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  • Jun 18 / 2011
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Electrical, Electronics

Understanding a Mobile charger /AC-DC adapter

The ac-dc adapters of gadgets are very light and ultra portable now-a-days. They are used in mobile chargers ,DSL modem adapter, TFT LED adapter etc,

How does it work?

All electronics graduates know about the transformer based bridge rectifiers. But they have heavy transformers, working at 50Hz.(Well,we studied the half-wave/full wave rectifier from 10th,I guess)

The basic principle is that as the frequency increases the size of transformer decreases, and the required inductance can be reached by lower no of turns & better core materials are available, which has better permeability,and low hysteresis loss.

Every ac-dc adapter is a flyback or forward converter. There are isolated or non-isolated type converter.

From 230V AC, it is rectified to 230V DC(The peak value is 230*ROOT2) which is about 315V. A high voltage electrolytic cap, typically 2.2-10uF is used for filtering. This is enough for a bridge rectifier output at current about 10mA.

The difference between “different-makes” start after this stage.

Typically a ac-dc adapter works at 30-200kHz,which is generated from this 315V DC. So how this frequency is generated?

There are 2 methods;

1. Using a dedicated controller chip

An isolated AC to DC controller with dedicated controller

The dedicated controller chip will give complete isolation. That means, even if everything goes bad, your phone and you are safe safe. It separate the high voltage section and low voltage section in the circuit. A transformer provides isolation between high voltage ac and output dc in the power output path and an opto-isolator provides isolation in the feedback path.

2. Using fly-back circuit or RC oscillator for generating frequency

This is the insecure way to do the same. There is no isolation in the feedback path. Its a cheap solution as it doesn’t need any dedicated controller. But if some component fails, then it can harm everything, ie you and your phone. Use at your own risk.

What should we care, as a consumer?

Use only fully isolated charger – that means,branded are preferred

So that it will be safe, even if some thing goes wrong. Electronic components work for ever and ever IFF(if and only if),No over-voltage or No thermal issues. If a spike comes(That means if some one turns on simple motor pump or a welding machine , its enough to make more than 1000V). But the optical isolated feedback and transformers will protect the isolation to your mobile.

You may have heard of someone died out of using mobile phone while charging. They are non-isolated chargers.

Plug your charger first and turn it on, before you plug it to your phone.

Every oscillating circuit needs some time to get into stability. Give it a second,to get stabilized, before putting it into device. The Nokia’s new charger AC-8E has implemented it.It will not work if the power comes after the device.

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