USB Ni-MH Battery Charger Part 2-The Current Source

Sunday, March 11, 2018

USB Ni-MH Battery Charger Part 2-The Current Source

So I’ve looked around on the Web for a suitable design for my controllable high-side current source. I would like it to be as simple and cheap as possible. The first one is the usual Op Amp and FET combination current source circuit. But to power up an op amp using USB 5V I would need a rail-to-rail type, which is not that cheap. I stumbled upon this design. The answer also improved the initial design by adding a diode.

The idea is that, the voltage drop across R10 will be one diode drop, ~0.7V, so the current will be determined by the resistance of R10. Q1 is the ‘switch’ to control the current source from BQ2002. Q2 will adjust Vce accordingly to pass the current determined by R10. This also means that Q2 will have a big voltage across CE junction. 5V minus ~1.6V from battery, and 0.7V across R10, there’s still 2.7V across Q2, and we are passing around 0.7A. So that’s ~2W of power loss on Q2. This is not an efficient design, but it’s the simplest and easiest to be prototyped with my current stocks.

I needed to try this before buying any Power PNP. I used a configuration known as Quasi Complementary Pair to try my circuit. With a jellybean PNP and a Power NPN, they act as a Power PNP transistor. The circuit was commonly used during the old times when Power PNP had lower quality due to process technology. Anyway, my circuit worked, and I proceeded to buying a couple of TIP32 (as they are the most common parts and the cheapest….) and some BQ2002.