[Amps] diode junction temps and thermal resistance

[Carl]

My comment was directed to using a 50W zener.

Carl


----- Original Message ----- 
From: "Kevin Normoyle" <knormoyle at surfnetusa.com>
To: <AMPS at contesting.com>
Sent: Tuesday, October 19, 2010 11:34 PM
Subject: [Amps] diode junction temps and thermal resistance


>
>
> Carl: "There is an ample front to rear aluminum shield that is more than 
> adequate to dissipate the 8W or so in normal use."
>
> All heat gets dissipated eventually. The question is: what is the junction 
> temperature inside the diode and is it over
> specification as a result of the thermal issues?
>
> The 1N2971A data sheet says 10 deg C/W thermal resistance for junction to 
> stud. That can't change since it's the part
> packaging. The maximum allowed junction temp for the part is 175C (from 
> the spec)
>
> The stud to aluminum thermal resistance is probably at least 2 deg C/W. So 
> that's 8W * (10+2) = 96 deg C rise (for the
> diode's junction)
>
> But we have to add the temp of the aluminum chassis wall (the max temp of 
> the wall relates to it's ability to radiate/be
> cooled by air). I'll guess the aluminum wall around the stud can be 
> measured with an IR thermometer to be 150 deg F.
> (after some hours of 15M RTTY contesting) In the summer, with room temp at 
> 90F, that's easily imaginable?
>
> 150F is 66 deg C, so the junction temperature would be 66 + 96 = 162 deg 
> C. (compared to 175C allowed)
>
> A poor path from stud to aluminum wall would be maybe 3 deg C/W. So that 
> would add 8 deg C. to get you to 170C.
> (remember max allowed is 175 C)
>
> I think it's fair to say that there is marginal headroom, if 8W occurs in 
> normal use. Does 8W get dissipated by each
> zener? Since they are right next to each other, they locally heat the 
> aluminum wall that's doing the dissipation. My 150
> deg F guess is just based on probable heating within the tube cavity (this 
> is just memory..may be wrong)
>
> The main point: you can do numbers on all this, and IR measurements..it's 
> not just guessing.
>
>
> Now: It may be true that none of these common zener failures are due to 
> overtemp. If so, then what is causing them? Is
> RF causing them? or what?
>
> I also noticed that Ameritron put the thermal compound between the case 
> and the solder lug washer..not just between the
> case and the insulator+washer. I Thought the thermal compound was 
> insulating? If so, than that interferes with the
> electrical connection between the diode case and the solder lug/washer.
>
> -kevin
> AD6Z
>
>
>
>
>
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