Analysis of output waveform of switching power supply
Switching power supply is a kind of power supply that uses modern power electronics technology to control the time ratio of switching tube on and off and maintain stable output voltage. Switching power supply is generally composed of pulse width modulation (PWM) control IC and MOSFET. With the development and innovation of power electronics technology, the switching power supply technology is constantly innovating. At present, switching power supply is widely used in almost all electronic equipment because of its small size, light weight and high efficiency, and it is an indispensable power supply mode for the rapid development of electronic information industry.
Composition of switching circuit
Switching power supply consists of four parts: main circuit, switching power supply control circuit, detection circuit and auxiliary power supply.
1. Main circuit
Impulse current limiting: limit the impulse current at the input side at the moment when the power supply is turned on.
Input filter: its function is to filter the clutter existing in the power grid and prevent the clutter generated by this machine from being fed back to the power grid.
Rectification and filtering: the AC power supply of the power grid is directly rectified into smoother DC power.
Inverter: Convert the rectified DC into high-frequency AC, which is the core part of high-frequency switching power supply.
Output rectification and filtering: provide stable and reliable DC power supply according to load requirements.
2. On the one hand, the control circuit samples from the output end, compares it with the set value, and then controls the inverter to change its pulse width or pulse frequency to make the output stable. On the other hand, according to the data provided by the test circuit, the control circuit is identified by the protection circuit to provide various protection measures for the power supply.
3. The detection circuit provides various parameters and various instrument data in operation in the protection circuit.
4. The auxiliary power supply realizes the software (remote) startup of the power supply and supplies power for the protection circuit and control circuit (PWM and other chips).
Analysis of output waveform of switching circuit
(1) CCM and DCM mode judgment basis
The judgment of CCM and DCM is not based on whether the primary current is continuous or not. But according to the current synthesis of primary and secondary. As long as the primary and secondary currents are different and zero, it is CCM mode. And if there is a state in which the primary and secondary currents are zero at the same time, it is DCM mode. In between is BCM mode.
(2) the difference between the two modes in waveform.
● Transformer primary current, CCM mode is trapezoidal wave, while DCM mode is triangular wave.
Low secondary rectifier current waveform, CCM mode is trapezoidal wave, DCM mode is triangular wave.
● Vds waveform of ●MOS, CCM mode, Vds remains on the platform of Vin Vf until the next cycle is opened. In DCM mode, before the next cycle is opened, Vds will descend from the platform of Vin Vf and have damping oscillation. (Vf secondary reflected to primary voltage). Therefore, we can easily see from the waveform whether the flyback power supply works in CCM or DCM.
