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Registered Member #5323
Joined: Fri Jun 15 2012, 02:14PM
Location:
Posts: 104
I want to build a full wave bridge rectifier in the 30-35kV range. I've thoroughly researched the vendors that I am familiar (Digikey, Mouser, Newark, etc even Ebay) in an attempt to locate HV diodes in that range, but not having much luck. The highest I've found are HMV16 rated at 16kV. I realize that I could series connect these to reach my target, but I'd much prefer finding single diodes that are rated at 30-35kV if such exist. Does anyone know of a vendor that handles HV diodes in the 30-35kV range? Thanks in advance.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
At this voltage range the diodes must be quite long, so a series association is a good idea. Three microwave oven diodes in series would be enough and very safe. And they are easy to find.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Depends on the current you need, but here 30 kV "sticks" are pretty common, they were used in B/W TVs to rectify the flyback. The current rating is usually only around 5 miliamps.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Do you mean 30-35kV RMS working voltage or PIV?
If you intend that the rectified output across a smoothing capacitor should be 30 - 35 kV, then you should choose diodes with a PIV double or even triple that figure to give good immunity to spikes, and an acceptable MTBF. If you'd be happy if your circuit just worked for an hour or two then you could use less conservative PIV values.
If you plan to series the diodes, then you should derate them quite a lot i.e. Three 10kV diodes in series does not make one 30kV diode - 20 kV would be a more realistic figure for long term reliability.
Ebay sellers in Hong Kong and mainland China offer by far the best deals on HV diodes, and many here on 4HV including myself have bought from them without problems over these last years. Here's a few examples that I found in a couple of minutes:
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Proud Mary wrote ...
If you plan to series the diodes, then you should derate them quite a lot i.e. Three 10kV diodes in series does not make one 30kV diode - 20 kV would be a more realistic figure for long term reliability.
Agreed. Dr. Slack recently posted something regarding this in another thread.
This probably explains why I have managed to kill a number of HV diodes that were being used in series on the HV side of an inverter I built.
Dr. Slack wrote ...
Diodes have lots of other parameters, parameters you have no right to expect to be well matched between two random diodes. The one that really bites you in the bum is reverse recovery time. A forward biased diode stores a lot of charge, a bit like a large, poorly specified capacitor. As the two series diodes get reverse biased, stored charge flows out of the junctions, maintaining a very low voltage across the diodes. Eventually, one runs out of charge, and gets the full reverse voltage slapped across it. Ouch. Unless it's avalanche rated, exceeding the PIV by a factor of 2 doesn't do it a whole lot of good.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Yes, that's why you should use avalanche rated diodes, then the ratings multiply
PS. HV diodes usually contain more junctions in series, which must (or at least should be) avalanche rated. So seriesing HV diodes should work all right.
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Dr. Dark Current wrote ...
Yes, that's why you should use avalanche rated diodes, then the ratings multiply
PS. HV diodes usually contain more junctions in series, which must (or at least should be) avalanche rated. So seriesing HV diodes should work all right.
I actually was using avalanche rated diodes, but they still ended up in pieces.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
From my own experience with combinations of diodes at the voltages we're talking about here, any sort of flashover means dead diodes.
A tracking-type surface discharge can start off invisible to the naked eye, and then suddenly go flash-bang when it opens up a good ionisation channel for itself.
A sound like a quick "dink" will sometimes warn that something bad is happening.
Surface discharges - such as across the outside surface of a diode - have got ideas of their own, and rules about breakdown voltages in air gaps don't apply to them.
So however you make your bridge, stick it in some oil - ordinary kitchen sunflower seed oil is both effective and eco-friendly.
Registered Member #5323
Joined: Fri Jun 15 2012, 02:14PM
Location:
Posts: 104
Thanks. Good advice from all. So, if I series connected three HVM12 (rated at 12kV each) I shouldn't count on their collective specs to be 36kV, but more in the range of 20kV (+/-) ?
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30-35kV diodes for full wave bridge rectifier
