Mechanism of AC-DC converter

・In Japanese
Premise knowledge
・Alternating current
・Transformer
・Rectifier circuit

■What is an AC-DC converter?

An AC-DC converter is a circuit that has the function of converting alternating current to direct current. A rectifier circuit is fine if it simply converts alternating current to direct current, but an AC-DC converter has the function of converting the voltage in addition to the rectifying function.

The application is to convert alternating current obtained from household electrical outlets and automobile alternators into direct current. The adapter that comes with electrical appliances is called an AC adapter, which is a general term for a device that extracts the desired direct current according to the device, and is the same as an AC-DC converter.

There are two main types of AC-DC conversion methods: transformer type and switching type.

■Transformer type

A transformer and rectifier circuit are combined as shown below.

■Switching type

Below is the circuit diagram of the switching type. The transformer type transforms the voltage and then rectifies it. After the switching type is rectified, it is turned on and off at high speed to generate a square wave, and then rectified again.

In this way, the switching type has a more complex circuit than the transformer type, but its advantages are that it is highly efficient and can be made smaller. You might think that the more complex the circuit, the more difficult it is to miniaturize it, but the transformer section (coil) occupies the most volume of this circuit, and the smaller the frequency, the larger the transformer is needed (The reason is explained here). The switching type can generate high frequency current by turning the transistor on and off quickly, making it possible to make the transformer smaller.

In the past, adapters that used a transformer type were large and heavy, but these days, switching type adapters are used and are smaller.

The disadvantage of the switching type is that the circuit is expensive because it uses transistors and requires a circuit that is not affected by noise during switching.

■Charger and quick charging mechanism

A charger has basically the same circuit as an AC adapter. As shown in the diagram below, the battery and AC adapter are connected in parallel, and will be charged when the battery voltage is low. In addition, high-speed charging is achieved by rapidly turning the switch circuit ON/OFF to increase the voltage. This method of controlling voltage by changing the ON/OFF interval is called PWM (Pulse Width Modulation) control.

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