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Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Does anybody ever worry about temperature rise in their multiplier diodes?
Or maybe most of the multipliers built by people here have a small duty cycle, so it's not much of an issue?
If a multiplier is putting out 50 kv @ 20 or 30 ma, I can see the diode strings dissipating a few watts of power. I was going to pot the whole multiplier in silicone rubber but I'm having second thoughts now because I don't have a good feeling about heat removal from the diodes.
The only two options I can think of would be to either use a potting compound with a relatively high thermal conductivity, or to not pot it at all and blow air over it with a fan.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Yes -- The oil strikes back! All you solid encapsulant pansies.
You are right in the two points you mention Jp, first most multipliers ive seen are low power, low duty cycle, so heat isnt a problem for many of the common apps.
However for my own upcoming high power, high duty cycle (0.90), (70nS Trr, D's) and HF (100Khz) 20 kv input, 50-60 kv output multipiler ive done the math for air and their spec heat capability, adjusted for my wave shape and my power in terms of (V x I) x (0.9t), with the Amps graph instaneously multiplied by instaneous V, then all integrated to find the area under the curve which is power dissipated, thus under oil i can be sure of how much i demand of the diode string. Lots of math, differential measurements with Hv probes to my scope are still planned to confirm all the math. (Hopefully my Hv probes will work when i finish them)
i have to do all this convoluded math because the diode makers cant and dont tell us any info on power dissipated for anything except the simple cases like pulsing DC and Sine wave rectifying. For high power and more demanding (odd waves, odd DC's) applications, these simple approximations of power dissipated Vs power passed is pretty much worthless. We have to figure out what their diodes can withstand in our applications. Math and a prototype device are the only credilbe means ive seen to do all this.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
I'd be happy if the Chinese diode manufacturers would just supply some unambiguous basic specs on their parts.
I bought some 2CLG10KV/1A diodes (10kv/1amp) that were advertised as having Vf = 12v, (but no mention of the value of the current at that point). Then later, after I bought them, I saw another "spec" wherein Vf = 20v. Now I'm wondering, well which is it? I wish they'd make up their mind.
Anyway, in light of my power dissipation concerns, I may have to condescend down to the level of you "oilers" and use oil...although it's just not the kind of thing I like to talk about in polite company...
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
For a string of 50 UF4007's (with a forward voltage of ~85V) at 30 mA, you dissipate ~3W...an average of 60 mW per diode. Considering they can dissipate 2W in air, I don't think diode heating will be a problem...I mean, you're putting 30mA through a 1A device.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
If I was using a long string of 1 kv diodes I wouldn't be worried so much because the power dissipation would be physically spread out over a large area. What worries me are strings of 3 2CLG10KV/1A diodes in close proximity to each other, with each diode dissipating between 1 and 2 watts, with the whole assembly encapsulated in a silicone rubber that doesn't have very good thermal conductivity.
Even if it's not enough heat to cause immediate failure I don't want to brutally thermally cycle it, if possible.
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Power Dissipation In Multiplier Diodes
