# How does this charger work?

I just got this charger circuit from my broken charger. I would like to know about the circuit of the charger.

When you see the circuit, the AC line is directly connected to a bridge rectifier which will convert AC to DC. It goes to a step down transformer.

What I don't understand is that transformers only work on AC but the DC is going to the transformer in this circuit.

I found a sample charger circuit where the rectifier is attached at the output of the transformer.

This should be a normal way but how does my charger's circuit work?

Is it possible to put DC in to a transformer input?

Modern power supplies typically don't use the traditional transformer->rectifier->linear reg approach as the mentioned schematic.

Instead a switching regulator (SMPS) is used. When converting from mains voltage to something like 5V the SMPS typically rectify the mains voltage directly, creating around 300 VDC (for 230V input).

Then this is chopped up at high frequency using a transistor, and (the high frequency AC) is fed into a transformer, rectified (again) and filtered into DC. The duty cycle of the switched voltage is regulated to generate the exact voltage output, eliminating the need for a regulator after the transformer.

This is done mainly because transformers for high frequency can be made much smaller and thus, cheaper than the traditional bulky 50 Hz transformers.

The device has a switch mode power supply and your drawing shows a diagram of a linear power supply.

They are two completely different power supply topologies.

In a switch mode supply, there is no DC sent to transformer, the high voltage DC is converted to high frequency AC that is sent to transformer which enables the use of a small transformer.

The linear power supply has mains frequency AC in which requires a larger transformer.

I would like to know about the circuit of the charger.

It's called a flyback converter similar to this design from power integrations: -

What I don't understand is that transformers only work on AC but the DC is going to the transformer in this circuit.

The rectified DC is being "chopped" into AC by U3 in my reference circuit above. This is done at high frequencies and ensures that the transformer primary receives an AC voltage.

Is it possible to put DC in to a transformer input?

Well, yes but it doesn't get you very far because the magnetic core will rapidly saturate. Anyway, a flyback transformer circuit doesn't work like this.

Alternative design from here: -