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Since my previous charger said farewell today, I have decided to build a 48V 100A solar PWM charger (for LFP cells, ie. CC-CV) myself. I don't need MPPT because I have already well-balanced PV and battery voltage. I would have to operate an MPPT charger for decades until the additional cost/effort for MMPT pays off through its modest efficiency gain.

So I would like to educate myself about PWM chargers. In principle the charger's job seems pretty straightforward:

  1. connect the PV modules directly to the battery if its voltage is below its final value (CC mode)
  2. disconnect the battery as soon as final charging voltage is reached
  3. reconnect the battery with some hysteresis if it falls slightly below the upper limit (CV mode)

It is simple enough so that I could do these steps by manually operating the main battery switch, but even though my time is not too precious, I would like to spend some time with my family every now and then. You get the idea, I don't require 1 MHz switching frequency.

Choosing a microcontroller and an appropriately sized MOSFET doesn't seem to be too difficult. Soldering the latter to big copper bars or mounting them to an even bigger heat sink, neither. Yet I would still need some education about how to properly control a 10V MOSFET from a 5V micro (sadly, there don't seem to be too many logic-level MOSFETS at that performance level). Over-/undervoltage protection would not be an issue, because I already own a separate BMS. The only safety thing I would strongly prioritize is that the charger does not burn my house down, nor that it triggers my neighbour's love toys.

Is there a single source (book, website) where I can find that basic knowledge about a PWM plain vanilla "reference design" with appropriate electric safety?

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  • \$\begingroup\$ It seems like you've got the basic idea already. I am not sure how well this site handles questions of safety. \$\endgroup\$
    – user253751
    Mar 9, 2023 at 22:25
  • \$\begingroup\$ Requests for off-site resources are not allowed due to link rot. \$\endgroup\$
    – user253751
    Mar 9, 2023 at 22:26
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    \$\begingroup\$ If you use PWM, you can add a coil + diode and you have a much better solution. If the voltage of the PV-module is higher than the voltage of the battery, you build a step-down voltage regulator. If it is lower you build a step-up regulator. But mostly they use a voltage inverter, so that it is possible to use N-Channel MosFETs with a cheap driver. If you have high a current flow, you need a fast driver. There are simple driver possible or as a little chip. If you use a shunt + OPV, then you can measure the current and with an Arduino/STM32 you can build a good regulation. \$\endgroup\$
    – MikroPower
    Mar 10, 2023 at 0:03
  • \$\begingroup\$ I have seen the coil and diode arrangement, but to be honest, I did not understand it. Is it for damping transients? My inverter has a big input capacitor (because I can see that it still runs for several seconds after switching off the battery), and wouldn't that render additional transient treatment unnecessary? That is one example I would be happy to read about. I can't imagine that everyone who builds a PWM controller does/learns everything from scratch. If links aren't allowed in an answer, maybe it is in the comments... \$\endgroup\$
    – oliver
    Mar 10, 2023 at 21:56

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I have decided to use this design as a proven starting point of my endeavor. Of course this is only designed for 6V/12V, so the first thing to do will be replacing the MOSFET by one that is capable of the 74V open-loop voltage of my PV array. Furthermore, the MOSFET need to be capable of carrying at least the 78 A (@MPP, currently for my 5 kWp array) or better 156 A (preferrable if I want to double my PV power to 10 kWp later on, my PV cables are already 50mm2). If the MOSFET has RDS(on)=2mOhms, the maximum loss (at 156 A) will be ~50 Watts, hmm... quite challenging, I think, but still only 50W/10000W=0.5% inefficiency, which would be rather good I think. Of course, normal PCB traces will be too thin for carrying 156 A, so drain an source should probably be soldered directly to a conductor bar. I have opened another question about that point.

I won't need the load control circuit (one of the, presumably expensive, MOSFETs can be saved), because I have a BMS for worst case, and an inverter which will turn to grid if battery falls below a threshold voltage. The inverter will be connected directly to the battery, so the charger will be a charger only.

I am not so sure about the driver BJT for the MOSFET, because it acts over 10kOhms impedance, which looks a little generous even if I don't need lightning fast switching. I need to do a calculation/research as to what that means for maximum switching frequency vs. power dissipation.

Placing a hall sensor near the conductor bar for measuring current and gauging it will not be a major obstacle, I guess.

One remaining challenge will be the Schottky reverse blocking diode, which would be dissipating another 1V*156A=156W (good in Winter time for heating the installation room, not so good for thermal management and charging efficiency). Maybe I should rather use an ideal diode circuit for that, but I am a complete noob at that. I imaging that it'd at least require another MOSFET for that, so a point left open to me is if I can just cleverly rearrange the one that is already in the circuit for the reverse protection function.

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