On Friday, June 15, 2012 05:22:22 PM Jon Elson did opine: > gene heskett wrote: > > Generally speaking, the igbt stuff I have observed has a noticeably > > slower turnon/turnoff times, which when coupled with their quite a > > bit higher "on" resistance, does lead to heat problems. > > Right, IGBTs don't make sense for anything under 200 Volts. The MOSFETs > are getting > much better, with insanely low on resistance, like 9 milli-Ohms. The > IGBT is a bipolar > transistor, so it will never get below 0.7 V voltage drop. And, as you > note, they are quite > a bit slower than FETs. > > Jon
Thanks Jon. I felt like a wee bit of babbling about the diffs was in order, and in this case needed to be said. Mosfets are quite capable to handling something in the 50-100 amp range, an with IR having some with kilovolt ratings, I don't see any need to entertain the use of a clearly inferior technology. But, I think it also needs to be said that the driver design needed to successfully drive these higher powered hexfets, is not something that I with my somewhat dated design thinking, would consider. To start with mosfet/hexfet's have one very non-endearing feature. Because they are effectively the perfect equivalent to a vacuum tube triode, they also share a attribute known as miller effect in the tube world. This makes them look as if there is a large capacitor from drain to gate, which is in fact the gates input capacitance, which for some devices can exceed 25,000pf, but since a goodly amount of that is drain-gate capacitance, it is then multiplied by the transconductance of the device, some of which exceed 25 mhos! It can't switch any faster that the gate driver circuit can charge or discharge this amplified capacitance. So the gates driver stage, in addition to being floated at the src point for the 2 bugs at the top of the bridge, must itself be capable of delivering that current fast enough to achieve the switching speed desired. That could easily be 100 amps, for that 10 to 15 nanoseconds. In that sense, whomever is trying to design such a puppy, will often need to think in terms of microstrip surge impedance's in order to be able to make the circuit perform at that level. My practical education in such tells me I'd likely fail, destroying the hexfet's from SOA effects. That last time I tried to use big hexfets as an analog voltage regulator worked well, for about a month at a time, but the hexfets gradually converted themselves to big power resistors, so I had to put the 1950's GE design, using type 7094 vacuum tubes back in as it was more dependable. It never did work WELL, but within its limits, it did work for long periods of time. 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> Time to be aggressive. Go after a tattooed Virgo. ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Emc-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-developers
