Design Method of AC/DC PWM Flyback Converter

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Design Method of AC/DC PWM Flyback Converter [Copy link]

First, let's briefly explain switching-type AC/DC conversion. Please refer to 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 first rectified by a bridge diode. This is full-wave rectification. 100VAC is directly rectified, so the bridge diode must be rated for high voltage. The peak value of 100VAC is about 140V.

Then use capacitors to smooth it out. The specifications of the capacitors must also be able to withstand high voltages.

According to the conversion principle, AC/DC conversion will be performed at this time, but since it 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 afterwards.

The high-voltage DC voltage converted by the rectifier and capacitor is chopped (divided) by the ON/OFF 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 is the switching (chopping) frequency, for example, 50kHz, which is much higher than the original 50/60Hz. In addition, the chopped DC voltage will become AC showing the square wave in the figure above.

The high-frequency AC voltage is rectified by the rectifier diode on the secondary side, and after being smoothed by the capacitor, it is converted into 12VDC if the DC output voltage setting value is 12V. The rectified waveform of the high-frequency AC voltage is omitted in the picture, but it is half-wave rectification using one diode.

The process and working conditions of converting AC to DC are sorted out. The primary side directly rectifies and smoothes 100VAC and converts it into a high-voltage DC voltage. Then, a switching element is used to convert the high-voltage DC voltage into a high-frequency square wave, which is then transmitted 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 it into the desired DC voltage.

In this method, 100VAC is converted into 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, the AC/DC conversion using the switching method is chosen.

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