High voltage diode blowing mysteriously

I have a high voltage power supply,

formatting link
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:

formatting link

Reply to
Tim Williams
Loading thread data ...

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

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.

Reply to
Jan Panteltje

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

Reply to
Ban

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.)

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.

--
Regards, Joerg

http://www.analogconsultants.com/
 Click to see the full signature
Reply to
Joerg

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.)

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.

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

John

Reply to
John Larkin

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.)

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.

Sir, yes, Sir!

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/
 Click to see the full signature
Reply to
Joerg

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

Reply to
Nunya

Snipped incorrectly formatted post body

Cut your line length to the Usenet standard, Mr. Ignore the rest of the world. Some folks need to be whacked upside da head.

In HV, you can have multiplier caps that 'appear' to be good, but they are not able to be tested properly at low voltages. I would trade out ALL of the HV multiplier caps.

Reply to
Nunya

On Sun, 04 Jul 2010 17:33:14 -0700, Joerg wrote: (snip)

(snip rest)

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

Reply to
fly on the wall

Not even worth the plonk.

Tim

--=20 Deep Friar: a very philos> I have a

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

Reply to
Tim Williams

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:

formatting link

Tim

--=20 Deep Friar: a very philosophical monk. Website:

formatting link

Reply to
Tim Williams

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.

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:

formatting link

Reply to
Tim Williams

where

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

"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

--=20 Deep Friar: a very philosophical monk. Website:

formatting link

Reply to
Tim Williams

3A heaters, fractional mA at -2kV, 100mA at +230V, 35mA at -230V). =A0It r= 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. =A0One of the negative output diodes is failing shorted. =A0(G= ood thing the current limit keeps it from nuking the transistors.)

from the snubbers). =A0The waveforms show 120V overshoot, which is well wi= thin ratings (1000V diode with about 600V peak reverse). =A0I can't imagine= it's an avalanche thing, as the reverse voltage is low and, until failure,= the diodes run cool. =A0I'm still more confused that it's consistently the= negative side diode (three have died so far), which is the lighter loaded = side.

e

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".

Reply to
Nunya

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.

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

Reply to
Ban

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

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

Nice, some HV coil design.

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.

Reply to
Jan Panteltje

On a sunny day (Sun, 4 Jul 2010 19:57:32 -0700 (PDT)) it happened Nunya wrote in :

What is your problem, there is no limit to line length in Usenet, so it is up to the artistic capabilities of the creator of the posting to format the text anyway they want, in fact newsreaders who garble that text by reformatting violate the principle of conservation of of creativity, so take that. . .

Reply to
Jan Panteltje

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.

way of=20

probe,=20

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:

formatting link
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:

formatting link

Reply to
Tim Williams

Well, considering you seem to have writing/comprehension problems, I'll = go easy on you and give you a visual aid. :-)

formatting link

Tim

--=20 Deep Friar: a very philosophical monk. Website:

formatting link

Reply to
Tim Williams

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:

formatting link
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.

Reply to
Ban

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.