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From: Winfield Hill
Subject: Re: Help - Power mosfets - difficult load
Date: 29 Oct 2002 06:37:52 -0800
Organization: Rowland Institute
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Bill Allison wrote
> Tony, John
> I know that discontinuous low frequency operation will be inefficient,
> 1) Nobody has disagreed with my contention that those large low
> frequency impulses will be *very* useful in overcoming stiction and
> allowing the motor to be controlled smoothly at start-up and at very
> low speed. ...
You could force a 50% 50Hz duty cycle on top of the high-frequency
PWM, but only for the lowest PWM ratios, to create vibration.
> 2) *Provided* I can keep within fet ratings, I don't care too much
> about efficiency - the unit is going to be powered up for only 10
> seconds at most every 5 minutes at least so heating isn't a huge
> problem and irrespective of frequency, at full throttle (i.e 100%
> duty cycle) losses will be due only to RDSon.
I showed in detail how you'll be heating the FETs with 4x higher
dissipation by using a 50Hz PWM rate, and running them near or past
their specs in the 25% to 100% PWM region. A FET junction's thermal
time constant is well under a second, more like 0.1 second, so your
10-second operating time may not be of much help, if you intend to
keep the FET junction below the recommended temperature limits.
> If tests show this isn't do-able (fets-wise) at 50 Hz then circuitry
> will need to be devised to produce a 500Hz PWM drive from the 50Hz
> control signal but I want to avoid that complication if possible.
A easy way to do this would be to use a pot-operated PWM-generator
circuit, and run the pot from your RC servo output shaft. :-)
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