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From: John Popelish
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Subject: Re: Help - Power mosfets - difficult load
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Date: Wed, 23 Oct 2002 00:36:10 GMT
NNTP-Posting-Date: Tue, 22 Oct 2002 20:36:10 EDT
Bill Allison wrote:
> I am encouraged by the fact that Futaba Corporation have made millions
> selling servos driven at 50Hz. I am further encouraged by their choice
> of a frequency as low as 120Hz for their newest servos in which (they
> are microcontroller-based) they could have chosen virtually any
> We may to an extent be at cross purposes here. I am concerned with fet
> switching losses, because I know from the graphs that if I am not
> careful in this area I risk straying outside SOA. The less time the
> fet spends in the switching region the better from the point of view
> of heat generation in it. When fully on the power dissipated will be
> I^2 * R where R = 0.0033 ohm for the pair of FB180SA10 I will use i.e.
> 33 watt at 100 amp and of course when fully off the power dissipated
> in the fet will be zero. But when switching, the power dissipated,
> ignoring inductive lag, will be highest at the midpoint when 6 volts *
> 50 amps = 300w. So I want to be in that switching zone as infrequently
> as practicable. I am not concerned with motor or overall efficiency,
> because the motor has power aplenty for the job and battery capacity
> is not an issue either.
> I do accept that the *system* will operate more efficiently at higher
> frequencies (although the motor vibration from 50 Hz excitation is
> undoubtedly useful to overcome brush stiction and improve low speed
> starting). But as the control input available to me is a 50 Hz pulse
> train it is simplest and makes sense to stay with comparing that with
> a similar train derived from towline tension and expand the difference
> to (via a suitable drive IC) gate my fets.
> So I'm going with 50 Hz. It's the practical solution and will work.
The point you miss is that during a long on time, the current climbs
much higher than the average current, so the switch off losses are
quite a bit higher per pulse than if the ripple current were kept
lower. This is the reason for the sweet spot around a kilohertz or
two for line voltage motors. The lowest loss point may be lower for a
big 12 volt motor but I am willing to bet a dollar that it is higher
than 50 Hz.
But you can find this out by experiment after you have a functional
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