JPH03256558A - Switching power supply - Google Patents

Abstract

PURPOSE:To block dummy current for preventing interruption of oscillation, to lower the frequency and to reduce the size of a transformer by providing a switching pulse detecting circuit and a circuit for opening/closing an RCC system switching power supply control circuit according to the detected state. CONSTITUTION:A current limiting resistor R4 and a triac light emitting element(PT)1-1/2 are connected to the output of a diode bridge DB1. When an output potential exists on the secondary winding S1, primary PT1-2/2 is triggered to connect the output of a control circuit CT1 to the base of a switching transistor Q1. When the load current of a switching power supply decreases, switching frequency of an RCC system increases thus terminating self-oscillation. Consequently, a control circuit CKT1 is isolated and the switching transistor Q1 is turned ON by means of a starting resistor R1 thus turning the PT1 ON and sustaining the switching state.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a switching power supply.

[Conventional technology]

Conventional switching power supplies take measures to prevent oscillation interruption by running a dummy current to avoid light loads, and by setting the switching frequency low during heavy loads.

Next, a conventional example will be explained with reference to the drawings.

FIG. 3 is a circuit diagram showing a first conventional example.

The switching power supply shown in Fig. 3 has a pair of primary windings Pi
, P2 and a secondary winding Sl, a smoothing capacitor c2, and a rectifier diode D.
2, a starting resistor R1, a switching transistor Q1, and a control circuit CKT1 for controlling switching.

The output of the control circuit CKTI is the switching transistor Q
Bidirectionally connected to the base of 1.

When the load current of the switching power supply decreases, in the case of the PCC system, the switching frequency gradually increases, and due to the delay time inside the control circuit CKT1, self-oscillation cannot be maintained and oscillation stops repeatedly.

Further, in conventional switching power supplies, a back electromotive force absorption circuit is connected to both ends of the winding of a switching transformer that generates a back electromotive force.

FIG. 4 is a circuit diagram showing a second conventional example.

The back electromotive force absorption circuit in the switching power supply shown in FIG. 4 is composed of a discharge resistor R2, a back electromotive force absorption capacitor C3, and a back electromotive force blocking diode D3.

Therefore, the rated voltage of these components becomes large.

[Problem to be solved by the invention]

However, the above-mentioned conventional switching power supply has disadvantages in that it generates a large amount of heat because a dummy current is passed in order to prevent interruption of oscillation, and that the transformer becomes large in order to lower the minimum frequency.

Further, the above-mentioned conventional switching power supply has a disadvantage in that the back electromotive force absorption circuit is connected to both ends of the winding, so each component of the circuit needs to have a high voltage rating.

[Means to solve the problem]

In the switching power supply of the present invention, the oscillation interruption prevention circuit uses the switching pulse detection circuit and its detection state to perform RC
It is configured with a circuit that opens/closes the control circuit of the C-type switching power supply.

Further, in the switching power supply of the present invention, the back electromotive force absorption circuit is configured such that a center tap is provided in the winding that generates the back electromotive force of the switching transformer, and the back electromotive force absorption circuit is also connected to the center tap.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

The diode bridge DBI is a diode bridge that performs full-wave rectification of the output pulse of the secondary winding Sl.

Since the output of this diode bridge DBI is connected to the phototriac PTI light emitting element PT1-172 by the current limiting resistor R2, it is always connected to the phototriac PTI's light emitting element PT1-172 while the output potential of the secondary winding S1 is present. The light emitting diode of the phototriac PTI emits light, and the phototriac light receiving element PTI- which is the light receiving element
2/2 continues to be triggered.

Therefore, the output of the control circuit CKT1 is bidirectionally connected to the base of the switching transistor Q1.

When the load current of the switching power supply decreases, in the case of the RCC system, the switching frequency gradually increases, and due to the delay time inside the control circuit CKT1, self-oscillation cannot be maintained and oscillation stops repeatedly.

When the oscillation stops, the phototriac PTI is turned off because the light emission condition is not met. When the phototriac PTI is turned off, the control circuit CKT1 is disconnected, the switching transistor Q1 is turned on by the starting resistor R1, and this R
The initial state of the CC switching power supply is realized. The initial pulse will turn on the phototry book PTI again and maintain the switching state.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention.

Primary windings P11 and PI3 of switching transformer T1
is precisely divided into three equal parts by the center tap.

Therefore, the voltage of the back electromotive force appearing at this center tap is accurately divided into equal parts.

Therefore, the back electromotive force absorption circuit shown as an example is composed of the discharge resistor R2, the back electromotive force absorption capacitor C3, and the back electromotive force blocking diode D3, and the discharge resistor R3, the back electromotive force absorption capacitor C4, and the back electromotive force blocking diode. Accurately divided back electromotive force voltage is applied to another back electromotive force absorption circuit constituted by D4.

Therefore, the discharge resistors R2 and R3, the back electromotive force absorption capacitors C3 and C4, and the reverse voltage blocking diodes D3 and D4 can have equal rated voltages.

〔Effect of the invention〕

By preventing interruption of oscillation, the switching power supply of the present invention has the effect of eliminating heat generation due to dummy current and increase in size and cost due to dummy resistors, and making it possible to downsize the switching transformer.

Furthermore, in the switching power supply of the present invention, by providing a center tap in the winding that generates the back electromotive force of the switching transformer and configuring the back electromotive force absorption circuit, each component of the back electromotive force absorption circuit has a low rated voltage. It has the effect of being able to use things.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention, FIG. 2 is a circuit diagram showing a second embodiment of the present invention, and FIG. 3 is a circuit diagram showing a conventional first example. FIG. 4 is a circuit diagram showing a second conventional example. TI, T2. T3...Switching transformer,
Capacitor, Ql, Ql 1... Switching transistor, C3, C4... Back electromotive force absorption capacitor, R1... Starting resistor, R2, R3...
...Discharge resistance, D3, D4... Reverse voltage blocking diode, D2... Rectifier diode, CKTl.
...Control circuit, PTI...Phototriac, PTI, 1/2...Phototriac light emitting element, PT1*2/2...Phototriac light receiving element, DBI...Diode bridge, R4...Current limiting resistor.

Claims (2)

[Claims] (1) A switching transformer having a primary winding and a secondary winding, a phototriac light emitting element that detects a switching pulse generated in the secondary wire and emits light, and a phototriac light receiving element that receives the emitted light. A switching power supply comprising: a phototriac, and a control circuit that enters an initial state when the phototriac stops emitting and receiving light in response to the stoppage of the switching pulse. (2) Primary winding divided into equal parts by center tap and 2
A switching power supply comprising: a switching transformer having a secondary winding; and a pair of reverse starting power absorption circuits having one end connected to the center tap and the other end connected to each primary winding.