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| David Hess:
--- Quote from: bob91343 on June 07, 2019, 05:58:11 am ---You can parallel two or even four diodes. Trouble is, diode voltage drop goes down as it warms so unbalance gets worse with higher currents. You could select the diodes and put them in a series parallel arrangement with a high and a low in each side. --- End quote --- If the diodes are mounted to the same substrate like with a bridge rectifier or dual diode, then they can match pretty well over temperature. --- Quote from: T3sl4co1l on June 07, 2019, 09:46:00 pm ---Wouldn't recommend more than 35A for a single diode in a FWB. They're amazingly robust devices, but they're still made like a fullpack device -- plastic molding provides heatsinking. --- End quote --- I would current derate them to 1/2 or 2/3 as well but I do that with all power diodes. DC operation is actually a little easier on the diode since the average specification takes into account greater losses from a poorer crest factor. It seems like older bridge rectifiers more commonly used a metal substrate with a plastic encapsulation. If I had a choice, I would choose that kind of package for adverse operating conditions. I like the SOD-57 and SOD-64 packages for the same reason. Glass DO-41 packages used to be available but now they are all plastic. |
| exe:
--- Quote from: David Hess on June 08, 2019, 03:21:39 am ---DC operation is actually a little easier on the diode since the average specification takes into account greater losses from a poorer crest factor. --- End quote --- Interesting, I thought the opposite: losses (Vf) grow slower than current: see "Fig. 4 - Typical Instantaneous Forward Characteristics" in https://www.vishay.com/docs/88503/1n4001.pdf , for example. What do you think? Or did you mean switching losses? |
| T3sl4co1l:
I was thinking, because only one diode out of the four is active at a time. The 50A rating is for alternating pairs, averaging 25A per diode. This buys a free, about less than two times, margin over DC, at the expense of somewhat higher voltage drop due to the crest factor. So i was figuring 35A as a compromise between voltage drop and thermal capacity per diode. FWIW, the two AC pins can be connected in parallel, utilizing half the FWB as a single diode. Don't expect the rating to be fully double though (i.e., the full ~50A DC from the module, give or take), because of current sharing issues. Tim |
| David Hess:
--- Quote from: exe on June 08, 2019, 06:27:20 am --- --- Quote from: David Hess on June 08, 2019, 03:21:39 am ---DC operation is actually a little easier on the diode since the average specification takes into account greater losses from a poorer crest factor. --- End quote --- Interesting, I thought the opposite: losses (Vf) grow slower than current: see "Fig. 4 - Typical Instantaneous Forward Characteristics" in https://www.vishay.com/docs/88503/1n4001.pdf , for example. What do you think? Or did you mean switching losses? --- End quote --- The forward voltage increases at the typical 60 millivolts per decade of current plus the contribution from series resistance but I^2R losses still apply so for the same average current, a higher crest factor still results in higher losses. This is reflected in the surge current ratings, if given in enough detail, which allow higher average current at higher duty cycles. --- Quote from: T3sl4co1l on June 08, 2019, 12:52:13 pm ---I was thinking, because only one diode out of the four is active at a time. The 50A rating is for alternating pairs, averaging 25A per diode. This buys a free, about less than two times, margin over DC, at the expense of somewhat higher voltage drop due to the crest factor. So i was figuring 35A as a compromise between voltage drop and thermal capacity per diode. FWIW, the two AC pins can be connected in parallel, utilizing half the FWB as a single diode. Don't expect the rating to be fully double though (i.e., the full ~50A DC from the module, give or take), because of current sharing issues. --- End quote --- The datasheet is unfortunately not specific enough to give test conditions but I suspect you are right. Distributing the current over two diodes per cycle halves the junction-to-case thermal resistance. I suspect using two diodes in parallel and the lower loss at DC more than makes up for the difference and any issues with current sharing. |
| calexanian:
Just doing a casual search turned up a whole new crop of reasonably priced Schottkey rectifiers in that current range. There most have been a new process developed recently because some of them are just a few dollars in single piece quantity while others are well in excess of ten dollars and they have similar specs. Leads me to believe the new parts have better fab processes. |
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