High voltage diode blowing mysteriously
in [Design]
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From: Tim Williams on 4 Jul 2010 16:52 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
From: Jan Panteltje on 4 Jul 2010 17:56 On a sunny day (Sun, 4 Jul 2010 15:52:50 -0500) it happened "Tim Williams" <tmoranwms(a)charter.net> wrote in <5x6Yn.15761$dx7.222(a)newsfe21.iad>: >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. 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. >Tim > >Deep Friar: a very philosophical monk. >Website: http://webpages.charter.net/dawill/tmoranwms >
From: Ban on 4 Jul 2010 18:05 Tim Williams wrote: > 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
From: Joerg on 4 Jul 2010 18:28 Tim Williams wrote: > 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. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 4 Jul 2010 20:23
On Sun, 04 Jul 2010 15:28:01 -0700, Joerg <invalid(a)invalid.invalid> wrote: >Tim Williams wrote: >> 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. TR? A transistor is Q. A transformer is T. John |