Assuming that the switch K is repeatedly turned on and off at a certain time interval, when the switch K is closed, the input power supply E supplies power to the load RL through the switch K and the filter circuit, and the power supply E supplies energy to the load during the whole turn-on period; When the switch K is turned off, the input power supply E interrupts the energy supply. Therefore, the input power supply intermittently supplies energy to the load. High-frequency switching power supply must design a set of its own energy storage facilities in order to enable the power supply to provide continuous energy to the load, so as to store some energy when the switch is turned on and continue to provide electric energy to the load when the switch is turned off. Changing the ratio of ON time TON to duty ratio T, that is, changing the duty ratio of pulse, is called "time ratio control" TRC.
According to the TRC control principle, there are three ways:
The switching period of (1) PWM is constant, and the duty cycle is changed by changing the pulse width.
(2) The pulse width of 2)PFM is constant, and the duty ratio is changed by changing the switching frequency.
(3) The on-pulse width and switching frequency of mixed modulation are not fixed and can be changed with each other.
The characteristics of PWM circuit: it can get the output voltage and current quite close to sine wave, so it is also called sine wave pulse width modulation SPWM. PWM modulation is to control the output voltage by adjusting the pulse width, and low-order harmonics can be suppressed or eliminated by using PWM inverter. By modulating the width of each pulse according to certain rules, the output voltage of the inverter circuit and the output frequency can be changed. Using the existing devices and circuit technology, PWM switching power supply can generally work at the switching frequency of tens of kHz to hundreds of kHz. The power supply equipment can be considered as the best in terms of weight, efficiency, reliability, price and overall size, and is suitable for medium and high power applications. Moreover, the general PWM modulation mode is used, and the requirements for components are not too high, which is very suitable for realizing the high cost performance of equipment.
Pulse width modulation (PWM) converter converts a DC voltage (current) into a high-frequency square wave voltage (current) by repeatedly turning on/off the switch, and then converts it into another DC voltage after rectification and smoothing. PWM converter consists of power switch tube, rectifier diode and filter circuit. When electrical isolation is needed between input and output, transformers can be used for isolation and step-up/step-down. The working principle of PWM converter is shown in Figure 2- 12. Due to the increase of switching frequency, magnetic components such as filter inductance L, transformer T and filter capacitor C can be miniaturized.
For PWM converter, the waveforms of voltage arc at both ends of switch tube J 1 and current flowing through J 1 are approximately square waves, as shown in Figure 2- 12.
Figure 2- 12? Basic working principle of PWM converter
Figure 2- 13? Waveform of converter switching operation
The converter has two working modes. One is the pulse width modulation (PWM) mode, which keeps the switch duty ratio Ts unchanged and controls the on-time ton of the switch, and the other is the pulse frequency modulation (PFM) mode, which keeps the on-time ton unchanged and changes the switch duty ratio Ts.
2.4? Control structure of output voltage
IN400x and IN540x series rectifier diodes are suitable for low frequency. In the high-frequency switching circuit, because the reverse recovery time is not short enough (simply understood as from the forward conduction state in the first half to the reverse cut-off state in the second half, it can't keep up with the frequency conversion of alternating current in time, and vice versa), the rectification efficiency is not high, and the lamp tube is easy to heat up. In high frequency switching circuits, it is best to use fast recovery diodes, such as FR series; Or high-frequency high-efficiency rectifier diodes, such as HER series.
Let's briefly analyze the control principle of output voltage.
Figure 2- 14? Output voltage control principle
Because the current flowing through the choke L 1 is continuous, there is
△i2=△i3
Then the following formula holds: ton * (v2-v0)/l1= toff * VO/l1.
After completion, the output voltage Vo can be expressed as:
VO = ton * Vs/(ton+toff)= ton * f * Vin * Ns/Np
Where: switching frequency f= 1/(ton+toff).
The above formula accurately expresses the working principle of forward converter. It shows that the average value of transformer secondary terminal voltage Vs can represent the output voltage Vo. Therefore, if the frequency is fixed and the input voltage Vin decreases, ton will become wider; On the contrary, if Vin increases, toff will become narrower, so Vo can always remain the same.
In other words, within 1 cycle T=tON+toff, by changing the ratio of the on-time of the transistor, that is, the duty ratio:
D=ton/T=ton/(ton+toff)
Can play a role in controlling the stability of output voltage, which we call Pulse modulation (pulse? Width? Modulation, pulse width modulation).
If ton is constant, the method of controlling the duty ratio by changing the period or frequency is called Pulse frequency modulation (Pulse? Frequency? Modulation, PFM).
Figure 2- 15? Voltage frequency control mode