High voltage diode blowing mysteriously
in [Design]
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From: Kevin McMurtrie on 5 Jul 2010 17:14 In article <5x6Yn.15761$dx7.222(a)newsfe21.iad>, "Tim Williams" <tmoranwms(a)charter.net> 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 > > 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? -- I won't see Google Groups replies because I must filter them as spam
From: Tim Williams on 5 Jul 2010 19:04 "Hammy" <spam(a)spam.com> wrote in message news:0qk336ptcj6mn9bso5omb6mu1l95ttgqp0(a)4ax.com... > 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 -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Tim Williams on 5 Jul 2010 19:09 "Joerg" <invalid(a)invalid.invalid> wrote in message news:89elf4Fa7sU2(a)mid.individual.net... > 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 -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Tim Williams on 5 Jul 2010 19:11 "Kevin McMurtrie" <mcmurtrie(a)pixelmemory.us> wrote in message news:4c324b1d$0$22128$742ec2ed(a)news.sonic.net... > 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 -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Joerg on 5 Jul 2010 20:21
Tim Williams wrote: > "Joerg" <invalid(a)invalid.invalid> wrote in message > news:89elf4Fa7sU2(a)mid.individual.net... >> Nope. No capacitance values, no resistor values. > > Not even in the note that says "5.6k + 47pF"??? > Whoops, didn't see that, was looking in your text. And shouldn't it say snubbered, snubberated or snubberificated? :-) Best to put them on the schematics. 47pF is almost nothing. Are you sure this quenches ringing enough? >> 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. ... That's for non-repetitive. > ... They aren't heating up very much, > at least until failure. Do you think it could be peak current? > Possibly, because at 75nsec trr these are fairly slow. Since you probably don't want to spring for four of those Cree super thingamagics which would totally raid the beer kitty, why the 2x10mH common mode choke? Tried separate chokes yet? Of course, if you came into a wad of cash: http://www.cree.com/products/pdf/C2D05120.pdf BTW, for better schematics drawings you can use the free edition of Cadsoft Eagle, as long as it's hobby and not for profit use. Later when you want to buy it for business it's rather inexpensive. Not like Orcad. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |