Popular science knowledge of switching power supply
Through the circuit, the switch tube is controlled to make high-speed on and off. The direct current is converted into high-frequency alternating current and supplied to the transformer for transformation, thus generating one or more groups of voltages needed!
Switching power supply can be roughly divided into two types: isolated and non-isolated. The isolated type must have a switching transformer, but the non-isolated type may not necessarily have it.
The working principle of switching power supply is:
1. AC power input is rectified and filtered into DC;
2. Control the switch tube by high frequency PWM (pulse width modulation) signal, and add that DC to the primary of the switching transformer;
3. The secondary of the switching transformer induces high-frequency voltage, which is supplied to the load through rectification and filtering;
4. The output part is fed back to the control circuit through a certain circuit to control the PWM duty ratio, so as to achieve the purpose of stable output. When AC power is input, it usually goes through something like the current loop to filter out the interference on the power-off network, and at the same time, it also filters out the interference of the power supply on the power grid;
At the same power, the higher the switching frequency, the smaller the volume of the switching transformer, but the higher the requirements for the switching tube;
The secondary of the switching transformer can have multiple windings or one winding has multiple taps to get the required output;
Generally, some protection circuits should be added, such as no-load and short circuit protection, otherwise the switching power supply may be burned.
Popular science knowledge of switching power supply
Main components of ATX power supply
EMI filter circuit: The main function of EMI filter circuit is to filter out the interference of high-frequency pulse of external power grid to the power supply, and at the same time reduce the electromagnetic interference of switching power supply itself to the outside world. Generally, there are two-pole EMI filter circuits in * * power supply. There are thousands of high-tech (AC-DC, DC-DC, DC-AC) high-frequency switching power supplies and modular power supplies.
EMI circuit: the AC power socket is welded with **EMI power filter circuit, which is an independent circuit board and a * * group of circuits after AC power input. This low-pass network consisting of choke and capacitor can filter out high-frequency clutter and in-phase interference signals on the power line, and at the same time shield the interference signals inside the power supply, thus forming the anti-electromagnetic stem of the power supply.
The first line of defense.
Secondary EMI circuit: after the mains power enters the power board, it first passes through the power fuse, then passes through the * * channel EMI circuit composed of inductor and capacitor to fully filter out high-frequency clutter, and then passes through the current-limiting resistor to enter the high-voltage rectification filter circuit. The fuse can be blown when the power supply is too high or the components are short-circuited, so as to protect the internal components of the power supply. The current-limiting resistor contains metal oxide components, which can limit the instantaneous large current and reduce the current impact of the power supply on the internal components.
Bridge rectifier and high-voltage filtering: the commercial power filtered by EMI is converted into high-voltage DC after full-bridge rectification and capacitor filtering. At present, there are two ways to convert alternating current at the input end into pulsed direct current. One is to package four diodes together in a full bridge, and the other is to form a bridge rectifier circuit with four discrete diodes, which has the same effect and the same effect.
Generally speaking, there should be two or more tall barrel-shaped components near the whole bridge, that is, high-voltage electrolytic capacitors, which are used to filter out AC components of pulsating DC and output relatively stable DC. The use of high-voltage electrolytic capacitor is closely related to the design of switching circuit, and its capacity is often the focus of power supply evaluation in the past, but in fact its capacity has nothing to do with the power supply, but increasing its capacity will reduce the ripple interference of power supply and improve the current output quality of power supply.
