Replacement for MUR860 (ChipAmp Power Supply)

[pyromaniac_]

Hi, got an PCB from ChipAmp.com for a mono LM3886 module with PSU which I'm going to build. However I can't find thoose MUR860 diodes for the power supply. But I've got some MBR1045 diodes (which also comes in TO-220 package), I thought it might work satisfying with them as well, instead of the MUR860, but what do say?

MBR1045 datasheet:
http://www.fairchildsemi.com/ds/MB/MBR1045.pdf

[Corax]

The MUR860 has a repetitive reverse voltage of 600V and the MBR1045 of only 45V.
Concerning current capabilities, 8A or 10A, they're almost identical.
Since the MUR860's are used as rectifier diodes in the power supply I suggest you better find other diodes with a higher voltage rating. You need at least a voltage rating of twice the peak rectified ac voltage of your transformer secondary winding (2x 24Vac => 2 * 24Vac * SQRT(2) = 68Vdc) or the peak rectified voltage of your center-tapped secondary winding (2x 24Vac => 48Vac_ct * SQRT(2) = 68Vdc). Considering some mains voltage fluctuations of +/-10% you end up with a voltage rating for the diodes of not less than 80V. If you're using other output voltages of your transformer feel free to recalculate the necessary voltage rating.
Besides, consider also that the voltage rating of a transformer is given under the specified power rating. If no load or just a small idling current for the LM3886 is consumed the output voltage of a transformer is higher than under full load. You may find the factor in the datasheet of your chosen transformer. As a rule of thumb you can say that the bigger the transformer the closer will the factor be to 1. As an example: A 150VA rated transformer has a factor of approx. 1.1 which means the output voltage is with no load 1.1 times higher than with a load of 150VA.
Finally I would suggest you look for diodes having a voltage rating of not less than 100V.

[pyromaniac_]

Ok, I didn't thought of that, thanks for telling me.

So i find these other diodes, BYW29E. The datasheet describes them as "ultrafast, rectifier diodes"-something like that, and they're rated at 8A 200V. Now they should do the job, or is there something else I've missed?

BYW29E datasheet;
http://www.electrokit.se/download/BYW29E_SERIES_4.pdf

[Corax]

No, you missed nothing more I guess - they have the same pin assignment, same ampere rating, ... .
They should do the job!

The only thing you have to take care about is the postfix at the type designation. I mean the three digit number following the "BYW29E-...". Only the "BYW29E-200" has a voltage rating of 200V while the "...-100" and the "...-150" has the corresponding rating that these three digits represent. Worst case would be the BYW29E-100 but that could be sufficient in your case - but that all depends (review the calculations in my last post).

Happy soldering. ;-)

[Cristi]

use shottky diodes. there are 150-200V models available. MBR10150, MBR10200 and many others. they feature very low voltage drop and no reverse recovery (at mains freq. doesnt's matter this anyway)
they will be just cooler than normal or fast diodes. if your current requirement is high, u must use a heatsink. don't let them to get hot as the lk. current increase with temperature.

[pyromaniac_]

Yeah, I was thinking of them too. MBR-10100 (10A 100V) and BYW29E-200 (8A 200) cost about the same too but I think I'll go with the BYW29E, because of the higher voltage rating. The current will probably not be a problem anyway (two bridge rectifiers driving one LM3886 chip), so they probably not even get hot.