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high-side mosfet scheme

I used to work on a bike with an old-fashioned generator that had a segmented commutator and put out dc. That means that a voltage regulator has to have a rectifier in it, to block the battery from discharging into the armature when you turn the bike off (they used a relay in the old days). I built a solid state regulator for the bike using a shottky rectifier, but now I'm thinking about ways to use a low-resistance mosfet in the high side so it doesn't require a heat sink. The suggested current limit for this generator is ten amps. A ten-mOhm mosfet would only make one watt of heat. The MBR3035PT shottky created five and a half watts at ten amps. So now I"m trying to figure out how to control a mosfet in the high side as a blocking diode and have it turn on and off, like a syncronous rectifier. The circuit will have a comparator that controls the field current, so I figured I might use a comparator for controlling the power mosfet as well. Since I'll have a charge pump for driving the mosfet gate, I can run the comparator off it. This means I can use an ordinary comparator to sense the voltage across the high-side mosfet. Here's an illustration of the basic topology I envision

,-----------, | | ||-+ | ||---+-R1-+-||-+ | ^ | | | | | | | | | | | | | | | | | | | generator | | | | armature--+----|----|----' | | | | | R2 | '------, | | | | | | +-R4-, | | | | | | | +----|-||-+ | | | |\ | | | | '-|-\| | | | | \ | c | LM393| >---+-----b | | / e | ,-|+/| | | | |/ | gnd | | gnd | '--------------R3----------+ | | + ----- --- ----- --- ----- --- | - | gnd

The only problem is that the comparator may chatter at low charging currents. For example, if the comparator has an offset voltage of -5mV and the mosfet has 5mOhm Rds(on), then at currents less than an amp it will oscillate. I put the negative feedback capacitor there to alleviate it as much as possible. And though it's unlikely the generator will have to put out very low charging currents for any length of time, I still would like to find a way around the oscillation. Any suggestions?

Reply to
Tolstoy
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Why does it need to be such a sensitive switch? Couldn't you use an absolute reference for the turn-on, depending on the armature being well above some threshold while it's working?

-- John

Reply to
John O'Flaherty

It seems like they would have done it that way back in the day they used relays. They must have set the relay to engage at 15 volts or something on the armature and cut out at say 11.5, or whatever the armature would drag a fully charged battery down to, once the generator stops. I don't actually know how the cutoff relays in the old mechanical regualtors worked. Anyhow, taking that approach would call for a fixed voltge reference and a comparator with hysteresis. I'll have to think about it.

Reply to
Tolstoy

Yes - add a resistor for hysteresis. See

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Ed

Reply to
ehsjr

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