# How to calculate power dissipation in a diode

#### mike _Jacobs

Joined Jun 9, 2021
126
So obviously everyone knows how to calculate the power in a Diode.
Just the VF x current

but, who do you figure out how much power the package can safely dissipate?

So take any Diode and the data sheet says it can run an absolute max of 20A

ok great, we all know that is under very specific conditions.
Let’s say I want to run 10A through it and we know the Vf is .5

ok so 5 watts
How can I prove that the part is under its thermal limit? Is that also listed in the data sheet?

Joined Feb 20, 2016
4,426
Generally the max temp is in the data sheet, and those diodes that mount on heat sinks have thermal transfer info I think, as do the various heat sinks them selves. So, some data sheet study may be in order, or just mount the diode with good thermal transfer to a large heat sink and see how it goes.

#### mike _Jacobs

Joined Jun 9, 2021
126
Generally the max temp is in the data sheet, and those diodes that mount on heat sinks have thermal transfer info I think, as do the various heat sinks them selves. So, some data sheet study may be in order, or just mount the diode with good thermal transfer to a large heat sink and see how it goes.
So is that the operating temp in the data sheet?
I see in a few different data sheets there is an operating temp and a coefficient.
is that r theta jc?
it says 1.25 c/watts

that seems really low.
so going by the above example, of 5 watts I would only see a temp rise of 1.25 x 5? That doesn’t seem right ?

Joined Feb 20, 2016
4,426

#### crutschow

Joined Mar 14, 2008
34,047
it says 1.25 c/watts

that seems really low.
That's the junction to the case, not junction to air.
The bare case to air would likely be more like 100°C/W.
You have to add in the thermal resistance of whatever heatsink the case will be attached to.

#### mike _Jacobs

Joined Jun 9, 2021
126
So let’s assume no heat sink for a second which I know is not idea but how do you calculate the max without one

#### mike _Jacobs

Joined Jun 9, 2021
126
That's the junction to the case, not junction to air.
The bare case to air would likely be more like 100°C/W.
You have to add in the thermal resistance of whatever heatsink the case will be attached to.
So where is the max operating temp specified at? Like where physically I mean? So i have seen a term TOP or operating temp.
How do I know if I’m going over that mathematically

#### WBahn

Joined Mar 31, 2012
29,857

#### mike _Jacobs

Joined Jun 9, 2021
126
I don’t even see a rtjc on that sheet
So how do I figure out how hot that one will get with say 5 A going through it

#### dl324

Joined Mar 30, 2015
16,679
Do you have a fav Diode? I can go grab the data sheet to use as an example for us.
Since you don't want to provide the datasheet for the diode you referred to in your original post, and since you mentioned a 5V "forward" voltage so it must be a zener, here's information for 1N5xxx zener diodes from Microsemi for reference:

The diodes are rated for 500mW, but that is derated according to how the diodes are mounted (lead length, copper thickness, and trace width) and ambient temperature.

What they didn't mention is that the maximum junction temperature is 200C (taken from a Fairchild datasheet that didn't mention thermal resistance for junction to lead).

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#### mike _Jacobs

Joined Jun 9, 2021
126
Since you don't want to provide the datasheet for the diode you referred to in your original post, and since you mentioned a 5V "forward" voltage so it must be a zener, here's information for 1N5xxx zener diodes from Microsemi for reference:
View attachment 306312View attachment 306313
The diodes are rated for 500mW, but that is derated according to how the diodes are mounted (lead length, copper thickness, and trace width) and ambient temperature.

What they didn't mention is that the maximum junction temperature is 200C (taken from a Fairchild datasheet that didn't mention thermal resistance for junction to lead).
I said above .5v but
Your to busy giving me a hard time
I just made up a number then I supplied a datasheet as an example one to work just off Digi-Key. No where did I mention a zener if 5v vf

#### dl324

Joined Mar 30, 2015
16,679
I said above .5v but
I missed the decimal point. We put a 0 in front of decimal points to avoid issues like that.

It would have been helpful if you had mentioned that it was a Schottky diode.

Maximum junction temperature is specified as 150C.

Thermal resistance is:

Derating curve:

You calculate junction temperature using the junction to ambient thermal resistance and stay under 150C.

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#### WBahn

Joined Mar 31, 2012
29,857
I don’t even see a rtjc on that sheet
So how do I figure out how hot that one will get with say 5 A going through it
At the top of the second page is the following:

Elsewhere in the data sheet it says that the TYPICAL forward voltage drop at 5 A is 513 mV. You really want to work with the max voltage drop, which isn't given, but based on other things can be inferred to be in the vicinity of 550 mV. On the other hand, the voltage drop goes down with temperature and you are probably in the 500 mV range at a max when at max junction temperature (which is 150°C, but I'd recommend not designing it to exceed 125°C).

So you are looking at 2.5 W of dissipation. If the ambient is 25°C and you are allowing an increase of up to 100°C, then your junction-to-ambient thermal resistance can't exceed 40 °C/W,

So you need at least as good a thermal mounting as what they show as their "Maximum Land Pattern".