Wednesday, October 28, 2015

FAQ-2 How does mobile phone or Laptop charger Work?

Again good evening and welcome back, in this Frequently asked question post I will try to answer questions that are asked by people very often so here is another post and this one is about how mobile phone and laptop chargers work.

Laptop and mobile phone charges are something  that we use everyday and they have improved a lot over the last couple years and main improvements include cleaner power and higher efficiency. Almost all of these chargers that we see today have switched mode design and they don't use line frequency transformer which makes it possible to be small in size but pretty powerful. So let's take an example and break it down how it works. In this particular post I will be talking mainly about a mobile phone charger but laptop chargers or even computer power supplies are pretty much the same these days, they utilize same technology.

So, in the picture what you see is a charger provided with the Asus Nexus 7 2013 tablet, this same charger is also provided with their first generation of Zenfone too.


So how does it work? Well first let's talk about input and output. If we look closely we can see that the

Input here is 100-240V AC 0.25A 50-60Hz, so it is global, which means it will work anywhere in the world as you might know some countries use 100-120V 60Hz and some other countries use 220-240V 50Hz in their household.
Let's take a look at the Output now, which is 5.2V 1.35A DC so you can pretty much charge any phone with it, time might vary according to that phone's battery and charging circuit.

So this wall charger is taking in 100-240V AC and converting it to 5.2V DC. Our question is How?

Well first stop,it takes in the AC voltage via the two prongs and then Converts the AC voltage to DC voltage with normal diodes.

Then the high voltage DC is filtered with capacitor, usually electrolytic type and capacitance depends on the power of the charger. It lowers the ripple voltage too.

Next this high voltage DC is fed into a circuit that converts it to a very high frequency and high voltage AC using components like integrated circuit, MOSFETs etc. The frequency of this AC voltage is about couple hundred kilohertz.

Then that high frequency high voltage AC drives a transformer (As the transformer of this charger is using very high frequency it doesn't need to be bulky).

On the other side or the secondary side of the transformer we get low voltage high frequency AC.
That AC is converted to DC via diode but not normal one, this time a schottky barrier diode. The reason for using this type of diode is mainly because it has to make sure that the conversion of AC to DC is good because normal diodes don't work that great with such high frequency.

Then again it is filtered with more capacitors, this time low ESR capacitor. ESR stands for equivalent series resistance, with lower resistance more AC component can flow through it which ensures that even if there is some of the high frequency harmonics available after the final conversion, it will be omitted. These capacitors also lowers the voltage ripple and keeps the voltage stable.

Hence we get the low voltage rectified DC which in this case 5.2V.

There are other things inside of these type of chargers like the output is monitored and a feedback is given to the High voltage DC to high voltage high frequency AC circuit to make sure that the voltage stays stable. Usually with low current consumption voltage tends to rise and this information is sent to the control unit which usually lowers the frequency of switching or the duty cycle to keep the voltage low as well and vice versa.






There might be some temperature sensors too, if the temperature goes beyond a specific value it might turn off automatically. 

There are also some X and Y capacitors for safety reasons. And there are some other capacitor to suppress the noise from AC main line.




This is how most modern laptop and mobile chargers work although there are some newer chargers that are "smart" as well. Smart in a sense that it can sense the device connected to it and might change the power profile. For example the new Asus Zenfone comes with a charger that can deliver 5.2V 1.35A DC in normal condition but if you connect it to a phone that can take up high amount of power it can deliver 9V 2A so in normal case it can deliver around 7W and in special case it can deliver 18W which helps the phone to charge their batteries faster. New Samsung phone also have this type of charger. So in this case there is some added circuitry in the charger that allows the charger to communicate with the phone or laptop, new Lenovo laptops also has this option too and upon sensing this the charger changes the power profile and switch to a different power profile and provide that. This option allows the charger to be used with all standard devices and at the same time it makes it possible to utilize faster charging for specified devices.

This is how modern chargers for mobile, laptop or router work, they are not as simple as the linear ones mainly because they need to be more efficient and compact. 


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Have a great day.

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