Kirk Wallace wrote: > Is this the high side bootstrap that creates the high side FET gate > control voltage? No, not really. That may be where John Kasunich got the name to call it the "bootstrap" parameter of the driver (I didn't supply that name to him). These servo amps do have such a bootstrap scheme to supply the high side bias. But, what this one pulse each direction thing does is to reset the shutdown latches in the IR2113S FET driver chips. I would have been fine without such a latch, but it is a feature of the chip, I can't change it. When the positioning loop starts up, EMC may decide the motor needs to move in one direction to null the error, and if the amp hasn't seen at least one tiny pulse in the opposite direction, it won't produce any output. > 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. > I really don't see it as a problem. It does mean you can't go to 100.0% duty cycle, somewhere around 95% is a good limit. I did put a high-side supply on my first generation servo amp. It required winding a custom transformer, and having a power driver circuit for it. The reason is this is not just a fixed voltage a little above the motor supply voltage, it is a FLOATING supply that tracks the source terminal of the high-side FET. So, this whole power supply swings the entire range of the motor supply in 200 ns or so, every time the transistors switch. And, you need TWO of them, one for each high side transistor. The capacitive coupling between the drive winding and the output windings causes problems with blowing out the driver chip, so isolation of the windings is needed. A TOTAL pain! So, that's why these bootstrap circuits are so popular, they work and are elegantly simple.
Using P-channel FETS would work, and avoid the loating supplies, but..... I use 40 milli-Ohm transistors in my drives. Try to find a 200 V 40 mOhm P-channel FET. They don't exist. 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
