor 26.
[0024]
When the voltage Vc2 at the series connection point of the resistors 25 and 26 exceeds the second overvoltage detection reference voltage Vref2 of the reference voltage source 29, the output of the comparator 28 changes from low level to high level, 22 is latched.
[0025]
As a result, the output voltage of the latch circuit 22, that is, the control circuit stop signal Vg changes from the time t1 when the phototransistor 20b is turned on (ie, the time when the overvoltage is detected) to the time when the above-mentioned voltage Vc2 is detected by the second overvoltage detection. After a lapse of a predetermined time T until a time point t2 when the voltage exceeds the use reference voltage Vref2, the signal is latched from low level to high level. As a result, the PWM control of the control circuit 14 is latched off, the output voltages of the output terminals 10 and 11, which are clamped at the clamp level Vm, decrease from the time point t3, and the overvoltage state is eliminated.
[0026]
As described above, when an overvoltage is detected, the output voltage is clamped to the clamp level by the first protection unit 15A. Then, when the clamp operation by the first protection means 15A continues for a certain period of time T or more, the PWM control of the control circuit 14 is stopped by the second protection means 15B, and the overvoltage state is eliminated. When the overvoltage state is not detected within the above-mentioned fixed time (the above-mentioned time T), the stabilized power supply automatically returns to the normal operation.
[0027]
As a result, the latch can be stopped at the time of a true overvoltage, and does not operate at the time of an overvoltage detection malfunction due to other external factors (for example, noise, lightning surge, droop release, etc.), so that the reliability of the system is improved. In addition, the above-described protection operation is not affected by the control response of the conventional switching power supply, and an excessive voltage is not applied to the load.
[0028]
As described above, the embodiment of the present invention has been described. However, the present invention is not limited to this, and various modifications and applications are possible.
[0029]
For example, in the above-described embodiment, the latch circuit 28 outputs a high-level latch signal when a high level is applied to the input. However, depending on the logical relationship with other components, The output can be configured to be any combination of low level and high level.
[0030]
In the above-described embodiment, the control circuit 14 stops operating when a high-level control voltage Vg is applied to the operation control port. The operation may be stopped when a low-level control voltage Vg is applied to the operation control port.
[0031]
In the above-described embodiment, the overvoltage protection in the synchronous rectification type forward switching power supply has been described.
[0032]
【The invention's effect】
According to the present invention, highly reliable overvoltage protection that operates only when a true overvoltage is detected and does not operate under other external factors (for example, noise, lightning surge, drop-opening, and the like) is performed.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing an embodiment of a protection circuit in a stabilized power supply according to the present invention.
FIG. 2 is a timing chart of signals of respective units in the circuit diagram of FIG. 1;
FIG. 3 is a circuit diagram showing an embodiment of a protection circuit in a conventional stabilized power supply.
FIG. 4 is a timing chart of signals of respective units in the circuit diagram of FIG. 3;
[Explanation of symbols]
15 Overvoltage protection circuit 15A First protection means 15b Second protection means

Claims (1)

First protection means for clamping the output voltage of the stabilized power supply to a predetermined voltage equal to or lower than the overvoltage set voltage at the time of overvoltage detection, and when the clamp operation by the first protection means has continued for a predetermined time or more, the stabilized power supply And second protection means for stopping the operation of the latch.
The first protection means is turned on / off by a comparator for detecting that the output voltage has exceeded an overvoltage set voltage and an output of the comparator connected between both output terminals, and clamps the output voltage. And a transistor that performs
The second protection means detects, based on the output value of the comparator in the first protection means, that the clamp operation by the first protection means has continued for a predetermined time or more, and performs a switching operation of the stabilized power supply . possible to latch stop,
An overvoltage protection circuit in a stabilized power supply.

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