High voltage diode blowing mysteriously

[Tim Williams]

I have a high voltage power supply,
http://myweb.msoe.edu/williamstm/Images/Tubescope_Supply2.png
under reasonable load (140VDC link voltage; Vadj set for 230V output; 3A heaters, fractional mA at -2kV, 100mA at +230V, 35mA at -230V). It runs cool and smooth for about a minute (aside from the snubbers, which get quite hot), then suddenly the output drops dead and the current limit starts squealing. One of the negative output diodes is failing shorted. (Good thing the current limit keeps it from nuking the transistors.)

Until failure, the diodes run cool (aside from what heat they pick up from the snubbers). The waveforms show 120V overshoot, which is well within ratings (1000V diode with about 600V peak reverse). I can't imagine it's an avalanche thing, as the reverse voltage is low and, until failure, the diodes run cool. I'm still more confused that it's consistently the negative side diode (three have died so far), which is the lighter loaded side.

Tim

Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

 

[Ban]

I have a high voltage power supply,
http://myweb.msoe.edu/williamstm/Images/Tubescope_Supply2.png
under reasonable load (140VDC link voltage; Vadj set for 230V output;
3A heaters, fractional mA at -2kV, 100mA at +230V, 35mA at -230V).
It runs cool and smooth for about a minute (aside from the snubbers,
which get quite hot), then suddenly the output drops dead and the
current limit starts squealing. One of the negative output diodes is
failing shorted. (Good thing the current limit keeps it from nuking
the transistors.)

Until failure, the diodes run cool (aside from what heat they pick up
from the snubbers). The waveforms show 120V overshoot, which is well
within ratings (1000V diode with about 600V peak reverse). I can't
imagine it's an avalanche thing, as the reverse voltage is low and,
until failure, the diodes run cool. I'm still more confused that
it's consistently the negative side diode (three have died so far),
which is the lighter loaded side.

Tim

Your transformer seems to have a lot of stray-inductance, a very simple way
to suppress those high spikes is to simply put a resistor across the
secondary around 500k/1W should be a starting point. The snubbers will not
help much, since the secondary floats when all diodes are cutoff.

ciao Ban

 

[Joerg]

I have a high voltage power supply,
http://myweb.msoe.edu/williamstm/Images/Tubescope_Supply2.png
under reasonable load (140VDC link voltage; Vadj set for 230V output; 3A heaters, fractional mA at -2kV, 100mA at +230V, 35mA at -230V). It runs cool and smooth for about a minute (aside from the snubbers, which get quite hot), then suddenly the output drops dead and the current limit starts squealing. One of the negative output diodes is failing shorted. (Good thing the current limit keeps it from nuking the transistors.)

Until failure, the diodes run cool (aside from what heat they pick up from the snubbers). The waveforms show 120V overshoot, which is well within ratings (1000V diode with about 600V peak reverse). I can't imagine it's an avalanche thing, as the reverse voltage is low and, until failure, the diodes run cool. I'm still more confused that it's consistently the negative side diode (three have died so far), which is the lighter loaded side.

I don't see any snubbers of clamps on the upper right transformer where
it says 102T.

BTW, it helps to turn on designators. TR1, Q5, R6, and so on.

 

[Joerg]

TR? A transistor is Q. A transformer is T.

John

Sir, yes, Sir! <clicking heels, saluting>

On mil schematics that's the case but civilian ones are all over the map
in that respect. You should see the new DIN or whatever standard WRT
designators, it's the epitome of bureaucratic nonsense. Designators
different for the same type of component and depending on its function.

I just had one that said TR, another one XFMR.

 

[Tim Williams]

Not even worth the plonk.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

I have a

STOP posting text with greater than 72 character line length, you
retarded Usenet abusing IDIOT!

 

[Tim Williams]

Capacitors? Is not 10uF / 250 V too low for 250 V output?
Maybe a cap starts drawing a lot of current?
Also check the .47 uF.

Err, choke input, forward coverter.

Peak current is not much greater than average, there are gobs of inductance everywhere and only parasitic capacitance. The filter choke is bank wound thusly:
http://myweb.msoe.edu/williamstm/Images/Coil.jpg

Tim

 

[Tim Williams]

Your transformer seems to have a lot of stray-inductance, a very
simple way to suppress those high spikes is to simply put a resistor
across the secondary around 500k/1W should be a starting point.

That's not enough. As noted on the schematic, the diodes have a 5.6k + 47pF snubber across *each*, thus across the secondary as well. This dampens the stray inductance quite effectively, leaving only the overshoot that I noted.

The
snubbers will not help much, since the secondary floats when all
diodes are cutoff.

Err, the CT is grounded?? Filter choke current is continuous so the secondary voltage is held quite accurately at 0V during dead time.

Tim

 

[Tim Williams]

I don't see any snubbers of clamps on the upper right transformer where
it says 102T.

Ran out of room to draw them. The note just above indicates the nature of said snubbers.

BTW, it helps to turn on designators. TR1, Q5, R6, and so on.

"Turn on"? I draw these in Paint. Refdes are a hassle

Call 'em "high voltage diodes", and not "low voltage schottky", "EHV diodes" or "FWB". I'll know exactly what you're referring to, just as well as "D17-D20" or whatever.

Tim

 

[Ban]

Your transformer seems to have a lot of stray-inductance, a very
simple way to suppress those high spikes is to simply put a resistor
across the secondary around 500k/1W should be a starting point.

That's not enough. As noted on the schematic, the diodes have a 5.6k + 47pF
snubber across *each*, thus across the secondary as well. This dampens the
stray inductance quite effectively, leaving only the overshoot that I noted.

The
snubbers will not help much, since the secondary floats when all
diodes are cutoff.

Err, the CT is grounded?? Filter choke current is continuous so the
secondary voltage is held quite accurately at 0V during dead time.

Tim

If you look at your design, where does the current from the leakage
inductance flow? there is a big choke and no caps behind the rectifier and
on the other side is the transformer where the current comes from. No way of
going into the ground.
Try my suggestion and look at the voltage with a really high-z 100:1 probe,
then you will see the high peaks.
ciao Ban

 

[Tim Williams]

If you look at your design, where does the current from the leakage
inductance flow?

It twangs through the primary circuit (which is an approximate constant voltage source, making it series resonant with the winding's and choke's parasitic capacitance), which gets sucked up by the snubbers I said are there.

There is a measurable and finite overshoot, of about 120V as I stated. This puts the peak reverse voltage around 640V, which is within ratings. There is very little ringing; the damped Q is quite low, maybe 2. The resistors get warm.

there is a big choke and no caps behind the rectifier and
on the other side is the transformer where the current comes from. No way of
going into the ground.
Try my suggestion and look at the voltage with a really high-z 100:1 probe,
then you will see the high peaks.

All I have is a 10M probe. Fortunately, the impedance in the area is around 5.6kohms and 47pF (28kohms reactance at the fundamental; much less for harmonics), so this is sufficiently high.

Detail:
http://myweb.msoe.edu/williamstm/Images/Tubescope_Snubber.png
As you can see, there are in fact snubbers across the diodes.

Do you see anything that would kill the diodes?

Tim

 

[Tim Williams]

Do not drink while posting

Well, considering you seem to have writing/comprehension problems, I'll go easy on you and give you a visual aid.
http://myweb.msoe.edu/williamstm/Images/Tubescope_Snubber.png

Tim

 

[Ban]

If you look at your design, where does the current from the leakage
inductance flow?

It twangs through the primary circuit (which is an approximate constant
voltage source, making it series resonant with the winding's and choke's
parasitic capacitance), which gets sucked up by the snubbers I said are
there.

There is a measurable and finite overshoot, of about 120V as I stated. This
puts the peak reverse voltage around 640V, which is within ratings. There
is very little ringing; the damped Q is quite low, maybe 2. The resistors
get warm.

there is a big choke and no caps behind the rectifier and
on the other side is the transformer where the current comes from. No way
of
going into the ground.
Try my suggestion and look at the voltage with a really high-z 100:1
probe,
then you will see the high peaks.

All I have is a 10M probe. Fortunately, the impedance in the area is around
5.6kohms and 47pF (28kohms reactance at the fundamental; much less for
harmonics), so this is sufficiently high.

Detail:
http://myweb.msoe.edu/williamstm/Images/Tubescope_Snubber.png
As you can see, there are in fact snubbers across the diodes.

Do you see anything that would kill the diodes?

Yes, the puls from the leakage inductance cannot flow into the snubbers
because the 10mH inductance behind will block that current. If you put a
capacitor across the rectifier output, the spikes can be absorbed. Also this
will prevent this point being pulled negative by the 10mH choke, which will
double the voltage across the diodes.
Make some tests with 2 probes in differential mode to see the true voltage
across the diodes.

 

[Hammy]

It twangs through the primary circuit (which is an approximate constant
voltage source, making it series resonant with the winding's and choke's
parasitic capacitance), which gets sucked up by the snubbers I said are
there.

There is a measurable and finite overshoot, of about 120V as I stated. This
puts the peak reverse voltage around 640V, which is within ratings. There
is very little ringing; the damped Q is quite low, maybe 2. The resistors
get warm.


All I have is a 10M probe. Fortunately, the impedance in the area is around
5.6kohms and 47pF (28kohms reactance at the fundamental; much less for
harmonics), so this is sufficiently high.

Detail:
http://myweb.msoe.edu/williamstm/Images/Tubescope_Snubber.png
As you can see, there are in fact snubbers across the diodes.

Do you see anything that would kill the diodes?

Yes, the puls from the leakage inductance cannot flow into the snubbers
because the 10mH inductance behind will block that current. If you put a
capacitor across the rectifier output, the spikes can be absorbed. Also this
will prevent this point being pulled negative by the 10mH choke, which will
double the voltage across the diodes.
Make some tests with 2 probes in differential mode to see the true voltage
across the diodes.

You need to take differential measurments.

I plugged in some estimates based on an absouloute output voltage of
500V and they do indeed see 1.9KV.

http://i48.tinypic.com/2ec3z89.png

 

[Joerg]

And BOTH are incorrect designations. They can be used in item
descriptions (xfmr), but reference designators have an industry
standard
and your remark that they do not is noncorrect in all circles excpet
those
where some stupid dope like you refuses to use the industry
standard(s).
So, essentially you hang out in some stupid clics. Designators have
had variances based on device function. Diodes are a perfect example.
We see "D1" or "CR1", where "CR" was derived from "Cathode
Rectifier".

Just FWIW: What you call stupid clics are often rather large and
successful enterprises, the products of which even you will experience
on a regular basis. Like when you are going places ...

 

[StickThatInYourPipeAndSmokeIt]

Well, the idiot is you, just look up RFC977

No, the idiot is you.

http://james.apache.org/server/rfclist/nntp/rfc0977.txt

The idiot is you. It doesn't even get addressed.

I acually wrote a newsreader, while you are just a google sucker without even a clue about it.

No, you claim to have done so. Since it is not a current, mainstream
news client, I have serious doubts that you succeeded. And the fact that
you wrote that line with an excessive length proves that you do not know
the first thing about it.

snipped retarded link to retarded attempt at writing a real application.

The reset of your text ejaculations stink too.

No, asshole. Idiots like you that claim to be civil, yet refuse to
follow KNOWN, ESTABLISHED conventions that have been around for decades
prove that ejaculation was wasted by your entire bloodline. You
contaminate the human experience with animal stupidity.

That rfc addresses COMMAND line entries, idiot. It is about NNTP
servers, idiot. Is says NOTHING about Usenet posting protocols or
conventions, idiot.

Go stink somewhere else.

 

[Joerg]

Ran out of room to draw them. The note just above indicates the
nature of said snubbers.

Nope. No capacitance values, no resistor values.

"Turn on"? I draw these in Paint. Refdes are a hassle


Call 'em "high voltage diodes", and not "low voltage schottky", "EHV
diodes" or "FWB". I'll know exactly what you're referring to, just
as well as "D17-D20" or whatever.

Did you scope the voltage at the transformer secondary? And the current
into the diodes, with a current transformer?

 

[Kevin McMurtrie]

I have a high voltage power supply,
http://myweb.msoe.edu/williamstm/Images/Tubescope_Supply2.png
under reasonable load (140VDC link voltage; Vadj set for 230V output; 3A
heaters, fractional mA at -2kV, 100mA at +230V, 35mA at -230V). It runs cool
and smooth for about a minute (aside from the snubbers, which get quite hot),
then suddenly the output drops dead and the current limit starts squealing.
One of the negative output diodes is failing shorted. (Good thing the
current limit keeps it from nuking the transistors.)

Until failure, the diodes run cool (aside from what heat they pick up from
the snubbers). The waveforms show 120V overshoot, which is well within
ratings (1000V diode with about 600V peak reverse). I can't imagine it's an
avalanche thing, as the reverse voltage is low and, until failure, the diodes
run cool. I'm still more confused that it's consistently the negative side
diode (three have died so far), which is the lighter loaded side.

Tim

Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Is the part labeled "2 x 10mH 100mA 300T" causing problems when the +/-
250V outputs currents aren't balanced?

 

[Tim Williams]

You need to take differential measurments.

I plugged in some estimates based on an absouloute output voltage of
500V and they do indeed see 1.9KV.

http://i48.tinypic.com/2ec3z89.png

Well that's obviously wrong. The transformer only makes 300V peak output (600p-p, not counting LL), and it's obviously full wave, not half wave. The driver is clearly half bridge and the transformer ratio is clearly 1:4+4.
http://myweb.msoe.edu/williamstm/Images/Tubescope_Calc.png
These are my actual design constraints, actually, and the waveforms are very familiar indeed. Add some laggy parasitic C, and some springy LL, and you've got the real thing. The only difference between this circuit and the actual circuit is the center tap and split choke, which do not affect the simulation results, and serve only to split the resulting DC output in half, hence +/-250V.

Tim

 

[Tim Williams]

Nope. No capacitance values, no resistor values.

Not even in the note that says "5.6k + 47pF"???

Did you scope the voltage at the transformer secondary? And the current
into the diodes, with a current transformer?

Voltage on both sides, yes. Current, no. I soppose recovery current could be interesting; of course, that will be snubbed quite effectively by leakage, so it will certainly be finite...

Hmm, I know the peak overshoot. I should be able to estimate peak current from that, if I know LL and C. Meh, it'll probably be within 50% of "it all comes from the primary", which isn't accurate enough to estimate recovery.

UF4007 is rated for 20-30A peak. They aren't heating up very much, at least until failure. Do you think it could be peak current?

Tim

 

[Tim Williams]

Is the part labeled "2 x 10mH 100mA 300T" causing problems when the +/-
250V outputs currents aren't balanced?

It doesn't seem to be. The waveforms on either end look identical and inverted. The currents aren't aall that different. 30mA seems to be enough to cause continuous conduction mode, so it's not trying to play funny on that side.

Tim