Winfield Hill wrote...
Spehro Pefhany wrote...
I believe the Vce-monitoring catches much more than dead shorts.
Any time the IGBT's Vce is significantly in excess of the normal
saturation voltage, for an adjustable time, the IGBT error shutoff
will trigger. This occurs for Vce > 5.2V for the SKHI 23/12, with
Rce = 18k. The time is also user-set, with the Rce and Cce values.
Semikron's point is that if the time is short enough, the full IGBT
self-current-limit can be sustained at short circuit without damage.
Any current that forces the IGBT out of saturation beyond 5.2V for
more than the set time will trigger a fault shutoff for that cycle.
BTW, Semikron has a nice 269-page power IGBT manual on their website
(although it's broken into a million pieces). They discuss the Vce-
monitoring scene on page 210, and rules for protecting IGBTs against
shorts on page 207. "The following important boundary conditions
have to be fulfilled to guarantee safe operation:
- the short circuit has to be detected and turned off within 10 µs,
- the time between two short circuits has to be at least 1 second,
- the IGBT must not be subjected to more than 1000 short circuits
during its total operation time."
The technique is not able to function as a universal power-limit
detector. E.g., if the IGBT is desaturated, but under 5.2V, it can
still quickly exceed its thermal power limit. For example, consider
Ignoramus14838's Toshiba MG200Q2YS40 IGBT module, datasheet page 3.
With 15V gate drive, the collector current goes offscale at 400A
for Vce above 4 volts. The current at Vce = 5V (just below the 5.2V
trigger) could be as high as 550A, which would be a power dissipation
level of 2750 watts. The transient thermal resistance curves show
this power level can be sustained for only about 15-milli-seconds,
with a 100C junction-temperature rise. A cautious user could reduce
the Vce-fault value, e.g. 4V (400A, 1600W, 0.1s) with Rce = 14k, etc.,
but the IGBT could still be damaged with exactly the right conditions.
But this exact condition (a sustained current of more than say 250A,
but less than 400 to 500A) may be rather unusual and hard to create.