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Patrick

Sun Jun 13 2010, 06:07AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i would like an oppinon on calculating what a uf4007 diode in air dissipating 2.08 watts, and assuming air has thermal conductivity of 0.024 K (Wm/K) and FR 3 oil, from this datasheet...here.
]00092_fr3.pdf[/file]

so my simple question is, if i have a single diode, generating 2.08 watts, of heat, in air 20 deg C.
and then; holding elecrtrical and temperture constant, take that same diode out of air and into FR3 oil, what will the previous watt rating "effectively" increase too?

have included enuff info?

EDit: please include math and description if you can help please.

Sulaiman

Sun Jun 13 2010, 07:03AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I don't know the answer but I'm pretty sure that most of the thermal conductivity is via the leads/wires, so if you want to keep the junction temperature down heatsink the leads as close to the body as possible.
Typical numbers are 60 C/W junction to air and 15 C/W junction to leads.
I have a vague memory that the anode end is most important to heatsink.

Patrick

Sun Jun 13 2010, 07:08AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i want to submerge 240, uf4007 diodes into insulating oil, for a voltage multiplier. this should increase there current rating at least modestly. plus insulate them. im looking at my college physics book right now, related to thermodynamic exmples, but i need to get back to my calculus homework.

aslo, what the hell is the difference between "diode" and "rectifier" ?

Dr. Slack

Sun Jun 13 2010, 07:47AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Diode is quicker to type, and fewer syllables to say - otherwise there's no difference.

Diode means di = 2, ode = terminal, so 2 terminal device, which covers all applications. So you also get transient suppressor diodes, and DIACs.

The word rectifier started life in the field of power engineering, when diodes were stacks of selenium of cadmium, or vast ET-like bottles of mercury. You probably need to be over 50 to use "rectifier" with a straight face. Any diode that is called a rectifier is probably good for a few amps.

Patrick

Sun Jun 13 2010, 07:54AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

LOL, old people....lol..heheh.

mercury is so much fun to play with, until you go mad as a felt hatter, then die.

[Moderator edit: removed disrespectful and off-topic content]

Sulaiman

Sun Jun 13 2010, 11:24AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

As above, diode = rectifier
but I tend to use the word 'rectifier' when a diode is used at a current where voltage across it's internal resistance is significant compared to an 'ideal' diode

Antonio

Sun Jun 13 2010, 01:48PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

2 W in a diode means a current of about 3 A. The uf4007 is rated for 1 A only. Smoke...

Pinky's Brain

Sun Jun 13 2010, 02:38PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

These guys say in oil lead->oil dominates and you can run their diodes at around twice the rating ... no math involved, but I think doing anything less than full blown PCB level simulations is just fooling yourself. The amount of trace per diode, the amount of surrounding copper fill, buried layers and vias, etc all make too much of a difference AFAICS.

klugesmith

Sun Jun 13 2010, 04:49PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Patrick wrote ...
i want to submerge 240, uf4007 diodes into insulating oil, for a voltage multiplier. this should increase there current rating at least modestly.

For how many minutes does your oil-immersed multiplier need to dissipate modestly more than 500 watts of diode losses, plus whatever power is lost in the capacitors, and in transformer if that shares the tank? Your intended oil temperature is the "ambient" for component temperature calculations.

If you are intending to push the limits of UF4007 power dissipation and junction temperature, please:
1) use parts by a known manufacturer, from a respected distributor.
2) characterize the t_j rise of samples from the actual purchased lot, in a sample of your multiplier stick. Thermal resistance is strongly sensitive to lead length, how the leads are terminated, how closely the diodes are mounted to each other and to the supporting board, relative orientation of gravity, etc. You can estimate changes in junction temperature from the progressive drop in forward voltage at a representative DC test current. Note that a significant fraction of the t_j change happens in the first millisecond after a change in power at the junction.

radiotech

Sun Jun 13 2010, 06:13PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Rectifiers can change AC to DC. A single diode can be considered an element of a rectifier. No single element alone, germanium, silicon, selenium or copper oxide can do anything, conduct, block, or mix unless they are connected to a load impedance.

Also many rectifiers can heal after overload which mandated their use in such things as elevator controls. Few diodes heal. With luck, they may blow open like a fuse.

I believe alchemists used the word rectify followed closely by the word reverse and perhaps a few choice cuss words when their processes went too far.

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