On Sunday, August 01, 2010 12:56:35 am Kirk Wallace did opine: > On Sat, 2010-07-31 at 23:15 -0500, Jon Elson wrote: > > I know a number of people are using my PWM servo amps with Mesa > > controller boards. > > A feature of the dumb control logic on the servo amp is that it needs > > a short pulse in each direction > > to clear the shutdown latches on the FET driver chips. I built a > > little state machine into the driver to accomplish this, it is called > > the "bootstrap" parameter. It gives 5% duty cycle pulses in each > > direction on consecutive servo cycles, then goes to normal operation > > as commanded by the PWM input. If you don't do this, the drive can > > act like it is disabled until you attempt to move it both directions, > > then it will suddenly come "live". > > > > So, has anyone written up a couple lines of HAL to do this, or how > > else do you solve the problem? > > > > (This applies only to the brush version of the servo amp, the > > brushless amp has a CPLD that manages this function.) > > > > Thanks, > > > > Jon > > Is this the high side bootstrap that creates the high side FET gate > control voltage? This has always seemed to create more trouble than it > is worth. Why not have another supply with the proper voltage and not > have to deal with the bootstrap? Or use N and P FET's? I know the > bootstrap method may save a portion of the parts cost, but for the > product quantities for the CNC market, it doesn't seem worth it. I may > be showing my ignorance here.
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. 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. -- 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) marriage, n.: Convertible bonds. ------------------------------------------------------------------------------ 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