Analysis of the Principle of High-Frequency Switching Power Supply Circuit

High frequency switching power supply consists of the following parts:

1. Main circuit

The whole process from AC grid input to DC output includes:

1. Input filter: Its function is to filter out the noise in the power grid, and also to prevent the noise generated by the machine from being fed back to the public power grid.

2. Rectification and filtering: directly rectify the AC power of the grid into smoother DC power for the next level of conversion.

3. Inversion: Converting rectified DC into high-frequency AC, which is the core part of high-frequency switching power supply. The higher the frequency, the smaller the ratio of volume, weight and output power.

4. Output rectification and filtering: Provide stable and reliable DC power supply according to load requirements.

2. Control circuit

On the one hand, samples are taken from the output end, compared with the set standard, and then the inverter is controlled to change its frequency or pulse width to achieve output stability. On the other hand, based on the information provided by the test circuit and identified by the protection circuit, the control circuit is provided to take various protection measures for the entire machine.

3. Detection Circuit

In addition to providing various parameters of the protection circuit in operation, it also provides various display instrument data.

4. Auxiliary power supply

Provide power supply for different requirements of all single circuits.

Section 2 Switch Control Voltage Regulator Principle

The switch K is repeatedly turned on and off at certain time intervals. When the switch K is turned on, the input power supply E is provided to the load RL through the switch K and the filter circuit. During the entire period when the switch is on, the power supply E provides energy to the load; when the switch K is turned off, the input power supply E interrupts the supply of energy. It can be seen that the input power supply provides energy to the load intermittently. In order to enable the load to obtain continuous energy supply, the switching voltage-stabilized power supply must have a set of energy storage devices to store part of the energy when the switch is turned on and release it to the load when the switch is turned off. In the figure, the circuit composed of inductor L, capacitor C2 and diode D has this function. Inductor L is used to store energy. When the switch is turned off, the energy stored in inductor L is released to the load through diode D, so that the load obtains continuous and stable energy. Because diode D makes the load current continuous, it is called a freewheeling diode. The average value EAB of the voltage between AB can be expressed by the following formula

EAB=TON/T*E

Where TON is the time the switch is turned on each time, and T is the on-off duty cycle of the switch (i.e. the sum of the switch on time TON and the switch off time TOFF).

It can be seen from the formula that by changing the ratio of the switch on time and the duty cycle, the average value of the voltage between AB will also change. Therefore, by automatically adjusting the ratio of TON and T as the load and input power supply voltage change, the output voltage V0 can be kept unchanged. Changing the ratio of the on time TON and the duty cycle is to change the duty cycle of the pulse. This method is called "Time Ratio Control" (abbreviated as TRC).

According to the TRC control principle, there are three ways:

1. Pulse Width Modulation (PWM) The switching period is constant and the duty cycle is changed by changing the pulse width.

2. Pulse Frequency Modulation (PFM) keeps the on-pulse width constant and changes the duty cycle by changing the switching operating frequency.

3. Hybrid Modulation

The conduction pulse width and the switching operating frequency are not fixed, and both can be changed. It is a mixture of the above two methods.

Section 3 Development and trend of switching power supply

In 1955, the self-excited oscillation push-pull transistor single transformer DC converter invented by GH . Roger of the United States was the beginning of realizing high-frequency conversion control circuits . In 1957, Jen Sen of the United States invented the self-excited push-pull dual transformer. In 1964, American scientists proposed the idea of ​​eliminating the series connection of the power supply of the industrial frequency transformer, which provided a fundamental way for the power supply to reduce the volume and weight. In 1969, due to the improvement of components such as the increase in the withstand voltage of high-power silicon transistors and the shortening of the reverse recovery time of diodes , a 25 kHz switching power supply was finally made. At present, the switching power supply is widely used in almost all electronic equipment such as various terminal equipment and communication equipment dominated by electronic computers due to its small size, light weight and high efficiency . It is an indispensable power supply method for the rapid development of today's electronic information industry. Among the switching power supplies currently sold on the market, the 100kHz power supply made of bipolar transistors and the 500kHz power supply made of MOS-FET have been put into practical use, but their frequency needs to be further improved. To increase the switching frequency, the switching loss must be reduced, and to reduce the switching loss, high-speed switching components are required. However, when the switching speed is increased, the distributed inductance and capacitance in the circuit or the charge stored in the diode will generate surges or noise. This will not only affect the surrounding electronic equipment, but also greatly reduce the reliability of the power supply itself. Among them, to prevent the voltage surge that occurs with the switch opening and closing, RC or LC buffers can be used, and for the current surge caused by the charge stored in the diode, a magnetic buffer made of amorphous cores can be used. However, for high frequencies above 1MHz, a resonant circuit should be used to make the voltage on the switch or the current passing through the switch a sine wave, which can reduce the switching loss and control the occurrence of surges. This switching method is called a resonant switch. At present, research on this switching power supply is very active, because this method can theoretically reduce the switching loss to zero without significantly increasing the switching speed, and the noise is also small, which is expected to become a major way to increase the high frequency of the switching power supply. At present, many countries in the world are committed to the practical research of multi-megahertz converters.