Switching power supply basics

Filter 18.432M

Switching power supply is to use the circuit control switch tube for high-speed pass and cut. Converting DC power to high frequency AC power is supplied to the transformer for voltage transformation to produce one or more sets of voltages required!
The switching power supply can be roughly divided into two types: isolated and non-isolated. The isolated type must have a switching transformer, and the non-isolated one may not necessarily be.
The working principle of the switching power supply is:
1. The AC power input is rectified and filtered into DC;
2. The switching transistor is controlled by a high frequency PWM (Pulse Width Modulation) signal, and that DC is applied to the primary of the switching transformer;
3. The secondary of the switching transformer induces a 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 cycle to achieve the purpose of stable output. When the AC power input is input, it generally passes through something like the E-flow circle to filter out the interference on the power grid. Filter out the interference of the power supply to the grid;
When the power is the same, 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 may have multiple windings or one winding with multiple taps to obtain the desired output;
Generally, some protection circuits should be added, such as no-load, short-circuit, etc., otherwise the switching power supply may be burnt.
The main components of the ATX power supply
EMI filter circuit: The main function of the EMI filter circuit is to filter the interference of the high-frequency pulse of the power grid to the power supply. At the same time, it also reduces the electromagnetic interference of the switching power supply itself. In the high-quality power supply, there is generally a two-pole EMI filter circuit. Has thousands of high-tech (AC-DC, DC-DC, DC-AC) high-frequency switching power supply, module power supply.
First-class EMI circuit: The first-level EMI power filter circuit is soldered on the AC power socket. This is a separate circuit board. It is the first group of circuits that pass through the AC input. This is composed of a choke coil and a capacitor. The network can filter high frequency clutter and in-phase interference signals on the power line, and also shield the internal interference signal of the power supply to form the first line of defense against electromagnetic interference.
Secondary EMI circuit: After the mains enters the power board, it passes through the power fuse, and then passes through the second EMI circuit consisting of inductor and capacitor to fully filter out the high-frequency clutter, and then enters the high-voltage rectification filter circuit through the current limiting resistor. . The fuse can be blown to protect the components inside the power supply when the power is too high or the component is short-circuited. The current limiting resistor contains a metal oxide component, 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: After EMI filtering, the mains is converted to high voltage DC after full bridge rectification and capacitor filtering. Converting the AC power at the input to pulsed DC power, there are currently two forms, one is that the full bridge is to pack four diodes together, the other is to use four discrete diodes to form a bridge rectifier circuit, the same effect, the same effect .
In general, there should be two or more tall barrel-shaped components near the full bridge, namely high-voltage electrolytic capacitors, which function to filter the pulsating DC power to remove the AC component and output a relatively stable DC power. The use of high-voltage electrolytic capacitors is closely related to the design of the switching circuit. Its capacity is often the focus of power evaluation in the past, but in fact its capacity has nothing to do with the power of the power supply, but increasing its capacity will reduce the power supply. Ripple interference improves the current output quality of the power supply.
PFC circuit: The PFC circuit is called a power factor correction or compensation circuit. The higher the power factor, the greater the power utilization.
At present, there are two ways of PFC circuit, one is passive PFC, also known as passive PFC, and the other is active PFC, also known as active PFC. The passive PFC compensates the phase difference between the fundamental current and the voltage of the AC input through a power frequency inductor, forcing the current and voltage phases to be consistent. The passive PFC is inefficient, generally only 65%-70%, and the work used. The frequency inductance is large and cumbersome, but due to the low cost, there are still many ATX power supplies in this way. The active PFC is composed of electronic components, small in size and light in weight. The phase of the current waveform is adjusted by a dedicated IC, and the efficiency is greatly improved, up to 95%, but due to the high cost, it is usually only in advanced applications. Can you see it.
Switching triode and switching transformer: As the name implies, the switching power supply is the switch word. The switching transistor and the switching transformer are the core components of the switching power supply. The self-excited or other excitation mode causes the switching tube to operate in a saturated, cut-off (ie, open, closed) state, thereby inducing a high-frequency voltage on the secondary winding of the switching transformer. After rectification, filtering and voltage regulation, various DC voltages are output. Switching triode and switching transformer are the core components of ATX power supply. Its quality directly affects the quality and service life of the power supply. Especially the switching transistor, working under high back pressure, there is not enough protection circuit, it is easy to breakdown and burn. The quality of the switch directly determines the stability of the power supply. It is also the main heating element in the power supply. The two transistors on the main heat sink seen after the power is removed are the switch tubes.
Factors affecting the performance of high-frequency switching transformers include the efficiency of ferrite, the cross-sectional area of ​​the core and the width of the magnetic gap. Transformers with too small cross-sectional area are prone to magnetic saturation and cannot output large power, and the number of turns of each winding Directly affect the output voltage, usually we can not specifically grasp these parameters, so we can not accurately determine how much power the transformer can output, only through the electronic load machine measurement can be known, in addition, the output of the switching transformer, although many, but Some outputs use the same windings, such as +3.3VDC and +5VDC, so when +3.3VDC outputs the maximum current, +5VDC can't output a large current, so we can't put the output of each power supply. The power is simply accumulated.
In addition to the main transformer, there should be two small transformers in the general power supply. One of them will amplify the switching circuit control signal to drive the switching tube to work. At the same time, it can also operate the high voltage area of ​​the switching tube and the low voltage area of ​​the integrated circuit. Physical isolation. The other is a completely independent small switching power supply. This is what we call the standby circuit. The output voltage is the main circuit of the power supply. At the same time, the +5VStandBy output is output to the main board to realize the wake-up function.
Low-voltage rectification and filtering circuit: After the high-frequency transformer is stepped down, the ripple voltage is also rectified and filtered by diodes and capacitors. However, the operating frequency during rectification is very high, and Schottky rectifier diodes with fast recovery function must be used. The ordinary rectifier diode is difficult to do this, and the capacitor used in the rectification part can not have too much AC impedance, otherwise the high-frequency AC component cannot be filtered out, so the selected capacitor not only has a large capacity but also has a lower capacity. The AC resistance is also available. In addition, one or two large-sized inductors with a magnetic core can be seen, and the high-frequency AC component is filtered together with the filter capacitor to ensure the output of pure DC power.
Since the low-voltage rectifier terminal needs to output a large current, the rectifier diode also generates a large amount of heat. These diodes and the front switch tube require a separate heat sink for heat dissipation, and these components are fixed on the other heat sink in the power supply. Output from these components is the output current of various voltages.
Voltage regulation and protection circuit: The voltage stabilization circuit usually samples a part of the voltage from the output voltage of the power supply terminal and compares it with the standard voltage. The compared difference is amplified to drive the switching transistor and adjust the duty cycle of the switching tube. The voltage is stable. The function of the protection circuit is to detect the change of the output voltage or current at each end. When the output terminal is short-circuited, over-voltage, over-current, overload, under-voltage, etc., the protection circuit operates, and the excitation signal of the switch tube is cut off, so that the switch tube Stopping the vibration, the output voltage and current are zero, and it plays a protective role.