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Resistor power ratings

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OM222O:
I was shopping for some high power smd resistors with decent temp co and I came across these 2:
RHFH4Q001R0FE (data sheet at: https://www.mouser.co.uk/datasheet/2/611/rhf-series-1074351.pdf)
RW5S0FA1R00FE (data sheet at: https://www.mouser.co.uk/datasheet/2/303/res_rw-1265480.pdf)

to me this makes no sense  ??? how can the much much smaller package (DPAK , not even D2PAK!) be rated for 35 watts, while the much larger package, wirewound resistor be only rated for 5? similar packages are rated in the same region: 1 to 5 watts, so I'm not sure how they claim a 35 watt figure?
is that under the assumption of infinite heat sink or do they mean 35 watts without any heat sink (like any other manufacturer states the power ratings)?

even assuming a heat sink (a realistic one, not an infinite case under refrigerant or something), a DPAK would pretty much de solder itself under a 35 watt load, given how small the package is! Am I missing something here?

edit: after checking the notes on the datasheet, they claim 2 watts without heat sink, but I'm still not convinced it can pull 35 watts under any reasonable heat sink!

wraper:

--- Quote from: OM222O on May 27, 2019, 01:27:45 am ---is that under the assumption of infinite heat sink or do they mean 35 watts without any heat sink (like any other manufacturer states the power ratings)?

--- End quote ---
Do they? First thing I see in the datasheet:

--- Quote ---POWER RATING 1
(with heatsink)

1 2W on simple solder pad

--- End quote ---
And then again:

--- Quote ---Power Rating (with heatsink) 35 W ( 2W on Simple Solder Pad )
--- End quote ---
Did you even spend 30 seconds reading datasheet?

OM222O:
yes, my bad. I had already edited the first post though.
I'm still not convinced you can cool 35 watts on a DPAK with any heat sink!
I did a quick search on mouser (only stocked options) and the best ones they have are rated for about 3 to 4 c/w

that is about 140 degrees, assuming 0 thermal resistance from the resistor to the pads! add the package thermal resistance and you're already at double that, about 300c! as I suspected before, solder melts at those temperatures!

wraper:

--- Quote from: OM222O on May 27, 2019, 01:27:45 am ---edit: after checking the notes on the datasheet, they claim 2 watts without heat sink, but I'm still not convinced it can pull 35 watts under any reasonable heat sink!

--- End quote ---
If you attach in directly to heatsink (only leads soldered) it could do that just fine, though I would not recommend. Package itself is basically TO-220 with a screw tab cut off (still can be used with spring clip). Otherwise the best you could do is using aluminium PCB mounted on heatsink.

--- Quote ---that is about 140 degrees, assuming 0 thermal resistance from the resistor to the pads!
--- End quote ---
Thermal resistance is 3.3 K/W, That means 115.5oC rise at 35W. Max internal resistor temperature is 175°C. That's not unreal if you keep the tab cool. Though note that max spec is not something you should use constantly.

wraper:
Then also look on power derating curve, you can run it at full power if keep the tab below 50oC.

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