snubber resistor power rating?

In article <41AA96A8.B03ABEF8@rica.net>,
John Popelish <jpopelish@rica.net> wrote:
Harry Dellamano wrote:

Ken Smith wrote:

IRC (the company, I do Recall Correctly) make some great smt resistors.
And have peak pulse power curves.

Caddock does too:
http://www.caddock.com/Online_catalog/smt/smt.html

--
John Popelish

Caddock is a perfect example of a poor surge (Pulse) power resistor. They
are thin film and X1.5 rated peak power. A good surge rated resistor is
X5000 or maybe 5 Joules. Call Richard Caddock and see if they speak in
Joules.

But that 1.5 factor is allowed for 5 seconds. I was assuming that
from an I^2*t fusing effect that the dissipation capability would go
up quite a bit if the time was in milliseconds.
Click to expand...
No, X1.5 applies to 0.1 attoseconds to 1.5 seconds. 1.500001 applies below
0.1 attoseconds.

(or something like that)

Thermally the situation looks a lt like this:


Power in
= I in R1 R2 R3 R4 R5
-> ---/\/\/\------/\/\/----/\/\/----/\/\----/\/\--- GND
! ! ! ! !
--- --- --- --- ---
--- C1 ---C2 --- C3 --- C4 --- C5
! ! ! ! !
GND GND GND GND GND

C1 is the thermal mass of the actual resistive material.

R1 is the thermal conductivity of the resistive material

C2 is the thermal mass of the substrate the resistive material is on.

R2, R3 with C3, C4 are a lumped constant representation of the substrates
bulk.


C5 and R5 are for the outer package to the air.

Unless you have some feel for R1 and C1 you really can't extrapolate from
1.5 seconds down. I know this because I did it and ended up with 1206
open circuits after a few months of service. The power spike was only
300uS long.



--
--
kensmith@rahul.net forging knowledge
 
Ken Smith wrote:
In article <41AA96A8.B03ABEF8@rica.net>,
John Popelish <jpopelish@rica.net> wrote:

Harry Dellamano wrote:

Ken Smith wrote:

IRC (the company, I do Recall Correctly) make some great smt resistors.
And have peak pulse power curves.

Caddock does too:
http://www.caddock.com/Online_catalog/smt/smt.html

--
John Popelish

Caddock is a perfect example of a poor surge (Pulse) power resistor. They
are thin film and X1.5 rated peak power. A good surge rated resistor is
X5000 or maybe 5 Joules. Call Richard Caddock and see if they speak in
Joules.

But that 1.5 factor is allowed for 5 seconds. I was assuming that

from an I^2*t fusing effect that the dissipation capability would go

up quite a bit if the time was in milliseconds.


No, X1.5 applies to 0.1 attoseconds to 1.5 seconds. 1.500001 applies below
0.1 attoseconds.

(or something like that)

Thermally the situation looks a lt like this:


Power in
= I in R1 R2 R3 R4 R5
-> ---/\/\/\------/\/\/----/\/\/----/\/\----/\/\--- GND
! ! ! ! !
--- --- --- --- ---
--- C1 ---C2 --- C3 --- C4 --- C5
! ! ! ! !
GND GND GND GND GND

C1 is the thermal mass of the actual resistive material.

R1 is the thermal conductivity of the resistive material

C2 is the thermal mass of the substrate the resistive material is on.

R2, R3 with C3, C4 are a lumped constant representation of the substrates
bulk.


C5 and R5 are for the outer package to the air.

Unless you have some feel for R1 and C1 you really can't extrapolate from
1.5 seconds down. I know this because I did it and ended up with 1206
open circuits after a few months of service. The power spike was only
300uS long.
Click to expand...
Philips (or whatever the hell they are called now) have nice peak pulse
power curves in their discretes book. an 0603 RC21 5% resistor has an
average power rating of 63mW, and can take a repetitive 1ns 2W pulse
every 1us.

It would be nice if they just gave us the damn RC thermal model, instead
of maybe giving us peak pulse power curves. I got a nice surprise the
other day when looking at an Infineon FET - a complete thermal model, as
part of the datasheet. yay.

Cheers
Terry
 
John Popelish wrote:

"HARRY DELLAMANO" <harry_td@verizon.net> wrote in message news:<lCwqd.2588$wr6.2162@trnddc04>...

"John Popelish" <jpopelish@rica.net> wrote in message
news:41AA96A8.B03ABEF8@rica.net...

But that 1.5 factor is allowed for 5 seconds. I was assuming that
from an I^2*t fusing effect that the dissipation capability would go
up quite a bit if the time was in milliseconds.

A-huh, assuming.
Harry


Yep. A good data sheet is much better than an assumption:
http://www.irctt.com/pdf_files/HSF.pdf
Click to expand...
Wow, thats GREAT! pisses all over the CHP series!

I'll remember that part, thanks John.

Cheers
Terry
 
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