2W from a SOT-223 ?

[Adam. Seychell]

The thermal resistance from junction to "soldering point" of a
PZT2222/PZT2907 SOT-223 is 26 K/W. Say I were to soldering the SOT-223
tab to a copper strip, which in then clamps to a large heatsink.
Assuming Tj rise of 50°C then;

Pd_max = 50/26 = 2 Watts

Anyone else heatsinked a SOT-223 this way ?
I would use a device with a larger package but there isn't anything else
around with similar specs and/or availability. The PZT2222/PZT2907,
replace the old metal canned TO-39 2N2219/2N2905.

 

[Pooh Bear]

The thermal resistance from junction to "soldering point" of a
PZT2222/PZT2907 SOT-223 is 26 K/W. Say I were to soldering the SOT-223
tab to a copper strip, which in then clamps to a large heatsink.
Assuming Tj rise of 50°C then;

Pd_max = 50/26 = 2 Watts

Anyone else heatsinked a SOT-223 this way ?
I would use a device with a larger package but there isn't anything else
around with similar specs and/or availability. The PZT2222/PZT2907,
replace the old metal canned TO-39 2N2219/2N2905.

SOT-223 doesn't ring immediate bells with me.

I can't believe there's no other suitable substitute. How far did you look ?

Have you looked at D-pak for power dissipation ? IR have an app note on
using pcb foil area for heatsinking.

Graham

 

[Graham Holloway]

The thermal resistance from junction to "soldering point" of a
PZT2222/PZT2907 SOT-223 is 26 K/W. Say I were to soldering the SOT-223
tab to a copper strip, which in then clamps to a large heatsink.
Assuming Tj rise of 50°C then;

Pd_max = 50/26 = 2 Watts

Anyone else heatsinked a SOT-223 this way ?
I would use a device with a larger package but there isn't anything else
around with similar specs and/or availability. The PZT2222/PZT2907,
replace the old metal canned TO-39 2N2219/2N2905.

You are correct in your assumptions. I have used a SOT223 linear regulator
in a similar manner although I use thermal vias to conduct the heat to a
baseplate below the PCB.

Graham Holloway

 

[[email protected]]

The thermal resistance from junction to "soldering point" of a
PZT2222/PZT2907 SOT-223 is 26 K/W. Say I were to soldering the SOT-223
tab to a copper strip, which in then clamps to a large heatsink.
Assuming Tj rise of 50°C then;

Pd_max = 50/26 = 2 Watts

Anyone else heatsinked a SOT-223 this way ?
I would use a device with a larger package but there isn't anything else
around with similar specs and/or availability. The PZT2222/PZT2907,
replace the old metal canned TO-39 2N2219/2N2905.

That's 50deg rise above the heatsink temp, not above ambient.

 

[[email protected]]

Take into account that each square of 35µm copper foil has 70k/W
thermal resistance.
If your copper track to the heat sink is 4mm wide and 6mm long this
would make an additional 105k/W !!!
Do you need electrical insulation?
Can you drill a hole in the PCB and solder the collector directly to a
copper heat sink?
Frank

 

[Winfield Hill]

[email protected] wrote...

That's 50deg rise above the heatsink temp, not above ambient.

Yes, 50 C rise above the connection point. The thermal resistance
of the [ SOT-223 tab -> copper strip -> clamps to large heatsink ]
path is at issue, especially the 2oz PCB copper-strip portion.

Or does Adam really mean a mean old copper strip? That could work.

 

[SioL]

You are correct in your assumptions. I have used a SOT223 linear regulator
in a similar manner although I use thermal vias to conduct the heat to a
baseplate below the PCB.

Graham Holloway

How effective is this method in terms of conducting heat to the underside of the PCB?

Nowadays SMD packages are taking over even for power devices and it is often quite
difficult to heatsink them

 

[Graham Holloway]

How effective is this method in terms of conducting heat to the underside of the PCB?

Nowadays SMD packages are taking over even for power devices and it is often quite
difficult to heatsink them

Siol

I don't have any specific details for you. However, I would flood the area
beneath the device with copper and then add the vias. Say, 28mil holes on a
staggered rows/columns pitch of 30mil would be a good start. I used a thin
Berquist insulator between the underside of the PCB and a raised section of
a baseplate, but I've seen other people used the gapfiller quite effectively
to get the heat across several mm. I think you might need to experiment.
Also bear in mind the vias will fill with solder (extra conduction) unless
you take specific steps to avoid it.



Graham Holloway
WPS/Accuphon Electronics
(Tel/Fax 0(044)1233 662599)

 

[SioL]

Siol

I don't have any specific details for you. However, I would flood the area
beneath the device with copper and then add the vias. Say, 28mil holes on a
staggered rows/columns pitch of 30mil would be a good start. I used a thin
Berquist insulator between the underside of the PCB and a raised section of
a baseplate, but I've seen other people used the gapfiller quite effectively
to get the heat across several mm. I think you might need to experiment.
Also bear in mind the vias will fill with solder (extra conduction) unless
you take specific steps to avoid it.

Gapfiller = that white thermal conduction paste?

I have been using aluminum block under the pcb, lots of vias and lots of copper
on both sides. The problem with filling vias with solder is that it creates uneven
surface, which adds extra thermal resistance to the aluminum block underside the pcb.
It would be best to have a copper block under the pcb, a hole in the pcb and
solder the part directly to the block. But it might be unhealthy for the part, its hard to
get that block hot enough to solder and it cools slowly.

 

[Terry Given]

[email protected] wrote...

That's 50deg rise above the heatsink temp, not above ambient.

Yes, 50 C rise above the connection point. The thermal resistance
of the [ SOT-223 tab -> copper strip -> clamps to large heatsink ]
path is at issue, especially the 2oz PCB copper-strip portion.

Or does Adam really mean a mean old copper strip? That could work.

there was a nice paper published in the 1996 High Frequency Power
Conversion conference proceedings about using multilayer PCBs as
heatsinks, along with some detailed measurements. thermal performance is
actually pretty good, and transient behaviour is excellent.

Cheers
Terry

 

[John Larkin]

The thermal resistance from junction to "soldering point" of a
PZT2222/PZT2907 SOT-223 is 26 K/W. Say I were to soldering the SOT-223
tab to a copper strip, which in then clamps to a large heatsink.
Assuming Tj rise of 50°C then;

Pd_max = 50/26 = 2 Watts

Anyone else heatsinked a SOT-223 this way ?
I would use a device with a larger package but there isn't anything else
around with similar specs and/or availability. The PZT2222/PZT2907,
replace the old metal canned TO-39 2N2219/2N2905.

Yup. Two watts sounds feasible. If you can afford a couple of square
inches of copper top and bottom, and if there's some air flow, you
probably don't need the heatsink at all.

Keep in mind that 1 oz copper foil has a significant thermal sheet
resistance, about 70 K/W per square. So you've got to be concerned
with spreading the heat laterally. Put the package in the center of a
copper island and use lots of fat flooded (not thermal) vias to
another island on the flip side. Parallel that with inner layers if
available.

But the 26 K/W theta of the device itself seems high; that's throwing
away 52 degC of junction temp right there. You might consider
something with a bigger die.

See pic in a.b.s.e.

John

 

[Graham Holloway]

Gapfiller = that white thermal conduction paste?

No, the thermally conductive rubber that comes in various thicknesses, up to
about 6mm.

 

[John Larkin]

No, the thermally conductive rubber that comes in various thicknesses, up to
about 6mm.

It's not very "thermally conductive." The best thing, by far, is
classic messy silicone grease, which will squeeze down below 100
microinches if the surfaces are flat.

John

 

[Pooh Bear]

How effective is this method in terms of conducting heat to the underside of the PCB?

Suggest you take a look at this.

http://www.irf.com/technical-info/appnotes/an-994.pdf

I think IR may have another app note too but I didn't find it instantly using the site search.

Graham

 

[SioL]

Suggest you take a look at this.

http://www.irf.com/technical-info/appnotes/an-994.pdf

I think IR may have another app note too but I didn't find it instantly using the site search.

Graham

This one's a keeper, very usefull, thanks!

 

[SioL]

It's not very "thermally conductive." The best thing, by far, is
classic messy silicone grease, which will squeeze down below 100
microinches if the surfaces are flat.

John

I think even silicone grease is not very conductive unless it is squeezed out
almost entirely. If two surfaces are not reasonably flat, it doesn't help much.

 

[Pooh Bear]

I think even silicone grease is not very conductive unless it is squeezed out
almost entirely. If two surfaces are not reasonably flat, it doesn't help much.

You need Motorola's AN1040 or something !

2 secs I'll find it.

Even better - it's on my bookshelf in rev 6 of the power devices manual.

Should be compulsory reading for anyone involved with power semis.

I looked in the book. It is indeed AN1040. Does that make me a power semis nerd ?

Here's the link. Everyone should read this.

https://www.onsemi.com/pub/Collateral/AN1040-D.PDF

Graham

 

[SioL]

You need Motorola's AN1040 or something !

2 secs I'll find it.

Even better - it's on my bookshelf in rev 6 of the power devices manual.

Should be compulsory reading for anyone involved with power semis.

I looked in the book. It is indeed AN1040. Does that make me a power semis nerd ?

Here's the link. Everyone should read this.

https://www.onsemi.com/pub/Collateral/AN1040-D.PDF

Another keeper, definitely.

Thanks.

 

[Pooh Bear]

Another keeper, definitely.

Thanks.

You're *very* welcome.

There are some snippets of information out there that are absolutely invaluable and likely never ever to be
bettered.

I was lucky to find AN1040 early in my working life ! Before the internet and stuff - when you had still had to
ask for data manuals. How times have changed !

Graham

 

[Terry Given]

SioL wrote:




You need Motorola's AN1040 or something !

2 secs I'll find it.

Even better - it's on my bookshelf in rev 6 of the power devices manual.

Should be compulsory reading for anyone involved with power semis.

I looked in the book. It is indeed AN1040. Does that make me a power semis nerd ?

Here's the link. Everyone should read this.

https://www.onsemi.com/pub/Collateral/AN1040-D.PDF

Graham

yep, it does. welcome to the club. Become a senior member by casually
referring to Jim Williams AN47.....

Cheers
terry