Switching power supply is used to control the switching tube through the circuit to carry out high-speed road pass and cut-off. DC into high-frequency alternating current provided to the transformer to transform, thereby generating the required one or more groups of voltage! Huawei high-frequency alternating current is converted to high-frequency alternating current in the transformer to transform the efficiency of the circuit is much higher than 50Hz. So the switching transformer can be made very small, and work is not very hot! The cost is very low. If you do not change 50Hz to high frequency that switching power supply is meaningless.

Switching power supply can be roughly divided into isolated and non-isolated two kinds, isolated must have switching transformer, and non-isolated may not necessarily have.

The working principle of 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 the DC to the primary of the switching transformer.

3. The secondary switching transformer induces a high-frequency voltage, 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.

AC power input is generally to go through a current loop of something like, filter out the interference on the grid, but also filter out the power supply on the grid interference; in the same power, switching the power of the same, the switching of the power of the same.

In the same power, the higher the switching frequency, the smaller the size of the switching transformer, but the higher the requirements of the switching tube.

The secondary of the switching transformer can have more than one winding or one winding with more than one tap, in order to get the required output.

Generally, some protection circuits should be added, such as no-load, short-circuit protection, otherwise the switching power supply may be burned. 

The main components of the ATX power supply

EMI filter circuit: EMI filter circuit is mainly used to filter out the external power grid of high-frequency pulse interference with the power supply, but also play a role in reducing the switching power supply itself to the outside world of electromagnetic interference, in the high-quality power supply generally have two poles of EMI filter circuit. 

A level of EMI circuit: AC power socket welding is a level of EMI power filter circuit, which is an independent circuit board, is the first set of circuits after the input of AC power, this low-pass network composed of chokes and capacitors can filter out high-frequency clutter on the power line and in-phase interference signals, but also the power supply internal interference signals shielded, constituting the power supply anti-electromagnetic interference of the first line of defence. The second level EMI circuit: the mains power enters into the power supply line.

Secondary EMI circuit: the utility power into the power supply board first through the power supply fuse, and then again through the inductance and capacitance composed of the second EMI circuit to fully filter out high-frequency clutter, and then through the current limiting resistor into the high-voltage rectifier filter circuit. Fuse can be in the power supply power is too large or components of the short-circuit fuse to protect the internal components of the power supply, while the current limiting resistor contains metal oxide components, can limit the instantaneous high current, reduce the power supply on the internal components of the current impact.

Bridge Rectifier and High Voltage Filtering: After the EMI filtered utility power, and then after the full bridge rectifier and capacitor filtering into a high-voltage DC. The input AC power into a pulsed DC, there are two forms, one is the full bridge is the four diodes packaged together, one is to use four discrete diodes to form a bridge rectifier circuit, the role of the same, the same effect. 

Generally speaking, there should be two or more tall barrel-shaped components near the full bridge, that is, high-voltage electrolytic capacitors, whose role is to filter out the pulsating DC alternating current components and output a relatively smooth DC. The use of high-voltage electrolytic capacitors and switching circuit design has a close relationship with its capacity is often the focus of the previous power supply evaluation, but in fact its capacity and the power of the power supply has nothing to do, but increase its capacity will reduce the power supply ripple interference, improve the quality of the power supply current output.

PFC circuit: PFC circuit is called power factor correction or compensation circuit, the higher the power factor, the greater the power utilisation.

Currently there are two types of PFC circuits, a passive PFC, also known as passive PFC, an active PFC, also known as active PFC. passive PFC is through an industrial frequency inductor to compensate for the AC input fundamental current and voltage phase difference, forcing the current and the voltage in phase, passive PFC efficiency is low, generally only 65% -70%, and the use of the industrial frequency inductance is large and bulky, but due to the low cost of PFC circuit, the power factor correction circuit. Bulky, but because of the low cost, there are still many ATX power supplies use this method. Active PFC is composed of electronic components, small size, light weight, through a dedicated IC to adjust the phase of the current waveform, the efficiency is greatly improved, up to 95% or more, but because of the high cost, usually only in advanced applications can be seen.

Switching triode and switching transformer: switching power supply as the name suggests its core is the word switch. Switching transistor and switching transformer is the core component of switching power supply, through self-excited or other excitation to make the switching tube work in saturation, cut-off (i.e., on, off) state, so as to induct the high frequency voltage on the secondary winding of the switching transformer, and then through the rectifier, filtering and regulator to output a variety of DC voltages. Switching transistor and switching transformer are the core components of ATX power supply, and their quality directly affects the power supply and service life, especially the switching transistor, working in a high back-voltage state, there is no adequate protection circuit, it is easy to breakdown and burn. The quality of the switching tube directly determines the stability of the power supply, it is also the main heating element in the power supply, disassemble the power supply to see the main heat sink on the two transistors is the switching tube.

Factors affecting the performance of high-frequency switching transformer, including the efficiency of the ferrite, the size of the core cross-sectional area and the width of the magnetic gap, the cross-sectional area is too small transformer is prone to magnetic saturation and can not output a larger power, the number of turns of each winding directly affects the output voltage, usually we can not get a specific grasp of these parameters, so we can not accurately judge the transformer in the end how much power can be output, only by electronic load machine Measured by the electronic load machine to know, in addition, although many outputs of the switching transformer, but some of the outputs are the same winding, such as +3.3VDC and +5VDC is so, so when the +3.3VDC output the maximum current +5VDC can not output a large current, so we can not be a power supply of the power of the various outputs of the power supply is simple to add up.

In addition to the main transformer, the general power supply should have two small transformers, one of which will amplify the switching circuit control signals to drive the switching tube to work, but also the switching tube work of the high-voltage area and the integrated circuit work of the low-voltage area of physical isolation. The other is a completely independent set of small switching power supply, this is what we call standby circuit, its output voltage for the main circuit of the power supply, and at the same time through the +5V StandBy terminal output to the motherboard to achieve the wake-up function.

Low-voltage rectifier filter circuit: after the high-frequency open transformer step-down of the pulsating voltage should also use diodes and capacitors for rectification and filtering, but at this time the rectification of the operating frequency is very high, you must use the Schottky rectifier diode with a fast recovery function, ordinary rectifier diode is difficult to be the job, and the rectifier part of the use of capacitance can not be too much AC impedance, otherwise it will be unable to filter out the high-frequency AC components, so the choice of capacitors not only large capacity, but also have a low AC resistance to do, in addition to see 1 or 2 large inductance coil with a magnetic core, and filter capacitors together with high-frequency AC components to ensure that the output of pure DC.

As the low-voltage rectifier needs to output a large current, so the rectifier diode will also produce a lot of heat, these diodes and the front of the switching tube need a separate heat sink for heat dissipation, power supply in another heat sink is fixed on these components. Output from these components is a variety of different voltage output current.

Voltage regulator and protection circuit: voltage regulator circuit is usually from the output voltage of the power supply output voltage sampling part of the voltage and the standard voltage for comparison, compared to the difference between the amplified to drive the switching transistor, adjust the duty cycle of the switching tube, so as to achieve the stability of the voltage. The role of the protection circuit is to detect the changes in the output voltage or current at each end, when the output side of the short-circuit, over-voltage, over-current, overload, under-voltage and other phenomena, the protection circuit action, cut off the excitation signal of the switching tube, so that the switching tube stops vibrating, the output voltage and current is zero, play a protective role.