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6 Simple Switching Power Supply Circuit

Author: Iggy Date: 14 Apr 2021  795

Have you ever wondered what is meant by "switch" in switching power supply? To be specific, what is " switching on" and " switching off"?

As the name implies, the switching power supply uses electronic switching devices (such as transistors, field effect transistors, thyristors, etc.) to make the electronic switching devices "on" and "off" continuously through the control circuit. What happened next? Then, let the electronic switching device pulse modulate the input voltage to realize DC/AC, DC/DC voltage conversion, as well as adjustable output voltage and automatic voltage stabilization.

After you have an understanding of the basic definition of switching power supply, you may be willing to further explore its application and deepen your understanding. Therefore, in this blog, we will introduce 6 simple switching power supply circuit design schematics to you.

Of course, if you have any questions about the circuit diagram, please leave your questions in the comment, and we will provide you with enthusiastic and professional answers.

A video introducing basics of switching mode power supplys

Catalog

I What is switch mode power supply

II 6 switching power supply circuit diagrams

2.1 A simple switching power supply circuit

2.2 24V switching power supply circuit

2.3 Single-ended forward switching power supply circuit

2.4 Push-pull switching power supply circuit

2.5 Power feedback isolation circuit

2.6 Inverter and rectifier circuit

III Conclusion

I What is switch mode power supply

Switch Mode Power Supply (SMPS), also known as switching power supply and switching converter, is a high-frequency electric energy conversion device and a type of power supply. Its function is to convert a level of voltage into the voltage or current required by the user through different forms of architecture.

Name

Switching Mode Power Supply

Nature

Utilize modern power electronics technology

Method

Pulse width modulation

Features

Small size, light weight and high efficiency

II 6 switching power supply circuit diagrams

2.1 A simple switching power supply circuit

A simple switching power supply circuit

This circuit is not difficult and can work normally without too many requirements. Basically pay attention to the following points:

  1. Adjust C3 and R5 so that the oscillation frequency is 30KHz-45KHz;
  2. The output voltage needs to be stabilized;
  3. The output current can reach 500mA.
  4. The effective power is 8W and the efficiency is 87%.

2.2 24V switching power supply circuit

24V switching power supply circuit

24V switching power supply circuit

24V switching power supply is a type of high frequency inverter switching power supply. The switch tube is controlled by the circuit to conduct high-speed pass and cut-off, convert the direct current into high-frequency alternating current and provide it to the transformer for transformation, thereby generating the required one or more sets of voltages.

The working principle of 24V switching power supply is:

  1. The AC power input is rectified and filtered into DC;
  2. Control the switching tube by high-frequency PWM (pulse width modulation) signal, and add that DC to the primary of the switching transformer;
  3. High-frequency voltage is induced in the secondary of the switching transformer, which is rectified and filtered and supplied to the load;
  4. The output part is fed back to the control circuit through a certain circuit to control the PWM duty cycle to achieve the purpose of stable output.

2.3 Single-ended forward switching power supply circuit

Single-ended forward switching power supply circuit

The typical circuit of a single-ended forward switching power supply is shown in the figure below. This circuit is similar in form to a single-ended flyback circuit, but the working conditions are different:

When the switch tube VT1 is turned on, VD2 is also turned on. At this time, the grid transmits energy to the load, and the filter inductor L stores energy;

When the switch VT1 is turned off, the inductor L continues to release energy to the load through the freewheeling diode VD3.

There is also a clamping coil and diode VD2 in the circuit. The diode can limit the maximum voltage of the switch tube VT1 to between twice the power supply voltage. In order to meet the magnetic core reset condition, that is, the flux establishment and reset time should be equal, so the duty cycle of the pulse in the circuit cannot be greater than 50%.

Because this circuit transfers energy to the load through the transformer when the switch tube VT1 is turned on, the output power range is large, and it can output power of 50-200 W. However, there are few practical applications of this circuit. The reason is that the transformer used in this circuit has a complicated structure and a large volume.

2.4 Push-pull switching power supply circuit

Push-pull switching power supply circuit

The typical circuit of push-pull switching power supply is shown in the figure below. It is a double-ended conversion circuit, and the magnetic core of the high-frequency transformer works on both sides of the hysteresis loop. The circuit uses two switching tubes VT1 and VT2. The two switching tubes are switched on and off alternately under the control of an external excitation square wave signal. The square wave voltage is obtained in the secondary group of the transformer T, which is rectified and filtered into the required DC Voltage.

The advantage of this circuit is that the two switching tubes are easy to drive, and the main disadvantage is that the withstand voltage of the switching tubes must reach twice the peak voltage of the circuit. The output power of the circuit is relatively large, generally in the range of 100-500 W.

2.5 Power feedback isolation circuit

Power feedback isolation circuit

In the switching power supply, the power feedback isolation circuit is composed of a photocoupler such as PC817 and a shunt regulator TL431, and its typical application is shown in the following figure. When the output voltage fluctuates, the sampled voltage obtained after the resistor divider is compared with the 2.5V bandgap reference voltage in the TL431, and an error voltage is formed on the cathode. Subsequently, the LED operating current in the optical coupling device changes accordingly. In this way, the current size of the TOPSwitch control terminal can be changed through the optical coupling device, and then the output duty ratio can be adjusted, so that Uo can be kept unchanged to achieve the purpose of voltage stabilization.

The role and selection of the main components in the feedback loop: The main role of R1R4R5 is to work with the TL431 and the optical coupling device. Among them, R1 is the current limiting resistor of the optocoupler, and R4 and R5 are the voltage divider resistors of TL431, which provide the necessary working current to complete the protection of TL431.

2.6 Inverter and rectifier circuit

Inverter and rectifier circuit

The circuit takes the UC3842 oscillator chip as the core to form an inverter and rectifier circuit. UC3842 is a high-performance single-ended output current-controlled pulse width modulator chip. The AC220V power supply is introduced through the common mode filter L1, which can better suppress the high frequency interference from the power grid and the radiation from the power supply itself. The AC voltage is filtered by the bridge rectifier circuit and the capacitor C4 to become an unstable DC voltage of about 280V, which serves as an inverter circuit composed of the oscillating chip U1, the switching tube Q1, the switching transformer T1 and other components.

III Conclusion

The above are 6 simple switching power supply circuit diagrams that we have prepared for you. During the browsing process, is there anything that I don’t understand or have doubts about? If so, don't hesitate to leave your thoughts in the comment area.

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