DS2712 switch mode charger - Page 1
Forum section: electronics / beginners

author: Andrejka
author_rank: Contributor
timestamp: November 13, 2017, 06:56:47 pm
content: Hello,Can someone can consult me with DS2712 charging IC. Schematic was made as typical copy-paste from datasheet. Charger looks working, but transistors FCX718 are heating extensively, after 5 seconds temperature is so high, that is unable to holding finger on surface. All three transistors are affected with this problem.Maybe just heatsink is too small? Layout, schematic, and live view is attached.ThanksAndrej

author: Buriedcode
author_rank: Super Contributor
timestamp: November 14, 2017, 05:00:41 am
content: The schematic you provided is from the datasheet. I understand this is the one you copied, but perhaps it would be better to post the schematic you used for the layout - that way we can spot any possible mistakes you made there.When you say 'charger looks working', how have you tested it and what have you measured?As its a switching converter, there are a number of things that could be the problem, I would first checking the switching part (T3, and the output from L1/D2) to make sure the output is regulated to a sensible voltage, then the two pass transistors (T1, T2). Layout can be critical with switching supplies, but until we have *your* schematic, I don't think we can comment on the layout, although to me, it doesn't look to be much of an issue (no really long traces where the high current path is).If it isn't switching (T3 is left on) then the input to the pass-transistors (T1,2) would be almost 5V, and with a drained NiMH cell at 1.0V that means there will be 4v across each transistor - quite a lot if you're charging at any reasonable current. SOT89's IIRC can dissipate ~800mW on a board, which would limit the charging current to 0.8/4 = 200mA, assuming the input is 5V.So, to check:waveform on the switch node (D1/L1 connection).Voltage on the output of the buck converter (L1,D2).Current through T1 and T2.Also, clean up some of that flux, whilst it probably doesn't hurt it may hide shorts.

author: Andrejka
author_rank: Contributor
timestamp: November 14, 2017, 07:40:35 am
content: Thanks for answerI will check all these things at evening. But does not look strange that in pulse mode transistor base current is approx. 200ma (when collector 2,5A) - from data sheet. But DS2712 outputs only provide 20mA current, so transistor is not fully open.Andrej

author: Andrejka
author_rank: Contributor
timestamp: November 14, 2017, 06:34:59 pm
content: So attached,Waveform on the switch node (D1/L1 connection)Voltage on the output of the buck converter is approx 2,8VCurrent through T1 and T2 is approx 2Amps (in alternating mode, at one time only one transistor is conducting)Andrej

author: Buriedcode
author_rank: Super Contributor
timestamp: November 14, 2017, 08:17:48 pm
content: Well it seems its switching, so thats something. Also I do not see your schematic, which should also include part values.If the charging current is indeed 2 amps, and the output of the switching section is 2.8V, then if you are charging a single NiMH, the voltage across the pass transistors is ~1.8v, at 2 amps that means each transistor is dissipating 1.8x2 =3.6W peak when on. In alternate mode, assuming one transistor is on at all times, that's 1.8W per transistor, which is pretty much the maximum power dissipation of the SOT89 packages with a large copper pour. No wonder they are getting hot. So it seems its doing its job just fine, just requires better heat sinking on those transistors.With that said the switching section appears to be current controlled - it isn't really a true switching converter just uses a comparator and transistor/inductor/diode to keep the current relatively constant, but I would have thought the output voltage of this would be the cell voltage plus the saturation voltage of the pass transistors, which means about 100mV higher than cell voltage, rather than 1.8V.What is the waveform of Ichg?I would halve the charging current to 1A (which would mean each cell charges at 500mA) and check the temperature.As to the base current being 200mA rather than 20, I have no idea why that is the case.

author: Andrejka
author_rank: Contributor
timestamp: November 15, 2017, 06:08:47 pm
content: Thanks for suggestions!Schematic is as from datasheet with same components values. I only added EMI choke in power input and one additional decoupling cap.Voltage waveform across current sensing resistor is attached (when charging one cell), little bit noisy, but for 0,056ohm resistor there is up to 100mv swing.I also thinking that this datashet schematic is only for "educational" purposes and in practice transistors working beyond their limits. So next I plan to rebuild circuit with P-MOS transistors and limit charging current to 1A.ThanksAndrej