How Does Smartphone Charger Work?

We use our smartphone every day. But, have you ever wondered how your smartphone charger works? 

You would know that charger converts AC to DC, but it's not that straightforward. First, It converts AC to DC then again back to AC and then finally to DC. Today we are going to know how the charger does this, and why are there intermediate steps.

Construction Of Charger

Let us consider a normal charger that converts 220 volts AC to 5 volts DC.
Let's we know about every hardware components  used inside. There are diodes, capacitors, transistors and resistor, also there are resistors below the Printed Circuit Board  (PCB). There is also a transformer, and an optocoupler.

Arrangement of Components In Charger

Once power is supplied it turns on. To understand it better let's rearrange the circuit.

Now we can see all the components and connections. There are two wires used in circuit, red wire and black wire. The red wire is phase wire and the black wire is neutral.

First we have a resistor. By observing the color bands and reference table of resistors we can see it's 2.6 ohms. This is a fusible resistor which prevents damage from overloading. 

Now there is a bridge rectifier made by four of one n4007 diodes and a filter capacitor of 450 volts and 2.2 microfarad. This circuit converts AC to DC.

There is an oscillator circuit which converts DC back to high frequency AC of 15 to 50 kilohertz. We can see the values of the components written over them. Oscillating circuit consists following components:

There is a transistor s8050 and there is another transistor 13001,you can also see their pin configuration.

There is a diode which looks like Zener diode but it's a fast switching diode 1n4148 and a capacitor of 50 volts 22 microfarad.

There is AC to DC converter for the photo transistor in optocoupler, which forms a circuit.

There is the transformer which has three windings primary, secondary, and auxiliary winding wrapped around the core. It is used to step down the voltage. The auxiliary winding is used to run the oscillator circuit.

Then we have a Schottky diode 1n5819 with a capacitor of 10 volts 470 microfarad to convert AC to DC and a led for indication.

Also, there is a feedback circuit that consists of an optocoupler pc817c and 4.2 volt Zener diode.

There is an optocoupler which is used for transmission of signal without contact. On the right side we have an infrared led and on the left a photo transistor. When the led turns on, its light turns on the base of photo transistor turning it on.

There is another capacitor is of 102 nanofarad used for safety purposes. It is connected between primary and secondary grounds to stop electromagnetic interference.

Working Of Charger

Let's we turn on the circuit and see in action. The green wires carry the positive voltage and the blue wires carry the negative voltage or ground. Also, we can see the voltage in the circuit on the graphs. We have the input of 220 volts 50 hertz AC.

Now current passes through the bridge rectifier, it converts AC into fluctuating DC. Now the fluctuating DC filters from the capacitor and becomes almost pure DC. Now we can see we have as DC in the circuit.

Now this current passes from the 2 mega ohm resistor to the base of T1, turning it on. This transistor isn't fully turned on, because of the resistance it turns on partially. Due to partial turning on of the transistor, a low current passed from the primary winding of the transformer, this induces a low voltage in the auxiliary winding.

The induced voltage now charges the capacitor and then the capacitor fully turns on the transistor. As the transistor is now fully on it allows the current to flow through itself. Now this turns on the transistor T2, this shunts the base of the T1, turning it off. As the T1 turns off the flow of current to the T2 is cut off. Now the current flows to the base of the T1 and the cycle repeats. This happens at 15 to 50 kilohertz which is thousand times faster than the rectifier circuit. Hence, we can see that the rectifier circuit is stopped. 

At the same time the voltage from the auxiliary also turns the diode on and charges the capacitor and flows to the optocoupler. The diode and capacitor convert the AC from the auxiliary coil to DC for the optocoupler. The current is also induced in the secondary winding. This is converted into DC by a Schottky diode and a filter capacitor. It is indicated by the led. 

What if the voltage is more than 5 volts?

Now we, we have a feedback circuit. As we reach 4.2 volts, the Zener diode turns on allowing current to flow to the optocoupler. It also drops the voltage by 4.2 volts. Hence the led of the optocoupler doesn't turn on. The led requires 0.8 volt to turn on.

When the voltage reaches more than 5 volts this turns on the led of optocoupler. The light of the led turns on the phototransistor of the optocoupler allowing the current to flow to the transistor T2. This turns on the transistor T2 shunting the first and stopping the flow of current in the primary winding. Also, the voltage in the secondary side of transformer drops below 5 volts, turning off the Zener diode and optocoupler and the circuit continuous to run normally.

Why not directly convert AC to DC than this?

This is because for the normal power supply which is at 50 hertz, the size of transformer and the capacitors are large and they cannot be mounted in a small charger like this. Hence in the charger the 50 hertz frequency is converted to 50 kilohertz. This reduces the size of the transformer and capacitor required in the circuit. So to change the frequency of AC, first we have to convert it into DC and then again back to AC. Hope now you know how the charger that we use daily works.