richie00boy said:
Originally posted by Nigel Goodwin
Perhaps you should try using diodes that work?
Can you please clarify what you mean by that?
Just that you said "I like to use Schottky ones", but they aren't working - so why would you prefer to use diodes that don't work?. Notice that I included a smiley as well!.
For a slightly different take on the topic; Diodes are cheap, and I have always substantially over specified, both for current and voltage. Check the short term surge ratings. Controlled reverse breakdown characteristics help too.
The power line can bring evil stuff to your doorstep, not least during lightning storms. Motors can also be the source of voltage spikes.
I like to put surge protection after the diode bridge, using surge rated Zeners; if they ever conduct that puts a current surge through the bridge rectifiers until the transformer collapses or the fuse blows.
I spent a few years designing high power professional amplifiers; our president had previously repped an industry standard amplifier brand. His observation matched mine, which was that the diode bridges were perhaps the most common failure point, even though they were apparently spec'd well for the job. So we overspec'd ours significantly, and had very few failures. And the cost differential was not at all high.
And I agree with the comments above that Schottky Diodes are not appropriate for this position. Their speed is of no value in this application, and they are very fragile compared to the good ol' clunkers.
The power line can bring evil stuff to your doorstep, not least during lightning storms. Motors can also be the source of voltage spikes.
I like to put surge protection after the diode bridge, using surge rated Zeners; if they ever conduct that puts a current surge through the bridge rectifiers until the transformer collapses or the fuse blows.
I spent a few years designing high power professional amplifiers; our president had previously repped an industry standard amplifier brand. His observation matched mine, which was that the diode bridges were perhaps the most common failure point, even though they were apparently spec'd well for the job. So we overspec'd ours significantly, and had very few failures. And the cost differential was not at all high.
And I agree with the comments above that Schottky Diodes are not appropriate for this position. Their speed is of no value in this application, and they are very fragile compared to the good ol' clunkers.
Thanks for that, it was useful, especially the bit about Schottky's being fragile. I will try normal 1N4002 diodes and see how that goes. However, I'm still puzzled and would like to be sure why they blew. They are 1A rated parts and within voltage spec, IMO they have been overspecced compared to what they were doing.
Hi,
I'm using Schottkys MBR1060 (60V/10A) in my GCs ( transformer's secondary voltages are 25V) without any problem.
http://www.moxtone.com/3875.htm
Regards,
Milan
I'm using Schottkys MBR1060 (60V/10A) in my GCs ( transformer's secondary voltages are 25V) without any problem.
http://www.moxtone.com/3875.htm
Regards,
Milan
The General Semi Data sheet for the 1N5059 series is illustrative of what I have in mind. Non-repetitive reverse current capability is shown as a mJ max, and also dissipation vs surge duration.
I prefer avalanche breakdown rectifiers as they are guaranteed to conduct over the most to all of the die area during reverse breakdown. Non-avalanche may conduct over a small area, leading to failure at much lower currents due to the current concentration causing a local hot spot.
And some Schottkies are so rated; and they are good for forward surge current. But their "natural" application is low forward drop, low voltage, high speed rectification, as noted above, in switch mode power supplies.
But with the cost differential usually so low, I tend to use a 400V diode and not worry about reverse breakdown. And usually, a 1N4002 say, will actually have 400V reverse breakdown. Not that you should count on it of course. Modern semi manufacturing is so precise that yields will usually be close to 100% of the higher spec units in the series. On the other hand, a 100V Schottky is getting close to the maximum voltage attainable with that technology. That's why >100V units are hard to find. And there certainly won't be a 200-400% probable margin.
Before I designed power amplifiers, I designed telephone equipment that connected to the phone lines, and both for practical and regulatory reasons surge survival is vital. So I cheerfully admit to a bit stronger concern for these issues than most.
Taking a larger look at the failure issue; diodes fail for only two applications reasons; overvoltage and overcurrent. With that in mind, you should be able to trouble shoot your doubler circuit. There was a very interesting thread on counterfeit parts, although that seems unlikely with diodes.
And finally none of what I've written is intended to contradict Peranders or Milan, both of whom I agree with.
I prefer avalanche breakdown rectifiers as they are guaranteed to conduct over the most to all of the die area during reverse breakdown. Non-avalanche may conduct over a small area, leading to failure at much lower currents due to the current concentration causing a local hot spot.
And some Schottkies are so rated; and they are good for forward surge current. But their "natural" application is low forward drop, low voltage, high speed rectification, as noted above, in switch mode power supplies.
But with the cost differential usually so low, I tend to use a 400V diode and not worry about reverse breakdown. And usually, a 1N4002 say, will actually have 400V reverse breakdown. Not that you should count on it of course. Modern semi manufacturing is so precise that yields will usually be close to 100% of the higher spec units in the series. On the other hand, a 100V Schottky is getting close to the maximum voltage attainable with that technology. That's why >100V units are hard to find. And there certainly won't be a 200-400% probable margin.
Before I designed power amplifiers, I designed telephone equipment that connected to the phone lines, and both for practical and regulatory reasons surge survival is vital. So I cheerfully admit to a bit stronger concern for these issues than most.
Taking a larger look at the failure issue; diodes fail for only two applications reasons; overvoltage and overcurrent. With that in mind, you should be able to trouble shoot your doubler circuit. There was a very interesting thread on counterfeit parts, although that seems unlikely with diodes.
And finally none of what I've written is intended to contradict Peranders or Milan, both of whom I agree with.
Thanks for the informative post, that's the kind of info I like to read
I'm confused by the following though
as during reverse breakdown (I assume you mean the same thing as reverse bias basically) why would you care about what portion of the die conducts? As surely you don't want it to conduct at all during reverse bias?
Or are you saying that the reverse leakage current which will always be present, concentrates on hot spots with some types which causes them to blow?
I'm taking your hints but still at a loss as to why these diodes failed - they were not even at the edge of any of their parameter envelopes.
I'm confused by the following though
as during reverse breakdown (I assume you mean the same thing as reverse bias basically) why would you care about what portion of the die conducts? As surely you don't want it to conduct at all during reverse bias?
Or are you saying that the reverse leakage current which will always be present, concentrates on hot spots with some types which causes them to blow?
I'm taking your hints but still at a loss as to why these diodes failed - they were not even at the edge of any of their parameter envelopes.
When a nasty HV transient strikes, and the reverse voltage rating of the diode is exceeded, it will go into avalanche mode (like a very HV zener) and has a better chance of surviving the incident than an ordinary diode. Purely for surviving very rare events. Like lightning strike near the power lines.
I prefer to put a transient rated zener after the bridge; Varistors (which could go in front) have a bad noise reputation. So the bridge can see high voltage spikes.
I prefer to put a transient rated zener after the bridge; Varistors (which could go in front) have a bad noise reputation. So the bridge can see high voltage spikes.
richie00boy said:
When a diode goes into reverse conduction, the power that is dissipated can be very very high. If it starts conducting at a kilovolt, one milliamp causes a watt dissipation. This power by itself, is not much, but if it occurs only at the edge, and only along one mm of the edge, then the power density is sufficient to melt the silicon. That current, even if it is spread around the entire edge, could still be too high in power density. This contrasts with forward conduction, where most of the area of the die is conducting, spreading the power across the entire surface of the die.
Avalanch die are made to supress breakdown along the edge of the die, forcing the breakdown to occur in the bulk of the junction in a uniform fashion.
Power manu's sometimes bevel the edge at an angle, this also supresses the breakdown at the edge, but this can only be done with round silicon, so tends to be used only for 1 to 4 inch diameter diodes (25 mm to 100 mm devices).
Other manu's add structures along the edge to do the same..IR uses a racetrack looking structure for their power fets, microsemi has used a mesa etched thing.
Cheers, John
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.