Understanding solar charger circuity - Page 1
Forum section: electronics / projects

author: TomS_
author_rank: Frequent Contributor
timestamp: May 16, 2020, 01:59:23 pm
content: Hi everyone. Im doing a little reverse engineering of a failed solar charger. Im not sure what has happened to it, but it doesnt seem to be charging any more.Curious about how it works, particularly on the "power side" of things I traced out some of the pins between the MOSFETs and power terminals and came up with the attached image.Trying to get my head around exactly how Q1 and Q2 are interacting.Q3 I assume could be PWM'd to achieve a particular load output voltage, or could simply be switched on hard to connect the load directly to the battery. There does seem to be a means to turn the load on and off via some buttons, so either of those.Q1 I can imagine might also be PWM'd to achieve a charging voltage for the battery from the solar input voltage.But other than using the reverse body diode in Q2 to provide a path back to the solar panel, I cant quite work out why there is another MOSFET in this position.The gates of all three MOSFETs do seem to be connected to the microcontroller in some way, through a convoluted maze of zener diodes, resistors and transistors that I havent quite been able to trace out, so it would seem they are all actively controlled in some way.Can anyone share any insight on what they think is going on?All MOSFETs are FBM75N68's.Thanks.

author: Peabody
author_rank: Super Contributor
timestamp: May 16, 2020, 03:40:35 pm
content: Is the battery lithium ion? If so, Q1 and Q2 could possibly be part of the protection circuit similar to the one shown in the DW01 datasheet, which is a low-side switching design. But if that's the case, the mosfet orientation is wrong. I think one of them would need to be reversed so there's no clear path through both body diodes. Anyway, that's just a guess. I've never done any solar stuff.

author: TomS_
author_rank: Frequent Contributor
timestamp: May 16, 2020, 04:12:45 pm
content: The charger supports a couple of different battery types, but I dont recall if lithium was one of them.It is supposed to have a variety of different protections (over current, reverse current, etc etc) so I suppose it could be something like that.In this design the two MOSFETs are connected drain to drain, so there shouldnt be a path through both diodes, one MOSFET will need to be switched on to create a circuit. That seems to match the topology used with the DW01. Will have to do some reading about that to figure out how it works.

author: PTR_1275
author_rank: Frequent Contributor
timestamp: May 16, 2020, 04:20:00 pm
content: There shouldn’t be any PWM on the load mosfet, they will turn it on and off hard. Normally as a low voltage disconnect to stop over discharge of the battery. Some higher end ones allow pwm of the load terminal to allow for load dumping.The dual mosfets on the PV side is there to stop any leakage back into the panels from the battery when the panels are shaded or it’s night time.Using a single mosfet will allow back discharge through the body diode, so they use 2 in the configuration you’ve described to overcome this.Pretty standard PWM regulator to be honest

author: TomS_
author_rank: Frequent Contributor
timestamp: May 16, 2020, 07:27:27 pm
content: Quote from: PTR_1275 on May 16, 2020, 04:20:00 pmYeah. Everyone selling these types of chargers seems to bleat about their supposed "PWM technology" that is meant to increase battery life yadda yadda.Not sure if you know, but are the two MOSFETs that are connected back to back switched simultaneously, or alternating somehow?I suppose Im wondering why Q2 couldnt be replaced with a stand alone diode.edit: after a bit more reading it would seem this also has elements of reverse polarity protection in it too. Jet lag is preventing me from wrapping my head around it at the moment.

author: mikerj
author_rank: Super Contributor
timestamp: May 16, 2020, 08:11:56 pm
content: Quote from: TomS_ on May 16, 2020, 07:27:27 pmYou could but this introduces a diode Vf drop into the charging circuit which will increase power dissipation for high charging currents and raises the minimum panel voltage at which charging will start. MOSFETs are pretty cheap, little reason not to use one.