> I'm using a 9.1 zener diode with 115mA flowing through it. It is excessive
> heat.
>
> The datasheet shows the following information that does not match:
>
> 3 Watt Plastic Surface Mount Zener Voltage Regulator (1SMB5913BT3 Series)
> Vz: 9.1V
> Izt: 41.2mA
> Izmax(dc): 164mA
>
> Thus, the nominal power dissipation for this diode is Vz.Izmax= 1,5W and not
> 3W. Is the datasheet wrong?

Data sheet link is always helpful to speed up looking at what you are
asking about

           http://www.onsemi.com/pub_link/Collateral/1SMB5913BT3-D.PDF

That's the "good ole' maximum ratings above which damage occurs,
versus operating ratings issue.

They advertise it as a 3W part BUT all the zener rated specs seem to
run it at about 1.5W, as you note.

Whether it is "too hot" depends on the value of too :-).

You are running it as about 9 x 115 mA ~= .035 W say 1 Watt.

Rtj-l = 25C/W so you can  can expect a 25C lead to junction rise -
that sounds bearable.

*BUT* if you are running it in free air or close to it Rtj-a = 226 C/W
so you have a junction rise of 226 above ambient which is indeed 'too
hot" :-) (without looking at the spec sheet Tjmax :-) ).

What you need is some heatsinking on the leads. Tjmax is 150C - and
running it at less would be a good idea [tm].

If you get the leads/pads down to 75C say then Tj will be 75+25 = 100C
which is OK.

For say 25C ambient Rth_heastinkpads = (75-25)/1 Watt = 50 C/W which
is not too too hard to achieve. Many manufacturers actually specifi
pad sizes to achieve hald decent opration but OnSemi seem to have
decided to make life interesting for you.

Work calls, andbrowser locks up and dies, so I'll leave you with that
line of investigation. More if needed.


     Russell







>
> Thanks in advance.
>
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