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Rds

I need a blocking diode and charging current sensing into the positive terminal of a twelve volt battery. It occured to me that I could use a mosfet as the blocking diode and bias it on during charging, with the Rds serving as sense resistor... and the side benefit of eliminating a wasteful power diode. How accurate is the Rds listed in mosfet datasheets? They all seem to have a number with one or two sig figs and no tolerance spec. And important is the Rds temperature coefficient; supposed positive but I have no idea of the magnitude. Max charging current is ten amps in the application. If it works, I may use the same circuit on a 35 amp charger.

Reply to
kell
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But I don't think RDS(ON) is going to be a very reliable value.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
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Reply to
Jim Thompson

Rds is a function of a variety of things including process variations, current and temperature. There is not a single temperature coefficient on a FET but many which could result in significant variations. I can't see why you'd want to use one over a resistor thats explicitly designed to do a good job at this application.

I'm also not clear how you would use a MOSFET as a "blocking diode". If you are using a diode connected MOSFET with the gate and drain tied together you are still taking a Vgs drop in the FET which may be more than a Schottky diode drop.

Reply to
notme

My project has the same requirement, monitoring the battery charger current and isolating it from the charger to verify that the battery is still present. I did use a mosfet to isolate the battery from the charger and I chose one with a small Rds to limit power disipation as the charger current can be as high as 10A.

To monitor the current, I used a four terminal resistor of .005R. The one I used costs a couple of dollars as I chose a high precision device. The net result is that I can read the current with my 12bit ADC (0 to 10A) both positive and negative direction with better than .1Amp accuracy.

I agree that Rds is too variable of a parameter if you wish a measurement with accuracy. If you are looking for a crude measuremetn of wheter the charger is charging, it may be sufficient for your needs.

I would also recommend a snubber circuit across the mosfet!.

If you are interested in implementation details, contact me at the email address listed in the post header and we can discuss.

Reply to
Noway2

I'd use a p-channel mosfet with the source connected to the positive terminal of the battery. With the mosfet turned off, it would act like a diode, blocking the battery from discharging. Connecting the gate to ground (battery negative) during charging would turn the mosfet fully on and reduce the power dissipated in the mosfet, assuming I(charge) x Rds is less than Vf of the body diode The mosfet would be conducting in the opposite direction of usual.

Reply to
kell

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