JPS62293819A - Switching power supply protection circuit - Google Patents

Abstract

PURPOSE:To obtain high immunity from external noise to the operation of a titled circuit by controlling the cut-off of a main switching transistor (TR) by a TR to eliminated the need for a latch circuit. CONSTITUTION:The latch circuit is eliminated to directly drive a TR 10 by an output of comparator 5, a power voltage Vcc is supplied to a power voltage drop detection circuit 70 in place of the output of a latch circuit driving a TR 17 to detect the drop of the power voltage Vcc and its detection output is used for the control. If the short-circuiting of a load takes place, the potential of a capacitor 41 rises and if it exceeds the reference voltage of a reference voltage source 22, the TR 10 is electrified to cut off the output of the comparator 2. Thus, even if the short-circuit state of the load is released, the output of the comparator 4 remains at a high level and the capacitor 41 keeps the state charged by a power voltage. Thus, the electrified state of the TR 10 is maintained and the output of the comparator 2 keeps the off-state.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a load short-circuit protection circuit for a switching TL source.

[Prior Art] Conventionally, as this type of protection circuit, there is a short circuit protection circuit shown within a dotted line frame in FIG. 2, for example.

In FIG. 2, l is an oscillator, 2 is a comparator, 3 is an error amplifier, 4 and 5 are comparators, and 6 is a latch circuit. 10 is a latch circuit 6 which outputs the output of comparator 2.
11 to 13 are switching power supply transistors, l=l to 17 are protection circuit transistors, and 18 is a switching power supply rectifier diode. 20 to 22 are reference voltage sources. 30 and 31 are switching power supply transistors 11 and 1
2, 32 and 33 are resistors forming a voltage divider that detects the switching power supply output, 34 is a protection circuit resistor, and the short circuit detection output obtained from the connection point of the voltage divider resistors 32 and 33 is an error amplifier. is supplied to one input terminal of 3. IIO is a smoothing capacitor, and 41 is a charging capacitor. 50 is a switching 'rvLF transformer, 6
0 is a switching power supply output terminal.

Now, when the output terminal 60 and the ground are short-circuited, the output voltage of the error amplifier 3 is significantly lower than that during normal operation, and becomes lower than the reference voltage of the reference voltage source 21. As a result, the output of comparator 4 becomes high level, and transistor 1
6 is made conductive, and a constant current determined by the resistor 34 flows as a collector current of the transistor 15 through a current mirror circuit constituted by transistors 15 and 15, and charging of the capacitor 41 is started with this constant current.

Therefore, if the short-circuited state of the load continues for a certain period of time, the potential of the capacitor 41 exceeds the reference voltage of the reference voltage source 22, the output of the comparator 5 is inverted to high level, and the output Q of the latch circuit 6 is set to high level. . As a result, transistor 10 becomes conductive and main switching transistor 1
3 and protect it from destruction.

Furthermore, at the same time, the capacitor 11 is discharged by the transistor 17, and the output of the comparator 5 becomes low level, but the output cutoff state is maintained by the latch circuit 6.

[Problems to be solved by the invention] In this conventional circuit, there is a phenomenon in which the output of the latch circuit 6 is set to a high level and the output is cut off due to external noise etc. even though there is no load short circuit. This is seen as a major problem in actual use.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a switching power supply protection circuit which has the same function as the conventional short circuit protection and which greatly reduces the possibility of malfunction due to external noise.

[Means for determining problem III] In order to achieve such an object, the present invention reduces the possibility of malfunction of the short circuit protection circuit by eliminating the need for a latch circuit used in conventional circuits. Significantly reduced.

That is, the present invention includes a first means for detecting a short circuit in a load as an abnormality in the output voltage of an error amplifier, a second means for determining whether the abnormal state continues for a certain period of time or more, and a second means for determining whether the abnormal state continues for a certain period of time or more. A switching power supply protection circuit that protects the main switching transistor from short-circuiting by including a third means for regularly interrupting the main switching transistor when the above continues, the second means stops charging by the detection output from the first means. The third means includes a capacitor for starting and a comparator for comparing the charging voltage of the capacitor with a reference voltage, and the third means directly controls the capacitor 11 by the output of the comparator.
The main switching transistor is characterized in that it has a transistor, and the transistor controls the blocking of the main switching transistor.

[Operations] According to the present invention, since a latch circuit is not required, malfunctions due to external noise are extremely difficult to occur. Therefore, in actual use, the switching power supply output will not be cut off accidentally and unexpectedly due to external noise, and the reliability of the power supply circuit will be improved.

[Examples] Examples of the present invention will be described below in detail and specifically based on the drawings.

An embodiment of the present invention is shown in FIG.

In this example, unlike the conventional example, the latch circuit 6 shown in FIG.
and instead of controlling the transistor 17 with the output of the latch circuit 6, the power supply voltage Vcc is supplied to the power supply voltage drop detection circuit 70 to detect a drop in the power supply voltage Vcc, and the transistor 17 is controlled by the detected output. do.

If a load short circuit occurs here, the potential of the capacitor 41 will rise
- When the reference voltage of the reference voltage source 22 is exceeded, the transistor 10 becomes conductive and cuts off the output of the comparator 2.

When the output of the comparator 2 is cut off, the output of the comparator 4 remains at a high level even if the short-circuited state of the load is released, and the capacitor 41 remains charged to the power supply voltage.

Therefore, the conduction state of the transistor IO is maintained,
The output of comparator 2 also remains cut off.

Since this circuit does not include a latch circuit, the possibility of malfunction due to noise is extremely low. The reason is,
This is because the external noise is pulse-like and has a narrow pulse width, so that it hardly raises the potential of the capacitor 41 to the reference voltage of the reference voltage source 22.

In addition, in order to restart quickly, capacitor II
It is necessary to discharge l, and for this purpose, in this example,
The power supply voltage drop detection circuit 70 detects the power supply voltage Vcc, and the transistor 17 is driven by the detection output.

That is, the circuit power supply is once turned off for restarting, but the moment the power supply drops to a certain level, the output of the power supply voltage drop detection circuit 70 becomes high level, immediately making the transistor 17 conductive, and thereby the capacitor 41 is discharged. After this, when you turn the power back on,
The circuit will start up again.

[Effects of the Invention] As is clear from the above, according to the present invention, since a latch circuit is not required, malfunctions are extremely difficult to occur due to external noise. Therefore, in actual use, the output of the switching power supply will not be cut off accidentally and unexpectedly due to external noise, and the reliability of the power supply circuit will be improved.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 'fJ2 is a circuit diagram showing an example of a conventional short circuit protection circuit. DESCRIPTION OF SYMBOLS 1... Oscillator, 2... Comparator, 3... Error amplifier, 4.5... Comparator, 6... Latch circuit, 10-17... Transistor, 18... Diode, 20... 22...Reference voltage source, 30-34...Resistor, 40, 41...Capacitor, 50...Transformer, 60...Switching voltage (・Original output terminal, 70...
・Power supply voltage drop detection circuit.

Claims (1)

[Scope of Claims] 1) first means for detecting a short circuit in the load as an abnormality in the output voltage of the error amplifier; second means for determining whether or not the abnormal state continues for a certain period of time; and the abnormal state. a switching power supply protection circuit that protects the main switching transistor from short-circuiting, the switching power supply protection circuit comprising: third means for regularly cutting off the main switching transistor when the main switching transistor continues for a certain period of time or more; The third means includes a capacitor that starts charging based on the detection output, and a comparator that compares the charging voltage of the capacitor with a reference voltage, and the third means includes a transistor that is directly controlled by the output of the comparator, and the transistor A switching power supply protection circuit characterized by controlling the blocking of a main switching transistor. 2) The switching power supply protection circuit according to claim 1, wherein the switching power supply protection circuit controls discharge of the capacitor in response to a decrease in the power supply voltage of the protection circuit.