Diode Bridge Heat Sink - Page 1
Forum section: electronics / beginners

author: just_fib_it
author_rank: Contributor
timestamp: October 13, 2014, 11:28:29 pm
content: I've managed to mess up the placement of the main diode bridge on my PSUPCB designa bit. The plan was to have the diode bridge and the series pass transistors on a common heat sink, but the diode bridge is facing the wrong way. I.e. the front of the package faces the heat sink.The diode has a 4-pin SIP-stylepackage. There's no metal tab on the package, so at first glance I'd say it shouldn't matter which side is mated to the heat sink. However, I'm a bit worried that internally the die is connected only to the rear side of the package.Does it matter on which side the heat sink sits, will putting the heat sink on the front increase the thermal resistance?Thanks, just_fib_it

author: Strada916
author_rank: Frequent Contributor
timestamp: October 14, 2014, 12:02:42 am
content: Place it label out. Ie so you can see the wiring on the front

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 14, 2014, 03:44:51 am
content: It will still work, but it will have higher resistance, yes. I wouldn't suggest using it at ratings.Tim

author: johansen
author_rank: Super Contributor
timestamp: October 14, 2014, 04:00:24 am
content: at 3 amps you don't even need to bolt it to the heatsink

author: just_fib_it
author_rank: Contributor
timestamp: October 14, 2014, 08:31:07 am
content: Quote from: johansen on October 14, 2014, 04:00:24 amThe forward voltage of each diode is 1V, so at 3A there's 6W to be dissipated. Or is it less? I'm not quite sure how the AC voltage comes into play here..Assuming 6A, the junction-to-case thermal resistance is 18C/W, so that would be 108C above ambient. The diode bridge might actually survive that, but I'm more worried about the big capacitor that sits right behind the diode (my PCB is a bit cramped...).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 14, 2014, 02:00:54 pm
content: I guess it's a rectified transformer sort of situation?The actual condition is more like, 3A average but 12A (or more) peak. But that peak is only on for maybe 25% of the cycle. And at that current, you're doing maybe 1.5V drop. At any given time, two diodes are conducting, so you get maybe 3V drop at an average 3A = 9W (or to put that differently, 12A * 3V = 36W when conducting, but only 25% of the time = 9W).It's slightly self limiting, because Vf has a negative tempco. More it heats up, less heat it makes, to some extent.And it is rated to 4A at 45C ambient, in air (no heatsink). That doesn't leave much headroom for warm surroundings (especially not if one side is an already-warm heatsink), but it can be done, it says so.3A isn't really much in the grand scheme of things, but I'd make sure it's on the heatsink anyway. At this level, it probably doesn't matter so much which side. Think I'd be more worried above 4 or 5A, as far as putting it on the right side.Tim

author: sunnyhighway
author_rank: Frequent Contributor
timestamp: October 14, 2014, 03:36:07 pm
content: Trim or bend the legs if needed.

author: just_fib_it
author_rank: Contributor
timestamp: October 15, 2014, 12:12:50 am
content: Thanks for all the advice!I think I'll just mount the front of the rectifier to the heat sink and monitor the temperature. If it turns out that things get too toasty I'll have a go at bending the leads.