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| Resistor power ratings |
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| wraper:
BTW it's TO-263 = D2PAK, not DPAK. Seems they made a typo. |
| OM222O:
--- Quote from: wraper on May 27, 2019, 03:18:07 am --- --- Quote from: OM222O on May 27, 2019, 03:00:57 am ---under 35 watt load, with an ambient of 0c, with infinite heat sink, the resistor is already sitting at 115c, at which point it must be de rated to just about 50%! I had to use a D2PAK package (much much larger than DPAK) with a lot of thermal vias to cool just 3 watts without the device getting too hot and failing. this 35 rating doesn't make any sense with the numbers pulled out of the same datasheet! please do the calculations yourself and you'll realize this is not realistic at all. --- End quote --- Nonsense. It does not need to be derated unless TAB temperature is above 50oC. Yet again you replace one spec with another and make nonsense conclusions |O. --- End quote --- before answering with such certainty and aggression, I suggest you read the difference between different thermal resistance meanings :-+ I used the tab tempreture which can be calculated by "Tambient + (Rj-c + Rc-a) * Power" and assumed an ideal Rc-a of 0 as well as 0c ambinet. the tab will reach 115C under the limitations of the package itself, EXCLUDING any external factors. Have a nice day. A quick side note: Rj-c + Rc-a is the same as Rj-a. in case of this device, the Rj-c alone is 3.3c/w :-/O Here is an example from a random N channel fet data sheet: As you can see , the package itself has a limit of 0.52c/w. that would be your thermal resistance with an ideal heat sink! the more realistic figure would be the 62.5c/w which is calculated without any heat sink! This is really huge compared to any heat sink (again, about 3 or 4 c/w) so you can just add the 0.52 to the thermal resistance of your heat sink to get your overall thermal resistance. multiply that by the power and you will have your temperature rise. add that to your ambient temperature and there is your "tab temperature". Again, the tab is NOT isolated in most cases, which gives you a figure between 0.5 and 1c/w for junction to case resistance. in this case they are isolating the tab which leads to a 3 times higher figure of 3.3 ;) |
| T3sl4co1l:
--- Quote from: OM222O on May 27, 2019, 01:41:09 am ---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! --- End quote --- I can't say I would recommend it. But consider what they're really rating. Case Temperature = 25°C Doesn't matter how you get there, doesn't matter how you have to do it, if it's a bath of boiling freon (which most times, is exactly what they do!). Shitty, but that's how it is. Anyway, they can't give you an arbitrary number for just any heatsink, that wouldn't be very helpful either. What is useful is the RthJC, and with the Tj(max) and some maximum ambient, and some heatsink Rth, you can calculate the real power dissipation capacity, for any heatsink, given you have these numbers. Tim |
| OM222O:
--- Quote from: T3sl4co1l on May 27, 2019, 03:46:02 am --- --- Quote from: OM222O on May 27, 2019, 01:41:09 am ---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! --- End quote --- I can't say I would recommend it. But consider what they're really rating. Case Temperature = 25°C Doesn't matter how you get there, doesn't matter how you have to do it, if it's a bath of boiling freon (which most times, is exactly what they do!). Shitty, but that's how it is. Anyway, they can't give you an arbitrary number for just any heatsink, that wouldn't be very helpful either. What is useful is the RthJC, and with the Tj(max) and some maximum ambient, and some heatsink Rth, you can calculate the real power dissipation capacity, for any heatsink, given you have these numbers. Tim --- End quote --- I knew there was something fishy, even mentioned sub zero cooling :-DD It is very misleading and I was quite surprised when I first saw it. this is honestly the first time I see this done for a resistor since manufacturers use practical figures since they know if you have spent 2$ on a resistor, you don't want to be spending 50$ on a cooling setup! I'm by no means a professional, just a hobbyist. maybe this practice of over rating components is more common in the industry than I expected. I had only seen mostly on transistors before. |
| wraper:
--- Quote from: OM222O on May 27, 2019, 03:41:10 am ---before answering with such certainty and aggression, I suggest you read the difference between different thermal resistance meanings :-+ I used the tab tempreture which can be calculated by "Tambient + (Rj-c + Rc-a) * Power" and assumed an ideal Rc-a of 0 as well as 0c ambinet. the tab will reach 115C under the limitations of the package itself, EXCLUDING any external factors. Have a nice day. --- End quote --- Before answering, recheck if you are understanding what you are answering. It's funny how you used internal thermal resistance between tab and resistor to calculate tab temperature outside the device which has nothing to do with it. --- Quote ---A quick side note: Rj-c + Rc-a is the same as Rj-a. in case of this device, the Rj-c alone is 3.3c/w :-/O Here is an example from a random N channel fet data sheet: As you can see , the package itself has a limit of 0.52c/w. that would be your thermal resistance with an ideal heat sink! the more realistic figure would be the 62.5c/w which is calculated without any heat sink! This is really huge compared to any heat sink (again, about 3 or 4 c/w) so you can just add the 0.52 to the thermal resistance of your heat sink to get your overall thermal resistance. multiply that by the power and you will have your temperature rise. add that to your ambient temperature and there is your "tab temperature". Again, the tab is NOT isolated in most cases, which gives you a figure between 0.5 and 1c/w for junction to case resistance. in this case they are isolating the tab which leads to a 3 times higher figure of 3.3 ;) --- End quote --- You put different specs into one pile. To not confuse yourself, using max junction temperature and junction to case thermal resistance, calculate what max temperature tab is allowed to reach. Then select heatsink and thermal interface accordingly to keep it at lower temperature. Instead of calling part flapping in the breeze "realistic". |
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