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Smartphones are now essential part of our life. If you see these smartphones over the last one decade, then they got bigger and more powerful over the time and to charge these phones, today a fast charging has become the new norm. There was a time when even 10W charging was considered as the fast charging. Today, even 100W chargers are readily available for smartphones.
In this article we will understand, what is fast charging and how this technology has evolved over the years. We will also see different charging solutions or the charging technology which is provided by the different manufacturers. But before that, first of all, let’s understand the basics of battery charging. So, in the smartphones, the Li-ion or Li-polymer batteries are used. The nominal voltage of this Li-ion battery is in the range of 3.6 to 3.7V. While the maximum charging voltage varies from 4.1 V to 4.3V. The capacity of the batteries is measured in the mAh. For example, for one Li-ion battery, if the battery capacity is 4000 mAh, it means that it can provide 1A current for the 4 hours. On the other hand, if we draw 4A current from this battery then this battery will last for1 hour. But we cannot draw an indefinite amount of current from any battery. So, the maximum rate at which these batteries can be charged or discharged is measured in terms of the C-rate. For example, for some 4000 mAh battery, if the maximum discharging rate is 1C, it means that we can draw up to 4A current from the particular battery.
On the other hand, if the maximum charging rate is 0.5C, it means that this battery can be charged at the maximum rate of 2A current. So, typically, these Li-ion batteries can be charged at the rate of 0.8C to 1C. But the fast-charging batteries can be charged at even higher C-rates like 1.5C or even 2C. So, these Li-ion batteries can be charged in the two phases.
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Constant Current Phase:
One is the constant current phase and the second is the constant voltage phase. In the constant current phase, the battery is charged with some predetermined constant current. And during this time, the voltage across the battery gradually increases. Once the voltage reaches the maximum charging voltage, then the battery starts charging in the second phase.
Constant Voltage Phase:
So, in this constant voltage mode, the voltage across the battery is maintained at the same level but now the battery current reduces. This phase continues until the battery current reaches the charge termination current. Once the battery current reaches this charge termination current then we can say that now the battery is fully charged. So, this is another graph which shows the same thing.
Now, during the first phase itself, the battery gets its 70 to 80 percent of the full capacity charge. In the second phase, it gets the remaining charge. But this second phase can be as long as the first phase or even sometimes it is longer than the first phase of charging. Because in this phase of the charging, the battery or the Li-ion cell is not able to take the charge as fast as it used to take in the first phase of charging. This can be understood with the analogy of the sponge. So initially, when the sponge is dry then it can soak the water easily. But after one level, it will absorb the water very slowly. If we try to pour too much of water into it, then the water will spill out. The same thing also happens in the Li-ion batteries.
In fact, all the fast charging technologies take advantage of this first phase. And by increasing the battery current in this first phase, it is possible to charge the batteries from 0 to 50% even in 15 to 20 minutes. So, broadly the current fast charging solution or the fast charging technology can be classified in the two categories. In the first category, the phone is charged with the higher voltages, while in the second category, the phone is charged with the higher currents. But to actually understand, how this fast charging technology works, and to appreciate that technology, let’s briefly see how the smartphone charging technique has evolved over the few years.
So, if you see the standard USB 2.0 port, then it can provide 5V with the maximum current of 0.5A. That means it can provide a maximum power of 2.5W. And it is enough for small batteries.
But by altering this D+ and D- lines, it is possible to get the more current. For example, in this DCP, or the dedicated charging port, this D+ and D- lines are shorted together. So, this DCP port can provide 5V with a maximum current of 1.5A. The maximum power which can be provided by this DCP port is 7.5W. Now, when the smartphone is connected to such a charging adapter, then they used to communicate with each other using the communication link. When the smartphone finds that, this D+ and D- lines are shorted together, then it knows that this USB port is the DCP-port. It can provide 5V with a maximum current of 1.5A.
The Samsung and other companies manipulate this D+ and D- lines with their own circuitry and with this manipulation, they can provide even more current on the same USB cable. For example, with such manipulation, it is possible to provide 5V at 2A current. So, when the Samsung phone is connected to such a Samsung adapter then by sensing this D+ and D- line, the smartphone gets to know that, it is the Samsung adapter. This adapter can provide a maximum 5V at 2A. So, for this fast charging to work, the smartphone, the cable, as well as the charging adapter should be compatible with each other. If any of them is not compatible, then the smartphone will charge at the lower charging rate.
So, inside the smartphone, there is a switching regulator IC, which regulates the input bus voltage and the current and provides the required current and the voltage for the battery. Now, at the low voltages, these buck converters are highly efficient and provide good efficiency. So, with this configuration or with this buck converter IC, if we want to charge the battery at higher currents, for example at 3A or 4A, then this micro USB cable becomes the limitation. Because the maximum current limit of the micro USB cable is the 2A. So, to charge the batteries at the higher currents, the bus voltage needs to be increased. For example, with 9V of bus voltage, the same USB cable can charge the phone at the 18W.
So, inside the smartphone, this 9V of bus voltage is down-converted to the required battery voltage, and at the same time, the current is also increased accordingly. For example, Qualcomm’s quick charge or even Samsung’s adaptive fast charging technology charges the phone with the higher voltages. And if you see this Qualcomm’s quick charge 2.0, then it also supports different charging voltages. That means Quick Charge 2.0 supported adapters can charge the phone at the different charging voltages. So, when the Quick Charge 2.0 supported phone is charged with such an adapter, then by default, this charging adapter puts 5V on the bus line. The adapter communicate with the smart phone on this D+ and D- line, and it can request for the higher voltage. But at higher voltages, the efficiency of the buck converter reduces. Because at the higher voltages, the difference between the battery voltage and the bus voltage will increase. So, because of that, excessive energy is wasted in terms of heat. The smart phone overall temperature will be increases. The increase in temperature will increases the charging time and will reduces the battery life. So, this issue can be handled to some extent, by using the two buck converters in the master-slave configuration. So, in this case, these two converters charge the battery parallelly.
In this way, the battery current gets shared between the two converters. And Qualcomm’s dual charge technology is one such example. Moreover, from Quick charge 3.0 onwards, the Qualcomm also supports the variable bus voltage. Where charging IC inside the smartphone can intelligently negotiate the required optimum power for the charging. So, in Quick charge 3.0, the bus voltage can be changed in the step of 200 mV, while in the Quick charge 4.0 onwards, it can be changed in the step of 20 mV. it is possible to increase the overall charging speed of the battery and also improve thermal performance. So, this USB power delivery is another popular USB standard, which supports fast charging. So, this USB power delivery supports different power profiles. The current will increase upto 5A with this arrangement and it is also possible to increase the bus voltage up to 20V. The requires special type -C cables will be require to draw 5A current we.
So, when the USB- power delivery certified charger is used to charge the smartphone then, first of all, using the channel configuration pins, that is CC1 and CC2, the device communicates with the charger. And by default, the charger starts with the 5V/2A profile. And along with that, it also shares the different supported profiles by the charger. It can request for specific current and voltage from the charger if it supports USB- power delivery. Moreover, with the new revision, this power delivery also supports the programmable power supply. And this programmable power supply or this PPS capabilities allows the small step-wise change in the bus voltage and the current. So, if the phone is connected to the PPS capable charger, then it can request the charge to make the small changes in the voltage and the current. This feature is an effective way to reduce the conversion loss during the charging. So, as I mentioned earlier, when the battery is charged with this high voltage on the bus, then there is a conversion loss in the buck converter. And with this PPS capability, it is possible to directly charge the battery by bypassing this switching converter. That means when both charging an adapter and the smartphone supports this PPS protocol, and if the charging IC in the smartphone also supports this direct charging then it is possible to bypass this converter and it is possible to directly charge the battery from the bus voltage and the bus current. So, in this configuration, the charging IC can ask for the required voltage and the current to charge the battery. And based on the request, the charging adapter puts the desired voltage and the current on the bus line. But with this technique, the battery can be charged at the most at 3A. Because this USB type-c cable supports up to 3A current. But using the 5A type-c cable, it is possible to charge the battery at even higher currents. Now, modern smartphones use the charge pump circuit inside the smartphone to boost the battery current. So, this charge pump is basically a capacitive divide by 2 circuit, which divides the input voltage to the half and doubles the input current. And not only that, they are also very much efficient. So, using this charge pumps, it is possible to use 3A type-c cable, and still, it is possible to provide 6A current to the battery. Some companies like Oppo and Oneplus charge the phone with the higher currents. So, here the bus voltage is kept at the 5V but now the bus current is increased. And for charging the phone at the higher currents, they are using the proprietary cables. Now, if you see their 65 W charging solution, then charge the phone at 10V with a maximum current of 6.5A. So, inside the smartphone, they use two Li-ion cells instead of a single cell. And here these two cells are connected in the series connection. And this 6.5 A current is provided to these Li-ion cells which are connected in the series connection And if you see their recently launched 125W charging solution, then they charge the battery at 20V, with 6.25A current. But here using the charge pump, this 20V/6.25A is scaled down to 10V/12.5A. And then using the charging IC, it is applied to the series connection of the batteries.
So, in this type of charging solution, as most of the conversion happens outside the smartphone, So the smartphone won’t get hot while charging. But the charging adapters might get hot during
the charging. But nowadays, these charging adapters are based on the Gallium Nitride. So, this Gallium Nitride is more efficient in transferring the current. And because of this more efficiency, less heat will be produced during the charging. And because of the less heat, it is possible to reduce the size of the charging adapter. So, if you want to know more about this Gallium Nitired based charging or even about the USB power delivery, then let me know in the comment section.