Re: Trying to learn a lesson from failure (IGBT bridge)
- From: "Tim Williams" <tmoranwms@xxxxxxxxxxx>
- Date: Thu, 1 Dec 2005 16:21:09 -0600
"Ignoramus5455" <ignoramus5455@xxxxxxxxxxxxxxxxxxx> wrote in message
news:bepjf.2814$xf5.2309@xxxxxxxxxxxxxxxxxxxxxxxx
> If so, a simple way to prevent this would be to install a diode such
> that if the DC+ voltage drops below DC- voltage, it would instantly
> start conducting. Is that at all sensible?
So let me get this straight: when the bridge switches state, it creates a
[dV/dt or dI/dt?] that causes the power rail to swing below 0V?
With no potentials inherently below 0V, this requires a combination of L and
C, probably activated by the transistors (that is, the L and C are connected
together in such a way that voltage will swing under 0V, rather than already
being connected and the change in voltage/current exciting the resonance),
so that parasitic (or intentional!) stored energy is able to invert the
voltage like that.
Once the voltage is negative, who knows... if your IGBTs don't have internal
diodes (and/or the snubber boards don't have diodes), you could get all
sorts of troubles. Transistors generally don't appreciate negative
collector voltage, and I have no idea what that would do to an IGBT in
particular. (An NPN transistor would pull the base down (until the E-B
junction fails) because the B-C junction conducts in that direction, but
IGBTs are a bit more than a BJT.)
Tim
--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
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