High voltage diode blowing mysteriously

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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

Re: High voltage diode blowing mysteriously


On a sunny day (Sun, 4 Jul 2010 15:52:50 -0500) it happened "Tim Williams"

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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.


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Re: High voltage diode blowing mysteriously


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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

--20%
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Re: High voltage diode blowing mysteriously


On a sunny day (Mon, 5 Jul 2010 01:20:02 -0500) it happened "Tim Williams"

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Yes, so? I would not use  a 250 V elco on a 250 V supply.

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Nice, some HV coil design.


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Do not drink while posting :-)

I still am not sure what diode[s] yo uare referring to.
As somebody mentionioned, use some reference Q1 Q1, D1 D2, T1 T2 etc.
Else answering is a bit like playing the lotto.

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Re: High voltage diode blowing mysteriously


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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

--20%
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Re: High voltage diode blowing mysteriously


On a sunny day (Mon, 5 Jul 2010 05:30:03 -0500) it happened "Tim Williams"

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PK, great, well maybe that other poster has a point mentioning huge voltage
peaks.
I could imagine bottom of 102T spiking positive while left bottom connection of
300T spiking negative.
Maybe you think that is damped by the other positive diodes, but then that 300T
reverses the phase :-)
If top left of 300T goes positive, then also bottom left of 300T will go
negative.
But I would check caps first.


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Re: High voltage diode blowing mysteriously


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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



Re: High voltage diode blowing mysteriously


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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.

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

Tim

--20%
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Re: High voltage diode blowing mysteriously



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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.

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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



Re: High voltage diode blowing mysteriously


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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.

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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

--20%
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Re: High voltage diode blowing mysteriously



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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.

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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.



Re: High voltage diode blowing mysteriously



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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

Re: High voltage diode blowing mysteriously


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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

--20%
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Re: High voltage diode blowing mysteriously



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wave.  The driver is clearly half bridge and the transformer ratio is =
clearly 1:4+4.
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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.
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And your line of thought goes to heck in a handbasket due to the inductor
input filter!  Put some capacitance right at the output of the bridge,
470 pf ought to do.  I bet it cools off your snubbers too.

Re: High voltage diode blowing mysteriously


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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.)
Quoted text here. Click to load it
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.
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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.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: High voltage diode blowing mysteriously



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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.)
Quoted text here. Click to load it
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.
Quoted text here. Click to load it

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

John


Re: High voltage diode blowing mysteriously


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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.)
Quoted text here. Click to load it
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.
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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.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: High voltage diode blowing mysteriously


(snip)
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(snip rest)

No, RoHS and mandating CFL's share the epitome.  Everything else is
struggling for a place behind them.

Re: High voltage diode blowing mysteriously


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uns 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 start=
s squealing. A0%One of the negative output diodes is failing shorted. A0%(G=
ood thing the current limit keeps it from nuking the transistors.)
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thin ratings (1000V diode with about 600V peak reverse). A0%I can't imagine=
 it's an avalanche thing, as the reverse voltage is low and, until failure,=
 the diodes run cool. A0%I'm still more confused that it's consistently the=
 negative side diode (three have died so far), which is the lighter loaded =
side.
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  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".

Re: High voltage diode blowing mysteriously


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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.)
Quoted text here. Click to load it
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.
Quoted text here. Click to load it


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 ...

--
Regards, Joerg

http://www.analogconsultants.com /

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