JPS5831426A - Power supply unit - Google Patents

Abstract

PURPOSE:To protect an output transistor (TR) by supplying an output from a rectifying and smoothing circuit consisting of a rectifying diode and a smoothing capacitor to a load circuit through a fuse and connecting a resistor to the fuse in parallel. CONSTITUTION:When a load circuit 14 is in normal operating status, current flowing into the secondary side of a conveter transformer 4 is rectified by a diode 6 and supplied to the load circuit 14 through a fuse 8. When the load circuit 14 makes a short circuit, the fuse 8 is burnt out. At that time current flows into the load circuit 14 through the diode 6 and a resistor 16 without opening the secondary side of the converter transformer 4.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a switching regulator type power supply device, which protects an output transistor that performs a switching operation from a large current when a short circuit occurs in a load circuit, and protects the secondary side of a converter transformer. The purpose is to make it possible to use a /J\ size capacitor with withstand voltage as a smoothing capacitor inserted in the.

In this type, the output transistor is turned on and off by the output of an oscillator, and the pulse width of the on-period of the base voltage of the output transistor is controlled according to the output voltage, so that a constant DC output voltage is output to the load. In a power supply device, for example, when a circuit on the load side becomes completely or partially shodded, the load becomes lighter, and as a result, a large current flows through the output transistor, destroying the output transistor. Assuming such a case, a heater is inserted into the secondary side of the converter transformer, and if the load circuit is completely or partially shunted, the large current flowing to the secondary side will melt the heater and destroy the switching output transistor. It is more protected. However, in this case, the secondary side becomes open due to the fuse blowing out, and the output voltage on the secondary side increases rapidly. A smoothing capacitor is inserted on the secondary side to take out the DC output voltage, and as the secondary side voltage increases, the withstand voltage of the smoothing capacitor becomes a problem. There is no problem if a smoothing capacitor with a high withstand voltage is used, but using a capacitor with a withstand voltage higher than the rising voltage is not a good idea because of the large increase in cost and shape.

FIG. 1 shows the conventional circuit described above. When converting the pulse voltage obtained by the blocking oscillation circuit into DC and outputting it, the detection control circuit works to ensure that the output voltage remains constant despite fluctuations in the AC input voltage. This is a circuit that controls the pulse width. 1 in the diagram
is an AC power input terminal, 2 is a full-wave rectifier circuit, 3 is a smoothing capacitor, 4 is a converter transformer, 5 is an output transistor that constitutes a blocking oscillation circuit together with the primary winding, resistor, and capacitor of this converter transformer 4, 6
and 7 are rectifier diodes and smoothing capacitors for rectifying and smoothing the output voltage from the secondary winding of the converter transformer 4, and the rectified output voltage is applied to the load circuit via the fuse 8. 9 is a detection winding, which detects fluctuations in the output voltage, performs negative and negative rectification with diodes 1 and d, and then compares the rectified output with the Zener voltage of Zener diode 12 connected to the emitter of transistor 11. to change the voltage between the base and emitter of the transistor 110. 13Fi output transistor 5 base
A transistor inserted in parallel between the emitters divides the current (it + Ib) flowing into the base of the output transistor 50 according to the collector voltage of the transistor 11,
The pulse width of the on-period of the base voltage of the output transistor 5 is controlled, thereby keeping the DC voltage applied to the load circuit 14 constant. In addition, 15 is a trigger terminal, to which the horizontal output signal (15,75KHz) of the television receiver is applied.
The output transistor 5 performs an oscillation operation in synchronization with this horizontal output signal.

Here, the heater 8 is an element for protecting the output transistor, and when the load circuit 14 is overloaded, the heater 8 is blown out by the large current flowing through the diode 6 on the secondary side, and the output transistor is protected by a large current. More protective. However, in this case, as mentioned above, Heath 8
The withstand voltage of the smoothing capacitor 7 after the supply and disconnection of the smoothing capacitor 7 becomes a problem.

The present invention eliminates the above-mentioned conventional drawbacks.

An example of this will be explained below with reference to FIG. In the diagram, elements having the same functions as those in FIG. 1 are given the same numbers.

The difference between FIG. 2 and FIG. 1 is that a resistor 16 is inserted in parallel to both ends of a fuse 8 inserted into the secondary side of the converter transformer 4.

In the above configuration, when the load circuit 14 is in a normal operating state, the resistor 16 connected in parallel to the fuse 8 is short-circuited by the fuse 8, and therefore in this case, as in FIG. The main current flowing is rectified by the diode 6 and sent to the adding circuit 14 via the heater 8.
If a short-circuit accident occurs in the load circuit 14 and the current supplied to the load circuit 14 increases, the current flowing through the diode 6 will also increase, exceeding the rating of the heater 8 and causing the heater 8 to melt. do. In this case, since the resistor 16 is inserted between both ends of the heater 8, the current flowing to the load circuit 14 will flow through the diode 6 and the resistor 16 without the secondary side being open. That is, the resistor 16 is inserted in series in the poor load circuit 14, so that the output voltage on the secondary side does not rise suddenly, and therefore the voltage across the smoothing capacitor 7, F! The capacitor 7 can be kept at a low value, and a capacitor 7 with a low withstand voltage can be used.

As is clear from the above embodiments, according to the present invention, there is an output transistor that performs a switching operation with a large current when a load circuit is short-circuited by simply adding one resistor.

In addition, it is possible to use a smoothing capacitor with a low withstand voltage as the smoothing capacitor on the secondary side of the converter transformer, which is advantageous in terms of cost and shape, and its practical effects are extremely large.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a conventional power supply device, and FIG. 2 is a circuit diagram of a power supply device according to an embodiment of the present invention. 4. Converter transformer 5.
, an output transistor that performs a switching operation, 6,
,,,,. Rectifier diode, 7, Smoothing capacitor, 8, Fuse, 9, Detection winding, 11.13 and transistor, 12,
, , , Zener diode, 14... Load circuit, 16... Resistor.

Claims (1)

[Claims] a switching transistor, a converter transformer that converts the switching output of this transistor and takes it out to a secondary output winding, a rectifying diode and a smoothing capacitor connected to the secondary output winding, and the secondary output. A circuit for detecting a voltage proportional to the output voltage appearing on the winding and comparing it with a reference voltage, and means for controlling the switching transistor using the output voltage of this circuit to stabilize the DC output voltage, A power supply device in which the output of a rectifying and smoothing circuit consisting of a diode and a smoothing capacitor is supplied to a load circuit via a fuse, and a resistor is inserted in parallel with this fuse.