Q=C*U? where Q is constant, but C decreases (e.g 10%) under the influence of increasing U -> making U even higher (e.g. 10 times higher).
I didn't get that bit...
What I?had in mind is the capacitance-loss that comes with X5R material when DC-voltage increases: e.g. a 100nF cap becomes 90nF at 5V and 10nF at 12V. Now when this capacitor is charged at 5V the charge Q = 5 * 90nF = 450n. Now when stepping the 5V to 12V the capacitance suddenly becomes 10nF, but since we already had 450n charge the new voltage becomes a voltage spike of 450n / 10nF = 45V. Does this make sense? I could be mistaken.
Doesn't make sense to me. I know X5R capacitors suffer capacitance loss at increased voltage but 90% capacitance loss by 12V seems higher than I would expect; the bit about the voltage spike is not anything I've heard about before. And those capacitors are on low impedance supply rails, it's hard to imagine a harmful voltage spike developing out of that. Any other comments from people here, on that topic??
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So my question is still... Wouldn't just making the zener diode bigger fix the sensitivity to this unusual supply scenario? In my tests, with the standard circuit (500mW zeners) I was able to do 6V to 9V, 10V, 11V steps without issue and the zener ate the brief over-voltage. It even did on 6 to 12V steps but not every time. So given that it almost works even with the 500mW zeners isn't it worth trying a 5W zener say, being optimistic that it may be able to swallow that extra current for a millisecond? Then if QMX has a specified supply voltage range of 7V to 12V say, you'd be able to do anything at all with the supply voltage in that range without any fear of damage.
True, that using bigger diodes might fix it eventually, the transients are?short. But on the other hand with L101=1.5R at 12V there is a theoretical peak current of more than 4A (12-5.6)/1.5, peaking to more than 25W 4*(12-5.6), so 5W might not be sufficient.
I'm not sure of your calculation; there are three P-ch MOSFETs in series with the 12V supply too; furthermore the 330uH I measured here at 2.0 ohms.?
The 5W zener diode I am considering is?SMBJ5339B, whose datasheet specifies a maximum surge current of 13.4 A; where?
"The Surge Current (IZM) is Specified As The Maximum Peak of a Nonrecurring Sine Wave of 8.3 Milliseconds Duration."
This makes it look well capable enough to withstand a couple of amps for 1 or 2 milliseconds.?
