In order to test a Li-Ion battery charger on our board (the BQ24074) I built a battery simulator circuit which was initially designed by TI. The circuit "pretends" to be a battery and can provide different voltages to study the behaviour of a battery charger. I created a PCB with the following schematic:
I'm providing a variable voltage to the VIN input using an LM317 regulator. I connect the BAT output to the battery charger like suggested in the application note of TI. I've tried to measure the battery charging cycle of the BQ24074 PMIC, but while testing I came across a few problems regarding the power supply for the battery simulator.
Since I don't have a dual voltage supply to provide +/-12 V I tried to build a voltage rail splitter. I use a 24 V supply and the following circuit I found online and in other posts:
I first tested the battery simulator without connecting the battery charger. I measured the expected +12 V and -12V on the voltage rail splitter. I also measured the expected VBAT output voltage of the battery simulator. Until now everything is working fine.
When I connect the battery charger and actually start the real test I notice that Q1 (BD239) of the rail splitter gets very hot! It ramps up to 100 degrees celsius and I'm forced to turn off the 24 V supply. The battery charger has been setup with a charge current of 450 mA. From the datasheet of the BD239 I see that the collector current (Ic) is 2 A, so it should be able to handle this current?
I was actually using a BC548 for Q1 of the rail splitter but I've already burned 2 of them while testing. What is the reason that this NPN transistor is getting so hot and burns? Does it have to sink that much current from the battery charger? I have tried adding some schottky diodes to the +/-12 V lines to prevent potential back powering but it didn't help.
I haven't been able to solve this problem and was thinking of just using an old PSU to provide the circuit with +/-12 V. Any suggestions?