The switch k is repeatedly turned on and off at a certa […]
The switch k is repeatedly turned on and off at a certain time interval. When the switch k is turned on, the input power source e is provided to the load rl through the switch k and the filter circuit, and the power source e provides energy to the load during the entire switch on period; When the switch k is off, the input power e interrupts the energy supply. 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 power supply must have a set of energy storage devices, which will store part of the energy when the switch is turned on. When disconnected, release to the load.
The circuit composed of inductance l, capacitance c2 and diode d has this function. The inductor l is used to store energy. When the switch is off, the energy stored in the inductor l is released to the load through the diode d, so that the load can obtain continuous and stable energy. Because the diode d makes the load current continuous, it is called freewheeling. diode. The average voltage eab between ab can be expressed by the following formula: eab=ton/t*e, where ton is the time when the switch is turned on each time, and t is the working period of the switch on and off (that is, the switch on time ton and off The sum of the off time toff). It can be seen from the formula that changing the ratio of the switch-on time to the duty cycle will also change the average value of the voltage between ab. Therefore, automatically adjusting the ratio of ton and t with the change of the load and the input power supply voltage can make the output voltage v0 stay the same. Changing the turn-on time ton and the duty cycle ratio means changing the duty cycle of the pulse. This method is called "time ratio control" (time ratio control, abbreviated as trc). According to the principle of trc control, there are three ways:
1. Pulse width modulation (pulse width modulation, abbreviated as pwm): The switching period is constant, and the duty cycle is changed by changing the pulse width.
2. Pulse frequency modulation (pulse frequency modulation, abbreviated as pfm): The width of the on-pulse is constant, and the duty cycle is changed by changing the switching frequency.
3. Hybrid modulation: The on-pulse width and switching frequency are not fixed and can be changed. It is a mixture of the above two methods.