On Sat, 31 Jul 2010, Kirk Wallace wrote: > Date: Sat, 31 Jul 2010 21:46:44 -0700 > From: Kirk Wallace <kwall...@wallacecompany.com> > Reply-To: "Enhanced Machine Controller (EMC)" > <emc-users@lists.sourceforge.net> > To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net> > Subject: Re: [Emc-users] Use of Pico Systems PWM servo amp with Mesa 5i20 > controller > > 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. > -- > Kirk Wallace > http://www.wallacecompany.com/machine_shop/ > http://www.wallacecompany.com/E45/index.html > California, USA >
Yes, this is the high side gate supply voltage. We have PWM amps that use separate supplies (7I27) and bootstrap supplies (7I30,7I29, 8I20). The advantages of the bootstrap supply are that it is cheaper and has fewer components. Supplying the top MOSFET gate power is awkward because the power supply is floating on the high side MOSFETs (or IGBTs) source electrode. This source electrode is switching at ~10 to 100 KHz with full motor bus voltage so the supply must be isolated for the full motor power supply voltage switching with a high frequency square wave. Because the high side gate power supplies are referenced to each MOSFET source, each high side MOSFET gate driver needs a separate supply (so a HBridge needs 2 separate high side gate supplies and a 3 phase bridge needs 3). If separate high side gate supplies are used, this adds considerable cost and complexity (and lowers reliability). Its certainly possible to have a high voltage transformer coupled isolated DC supply for each high side gate driver (like our 7I27) but whether its a good design decision is open to arguement. The disadvantage of boostrap PWM is than you cannot have 100% output duty cycle. Normally you will be limited to about 95-98% (depending on switching speeds, PWM rates, etc) This is because each HBridge legs lower MOSFET must turn on for some portion of time to charge the boostrap capacitors. For 3 phase drives it is possible to get 100% output from a bootrap supplied bridge at medium-high motor speeds (where you may need the 100%) because the normal phase rotation will charge the bootstrap capacitors. Peter Wallace Mesa Electronics ------------------------------------------------------------------------------ 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
