Switching power supply refers to switch and power supply
Switching power supply in professional books refers to a kind of power supply that uses modern power technology to control the time ratio of switching transistor on and off and maintain stable output voltage. Switching power supply is generally composed of pulse width modulation (PWM) control IC and MOSFET.
The types of switching power supply can be divided into isolation and non isolation
The working principle of switching power supply is as follows:
1. The input of AC power supply (such as 50 Hz) is rectified and filtered to form DC power supply;
2. High frequency PWM (pulse width modulation) signal is used to control the switch tube to add DC power to the primary of switch transformer;
3. High frequency voltage is induced from the secondary of the switch transformer and 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, so as to achieve the purpose of stable output
When the AC power supply is input, it is usually filtered and rectified by the things like the current loop to filter out the mutual interference with the power grid;
When the power is the same, the higher the switching frequency is, the smaller the size of the switching transformer is, but the higher the requirement of the switch tube is;
The secondary of switching transformer can have multiple windings or one winding can have multiple taps to get the required output;
Should increase some protection circuit, such as no load, short circuit and other electrical protection, to prevent burning switching power supply
High frequency switching power supply is the development direction in the future, and has broad prospects. High frequency switching power supply makes it miniaturized, and makes it enter a wider range of application fields, especially in the field of high and new technology, which promotes the miniaturization and lightness of high and new technology products. In addition, the development and application of switching power supply is in the fields of energy saving, energy saving and energy saving Saving resources and protecting environment are of great significance
Switching power supply means that the AC power supply is first rectified and filtered into DC power supply, and the high-speed channel and cut-off are carried out by controlling the switch tube through the circuit to convert the DC power into high-frequency AC power and provide it to the transformer for transformation, The reason for converting to high-frequency AC is that the efficiency of high-frequency AC in transformer transformation circuit is much higher than that of 50 Hz, so the switching transformer can do very small, and the working time is not very hot. The cost is very low. If 50 Hz is not converted to high frequency, the switching power supply is meaningless
The working principle of switching power supply is as follows:
1. AC power input is rectified and filtered into DC power supply;
2. High frequency PWM (pulse width modulation) signal is used to control the switch tube to add DC to the primary of the switch transformer;
3. High frequency voltage is induced from the secondary of the switch transformer and 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, so as to achieve the purpose of stable output
Switching power supply circuit:
Switching power supply design: switching power supply can be divided into isolated type and non isolated type. The isolated type must have switching transformer, while the non isolated type may not. When the AC power supply is input, it generally needs to go through the current loop to filter out the interference on the power grid and the interference of the power supply on the power grid;
When the power is the same, the higher the switching frequency is, the smaller the size of the switching transformer is, but the higher the requirement of the switch tube is;
The secondary of the switching transformer can have multiple windings or one winding can have multiple taps to obtain the required voltage output;
Should also add some protection circuit, such as no-load, short circuit protection, or it may burn the switching power supply
Working principle of switching power supply
The main components of ATX power supply
EMI filter circuit: the main function of EMI filter circuit is to filter the interference of high frequency pulse of external power grid to the power supply, and also to reduce the electromagnetic interference of switching power supply itself to the outside world. Generally, there are two pole EMI filter circuits in high-quality power supply
Level 1 EMI circuit: the level 1 EMI power filter circuit is welded on the AC power socket, which is an independent circuit board and the first group of circuits after AC input. The low-pass network composed of choke and capacitor can filter the high-frequency clutter and in-phase interference signal on the power line, and shield the interference signal inside the power supply, Constitute the first line of defense against electromagnetic interference
Secondary EMI circuit: after the power supply enters the power board, it first goes through the power fuse, then goes through the second EMI circuit composed of inductance and capacitor to fully filter the high-frequency clutter, and then goes through the current limiting resistance to enter the high-voltage rectifier filter circuit. The fuse can fuse when the power supply is too high or the components are short circuited to protect the components inside the power supply, The current limiting resistor contains metal oxide, which can limit the instantaneous large current and reduce the current impact of power supply on internal components
Bridge rectifier and high-voltage filter: after EMI filtering, the power supply becomes high-voltage direct current after full bridge rectification and capacitor filtering. There are two forms to convert the alternating current at the input end into pulse direct current. One is full bridge, which encapsulates four diodes together, and the other is bridge rectifier circuit composed of four discrete diodes, which has the same function and effect
Generally speaking, there should be two or more large barrel shaped elements near the whole bridge, namely high voltage electrolytic capacitor, whose function is to filter the pulsating DC component 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 previous power supply evaluation, But in fact, its capacity has nothing to do with the power of the power supply, but increasing its capacity will reduce the ripple interference of the power supply and improve the current output quality of the power supply
PFC circuit: PFC circuit is called power factor correction or compensation circuit
At present, there are two ways of PFC circuit, one is passive PFC, also known as passive PFC, the other is active PFC, also known as active PFC. Passive PFC uses a power frequency inductor to compensate the phase difference between the fundamental current and voltage of AC input, forcing the phase of current and voltage to be consistent. The efficiency of passive PFC is low, generally only 65% - 70%, and the power frequency inductor used is large and bulky, but due to low cost, There are still many ATX power supply in this way. Active PFC is composed of electronic components, small size, light weight, through a special IC to adjust the phase of the current waveform, the efficiency is greatly improved, up to 95%, but due to the high cost, it can only be seen in advanced applications
Switching triode and switching transformer: as the name suggests, the core of switching power supply is switch. Switching triode and switching transformer are the core components of switching power supply. Through self excitation or other excitation, the switching tube works in saturation and cut-off (i.e. on and off) state, so as to induce high-frequency voltage on the secondary winding of switching transformer, and then through rectification and switching The switching triode and the switching transformer are the core components of ATX power supply. Their quality directly affects the quality and service life of the power supply, especially the switching triode. When working in the high back pressure state, there is not enough protection circuit and it is easy to be broken down. The quality of the switching transistor 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 are switch tubes
The factors that affect the performance of high-frequency switching transformer include the efficiency of ferrite, the size of cross-sectional area of magnetic core and the width of magnetic gap. If the cross-sectional area is too small, the transformer is prone to magnetic saturation and can not output large power. The turns of each winding directly affect the output voltage. Usually, we can not grasp these parameters, In addition, although there are many output terminals of switch transformer, some of them use the same winding, such as + 3.3vdc and + 5VDC, so when + 3.3vdc outputs the maximum current, + 5VDC cannot output a large current, Therefore, we can not simply accumulate the power of each output terminal of the power supply
In addition to the main transformer, there are two small transformers in the general power supply. One of them amplifies the control signal of the switch circuit to drive the switch tube to work. At the same time, it can physically isolate the high-voltage area where the switch tube works from the low-voltage area where the integrated circuit works. The other is a set of independent small switch power supply, which is what we call standby circuit, The output voltage supplies power for the main circuit of the power supply, and the + 5V standby terminal is output to the main board to realize the wake-up function
Low voltage rectifying and filtering circuit: the pulsating voltage after high-frequency transformer step-down also needs diode and capacitor for rectifying and filtering, but at this time, the working frequency of rectifying is very high, so Schottky rectifying diode with fast recovery function must be used. Ordinary rectifying diode is difficult to do this, and the capacitor used in rectifying part cannot have too large AC impedance, Otherwise, it is impossible to filter out the high-frequency AC component. Therefore, the selected capacitor should not only have large capacity, but also have low AC resistance. In addition, one or two large inductance coils with magnetic core can be seen to filter out the high-frequency AC component together with the filter capacitor to ensure the output of pure DC
Because the low-voltage rectifier needs to output a lot of current, the rectifier diodes will also generate a lot of heat. These diodes and the front switch tubes need separate heat sink for heat dissipation. These components are fixed on another heat sink in the power supply. The output current from these components is the output current of various voltages
Voltage stabilizing and protection circuit: the voltage stabilizing circuit usually samples part of the voltage from the output voltage of the power supply and compares it with the standard voltage. After the difference is amplified, it drives the switch triode to adjust the duty cycle of the switch, so as to achieve voltage stability. The function of the protection circuit is to detect the change of the output voltage or current at each end, when the output end is short-circuit, over-voltage When over-current, over load, under voltage and other phenomena occur, the protection circuit acts to cut off the excitation signal of the switch tube, so that the switch tube stops vibrating, and the output voltage and current are zero, which plays a protective role
