I want to charge a battery bank from a car alternator. Either side may have a higher voltage than the other, e.g.:
- Battery bank can be 14V while alternator-side is 12V (engine off, solar charging)
- Battery bank can be 12V while alternator-side is 14V. (engine on, solar not charging)
I thought of using a relay, but the battery may be able to draw a higher current than the alternator can support. Therefore, switching to control the current is essential.
The current is very high, e.g. 100A, and therefore fast switching is needed to reduce heat build-up. By using MOSFETs in parallel, the current and consequent heat is reduced significantly.
To reduce jolting action on the alternator when switching on, and therefore reduce stress and increase lifetime, I imagine the switching ought to be high-frequency. Maybe 1000Hz is sufficient?
I'm interested in using NPN, because they're much cheaper. I'll use a cheap DC-DC boost circuit to provide 25V for driving the gate.
1) Should the body diodes face towards eachother or away from eachother?
2) In the diagram, the connection is: Battery -> Vd -> Vs + Vs <- Vd <- Alternator
Will Vs be floating when both MOSFETs are off? E.g. is it possible that Vs becomes 0V and therefore violates the Vgs max rating?
I was thinking that by facing the body diodes towards eachother, Vs will never be less than the highest voltage minus the drop over the body diode.
Also, I'm wondering if current has to be able to flow from Vs? Again, in the current schematic, with body diodes against eachother, I don't see how Vs would allow any current to flow.
3) I'm in particular worried about violating the Vgs< +-20V rating.
- When Signal=5V then Vg=25V, max Vs=14V, max Vgs +14V.
- When Signal=0V then Vg=0, Vs=(0-14V?), max Vgs -14V.
Is this correct?