What is switching power supply

Switching power supply is through the circuit control switch tube for high-speed turn on and cut-off. The direct current into high-frequency alternating current is provided to the transformer for voltage transformation, so as to generate one or more groups of voltage required! The reason for conversion to high-frequency AC is that the efficiency of high-frequency AC in transformer transformer circuit is much higher than that in 50 Hz. Therefore, the switching transformer can do very small work

Turn on the high-speed or high-speed voltage transformer to provide a set of high-speed switching power supply! The reason for conversion to high-frequency AC is that the efficiency of high-frequency AC in transformer transformer circuit is much higher than that in 50 Hz. Therefore, the switching transformer can be done very small, and the work is not very hot!! The cost is very low. If you don't change 50 Hz to high frequency, then switching power supply is meaningless!! Switching transformer is not mysterious. It is an ordinary transformer! This is the switching power supply

Switching power supply can be divided into two types: isolated and non isolated

In short, the working principle of switching power supply is as follows:

1. The AC power input is converted into DC through rectification and filtering;

2. The switch tube is controlled by high frequency PWM (pulse width modulation) signal, and the DC is added to the primary of the switching transformer;

3. High frequency voltage is induced from the secondary of the switching transformer, 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 duty cycle of PWM, so as to achieve the purpose of stable output

When the AC power supply is input, it usually goes through the Ehrlich circle to filter out the interference from the power grid and the interference from the power supply to 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 switch tube;

The secondary of the switching transformer can have multiple windings or one winding has multiple taps to obtain the required output;

Generally, some protection circuits should be added, such as no-load, short-circuit and so on

The above is the general working principle of switching power supply

In fact, now there is a very high integration of dedicated chips, can make the peripheral circuit very simple, and even achieve debugging free

For example, the top series switching power supply chip (or module) can be made into a basic switching power supply as long as it is equipped with some resistance and capacitance components and a switching transformer

Pwn-pfm control circuit of switching power supply

Zou Huaian, Zhang Rui, Hu rongqiang

School of automation, Wuhan University of Technology

1. Introduction

Due to its advantages in volume, weight, efficiency and reliability, switching power supply has completely replaced the traditional linear regulated power supply in many fields such as computer, communication, household appliances, radar, space technology and so on. There are three kinds of control technology of switching power supply: (1) pulse width modulation (PWM); (2) pulse frequency modulation (PPN); (3) pulse width frequency modulation (pwn-pfh). Among them, PWM is the most widely used control method in switching power supply. Its characteristics are low noise, high efficiency of full load and can work in continuous conduction mode. There are many kinds of good performance and low price in the market now PFM has the advantages of low static power consumption, but it has no current limiting function and can not work in continuous conduction mode. The integrated chips with PFM function include max641 and tl497. Pwm-pfm has the advantages of both PWM and PFM, but there is no single chip integrated chip with pwm-pfm function Sales limit its use.

This paper introduces a switching power supply controlled by pwm-pfm. The principle of the control circuit pwm-pfm circuit is analyzed in detail. The control circuit has few components and is easy to realize.

2. Introduction of switching power supply and its pwh-pfh control circuit

The input of the switching power supply is three-phase AC, the input voltage range is 320 ~ 575v, the frequency is 45 ~ 65Hz, the output specification is 24 V / 10A, and the weight is only about 1.4kg. The main circuit topology of the switching power supply is a double transistor flyback converter. By using two switches in series to reduce the turn off voltage requirement of high power MOSFET.

The pwm-pfm control circuit is composed of three digital chips (4011, 4066, 40106) and a small number of resistors, capacitors, diodes and triodes. The circuit diagram is shown in Figure 1. Among them, 4011 is a four NAND gate with two inputs, 4066 is a four way analog switch, and 40106 is a six Schmidt trigger.

3. Principle analysis

As shown in the circuit diagram, it has four inputs. The signals come from the output of optocoupler 1, the output of optocoupler 2, the rectified high voltage DC end, and the coupling with the switch transformer. One output terminal is used to drive the high-power MOSFET. The core part of the circuit is a RS flip-flop composed of two NAND gates (in3a / in3b / out3 and in4a / in4b / out4). The flip-flop has two inputs: the 9th pin (in3b) and the 12th pin (in4a) of 40ll, and the 10th pin (out3) and the 11th pin (out4) of 401l. The output signal of out4 is transmitted to five parallel Schmidt flip flops (I1 / O1, I2 / O2, I3 / O3, I4 / O4 and i5 / O5). The function of Schmidt trigger is to adjust the output waveform of NAND gate and prevent the malfunction of circuit. The redundancy design of five Schmidt flip flops in parallel improves the current drive ability of the circuit and enhances the whole circuit