On Tue, 21 May 2013, Gene Heskett wrote:
> Date: Tue, 21 May 2013 23:18:28 -0400
> From: Gene Heskett <[email protected]>
> Reply-To: "Enhanced Machine Controller (EMC)"
> <[email protected]>
> To: [email protected]
> Subject: Re: [Emc-users] Superglue, or Loc-tite for that motors flywheel?
>
> On Tuesday 21 May 2013 23:09:36 Peter C. Wallace did opine:
>
>> On Tue, 21 May 2013, Gene Heskett wrote:
>>> Date: Tue, 21 May 2013 22:04:34 -0400
>>> From: Gene Heskett <[email protected]>
>>> Reply-To: "Enhanced Machine Controller (EMC)"
>>>
>>> <[email protected]>
>>>
>>> To: [email protected]
>>> Subject: Re: [Emc-users] Superglue, or Loc-tite for that motors
>>> flywheel?
>>>
>>> On Tuesday 21 May 2013 21:53:39 Jon Elson did opine:
>>>> Gene Heskett wrote:
>>>>> Humm, is there an IC that will function as the switch driver, and
>>>>> whose output could be controlled with a PWM mode? Something that
>>>>> would start at 15% duty and taper to 100% when the applied voltage
>>>>> had dropped to say 20 volts. Then the Q is how fast can these be
>>>>> switched, I get the impression I'm still looking at hexfets I can
>>>>> steal out of dead computer PSU's. These things speed (lack thereof)
>>>>> will cause their own SOA self destruction I expect.
>>>>
>>>> I use the IR2113 and similar chips to drive FETs. You'd need some op
>>>> amps and/or comparators to generate the PWM. Lack of speed? The
>>>> IRFB31N20D was so fast it caused problems with the dV/dT (and dI/dT)
>>>> affecting the rest of the
>>>> circuitry. Easily 100 ns turn-on/turn-off times, even with resistors
>>>> in the gate circuit.
>>>
>>> Those are problems you should like Jon, they are telling you that your
>>> ground & power planes need to be wider, and need more bypassing. Both
>>> of those translate to reduced switching losses & cooler devices.
>>> Often you will need both a .05 ceramic on very short leads or better
>>> yet, surface mount, AND a 10uf in order to get enough bypassing.
>>>
>>> Of course I'm far enough away its not worth getting in the car to come
>>> and beat some manners into me. :) I think.
>>>
>>> FWIW, resistors in the gate to slow the rise & fall times will raise
>>> the switching losses, in some cases enough to destroy the device long
>>> before your finger says its even getting warm.
>>>
>>> But you knew all that.
>>>
>>>> Jon
>>
>> In practical Hbridges, gate resistors for assymetrical gate drive (fast
>> off slow on) are often needed to prevent dVdT turn-on of the high side
>> by the low side switches or vice versa. If your choice is a little bit
>> of switching loss vs shoot-through, its easy to choose...
>
> Very true Peter, but would not this sort of problem be better treated with
> some logic to use the gate delay to delay the on swing? I'm thinking cmos,
> old style 4000 series because I personally used some of that sort of
> trickery to put colored outlines around a character in a character
> generator that didn't have that capability originally. Running on 28
> volts, that 4028 ran detectably warm, but in 2 years, never failed. Of
> coarse in those days we thought 200 ns was a short time. Now, 25ns is
> economy grade utility stuff. But the idea should be just as valid.
Unfortunately its not a delay issue but how fast the MOSFET that is switching
on turns on. At some point the rate of rise of voltage (dVdT) at the drain of
the off MOSFET caused by the other MOSFET turning on will cause the off MOSFET
to turn on (due to the drain-gate capacitance of the off MOSFET) Having a gate
driver with assymetrical drive (slow on fast
off) helps
>
>>> Cheers, Gene
>>>
>>> Ask somebody something.
>>> A pen in the hand of this president is far more
>>> dangerous than 200 million guns in the hands of
>>>
>>> law-abiding citizens.
>>>
Peter Wallace
Mesa Electronics
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