On Saturday 27 September 2008, Jon Elson wrote: >Gene Heskett wrote: >> I'll have to go look at the outputs of my xylotex again when I get a >> chance, but ISTR it was quite a bit faster than 200 ns. The undershoot I >> could see, but it was IIRC in the 8 or 9 ns region and about the limit of >> my scope to define (its only a 100mhz scope, with usable response to >> around 200), and only went down 1 Vbe below ground. The clipping caused >> by substrate diode conduction was pretty obvious to the trained eye, but >> looked to be relatively 'clean'. > >Oh, just to comment on the undershoot thing. My original PWM amp had no >freewheel diodes, other than the body diodes of the FETs. When that >side of the bridge had been sourcing 20 A to the load, and then switched >off, the current flowing in the output filter inductor had to come from >somewhere. With just the body diode, it would reach -12 V and blow the >driver chip. This voltage could last for microseconds! I put several >fast diodes across the low-side transistor before finding one that could >turn on FAST! By slowing the high-side transistor turn-off a bit, the >diode could come on in time to catch the inductive undershoot and hold >it to less than -4 V, which the driver withstood fine.
If it wasn't for the poor PIV ratings of the schotky's(sp?), that would be ideal, but I don't know of any power schotky's good for that sort of reverse voltage. The std power si diodes slow turnoff can be a huge problem child also. In an old GE UHF transmitter, we had 6 stacks of 48 each 20a, 400piv si diodes in DO-5 cases that were used in the 3 phase beam supply. Load was about 6 amps per klystron at 20kv. Because of the slow turnoff, the 3 phase power was welded phase to phase 180 times a second for about 30 u-secs a blip. We had short lamp life all over the place cuz we had 2+kv spikes sitting on the 120 volt lines at the duplexes on the wall. We considered replacing them with schotky's, till we totaled up the materials cost's, which at the time would have come to about 20k$. Eventually lightning got the 375kw sola transformer between us and the powerline and I had to bypass it. That reduced the powerline impedance and dropped those spikes to 75 volts. It also got rid of the rectifier noise in the video it had had since it went on the air, so that was another set of jumpers, 4 pieces of 750mcm/phase, 12 total, that never got pulled back out. Almost any full power UHF broadcaster is going to be the power companies biggest customer. :) >As you turn the >DC supply voltage up, the output rise/fall times can become quite a bit >shorter than the gate drive rise/fall times. I had expected with the >Miller effect, etc. that the output rise/fall times would be somewhat >slewrate limited, but not so, the slew rate speeds UP. > The driver output probably gets stiffer faster than the FET's with the increased supply voltage. FET's also will sweep out the majority carriers faster at higher voltages, but I'd think miller effect is there, just being swamped by nitros burning drivers. >Jon > >------------------------------------------------------------------------- >This SF.Net email is sponsored by the Moblin Your Move Developer's challenge >Build the coolest Linux based applications with Moblin SDK & win great > prizes Grand prize is a trip for two to an Open Source event anywhere in > the world http://moblin-contest.org/redirect.php?banner_id=100&url=/ >_______________________________________________ >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) Finagle's Seventh Law: The perversity of the universe tends toward a maximum. ------------------------------------------------------------------------- This SF.Net email is sponsored by the Moblin Your Move Developer's challenge Build the coolest Linux based applications with Moblin SDK & win great prizes Grand prize is a trip for two to an Open Source event anywhere in the world http://moblin-contest.org/redirect.php?banner_id=100&url=/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
