Magic Smoke escapes

Milled some nice little circuit boards on the light machines sherline
using EMC and a random dremel bit (in the sherline headstock). First
real part I've made with any of my CNC projects. I don't have a
powered Z axis, so I milled the traces as one unbroken path and put in
M00's at each through hole location so I could lower the Z by hand to
create a center hole that could later be drilled through. It actually
all worked pretty well, although I don't think I will try doing PCB
layout on graph paper and manually generating a toolpath again!

Unfortunately, the circuit doesn't work as well as the boards look, at
least not yet. Each one is a half bridge with an IR2104, bootstrap
capacitor and diode, two IRF530's, sense resistor, recirculation
diodes, and other stuff to make up essentially the circuit from the
first page of the 2104 data sheet. So far I've let the magic smoke
out of 2 or 3 FET's, exploded a sense resistor, exploded the 7812
regulator that was powering the 2104, and blown a power supply fuse!
Between all of that I did learn quite a bit though.

I suspect that despite the 2104's best efforts, there is some shoot
through occuring. The datasheet shows series resistors on the MOSFET
gates, I used 1k ohm as in the DPRG ("untested") discrete bridge
circuit. In looking on the scope I'm seeing a noticeable ramp down of
the high side gate, and I suspect it is still on when the low side
switches on. So one action item for tomorrow is to try much smaller
resistors. (Let's see, somewhere I heard I figure of about 1000 pf
for gate capacitance. RC for that and 1k is 1 us, wheras the
off-before-on anti-shoot through sequencing is on the order of a
hundred nanoseconds).

Another thing I'm trying to figure out is if I should put the 2104's
ground on the FET (high) side of the sense resistor, in which case it
may misinterpret input signals, or if I should connect it to the
actual ground and let the resistor voltage potentialy add to or
subtract from the gate drive.

I'm probably going to take it in to work and hook it up to a power
supply with current limit, and try to find something with substantial
inductance that also has a high enough DC impedance to not fry things
if it gets left on 100% duty cycle.

Chris





--
Christopher C. Stratton, stratton@...
Instrument Maker, Horn Player & Engineer
22 Adrian Street, Somerville, MA 02143
http://www.mdc.net/~stratton
NEW PHONE NUMBER: (617) 628-1062 home, 253-2606 MIT