On Wednesday 22 May 2013 01:11:07 Peter C. Wallace did opine: > 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 > So the problem isn't amenable to just delaying the on drive. I take it that a hard and stiff off bias is simply swamped by the DvDt induced capacitance charge from drain to gate.
I also have to assume that tech writers 20 years ago didn't always understand the phenomenon they were writing about, because then the holy grail was just getting rid of any both on at the same time, eg they weren't sure of the causal mechanism. So my knowledge expands a bit. Now I wander into theory Peter, because it seems to me that with low enough impedance in the drive, if it getting turned back on for a microsec or so, that should be something that a better and lower resistance from the gate terminal to the gate area of the die ought to be able to absorb that. Perhaps the next generation of devices that may come out of the next shrink might find a better geometry to alleviate this? Food for thought, thats for sure. And I expect the chip makers are staying up late looking for that better way. Thanks Peter. Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> is up! My views <http://www.armchairpatriot.com/What%20Has%20America%20Become.shtml> Where's th' DAFFY DUCK EXHIBIT?? A pen in the hand of this president is far more dangerous than 200 million guns in the hands of law-abiding citizens. ------------------------------------------------------------------------------ Try New Relic Now & We'll Send You this Cool Shirt New Relic is the only SaaS-based application performance monitoring service that delivers powerful full stack analytics. Optimize and monitor your browser, app, & servers with just a few lines of code. Try New Relic and get this awesome Nerd Life shirt! http://p.sf.net/sfu/newrelic_d2d_may _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
