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Switching Method (AC-DC Converter)

AC-DC Conversion Method

There is a transformer method and switching method for AC-DC conversion.
Here we will cover the Switching method.

Switching Method

Shown here is the switching circuit configuration of a typical AC-DC converter.

【Switching-Type Circuit Configuration Example】

Switching-Type Circuit Configuration Example

The figure below shows the transition of the voltage waveform using the switching method.
Unlike the transformer method, which first performs step-down AC-AC in the transformer block, with the switching method the input AC voltage is first rectified as-is by the diode bridge circuit. In the case of general households, this input is normally between 100VAC and 200VAC, requiring a diode bridge that can handle large voltages.
Next the DC waveform is smoothed using a capacitor. Similarly, a high-voltage capacitor is needed.
Subsequently, this high DC voltage is chopped (separated) by turning ON/OFF the switching element, then transmitted to the secondary side via step-down operation using the high-frequency transformer. At this time, the chopped waveform becomes a square wave.
The frequency of the switching element is higher than that used in households (i.e. 100kHz vs 50/60kHz). Increasing the frequency allows the use of smaller, lighter transformers.

【Waveform Transition (Switching Method)】

Waveform Transition (Switching Method)

On the secondary side, the stepped down square wave is half-wave rectified using a rectifier diode, then smoothed with a capacitor before finally being output as DC voltage.
In the switching system, a control circuit controls the switching element, making it possible to obtain a stable, predetermined DC output (i.e. 12VDC).
In contrast to the transformer method, the switching method requires a more complicated circuit configuration utilizing a switching element and control circuit, but high frequency control supports the use of smaller transformers, providing a distinct advantage by reducing set size.

Basics of AC/DC Converter and Design Procedures