Wow, I have made a big screw-up. My PWM servo amps have RC
snubber networks on the junction between the high-side
source and low-side drains (output terminals) of the
half-bridges. The current values are 10 Ohms and 4700 pF.
I have a guy who wanted to run 160 V DC supply. All the
parts are rated with enough margin to do it. (200 - 250 V
ratings on various parts.) I never worked out the power
dissipation in the snubber resistors. I used 3/4 W SMT
resistors and THOUGHT that ought to be big enough.
Well, that guy kept having the resistors burn up, and then
the amps get flaky and trip falsely on overcurrent due to
too-high dv/dt affecting the sensing circuits.
Well, I finally wrote up a tiny program to numerically
integrate the energy in the resistors, and was unpleasantly
surprised that I'd vastly underdesigned that part. At 160 V
DC, and 50 KHz PWM (that's 100K charging events/second) I
get over 6 W dissipation (in a 3/4 W resistor)! I already
have a 2 W resistor on order, however the space on the board
is not going to dissipate the heat all that much better than
it did before, so replacing the resistor won't help remove
the heat much better. The customer is getting a buck
transformer to drop the DC voltage to about 142 V, and if he
also drops the PWM frequency 20 KHz (from 50K) it will
reduce the loss in the resistor to 1.9 W.
The problem happens on his Z axis, which is just sitting
there keeping the machine's head from dropping. So, that is
consistent, it sits for a long time on the same commutation
position of the motor, so one resistor at a time gets hot.
Any comments?
Jon
------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
Emc-developers mailing list
Emc-developers@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/emc-developers
