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.

FAQ-1 Battery Capacity mAh, Ah, Wh

Good evening everyone, although it is about 3 a.m. but whatever I will be starting a new segment where I will try to answer questions that are often asked. So the first one is a bit about battery capacity. Many people ask me about it and I see many people ask someone else about it so I thought why not just try and answer it simply! So here it goes!

mAh

Let's talk about mAh first because you will see this unit very often. Your mobile phone's battery, laptop's battery or portable media player's battery might have this rating printed on it. So what does that mean? Well mAh has two parts one is the mA or Milli-ampere and the second part is hour so in total it means how much current can the battery provide for an hour. For example if you have a cell that has a rating of 1000mAh it means if you draw 1000mA from it it will last for an hour. Theoretically it also suggests that if you draw more, say 2000mA from it, it will just give you 30 minutes and if you draw less, say 500mA it will give you 2 hours. So, I said theoretically why? Because battery has internal resistance which can be realized as a resistor in series with a power source, so more current you draw more power will be wasted on the resistor as we know that power dissipated on the resistor is equal to the value of resistance multiplied by the square of the current that is flowing through it. So more current, more waste and that's the reason why batteries heat up with higher current draw. There is one more thing, as I said earlier a 1000mAh cell can deliver 1000mA for one hour, does that mean it can provide 60000mA/60A for a minute? No, not really. Why? Because as I said there is an internal resistance that can be assumed as a series resistance with the source, so it will limit the current and each battery chemistry and construction has it's limit so 1000mAh does not indicate it can provide 60000mA/60A for even a minute.

Ah

Next stop Ah, there is not much to say about it really because it is almost the same as mAh which you might have guessed. The reason for using this unit is sometimes for powerful batteries if you need to express it in mAh, the numerical digit might become huge for example if you think about a car battery it is much easier to write 100Ah rather than writing 100000mAh. So to make life a little easier this unit is used with batteries that has higher capacity. Even for slightly smaller cells too like a normal 1200VA computer UPS usually comes with two 7Ah battery.

Wh

So far I have talked about mAh and Ah which are used all over the world for understanding battery capacity but there is one more unit, Wh or Watt hour. Why do we need another one? Well let's break it down, this unit has two parts like the other two, one the Watt or unit of power and other being the hour or unit of time. So Wh will simply tell you how much power you can get out of a battery for one hour but why do we need that? Because the mAh/Ah doesn't give you any indication of power. Let's think about it, how much power can a 1000mAh battery provide? Sadly we can't answer that question because power is the product of voltage and current and in this case we don't know the voltage so we can not answer this question. Another problem is if someone ask you that are all 1000mAh battery the same? Again we don't know the Wh so we can not tell. Let's look at an example, say a NiMH cell has a capacity of 2000mAh and a lithium ion with the same capacity, 2000mAh, are they same? No because the lithium ion can deliver more power than the NiMH because lithium ion has a nominal voltage of 3.7V where the NiMH has 1.2V so the Lithium ion will have 7.4Wh as opposed to the 2.4Wh of the NiMH. Let's look at an interesting thing here, even if I assume the lithium ion to be of 1000mAh capacity, it will still have 3.7Wh rating where even with 2000mAh rating the NiMH has only 2.4Wh rating. So recently many devices comes with Wh rating too, like the Lenovo laptop that I'm using right now has a 54Wh battery.

Like mAh and Ah there is a smaller unit of Wh too, which is mWh essentially the similar thing and I guess you know the simple math already, 1000mWh is equal to 1Wh.

If you don't have the information about voltage say you only know about the mAh rating and type of the battery, can you determine the Wh rating? Yes in many cases you can. Let's look at the chart to understand this better.

The table itself is self explanatory I think, so if you know the battery type and mAh/Ah rating you can easily find the Wh rating using this table. There are many other types of batteries out there but these are the most common ones so to keep things simple I have only mentioned them.

1. Ampere Hour
2. kilowatt Hour
3. Battery type Comparison