Power resistor in a DDPAK - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 05, 2023, 05:27:07 pm
content: I'm planning to use ahigh-power DDPAK resistorin my application because it's the only one that has low resistances and offers a high power rating. I've never used them before, so in this DDPAK configuration, how is it meant to be laid out? You have two terminals for the resistor (I assume), but what is the large pad meant for? Where do you solder it to?

author: canadaboy25
author_rank: Regular Contributor
timestamp: October 05, 2023, 05:32:36 pm
content: The backplate would just be for mounting/thermal dissipation. In the features section at the top of the datasheet, it says the backplate is electrically isolated from the resistor element. So it has no electrical function.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 05, 2023, 05:39:54 pm
content: Thanks. I see that now. Given that, I guess it goes on its own isolated copper island, or would it be better to mount it to a ground plane if doing so will help with heat dissipation?

author: srb1954
author_rank: Super Contributor
timestamp: October 05, 2023, 07:04:43 pm
content: Quote from: LoveLaika on October 05, 2023, 05:39:54 pmYou will need extensive heatsinking to this resistor if you are hoping to achieve anything like the specified power dissipation. The power rating of 35W is misleading as it is specified at a fixed 25℃ case temperature so effectively can only be achieved with an infinite heatsink or active cooling.You can calculate a more realistic power rating if you know your actual heatsink thermal resistance, the Rthjthermal resistance of 3.7℃/W and the maximum element temperature of 155℃. For example, if you can somehow attach this package to a discrete heatsink of 10℃/W you would only be able to achieve about 9.5W dissipation at a 25℃ ambient temperature.The recommended PCB pad alone would be hopelessly inadequate for transferring heat away from the package so you may need to use an aluminium core PCB to spread the heat out, possibly with an additional heatsink affixed to the back of the PCB.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 05, 2023, 07:20:37 pm
content: Really, it's only temporary for limiting the inrush current. It's not going to consistently pass a high current load for a long period of time. Looking at my sim, the max current passing through the resistor is approximately 1.7 amps for ~10 microseconds. Given a resistance of 8-ohms, that's a bit over 23 watts, well within the rated 35-watt limit. I wasn't planning to attach it to a heat sink. Worst case, I just use a larger resistor.

author: langwadt
author_rank: Super Contributor
timestamp: October 05, 2023, 08:03:48 pm
content: Quote from: LoveLaika on October 05, 2023, 07:20:37 pmat that time scale a heatsink doesn't matter

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 05, 2023, 08:20:30 pm
content: Glad to hear it. I just wanted to cover my bases. Hopefully that lone unconnected pad will be fine by itself. It's really odd to think of things in terms of such a time scale where milliseconds can be very long or very short, or both depending on how you look at it.

author: langwadt
author_rank: Super Contributor
timestamp: October 05, 2023, 08:25:05 pm
content: Quote from: LoveLaika on October 05, 2023, 08:20:30 pmpage of the datasheet has an overload rating with time and joules

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 05, 2023, 09:05:40 pm
content: Thanks for that. The chart was very helpful.