Thanks. I'll start working on it today. However doesn't the fet require 6-7 volts between gate and source? If so my power supply needs to be that much higher than the highest battery voltage?You are just gong to have to do some level shifting with normal transistors to get a 5 volt signal to control a 20 volt supply.
Because pumping 20 volts at 15 amps into a 300mah 3s lipo will cause it to explode and I need the 15 amps for my larger packs. PWM will be used to limit current and manage the constant voltage portion of the charge.Why are you using PWM to charge a battery?
output filter capacitors will smooth out the pwm. using twin 470uf and single 0.1uf tantalum with pwm freq of 20khz.PWM will not limit the current to the batteries. That is why I asked. It will limit the average current, but the peak current will be the same as if there was no PWM, and you will probably start a fire. Batteries must be charged by a constant current.
You ever heard of switching power supplies? They use pwm and filter caps and their output is very smooth, the voltage spikes that make it past the caps are not powerful enough to harm batteries or the circuits they drive.PWM will not limit the current to the batteries. That is why I asked. It will limit the average current, but the peak current will be the same as if there was no PWM, and you will probably start a fire. Batteries must be charged by a constant current.
Edit: PWM is usable in the context of a buck converter that controls a constant current source.
Only if you insist on using an N channel mosfet...which again brings up the question; If you can't tell the difference between an N-channel and a P-channel transistor, why do you think you can do this safely?doesn't the fet require 6-7 volts between gate and source? If so my power supply needs to be that much higher than the highest battery voltage?
If you came up with those circuits yourself we have every reason to be concerned. If you got them from somewhere else you need to find better sources of circuit ideas. People can and do get hurt doing silly things with voltage and current and chemistry. There is even the posthumous Darwin Award for scientist/engineer/inventors who like to tinker with stuff.
So it might be more efficient to research actual circuit designs rather than trying all possible combinations of hooking up components in the vain hope that you will stumble on one that works. You might want to start with a basic principle of N-channel FET operation that Vgs must be greater than Vgsth, the gate threshold voltage, in order to turn the device on. That one fact will prevent you from doing some things that have no chance of working.
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