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Instance production and design points of single-tube switching power supply

Posted by: Yoyokuo 2022-05-20 Comments Off on Instance production and design points of single-tube switching power supply

The single-tube switching power supply has been widely used in many small household appliances due to its simple components and low cost, such as LED lights, chargers, remote controls, doorbells, etc.

This article shares the key points of example production and design of single-tube switching power supply.

1. Brief description of single-tube switching power supply

The single-tube switching power supply has been widely used in many small household appliances due to its simple components and low cost, such as LED lights, chargers, remote controls, doorbells, etc.

The following is the circuit of this single-tube switching power supply.

Instance production and design points of single-tube switching power supply

Figure 1. Single-tube switching power supply circuit diagram

2. Component composition and circuit principle of single-tube switching power supply

1. Main components

a. Switch tube.

Instance production and design points of single-tube switching power supply

Figure 2 MJE13005

The switch tube can use bipolar transistors and field effect transistors. This circuit uses bipolar transistors, model MJE13005. This type of triode is a high-back pressure high-speed switching triode, which is mainly used for high-power energy-saving lamps, switching power supplies, Power converters, etc., which have the advantages of high voltage resistance, fast switching speed, and wide safe working area. Its main parameters are as follows:

Collector-base reverse withstand voltage up to 700V

Collector-transmitter maximum reverse withstand voltage reaches 400V

The maximum allowable current of the collector reaches 5A

The maximum power dissipation of the collector reaches 75W

MJE series ranges from 13001 to 13009. In actual products, models with appropriate parameters can be selected according to actual requirements.

b. Switching power transformer.

The role of the switching power supply transformer is to provide energy for the load circuit, while providing isolation for the front and rear circuits. There are EE and EC types of switching power supply transformers, and this circuit adopts EE type.

The switching power supply transformer has a primary winding and a secondary winding. The primary winding of the transformer in this circuit includes an energy storage winding and a positive feedback winding, and the turns ratio is 210:9; the secondary winding is center-tapped, and the turns ratio is 13:13. The ratio can be adjusted according to specific needs.

2. Circuit principle

a. Rectifier filter circuit. The 200V AC power is rectified and filtered by the rectifier bridge B1 (can be composed of 1N4007 x 4) and the capacitor C1 to output 300V DC voltage and enter the single-tube self-excited oscillation circuit.

Instance production and design points of single-tube switching power supply

Figure 3 Rectifier filter circuit

b. Switch self-oscillating circuit. One way of 300V DC voltage is added to the collector of the switch tube through the primary winding of the transformer T1, and the other way is to provide the base voltage to the switch tube through R4. Since the voltage induced by the positive feedback winding is added to the base of the switch tube through R5 and C3, the switch tube quickly enters a saturated conduction state. Since the switch tube is turned on, the positive feedback stops, the switch tube enters the amplifying state, and the collector current decreases, so that a reverse current is generated on the positive feedback winding and added to the base of the switch tube, so that the switch tube enters the cut-off state. Once the switch tube is turned off, the primary winding has no current, and the positive feedback winding has no voltage. At this time, the switch tube enters the initial state again, that is, the power supply provides it with voltage, thereby entering the conduction state again. The cycle goes on and on.

Instance production and design points of single-tube switching power supply

Figure 4 Self-excited oscillation circuit

c. Protection circuit. The protection circuit consists of C3, R5 and D1. The main function is to slow down the voltage change rate on the primary winding, so that the switch tube works in a safe area. When the switch tube is turned from saturation to cut-off, the voltage on the primary winding becomes reverse, at this time D1 is turned on, and the capacitor C5 is charged, so that the collector voltage of the switch tube rises slowly to achieve the purpose of protecting the switch tube. When the switch tube is turned on again, the charge on C5 is released through R5, thereby ensuring that the protection circuit is effective again and again.

Instance production and design points of single-tube switching power supply

Figure 5 Protection circuit

d. Secondary rectifier output circuit. When the switch tube is turned on, the primary winding is positive and the lower is negative, and D3 in the secondary winding is turned on; when the switch tube is turned off, the polarity of the primary winding becomes upper negative and lower positive, and D2 in the secondary winding is turned on, thus completing the energy conversion.

Instance production and design points of single-tube switching power supply

Figure 6 Secondary rectifier output circuit

e. Voltage regulator circuit. In this circuit, D6 is a voltage regulator tube, which provides a stable voltage for the load. If higher voltage regulation performance is required, a three-terminal voltage regulator can be used instead.

3. Summary

The single-tube switching power supply circuit is simple, easy to manufacture and maintain, but also has unstable performance and lack of safety design. In the next article, we will introduce the improvement method of the single-tube switching power supply, so that the single-tube switching power supply can work more reliably.

Below is the simulation result of the circuit of Figure 1.

Instance production and design points of single-tube switching power supply

Figure 7 Simulation diagram of single-tube switching power supply

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