High voltage diode blowing mysteriously
in [Design]
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From: Tim Williams on 5 Jul 2010 02:20 "Jan Panteltje" <pNaonStpealmtje(a)yahoo.com> wrote in message news:i0r025$ofk$1(a)news.albasani.net... > Capacitors? Is not 10uF / 250 V too low for 250 V output? > Maybe a cap starts drawing a lot of current? > Also check the .47 uF. Err, choke input, forward coverter. Peak current is not much greater than average, there are gobs of inductance everywhere and only parasitic capacitance. The filter choke is bank wound thusly: http://myweb.msoe.edu/williamstm/Images/Coil.jpg Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Tim Williams on 5 Jul 2010 02:22 "Ban" <bansuri(a)web.de> wrote in message news:i0r0j2$p4q$1(a)news.eternal-september.org... > 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 -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Tim Williams on 5 Jul 2010 02:25 "Joerg" <invalid(a)invalid.invalid> wrote in message news:89cg7iFchaU3(a)mid.individual.net... > 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 -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Nunya on 5 Jul 2010 03:12 On Jul 4, 5:33 pm, Joerg <inva...(a)invalid.invalid> wrote: > John Larkin wrote: > > On Sun, 04 Jul 2010 15:28:01 -0700, Joerg <inva...(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 > > 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. > 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".
From: Ban on 5 Jul 2010 05:41
"Tim Williams" <tmoranwms(a)charter.net> schrieb im Newsbeitrag news:XSeYn.7090$RC5.1226(a)newsfe08.iad... "Ban" <bansuri(a)web.de> wrote in message news:i0r0j2$p4q$1(a)news.eternal-september.org... > 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 |