Power supply knowledge summary: Understand the main control switch of the power supply

The main switch of the power supply has only two state modes, one is "open (conduction)" and the other is "off (non-conduction)". It is used to conduct and cut off the DC signal output from the filter capacitor. The pulse signal is formed by closing the switch, and the input voltage determines the amplitude of the pulse signal. The switching regulator controller adjusts the duty cycle. Therefore, the DC signal is converted into a square wave form and input into the main transformer of the power supply.

The duty cycle is the unknown ratio between the pulse duration and the pulse period.

After the DC voltage is adjusted by the main transformer of the power supply, it is output to the secondary side of the voltage (the output voltage is: +12V, 5V, 3.3V, 5VSB and -12V). Therefore, the transformer plays a very important role between the primary and secondary sides of the voltage.

When the main switch is on, the voltage across it is zero (theoretically); and when the main switch is off, the current across it is zero, which means that there should be no energy loss in the main switch.

However, this is just an ideal state. In reality, no transistor or MOSFET can achieve zero loss. For the transistor of the main switch, there is always a short period of time when voltage and current exist at the same time between turning on and off, so the product of voltage and current cannot be zero, which results in power loss, or "heat". Because all MOSFET tubes in the power supply are cooled by fans and heat sinks.