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RE: 3 Phase Full rectification, doide protection, HV Inductor needed, advise urged.!
Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>
At 03:12 PM 9/21/2006, you wrote:
Original poster: "Leigh Copp" <Leigh.Copp@xxxxxxxxxxx>
Jim (both of you I guess),
A few thoughts on the HV rectifier game:
I'll agree that while the tubes are a little more resilient, they are
pretty expensive to replace.
Not putting voltage sharing networks across the individual diodes
however would not be something I would recommend, but your mileage may
vary ;)
Unless you measure your diodes to determine their reverse breakdown
characteristic, and ensure that they are all at the same temperature,
they will not share voltage, and in a fault condition when the first
diode in the string fails, the little inductive snap and the step change
in voltage across the rest of the string will cause the spike to go
through the rest of your series devices like poop through a goose.
These days, diodes have very consistent reverse breakdown, or, more
important, reverse leakage properties, but that's actually not the
problem. If you over design, none of them will breakdown anyway,
even with some imbalance. All you care is that the least leaky diode
(which will have the most voltage) is still below it's rating.
The internal junction capacitance and resistance of these guys is not
necessarily consistent either - yet another reason to balance the
unequal impedances with relatively low impedance parallel paths.
I think they are more consistent these days than you think.
If my hastily rigged voltage divider and scope had survived at the time
(young, poor, and stupid at the time - enough said) I could show you the
waveform from one particular episode of the sort I am describing above.
And speaking of transients, and reverse recovery time, an R-C snubber
circuit across the individual diodes also reduces dv/dt handily to help
protect against such things.
100k ohms in parallel with 0.1 uF is very common when we have a 6-40 amp
DC supply to work with (BTW: my frame of reference is 50-800kW plate
supplies). The resulting losses may be unacceptable with the size of
your dc supply (1.4 amp diodes were mentioned?), however, so you may
need to increase "R" and "C".
Exactly.. Say you consider a 5 kW supply (1/10th-1/100th the size
which you cite), so say you increase the impedances by the same
factor.. 1-10 Meg, 0.01-0.001 uF. The leakage current through the
diodes will start to be more than the current through the equalizing resistors.
Not that one can't design appropriate snubbers, but I think that for
the vast majority of non-ragged edge applications, they might
actually cause more problems than they solve.
As I recall, the purpose of the C is to slow down the rise time of
the reverse voltage as the diode starts to turn off. Modern HV
diodes have slow (but consistent) recovery (aka "soft") for just this
reason. If you do have the parallel C, it's just stored energy next
to the diode to help cook it, if it does breakdown.
The other thing is that modern HV diodes are designed to safely
operate in the avalanche (Zener) mode for a short time (while the
rest of the string turns off). For normal sized power supplies (few
kW max, I would imagine) you can calculate the amount of energy
deposited in the junction during the turn off event, and it's small
enough that the device won't fail. In fact, putting resistors around
every diode increases the current through the diodes that turn off
early, actually making the overall string less reliable.
The other issue is that you should trade off using more diodes vs the
cost of resistors and capacitors. After all, the resistors will have
to dissipate the reverse current, and both R and C have to be rated
for the voltage and current they'll see.. they'll probably be more
expensive than the HV diodes, which are truly cheap.
Just as an example, if you put 100k and 0.1 uF across each 1kV diode,
you'll be dissipating 10W in the resistor (while reverse biased)...
in reality, it will be more like a few watts average (half cycle, rms
vs pk). And, the impedance of 0.1uF at 60 Hz is about 26k ohms,
which will appear like an extra load of about 30-40 mA.
Just goes to show that this is something worth analyzing, rather than
blindly following a suggestion from any of us.
There was a discussion of this on the list some years ago. The semi
mfrs data book (Philips makes HV diodes intended for series strings)
goes into some detail on it, as well. The only thing to be careful
about is to check the date on the mfr ap notes. What was common
practice in 1985 or 1990 may not be so common any more.