SOT223 package thermal resistance?

Papabravo

Well-Known Member

• Jan 4, 2024

• #2

It is spot on for that package. A TO-220 is a whole different ball game when you consider it benefits from radiation conduction and convection. An SMT part, flat on a board, doesn't get much benefit from radiation or conduction, so it must rely on convection.

 

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For The Popcorn

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Most Helpful Member

• Jan 4, 2024

• #3

Here's a thought – compare to other devices in SOT223 packages to see if it's within reason.

 

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Pommie

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Most Helpful Member

• Jan 4, 2024

• #4

I'm guessing that the top of the device gets the hottest. Hence Rth(J-case top) is the same as Rth(j-c).

Mike.

 

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Flyback

Well-Known Member

• Jan 4, 2024

• #5

Thanks, but ayk, Rth(j-c) is to the metal tab, not the plastic case top. I doubt its the same.
Does anyone know Rth(j-case top) for any SMD device wth plastic case top?

Here's a thought – compare to other devices in SOT223 packages to see if it's within reason.

Click to expand...

Thanks, agreed, though no other SOT223 datasheet states Rth(j-case Top).

In fact, there are no figures for Rth(j-case top) for any SMD device anywhere on the web.
This is amazing when you think of the large amount of SMD devices which get cooled like this in real products.

 

Last edited: Jan 4, 2024

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For The Popcorn

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Most Helpful Member

• Jan 4, 2024

• #6

Try this.

Spoiler

This Richtek paper looks pretty useful and it was the first link in the search.

 

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Papabravo

Well-Known Member

• Jan 5, 2024

• #7

For The Popcorn said:

Try this.

Spoiler

This Richtek paper looks pretty useful and it was the first link in the search.

Click to expand...

This article may be worth a look since the TS references the datasheet of a TI part.

https://www.ti.com/lit/an/spra953c/spra953c.pdf?ts=1704442399685

 

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Flyback

Well-Known Member

• Jan 5, 2024

• #8

Thanks but your kindly given doc doesnt show Rth(j-case top) for any SMD device wth plastic case top.
Ayk thru case top is not the accepted method by semico's , so they dont tell this...even though its a majorly popular way in the actual world.

 

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For The Popcorn

Well-Known Member

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• Jan 5, 2024

• #9

From my post above. I do believe this is the information you seek.

 

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Flyback

Well-Known Member

• Jan 5, 2024

• #10

Thanks, but unfortunately, that fig of 15c/w is for junction to the metal tab bit.
Thanks though i am familiar with that figure.
The junction to plastic case top isnt stated.

 

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Tony Stewart

Well-Known Member

Most Helpful Member

• Jan 6, 2024

• #11

Thermal resistance between junction and ambient, called Rth(j-a), depends on the copper surface used under the tab. The following figures R(thj-a) versus copper area on PCB for versus copper area on PCB for provide the relationship between Rth(j-a) and the copper surface under the tab for a FR4 board (35 μm copper thickness) for various surface mount packages.

 

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Tony Stewart

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• Jan 6, 2024

• #12

The area is a power law and easily shown on a log Y scale using spreadsheet insert power trendline.

 

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Flyback

Well-Known Member

• Jan 6, 2024

• #13

From the above it seems to suggest that its possible to estimate Rth(j-case Top) from the value of
Rth(ja) and a knowledge of the Cu area of the a-joined Cu pad on the PCB.
Though i am not certain how that follows.

 

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Tony Stewart

Well-Known Member

Most Helpful Member

• Jan 6, 2024

• #14

Use Ohm's Law and solve for Tc.

 

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