Working Principle of Industrial Frequency Transformer and Switching Power Supply
The working principle of industrial frequency transformer is relatively simple, by the primary coil input frequency AC voltage is converted into a magnetic field, through the magnetic conductive material (usually silicon steel sheet) transmitted to the secondary coil induced voltage. Output for the frequency and input frequency is the same, the voltage in accordance with the initial stage of the coil turns than the reduced voltage (if the number of turns of the secondary is more boost). As the transformer output is alternating current, and most of the electrical circuits used for direct current, the transformer output voltage also needs to be rectified, filtered, regulated and other circuits, into a relatively smooth and stable voltage for the load circuit part of the work.
Switching power supply is still the core of the transformer component, and also follow the rules of the voltage ratio is equal to the number of turns ratio. Unlike industrial transformers, switching power supplies need to increase the operating frequency, that is, the need to change the low-frequency AC voltage into high-frequency AC voltage, which requires additional control circuitry to achieve. Because the circuit requires DC power to operate, the input AC voltage has to be rectified and changed to DC voltage before it can be controlled by the circuit behind it. The following is an example of a commonly used mobile phone charger circuit to briefly understand the working principle of switching power supply.
Input 220V AC voltage after rectification and filtering, will become about 310V DC voltage (that is, 220V AC voltage peak), the following need to turn this DC into high-frequency AC. Want to turn this voltage into high-frequency alternating current, the simplest way is to use a switch, so that the switch is quickly disconnected and closed, so that the DC power into a high-speed pulse DC voltage, the realization of this switch is the component transistor. Transistors, including commonly used transistors and field effect tubes, etc., these two components can be used as an electronic switch, that is, through the voltage control of a pin (the base of the transistor as well as the gate of the field effect tube), you can make the other two pins to achieve the on-off control.
With the switch, the next need to control the switch circuit, the role of this circuit is to output high-speed switching signals to control the switching tube conduction and cut-off, this circuit is called the oscillation circuit. Switching power supply oscillator circuit is divided into many kinds of, no matter which one, the role is to provide control signals to the switching tube.
After the control circuit control, the input voltage from the low-frequency alternating current into a high-frequency pulsed DC voltage, input to the transformer for step-down, the voltage output from the transformer will also be rectified, filtered into DC output, provided to the load work. With the industrial frequency transformer is different, switching power supply is also more part of the voltage detection circuit, it will output the voltage signal through the detection of the primary transformer control circuit after feedback to the regulator, which makes the switching power supply to improve the stability of the output voltage, and can have a very wide range of input voltage. So the working process of switching power supply is actually realised by several processes of AC-DC, DC-AC and then AC-DC.
Here there may be a question, the transformer is not only through the alternating current, why the switching power supply DC can also be transformed through the transformer voltage? Transformer is indeed only through the alternating current, to be more specific is the need to have a change in magnetic flux, industrial frequency alternating current because it is sinusoidal, and the existence of positive and negative half-week, which will produce a change in magnetic flux. A switching power supply is made of switching tubes that turn DC into pulsed DC, and the switching tubes go from cutoff to conduction, and then from conduction to cutoff, which also produces a change in magnetic flux.
