Axial through hole power diode lead length vs parasitic inductance - Page 1
Forum section: electronics / projects

author: theleakydiode
author_rank: Regular Contributor
timestamp: November 23, 2022, 05:15:39 am
content: I'm trying to minimize parasitic inductance and capacitance of a little flyback converter I've made but I'm wondering how to approach the rectifier diodes (the ones recifiying high freqency switching stuff) when it comes to minimizing lead length whilst still allowing for diode cooling. The diodes are 4 amp axial types pulled from something (GUR460) but that's only if kept around 25c so they're currently standing up proud of the PCB for airflow but this also increases the parasitics of the circuit due to long untrimmed lead lengths.What's the best way to approach this? They'll be passing around 3 amps on average, if I mount it flush with the PCB then I assume it'll run hotter?

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 23, 2022, 05:22:19 am
content: The rectifier leads will add nH and pF, should be negligible unless using really high frequencies, rarely problem for 50-200KHz.Lots of PSUs have the diodes this way for better heat dissipation, it's fine.Yeah, trimming them down will rise working temps, but could also affect something else in the board, it's common to find dried electrolitics because they were mounted close to a regulator/rectifier dumping lots of heat.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 23, 2022, 08:44:07 am
content: Quote from: DavidAlfa on November 23, 2022, 05:22:19 amNot...really... Like, another cm (which is to say, ~10nH) here or there, can make or break a design! It's a valid concern.Keeping the paths low and wide (think: pours over ground plane) helps a lot, and also spreads out the heat. Don't underestimate SMT diodes. Well, if you're using pulls I guess that's not applicable anyway, but you could still clinch the leads very close to the body and use it the same way. That'd be, what, 5nH or so minimum?Heatsinking a planar design kinda sucks, but is doable. Soft squishy thermal pads can be clamped around the board, and that's feasible up to, oh, some tens of watts in a couple inch square size, say? If you need more, likely you need THTs bolted to a heatsink, and you just have to deal with the lead inductance.Heatsinks can be added to the diode directly; have seen many years ago something like a steel plate, with crinkled edges stamped into it, and a diode soldered into the middle. Or maybe it was brass or whatever, but it was tin plated for easy soldering. Just tacking on some flags of copper or brass sheet, adjacent to the diode body, can help out, have done that in a home-built PSU before.Better still, using schottkys, or synchronous rectification -- the best heatsink is not dissipating that watt in the first place!Tim

author: jonpaul
author_rank: Super Contributor
timestamp: November 23, 2022, 09:16:45 am
content: See single wire inductance formula.It is about 10 nH for 0.5" long 0.32" diahttp://www.consultrsr.net/resources/eis/induct5.htmJon

author: thm_w
author_rank: Super Contributor
timestamp: November 23, 2022, 10:18:14 pm
content: Quote from: T3sl4co1l on November 23, 2022, 08:44:07 amWe don't know what OP is using, it could be a LM2596.I wouldn't even think about it until they actually provide some information on the design.