JPS6216019A - Output short circuiting protection circuit for switching power source - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION FIELD OF INDUSTRIAL APPLICATION The present invention relates to a protection circuit for preventing destruction of a power supply device for driving electrical appliances such as television receivers due to a short circuit on the secondary side of the power supply device.

BACKGROUND ART In a typical switching power supply circuit, a resistor with a small resistance value is inserted into the emitter of a switching transistor, and a voltage drop across this resistor is detected to stop the oscillation of the drive section. Alternatively, an overcurrent protection circuit is provided on the primary side, so that the protection circuit is activated when the current on the primary side increases abnormally. Examples of these are shown in FIGS. 2a and 2b. In the figure, 1 is a drive unit including an oscillation circuit, Q5 is a switching transistor, and R17 is a resistor inserted into the emitter of the switching transistor Q5. When the secondary side current increases, the voltage across resistor R17 increases, so this voltage The protection circuit 2 is activated by detecting V. The overcurrent prevention circuit 3 is configured to detect an overcurrent of the primary side power supply current and activate the protection circuit 2.The problem to be solved by the invention, however, is that the second In the case of figure (a), a resistor must be inserted into the emitter, and if the secondary side load current is large, the voltage drop across this resistor will be large and the switching transistor Q5 will not operate properly.
If the switching current is large, it becomes inoperable. Although the case shown in FIG. 2(b) is reliable, it is expensive in terms of cost, and has the problem that it cannot be used in applications where cost is important.

In view of the above problems, it is an object of the present invention to provide an output short-circuit protection circuit for a switching power supply that can reliably protect against short-circuits on the secondary side, reduce losses, and perform normal switching operations at low cost. purpose.

Means for Solving the Problem In order to achieve this object, the switching power supply output short-circuit protection circuit of the present invention prevents each voltage on the secondary side from decreasing or OV, create the output voltage of the reference output terminal among each voltage, and add it to the base of the output short-circuit protection transistor via the Zener diode and current limiting resistor, as well as the collector of this protection transistor ( Under normal conditions, OV) is connected to the protection terminal of the switching oscillation circuit.

According to the present invention, the output voltage on the secondary side of the switching transformer is detected, and when the secondary side is short-circuited, the oscillation operation of the oscillation drive circuit is stopped via the protection transistor. Along with protection,
It has the advantage that it does not adversely affect the original operation of the switching power supply circuit, there is no power loss, and it can be constructed at low cost.

EXAMPLE An example of the present invention will be described below with reference to the drawings.

FIG. 1 shows a circuit block diagram of a switching power supply output short-circuit protection circuit according to an embodiment of the present invention. In Fig. 1, IC is an IC-based oscillation drive circuit 1Q that detects a short circuit or near-short circuit in the secondary DC output of the switching transformer T1, and prevents overcurrent from flowing when the secondary side is short-circuited. This is a detection unit that operates to stop the oscillation output of the oscillation drive circuit IC1 and stop the secondary side DC output. Transistors Q2Q3 are predrive circuits, C4 is a switching transistor, and D5
D6D7D8 are secondary side rectifier diodes provided via the switching transformer T1, so that current flows to the respective load circuits through smoothing capacitors c, C6, a, and C8. Q.

is a protection transistor, D is a Zener diode for output voltage detection, and R2 is a current limiting resistor. The moment when the charging current of C4 is output until transistor Q is turned on, transistor Q1 is turned off.
This is a resistor and a capacitor for N. R5 is a discharge resistor, and D2 is a diode for preventing reverse bias of the transistor Q1. R5 is a collector resistance of the transistor Q1, and C2 is a capacitor for a time constant of the collector potential of the transistor Q1. R15R14R15 is a reference voltage adjusting resistor and a volume. R16 and D9 are a reference voltage generating resistor and a diode for the oscillation drive circuit.

R12C9 is a time constant circuit that determines the oscillation frequency of the oscillation drive circuit IC4.

With the configuration shown in FIG. 1, the protection resistor R2 and the Zener diode D
1, a voltage of V□ shown in equation (1) is applied to the base of the transistor Q1.

VO(IXR2+Vo1)Thv+u--(1) In a normal state, a voltage that turns on the transistor Q1 is applied between the base and emitter.

Output voltage V that becomes the reference when any output on the secondary side is short-circuited or close to short-circuited. will be lower than Ov or the normal voltage. By doing so, Q
The voltage that turns on the base of , that is, the base-emitter voltage vBx, is expressed by the following equation (2).

Vo-(X x R2+va, ) < v811--
(According to Equation 21 (2), due to the voltage drop due to the Zener diode D and the voltage drop due to the resistor R2, the voltage applied to the transistor Q due to vo becomes lower than vBx of the transistor Q, and the transistor Q1 is turned off (
OFF), and the collector of transistor Q1 becomes high level (H level). This voltage is the oscillation drive circuit I
Turn on the W/Sat multi of C, and oscillation drive circuit IC.
, to stop the oscillation operation by changing the bias of the oscillation circuit inside.

Effects of the Invention As described above, according to the present invention, there is no need to include a resistor in the switching transistor, and there is no need for an expensive current limiting circuit, so that a stable switching power supply can be provided at low cost. Furthermore, the voltage at which the protection circuit operates can be freely set as shown in equation (1).

[Brief explanation of drawings]

FIG. 1 is a circuit block diagram of a switching power supply output short-circuit protection circuit according to an embodiment of the present invention, and FIG. 2 is a circuit block diagram of a conventional example. IC,...Oscillation drive circuit, Ql...
Protection transistor, D...Zener diode, 1o...detection section. Name of agent: Patent attorney Toshio Nakao and 1 other person 2nd
Figure (a-)

Claims (1)

[Claims] The secondary side DC output of the switching power supply circuit is equipped with a detection means for detecting a secondary side short circuit or a state where the secondary side is close to a short circuit, and by detecting a drop in the secondary side voltage,
A switching power supply output short-circuit protection circuit configured to stop oscillation of a switching power supply drive section and stop secondary-side DC output to prevent overcurrent from flowing due to a secondary-side short circuit, etc.