that top area you mean? quite nice idea I must admit, although I'd be concerned of the long wires + maybe some interference with the pass elements...? This PSU is quite precise, only around 1-8mV absolute error from 0-24V, settable in 1mV steps and I wouldn't want to introduce a huge error/interference source into that, if that'd even be a concern... (i honestly don't know).
BUT probably the biggest issue is, that there is very little clearance to the top of the case there, so hot rectifiers would very likely be melting the case there (assuming they'll still get considerably hot while being on such a heatsink, which they probably wouldn't)...
I doubt the wires would cause an issue at mains frequency, although you never know until you try! If the top is tight, my suggestion of an additional plate might work. Just cut a 2 or 3mm thick aluminum plate and attach part of it to the heatsink and have part of it hang over and screw the diodes on the bottom side, then insulate the topside of everything to keep the case cool You could even bend the plate down towards the board to keep the wires short.
Thanks! Is there some rule of thumb formula how I can calculate the conduction time and the peak current? I wasn't able to find anything on the internet, maybe I googled wrong...
What about a load which rapidly switches a high current (to be precise, I have a very cheap electronic load from aliexpress which does this)? That load seems to be switching to achieve the set current draw, I could imagine that this load could be maybe drawing loads of current more from the caps than it should which in turn causes the caps to be recharged much "harder" by the diodes?
I guess I'll give it another shot with the diodes, veeery likely with Schottkys like you suggested. Any advice on the reverse voltage rating with a 12/24VAC toroidal transformer?
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Rules of thumb are not all that accurate in a case like yours because there are too many unknowns--it's easier just to measure them. For example, how much does the peak voltage sag under full load? You would have to check with a scope to really know. But you should know that the AC RMS current will be higher than the DC current (1.5-3X) which has implications for your transformer selection and that the filter cap size does have an effect on this ratio, just not a big one in any design with reasonably sized capacitors. 1-2mF per ampere of output seems reasonable.
As far as your fast switching load, it shouldn't really put any significant stress on your capacitors or rectifiers, but the linear regulator may have an inadequate frequency response and may not be able to keep up, so you would want a decoupling capacitor right at the input of the switching supply. And some linear regulators don't like capacitive loads because it slows down their feedback response and can cause oscillation, so.... Again, check with a scope.
A reverse voltage of 40V would probably be just barely enough, but I'd go with 60V just for some safety margin. This increases your forward voltage and power dissipation, but probably not enough to matter.
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I doubt the wires would cause an issue at mains frequency, although you never know until you try! If the top is tight, my suggestion of an additional plate might work. Just cut a 2 or 3mm thick aluminum plate and attach part of it to the heatsink and have part of it hang over and screw the diodes on the bottom side, then insulate the topside of everything to keep the case cool You could even bend the plate down towards the board to keep the wires short.
Rules of thumb are not all that accurate in a case like yours because there are too many unknowns--it's easier just to measure them. For example, how much does the peak voltage sag under full load? You would have to check with a scope to really know. But you should know that the AC RMS current will be higher than the DC current (1.5-3X) which has implications for your transformer selection and that the filter cap size does have an effect on this ratio, just not a big one in any design with reasonably sized capacitors. 1-2mF per ampere of output seems reasonable.
As far as your fast switching load, it shouldn't really put any significant stress on your capacitors or rectifiers, but the linear regulator may have an inadequate frequency response and may not be able to keep up, so you would want a decoupling capacitor right at the input of the switching supply. And some linear regulators don't like capacitive loads because it slows down their feedback response and can cause oscillation, so.... Again, check with a scope.
A reverse voltage of 40V would probably be just barely enough, but I'd go with 60V just for some safety margin. This increases your forward voltage and power dissipation, but probably not enough to matter.
Ok, thanks! I'll try and develop a solution with five schottky diodes then
Those will work. There are two diodes in each case, but there's no equivalent common anode design so you may as well just parallel the two cathodes. One drawback of this part is that the metal tab is not insulated but is connected to pin 2, which makes attaching it to a heat sink a little trickier.
initially i was worried about thermal drift of the two diodes but then i figured there are many of those TO220 devices which have two diodes inside but don't mention that fact in the datasheet. I guess they are supposed to be used as a single diode and there are two in them to meet the Vf goal..? So I guess they'll be ok...
I was also looking for a complementary common anode device but wasn't able to find one either (would have saved some space). but then, more heat in one case so maybe it is better to utilize 5 devices (cases) anyway.
Would you recommend to utilize devices with a plastic package? I was deliberately looking for diodes with a metal tab to ensure better heat transfer. if that wouldn't be of any concern, plastic cases would make mounting easier (and cheaper), of course (i was planning to use quite expensive silpads, just like on the pass elements)...
edit: filtering for plastic packages, these were the cheapest diodes if i wanted single diodes in a case:
https://www.mouser.at/ProductDetail/Vishay-General-Semiconductor/VFT2080S-E3-4W?qs=HSPD0Bff7iaqHXrROfvV8A%3D%3DI'm really not sure whether paralleling two diodes within a case would be a good idea (if they are specified as two single diodes) in terms of thermal runaway and frying themselves eventually...