How to understand the switching AC-DC conversion in the power supply in an easy-to-understand way

“First, the AC/DC conversion of the switching method will be briefly explained. See the basic circuit on the right, and the waveform below.

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First, the AC/DC conversion of the switching method will be briefly explained. See the basic circuit on the right, and the waveform below.

Here, taking Japan as an example, the input voltage is set to 100VAC. This 100VAC is initially rectified with a bridge diode. This is full wave rectification. 100VAC is directly rectified, so the bridge diode must be high-voltage resistant. 100VAC peaks around 140V.

Then smooth it out with a capacitor. Capacitors must also be rated for high voltage.

In terms of the conversion principle, AC/DC conversion will be performed at this time, but since the conversion is converted into a DC voltage that can be used by a general DC drive circuit, there are still several steps that must be completed after that.

The high-voltage DC voltage converted by the rectifier and the capacitor is chopped (divided) through the ON/OFF of the switching element, and the electric energy is transmitted to the secondary side through the high-frequency transformer. The ON/OFF frequency of the switching element, that is, the switching (chopping) frequency, for example, the frequency is 50kHz, which is much higher than the original 50/60Hz. In addition, the chopped DC voltage will be AC ​​that exhibits the square wave shown above.

The high-frequency AC voltage is rectified by a rectifier diode on the secondary side, smoothed by a capacitor, and converted to 12VDC if the DC output voltage is set to 12V. The rectified waveform of the high frequency AC voltage is omitted from the picture, but it is half-wave rectification using 1 diode.

To sort out the process and working conditions of AC to DC conversion, the primary side will directly rectify – smooth 100VAC, and convert it into a high-voltage DC voltage. Then, using switching elements, the high-voltage DC voltage is converted into a high-frequency square wave, and then sent to the secondary side through a transformer.

A high frequency AC voltage is generated on the secondary side, so it is rectified-smoothed again to convert to the desired DC voltage.

In this way, the 100VAC will be converted to the desired DC voltage, and a control circuit (control IC and feedback circuit) must be used to control the ON/OFF time of the switching element. Basically, AC/DC conversion using switching method is chosen.