# Using switching to control load sharing of DC?

I'm not sure of the right terminology for what I am trying to ask about. Feel free to suggest a better question title.

Lets say that I have a fully charged LiFePO4 battery, which I do not want to charge any further, because it can damage the battery (e.g. float charging).

But the sun is shining, and the MPPT CC can provide 400w of energy, configurable at any voltage.

I want that the load draws as much power as possible from the CC, and the rest from the battery. I think the CC will drop the voltage when it doesn't have sufficient power, until it reaches an equilibrium voltage where the CC voltage is high enough to be the main source of current, but low enough that the battery provides the remaining current.

That solves the problem of load sharing with priority. I just set the CC voltage higher than the battery.

But because I don't want charging to occur, I either need to set the voltage lower (e.g. 12.7), or...?

I was thinking that perhaps I could set the voltage at 14.0V and switch the PMOS at a high frequency, and adjust the pulse-width so that current never flows into the battery (reverse polarity over the shunt). Without load, the pulse-width would be zero. With full load, the pulse-width would be 100%, the CC would automatically reduce its voltage while providing as much current as possible. But I'm also worried that some oscillation would happen, which would alternate between charging and not charging the battery. Perhaps I could program the controller, using averaging, so that an equilibrium state can emerge, with a pulse-width that doesn't allow any current to flow into the battery.

Is my thinking correct?

simulate this circuit – Schematic created using CircuitLab

• Uh is that a flaw in your circuit diagram? The gate is connected to the battery and the drain, and the mcu. That's a short. – Bradman175 Jan 2 '17 at 23:33
• I was trying to keep it simple, but I'm starting to realize that people on this forum prefer the full schematic :) – user95482301 Jan 2 '17 at 23:41
• I might also consider using an NMOS on the MPPT CC negative side, and using a driver circuit. But the question I'm really interested in, is whether the overall approach is correct. – user95482301 Jan 2 '17 at 23:48
• Yea ppl here like a proper schematic. Is the way the PMOS around intentional? The base of the BJT is basically a SHORT. And why are you referencing the gate of the pmos after the pmos? – Bradman175 Jan 2 '17 at 23:55

## 1 Answer

Can you detect Solar relative power levels with a PD or pulse off the PV to measure change of Voc with solar input, then Vmpt changes 82%+/-8% for most PV's , but you indicated Vmpt=75/80.2=93.2% which is higher than most.

Pls show PV specs.

This can be done several ways with a hunting algorithm power controller or a "feedforward" compensated shunt switching regulator regulates the input rather than the output voltage based on a PD solar level. This assumes neither PD and PV are blocked by shadows or dirt so several cheap \$.50 Vishay PD sensors may be used in parallel to get a logical "high man wins" current source into a fixed resistive load. The load can be a TIA or simply an Op Amp buffer or an inexpensive buffered PD Light sensor from Panasonic that runs of an LDO of say 5V (p/n AM103).

simulate this circuit – Schematic created using CircuitLab

Food for thought. Comments?

• Hi Tony. My question is more about how the voltage source can be limited with pulsed-switching, and whether it is a feasible method of supplying surplus sun power, while the battery bank remains connected, without undesired "charging". The alternative is to add a diode, so that the current cannot flow into the battery bank, but I was hoping that my approach would work. – user95482301 Jan 3 '17 at 9:31
• My approach is same as any Laptop or mobile. Load takes power from local DC-DC from battery and battery takes power from DC-DC from external charger. When charger exceeds load demand this means load always comes from PV. When load is greater than PV, not possible and when battery is drained and no PV , no load is possible either. Your model is incomplete. each function (module needs to be independent) yet as a system charger must always exceed load for excess situation to work. – Sunnyskyguy EE75 Jan 3 '17 at 16:31
• If fully charged and supply exceeds demand then Vpmt simply rises as underutilized available power. from output OVP protection. ( step up boost converter must have limits) Your method can work but less efficient unless you have MPT controller. – Sunnyskyguy EE75 Jan 3 '17 at 16:36
• Next question is how do you determine turns ratio, N for PWM inverter with Sin function so that source ESR deliver load regulation error needed at step power load e.g. 10% load error i.e. N² ESR' < 10xRload Where ESR' includes bridge and primary coil loss with battery ESR – Sunnyskyguy EE75 Jan 3 '17 at 16:44