On Wednesday 22 May 2013 01:30:27 Jon Elson did opine: > Gene Heskett wrote: > > 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. > > I went to a slightly larger transistor (moved up from the fast > IRFB31N20D to the slightly slower IRFB260N) and the problems > disappeared. The servo amp runs cool at rated current, so losses are > not a problem. I've got about as wide traces as the board can stand. > The boards is nearly all surface mount, the IR drivers are right next > to the transistors and the loop area of the power section is very > small. But, the output conductors run right under the logic on the > bottom of the board. I've sold 275 of these, now, so they are a pretty > mature design. > That speaks volumes Jon.
> > 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. > > The big problem with this is the IR driver chips have a parasitic diode > in them > that can't tolerate excessive negative voltage at the junction between > the high and low side transistors. That is an extremely common problem. :( > That was what I had to tame, that > junction going below ground. The body diode in the low-side transistor > takes several MICROseconds to turn on with 12 V forward bias! I put > ultra-fast power diodes across the low-side transistor to stop it from > going negative, but still wanted to slow down the fall-time to get > a bit more safety margin. Ultra fast, Si, or Schotkey? Si seems worthless but I have seen the amp or so rated Schotkey's make a difference. Their Vf is only about 2/3rds of a germanium diode, and a very low impedance once conduction starts. > Jon > > ------------------------------------------------------------------------ > ------ 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 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> It is a human characteristic to love little animals, especially if they're attractive in some way. -- McCoy, "The Trouble with Tribbles", stardate 4525.6 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
