John, One way to improve power dissipation a bit is to mount two resistors in series instead of one, forming inverted V ... It's not that ugly and will add some margin to available power dissipation..
Regards, Petr On Wed, Aug 23, 2017 at 6:30 PM, Jon Elson <el...@pico-systems.com> 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. 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 > -- Ing. Petr Ledvina ------------------------------------------------------------------------------ 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
