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Diode Lead Length

Hi,

I would like to use the 1N5283 diode in a certain application. Like some other applications in power electronics, this diode datasheet too mentions a certain Lead Length to be maintained.

(See

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Could someone please explain this? Why does this distance have to be maintained? (Its got to do with the thermal characteristics alright - but how?)

I also see thermal resistance versus lead length curve and a note which says "(MOST HEAT CONDUCTION IS THROUGH THE CATHODE LEAD)". How's this?

I tried searching and found some other datasheets mentioning 3/8" lead length, but no explanation.

Your inputs please?

Regards, Anand

Reply to
Anand P. Paralkar
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I assume that it's a minimum lead length that's not to be exceeded? They're probably counting on the circuit board to contribute to the heat dissipation from the part.

Does "most heat GENERATION happens IN the cathode" make more sense?

They're rating the parts for an installation into a board where the board itself is conducting heat away from the part.

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com
Reply to
Tim Wescott

Poorly stated... most heat flow is via the cathode lead, for mechanical reasons...

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[snip]

The same reason many I/C's now have a metal slug in the package, under the die and at least in thermal contact (and in many, it's also electrical... a quality ground). ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

I've never used one of those. But the app note sorta spells it out. The current is temperature dependent, The temperature depends on power dissipation and thermal conductivity, For best control keep power low (low forward voltage) and thermal conductivity low... short leads.

Jim nailed the cathode part.

George H.

Reply to
George Herold

Thanks Jim. That's something that I had all the factoids rolling around in my head, but never put two and two together. I'll remember now.

Does this apply to surface mount as well? Uniformly? Or might there be oddball cases where it's the anode that has the good contact?

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com
Reply to
Tim Wescott

I don't know for sure, but, for diodes and zeners, it would depend on the "starting material". If it's P diffused into N, it's the cathode that's on the header; and vice versa.

Most I/C's are P-type substrate (*), which is at the most negative potential (GND or -VEE or -VSS, etc)

(*) In the past, we played games and made two chip amplifiers, like the MC1524, where one chip was P-substrate, and one was N-substrate, to optimize vertical NPN's and PNP's... but I've not seen that sort of gimmick in years. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

"Anand P. Paralkar"

** No such requirement is in the data.

** There is no distance to be maintained.

FFS learn to read - it only says " keep leads short ".

.... Phil

Reply to
Phil Allison

As people have noted, most of the heat path is usually through the leads. Fig 2 makes the unreasonable assumption that both leads hit infinite heat sinks at some distance from the body. On a PC board, that's not the case, although soldering to some big copper pours is a rough approximation.

Current limiter diodes are expensive and not very accurate. They seem to be obsolete, or nearly so.

What's your application?

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

Axial power diodes are often soldered dual nailhead designs, thermally symmetric.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

they assume the full lead acts as a heatsink, and if you chop it off, you lose heatsinking, and power rating.

Of course they don't really account for a PCB with a trace also acting as a heatsink, and that's upto you to determine if it's less or more effective than a lead sticking out into the air.

I've seen things such as solder turrets used as heatsinks for power diodes.

Reply to
Cydrome Leader

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