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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Reducing contact resistance for low volt use?
Date: Mon, 16 Dec 2002 20:30:56 GMT
Organization: MindSpring Enterprises
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>>> I'll put some typical values to the components. Let's say you
>>> use five FET modules and use a 0.01-ohm shunt, for 20A max in
>>> each module. At the full current, each shunt will dissipate 4W.
>>> Digikey sells Ohmite's "10 series" resistors. Their 15FR010 is
>>> a good choice, a 0.01-ohm 1% 5W resistor at $1.61, quantity 10.
>>> That'll drop 200mV at 20A. Let's choose 10.0k for R4 R5 (this
>>> keeps the LT1013's 1.5nA input offset-current error low, under
>>> 15uV). You mentioned operation down to 50mA, or 10mA per FET,
>>> which would drop 0.1mV on our shunt. This minimum-current level
>>> fits nicely with the LT1013's 60uV typical input-offset error.
>>> We'll let the 1.25V maximum LM385-1.2 reference voltage drive
>>> the load overscale to 125A, so that means R2/R4 = 1.25/0.25 = 5
>>> and R2 R3 are 50k, or actually 49.9k for 1% parts. Each module
>>> looks like a 60k load to the current-setting pot, which is a 12k
>>> load for five modules. So you'd want to use a 1k to 10k pot max.
>>> What about loop stability? If we use a 0.01uF for C1 the loop
>>> bandwidth will be 1 / 2pi * 1E-8 * 1E4 = 1.6kHz. Let's say the
>>> FET's Ciss is a high 5nF. A 470-ohm gate resistor will isolate
>>> the opamp's output from the 5nF load, while making a 70kHz pole
>>> in the loop transfer function. Because this is far above the
>>> 1.6kHz loop bandwidth everything should be nice and stable.
>>> Now the power MOSFET. You'll need Rds(on) under say 0.6V/25A =
>>> 24 milli-ohms. An easy spec. How IR's elegant IRF1405 FET, a
>>> 5.3-milliohm TO-220 with only 0.95C/W of junction-to-heat-sink
>>> thermal resistance? This would have a modest 24C junction-temp
>>> rise at 20A * 1.25V = 25 watts. Digikey charges $3.30, qty 10.
>>> We've identified some nice components. I think I'll order some
>>> for the Institute's EE-Lab inventory!
Wow, thanks! I'm understanding the circuits better with the info you
added above (especially the LT1013 and loop stability stuff).
The IRF1405's thermal resistance is great! I was considering the
IRL1404 up until now (1.25, resistance to sink) but will go with the
IRF1405 even if it costs more. I'm only using a few, no volume
production costs to hold down.
I downloaded the data sheets for TI's and Natl. Semi.'s LT1013
(Digikey carries both) last night. The specs look close enough to not
worry about using either one...as they should be.
I've got some 4oz. copper blank PCB's that I need to think about a
bit...do I go with those to mount the MOSFETS (cutting out fat drain
and source traces) or with discrete 10AWG wires to the drain and
source leads directly (using perfboard to mount components and heat
sinks)? Time to think about the physical layout of the components.
Thanks again. If any of you kind folks are ever in NYC (especially
you Win), dinner and drinks are on me!
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