Fairchild interest alternatives

F5T 19n20C replacement

  • FQA19n20L

    Votes: 0 0.0%
  • FQA12n25C

    Votes: 10 83.3%
  • FQA32N20C

    Votes: 0 0.0%
  • FQA28N15

    Votes: 0 0.0%
  • FQP TO220

    Votes: 2 16.7%

  • Total voters
    12
  • Poll closed .
Status
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...But then... Why do "we" use a 3020?
...
My point is, I have'nt had one blow up, and I use 2535. 12.5 amps per leg and In ||(always) 25A with a .325 MaxVfwd!!!!
...
I just don't feel comfy with these "ultra fast", "soft-recovery" bla, bla,,, 1-2V
I'd love to make a comment firmly grounded in the wealth of knowledge and experience, however... I have neither...:eek:

So, if I may rather just ask a silly question or two?

Since I can get the "OnSemi" MBR2545 (Avg. 30A, 45V) locally at about 1/2 of the group-buy price for MBR3020 (!),

with this being a dual Schottky diode, (http://www.mantech.co.za/datasheets/products/MBR2545CT_ON.pdf):
- How would I configure them into a full-wave bridge? Short the legs 1 & 3 and then build up the 4-diode bridge as if they were single diodes?
- How do you derive the 0.325VFwd? The specsheet says .73-.82 (temp dependant), per leg. Does this mean that the voltage drop is halved because it's a dual diode?
 
All of this is educated guessing, first of all. The datasheet is giving typical and min and max numbers but as always Y.M.M.V.
You can get a good idea what the fwd drop will be by finding the Amp's current draw on the graph, probably the 100C line. Not 25C. Any hotter than 100 is really not a good idea. There is really alot more going on than a static current flowing obviously and it is what it is. I think we could easily expand this discussion into all the reasons and math but...
My amp is not doing 25A all the time. But, when you turn your amp on, the amount of current that flows can be huge without things like the thermistor and the R in your CRC.
Schotky diodes are generally not as rugged as some of these others processes.
No, these things don't have that physics magic about them that 2 in paralell 1/2s the voltage ;) What you can do is 1/2 the current before you go to that fwd voltage graph. Essentially that is what's happening.
 
Hmm, at F5s idle current draw, it looks like a no-brainer - MBR is half the VFwd/Vdrop of MUR. Half consumption - half dissipation?
Is it safe to assume that at power on the NTC and the "R" in CRC will protect the diodes from the surge? MBR2545 can withstand up to 300A turn-on surge, and up to 60A repetitive peaks.
If this all checks out and since Schottky speed and recovery characteristics are legendary, we are down to:
- is 45V enough for 25VDC requirement? How about 35VDC?

Have I missed anything?
 
- is 45V enough for 25VDC requirement? How about 35VDC?...

Down to the nitty gritty? If you mean a 19-20VRMS secondary for 25VDC out? I would use a higher than 45Vr rated device. The rule (of Thumb?) I beleive is, no less than double the secondary Vpk out. Don't forget to include a few percent high line in etc. etc..
Think about it. Look at the schemo of an N.P. supply. Take a look right off the Xfrmr is a diode to the 25V+ rail. When that node of the Xfrmr goes negative that diode needs to witstand +25V + -28Vpk = 53V. And that would be 0 margine for failure! This is an important Saftey issue to understand. You can easily be blinded or burnt by the stuff comin outta them wires. Not to mention the possible eye injuries from exploding plastic and silicon, and the unraveling capacitors that end up with AC on them etc. etc.
 
It is not, as simple as I made it sound. There is DC and rms there is the pos cycle, and negative cycle with mutually exclusive current, there diode 1/2 sine waves through each and lots of averaging and math but attention to peak surge behavior is important and only an educated guess at that! These and others behaviours I can't remember are the reason for a robust diode bridge component selection.
O.K. to be Honest I would be interested in an AndrewT or EUVL comment, not leaving out N.P. or any others but... The way things are in my industry to day... I Don't know anymore! I should stop now...
:Pawprint:
 
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