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. I use a 0.1 uF SMT ceramic cap for the bootstrap capacitor. 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.
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