Gene Heskett wrote: > 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. > > There are insanely fast turn-off diodes on the market. By trying out a few in a circuit already proven to provide a torture test, it was easy to find a part that also had a VERY fast turn-on. I didn't need anything super-fast, but it had to pick up 20 A in under 100 ns. > 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. :) > > Well, there's no way you can use standard rectifiers in a switching power supply, which any PWM servo drive is. These fast turn-off diodes may cause some EMI, but at least they don't have to be "re-charged" much after conducting. This is a big deal in 100 KHz+ power supplies.
Another big power co. load is electric refrigeration. Many buildings (or building complexes) have a 500 Hp electric turbine chiller (basically same as a home A/C except the compressor is a turbine instead of reciprocating). Newer ones run 6000 - 10,000 RPM with a VFD, too. I was shown an older one at work and there was a phone on the thing, next to the starting control panel. It had some early soft-start system. Next to the phone was a list of power plant engineers that had to be called, in order, one half hour before motor start. It had lots of exclamation points about doing this exactly right or the university would get a HUGE bill for not giving them adequate waning of starting a big load. >> 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. > The gate drive waveform has a distinct flat step when the transistor starts to turn on or off, as the Miller effect takes control of the gate. You can use this to tell what the gate threshold of the transistor is. 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
