On Saturday 27 September 2008, Jon Elson wrote: >Gene Heskett wrote: >> Maybe I'm being cautious, but if I hooked my scope to the FET's gate, and >> saw transition times noticeably slower than my 100mhz dual trace's own >> rise times for it to make that about 20 volt swing, then I would be >> looking for ways to speed up the driver. Even 30ns rise & fall times will >> be reflected in the increased temps of the FET's as the current rises >> toward the max ratings. > >Well, generally, you can't go that fast in full-bridge switching >applications. When feeding current to an inductive load, there is one >side of the bridge sourcing current. When you switch that side to >ground, the inductive effect draws current, and the common terminal >between the high-side and low-side transistors tends to go below ground >until the low-side transistor turns on completely. Most dual FET driver >chips can only handle a limited amount of negative voltage there. As >the Di/Dt rises, the voltages generated across even short, wide circuit >traces becomes enormous, it is easy to see 20 V across a 2" trace. >Circuit board layout only helps up to a point. When you reach the >limit, slowing down the transistors is the last fix. With relatively >slow PWM frequencies, a couple hundred ns rise and fall will not cause >excessive losses. When you get to the 100 KHz and up range, then you >have to work harder. > >Jon > Are you referring to FET's with free wheeling diodes, or the more common integrated H bridges (DMOS) like the xylotex & its A3977's?
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'. I'm not equipt to measure currents with the scope so I've NDI of the actual magnitudes involved. I believe the A3977 stages the switching to prevent any transition shorts, but don't recall being able to interpret the waveforms and confirm that without going to a resistive load. Not having a 15 ohm resistor of sufficient power rating, I hadn't even considered looking at it that way. Jeff I'm sure would frown on doing so. This is an interesting discussion though Jon. I blew the single chopper FET in the spindle motor of my little HF micromill, and when I looked up the number, I wasn't terribly impressed. 7 amps, 400 volts ISTR, in a TO-247 package yet! That is equivalent to floor sweepings. No such FET's are available at the local RS, so I opened up a couple of old computer psu's to see what might be available there. I picked one of those rated at about 2x the current and 2.5x the voltage & stuck it in. The heat sink runs a little cooler I think, and I have stalled the spindle for as much as 2 seconds several times since and it seems to shrug off such abuse without any ill effects. That will clear the fuse in that 2 seconds or less, but its only 2 amps anyway. >------------------------------------------------------------------------- >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) My mind is a potato field ... ------------------------------------------------------------------------- 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
