I need a blocking diode that will conduct about ten amps. I have a leftover comparator out of a quad, so I decided to use parts the circuit already has and run the comparator off the charge pump that drives the n-mos on the high side.
Now, if I try to string the comparator across the mosfet, the negative feedback coming from the voltage at the mosfet's source would cause oscillations.
But the circuit will also have voltage and current sensing, so I don't want the mosfet slamming on and off. I thought I'd try try something like this:
generator | | | NTP90N02 | | ,--|>|--, | | | +----+----+-+-, ,-+------, | | S | v | D | | 15K ------- | | | --- | | | | | | | | '--------+ Ich| | | | | R2 | | | | _|D V | | ,--+--+--||_ pmos | | | | | |S | | | 27K Zdiode| | | | | | | charge | | | '--+--+--+---o pump | +-|--10K---|----, ~24v | | | | | | | | |/-|-' | | | / | 1/4 | | '---+---< | LM339 | | | \\ | | | - |\\+|---10K----+ | ^ | | |D | | | '-- | | | nmos ||-+----|-----2K2-----+ ,-- | | | |S _/ | | +-----^ TL | _____ | |431 | 12v batt___ R1 | | | | | | | '-----+----+-------------+ | gnd
Setting the values of R1 and R2 in the constant current sink so that the voltage across R2 is .7 volts. The voltage across the NTP90N02 will reach .7 volts when it has between 1.5 and 2 amps flowing through the body diode. When that happens, the comparator goes low, turning the mosfet on and the current sink off. So now the comparator's inverting input sees the mosfet's source voltage. It won't turn off until the voltage across the mosfet drops to zero, +/- the offset of the comparator.
Let's say the comparator has a 9 mV max offset, and the mosfet has 4.5 mOhms Rds. If the comparator has the max offset in one direction, then the battery can discharge into the generator, and the comparator won't turn it off until the discharge current goes more than about 2 amps; if the generator windings have more than about 6 ohms resistance it won't turn off. With max offset in the other direction, we get back into oscillation. So I went looking for offset nulling circuits. This maxim appnote has some