Montag, 25. April 2016

Switching charger ICs - for higher charging rates

Next to LDO based charging ICs and usual desktop chargers there is a class of LiIon charging ICs that don't burn up energy, but efficiently put them into the LiIon batteries. Since I charge from USB and work with prototype board, there are only very few chips available that I can handle with conventional soldering.


These are CN3761 and EUP8202 which you can find cheap in low quantities on aliexpress. As I wondered how they behave compared to the more popular linear chargers like TP4056, LTC4054 and so on, I built a small board with each and tested their charging curves. Due to the necessary coils and shunt resistors as well as switching transistors, they need significantly more space.

Another advantage over the LDO based chargers is the higher current which these switching ICs can provide.

The charging curve of the CN3761 looks ok and shows that the IC has no problems using higher charging rates. During constant current phase the current drops significantly already, but a switch-over to constant voltage mode is clearly visible. It stops charging at ~1/5C.

This EUP8202 curve starts off at low charging rate. The spikes are from moving USB cables and only affect the CC charging phase.
This charger has some peculiarities: The circuit according to the data sheet isn't stable. I needed to add 47µF at Vcc and at the battery so the current set via shunt resistor is met.
Also a weird thing, but this is documented in the datasheet, is that the charger switches to "near end" at about 450mA charge current (1/3-1/4 of current set via shunt; 25µA on the LED which then is very dim) and then charges all the way up to just about 5mA before completely turning charging process off. This leads to a real full cell at 4.218V (no problem with that), and after disconnecting the voltage won't drop fast as usual. So it is safe to use, it switches off, but cells can be used already when the LED is dimmed.

These switching chargers are interesting as they allow for higher charging currents even in USB mode and don't produce much heat compared to the LDO based charging ICs.

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