On Wed, Aug 23, 2017, at 12:30 PM, Jon Elson wrote: > 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.
Numerical integration is unnecessary. Energy stored in a cap is 1/2*C*V^2, for 160V and 4700pF that is 60uJ. Times 100K events per second is 6W. (The cap energy is completely dissipated in the resistor, because the RC time constant is only 47nS, far shorter than the period of the PWM waveform. So the cap is fully charged and discharged during each cycle.) You could lower the capacitance to reduce the dissipation. Of course at some point the snubber will no longer be effective at damping oscillations. Did you put the snubber in there to deal with a specific oscillation or ringing problem, or just on general principles? If dealing with a specific problem, there are design techniques that can be used to minimize snubber dissipation. You need to know the ringing frequency and the impedance of the resonant circuit (either knowing the L or the C will do). I can go into more detail if it would be helpful. > 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 -- John Kasunich jmkasun...@fastmail.fm ------------------------------------------------------------------------------ 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
