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| PCB Desing, power rectifier advice |
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| Atom:
Hi to everyone, so now i'm making the pcbs of a bench psu and i'm struggling to make all fit, since i have a max dimensions of 100*100mm..(i want to stay inisde this dimensions so the pcbs will be much cheaper ). for the ones intrested the power supply is the one engineered by Blackdog. now the real problem is that i like to keep things cool so right now for testing i'm using a kbu6b diode bridge bolted to a generous heatsink, but that heatsink takes place so i need some way to reduce power disspation, i switched to discrete schottky diode bridge, i used 4 mbr20100.. they are cheap and the vf measured is 500mv at even 10A, so they are good, but there is still some power disspation, the thing is that i tought that i could use the pcb as heatsink by soldering the tabs to the copper, by doing so i have also a better path for the cathode of each diode and i can push more current through it (instead of a single to220 pin). Now we have the schottky diodes on the bottom and the capacitors on the top, the schottky are dumping heat into the pcb , and that is bad for the capacitors that will warm up. the power supply isn't going to supply more than 5 Amps but if i want i can push it to 8amps, so for calc purposes we can assume a 10A constant load , the capacitance on the output will be 30mF or even 40mf (capacitor of 35*50mm. thats why i don't have space). so what i'm seeking is an advice on how to keep the diodes cool and also the capacitors. thanks to everyone keep in mind that in the 3d "render" diodes and caps are placed randomly ..that is kind of what i want to do, but im here for advice |
| keymaster:
the only way to keep things cool ,is to have a fan to make airflow . Another option is to use mosfets for the bridge and a controller ,like the lt4320 (https://www.analog.com/media/en/technical-documentation/data-sheets/4320fb.pdf) (https://eu.mouser.com/new/Analog-Devices/adi-lt4320-controllers/) |
| Siwastaja:
You have probably reached a situation where increasing the efficiency is what you need: throw some hi-tech at it. At low voltages, traditional diode bridges are horribly inefficient. At line frequencies (50 or 60 Hz, I assume), doing synchronous rectification is trivial. You'll find control ICs for this purpose. Last time I checked, they were irritatingly expensive for what they really are, but that may not be a problem - it'll still be cheaper than a heatsink and/or a fan. Cooling things sucks and is bulky - consider it as a last resort and start by minimizing losses. Energy efficiency is then a bonus. Silicon is cheap today; cheaper than heatsinking. Since your frequency is so low, you won't have switching loss issues, so you can totally oversize your synchronous MOSFET sizes (minimize Rds(on)) to have arbitrarily low voltage drops over them. 10x efficiency improvement over your diode solution is easy. The capacitors won't heat up. However, such massive capacitance poses another issue: the inrush current might need some controlling, it may risk destroying your diodes (or MOSFETs if you go synchronous). |
| Atom:
Sorry for the late reply , yes i was aware of the lt4320 in fact the original designer uses it in the original desing , but it is still pricey for what it is.. i searched for alternatives and everything that i found is the LM74670..http://www.ti.com/lit/ds/snosd08a/snosd08a.pdf but it even more pricey....so anyone knows other alternatives? talking now on how to mount the pass transistors , i was thinking to make a board always (100*100max) with the pass transistor on one side and the controls an the other side , i have 2 transistor, and i was thinking of reusing old intel stock heaatsinks , they should be good and i can take away a lot more heat with a metal to metal contact instead of isolating the 2 with siliocon pads. i mean if we watch thermal conductivity we have aluminum 200W/m K vs the 1to5 W/m K of a silicon pad..well i think the choiche here is obvious. so i was thinking of making a 3mm hole on the heatsink tap it and bolt down the transistor, then i'm going to extend the legs whit some 2mm copper wire to the pcb and solder it there, for keeping the heatsinks in their places i'm going to 3d print some mounts. so any suggestions? |
| Atom:
So i searched the net for a bit and this is what i found ..http://projectcircuit4u.blogspot.com/2010/04/power-mosfet-active-bridge-rectifier.html i tried it on a breadboard and it works.. 20mohm p channel and 6mohm n channel, 1000uf output capacitance and a 1 amp load...the mosfet stayed cool as they should be ... any apparent flaws that a more experienced eye could catch? i'll solder the mosfet to a copper pcb and do some more power testing tomorrow let me know of any flaws on the circuit :-+ a discrete mosfet bridge even if it doesn't reach the lt4320 standard could be very good for "budget builds"...i mean 10 or so resistor and an old quad opamp |
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