On Sunday, August 01, 2010 05:19:09 pm Jon Elson did opine: > Gene Heskett wrote: > > By normal fab techniques, there is not a P type FET, they all need a + > > signal on the gate to turn them on. That said, a separate + and - 5 > > volt supply winding whose center tap rail is common to the FET's > > source rail is the much preferred method of deriving the high sides > > on pulse drive voltage. I generally detest the bootstrap methods > > because the voltage so developed isn't as dependable (IMO). The > > capacitors that are the isolation/storage elements of a bootstrapped > > circuit are usually common electrolytics, with their failure rates > > being 100x that of the semiconductors involved. When a semiconductor > > in one of these circuits fails, there is about a 100/1 chance a > > failing capacitor was the first circuit fault. > > 5 V isn't enough bias, I use 12 V. Does that not add to the stored charge, lengthening the turnoff times?
> I use a 0.1 uF SMT ceramic cap for > the bootstrap capacitor. I TBT was thinking along the lines of a small electrolytic in order to cover the frequency range of the VFD at slow speeds. Those should never be discussed in the same book as dependable. > That is plenty of charge as it is recharged every 20 us. I have never > had one of these caps fail. > The FET driver chips have an undervoltage lockout, so it will not even > turn on if the supply is too low. > If you go to 100% PWM duty cycle, this will eventually happen, and the > drive handles it gracefully > and just goes "limp". Hitting E-stop and resetting it clears the > trouble. > > > Driving a power FET is almost a separate chapter in the design tomes, > > as the gates in high powered versions of these can represent quite a > > large capacitance just from the sheer size of all the actual gates in > > the devices, with figures well above .05 microfarads, some of which > > gets amplified by miller feedback effects as its turned on and off. > > In order to minimize the junction heat during the on-off or off-on > > transitions, the driving waveform must be very fast, and capable of > > charging or discharging that large capacitance in nanoseconds. That > > implies a driver capable of several amps with rise & fall times of 10 > > or so nanoseconds. Many a power FET aka HEXFET has been destroyed by > > drivers that take a microsecond to make that nominally 9 volt swing. > > 5 volt + to fully turn them on, and about -4 to turn them absolutely > > off in the shortest time. > > > > I'm with Kirk on this one, we are a relatively small market, one that > > will never find a profit in 'simplicating' the right way out of a > > circuit, so it should be done right, not to consumer grade standards > > but better. > > I figure that I'd have to raise the price on my servo amps by $100 each > to cover the cost of winding the transformer by hand, and providing the > extra components for it. That's completely out of the question to solve > a problem that doesn't even exist. Despite the name of this PPMC driver > feature, it actually has NOTHING to do with bootstrap power supplies. That puts a different complexion on it entirely, thanks Jon. What I had in mind, and which does a bootstrap thing for both capacitor charge and transformer core reset, is a circuit similar to that which drives the H scan circuitry on older crt monitors. It is 'bootstrapped' in the sense that the bottom winding end of the driver transformers secondary, a teeny little ferrite thing, is tied to the hexfets src terminal, and IIRC a small, say 4.7 ohm resistor is used to limit gate current during the on period, the current then flowing in reverse through another diode when the driver transistor is turned off. Turn on time is about 15ns, turnoff time about 50ns. And its a none too reliable circuit that I'd not ever use in a fresh design. I think it could be made reliable, but I've not found that 'magic twanger' yet. It has 4 or 5 electrolytic caps in it, and any one of them developing an ESR above 2 ohms and the whole circuit goes up in smoke. > Jon > > ------------------------------------------------------------------------ > ------ The Palm PDK Hot Apps Program offers developers who use the > Plug-In Development Kit to bring their C/C++ apps to Palm for a share > of $1 Million in cash or HP Products. Visit us here for more details: > http://p.sf.net/sfu/dev2dev-palm > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > 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) A diplomat is a person who can tell you to go to hell in such a way that you actually look forward to the trip. -- Caskie Stinnett, "Out of the Red" ------------------------------------------------------------------------------ The Palm PDK Hot Apps Program offers developers who use the Plug-In Development Kit to bring their C/C++ apps to Palm for a share of $1 Million in cash or HP Products. Visit us here for more details: http://p.sf.net/sfu/dev2dev-palm _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
