This little BASTARD cap...
So I’m putting together my parts list for the 54645A, and I can’t find a schizzmatic or component level parts list to save my life.
Of COURSE, this is the side jammed up tight against the heat sink so I can’t tell what voltage, and parametric search on DigiKey returns bupkis for this value in the current P561 product line.
So I bite the bullet and remove the little stinkbomb; it’s a bit of a PITA due to silicone spooge entanglement, but not too bad. I keep telling myself.
I get my numbers, and I look for anything close available locally, but no. Gonna have to order. *sigh*
I go about cleaning up the pads on this and another bastard cap that was completely obfuscated with a heat-shrink wetsuit, and the ground pad is just a complete dicksore to get clear. Tried solder-wick multiple times, even dipped in flux, tried solder-sucker several times and finally got it by propping up the board on edge so I could get sucker on one side and big chisel iron on BOOST contacting the other side. That GND fill was just sucking away the heat like a vortex.
So now I want to put it all back together until parts arrive... and I realize I do NOT want to do that desoldering job ever again for fear of cooking the PCB to death.
Fuck me. With a shovel. Sideways.* ![Banging Head |O](https://www.eevblog.com/forum/Smileys/default/bangheadonwall.gif)
mnem
*in the room. I may be a perv, but I’m not suicidal. ![Tongue :P](https://www.eevblog.com/forum/Smileys/default/tongue.gif)
Okay... looking to find a sub for this little bastard; I was thinking of going to Y2 poly, and I've found a EPCOS MKP B3202 series that should just fit while having a lot more headroom. The manufacturer recommends this line for SMPS DC conversion applications, which is good, as this one is right across the gate/source of a big ol' IRFPC50 MOSFET in the first DC converter.
The only problem I see right now is that from experience these tend to have pretty low ESR, often similar to ceramic, so even for a 47nF I'd still expect ESR of 1Ω or less. The one I just took out measures 40Ω ESR. Can't tell from the datasheet if that's in the normal range or not.
I know these can be used as drop-in replacements for the dread RIFAs in AC line applications; I've seen it oodles of times. But here I'm concerned about low ESR affecting the stability of the SMPS at these higher frequencies.
Opinions?
mnem
![Broken! :-BROKE](https://www.eevblog.com/forum/Smileys/default/smiliey_scope_broke.gif)
Sounds like it might be a snubber with built in resistance of intentionally high ESR. I'd be very wary of replacing with anther type if you don't have full data. I've seen epoxy aed snubbers were you could see the resistor through the resin.
How about a pictue of the markings on the original?
EDIT
E.G. https://content.kemet.com/datasheets/KEM_F3024_PMR205.pdf
It’s not a specialist snubber. It’s PME261. https://kr.mouser.com/pdfdocs/KEM_F3294_PME261.pdf
Replace like with like lest you pick a fight with the SMPS gods. The only issue is it’s an old capacitor. New ones of the same type are fine.
47nF with a test frequency of around 100KHz turns up around 40 ohms reactance.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1044948;image)
Yup; just the ubiquitous PME261. Which KEMET has superseded to their own P561 product line; I attached both datasheets to my original post.
The IRFPC50 has the usual integral clamping diode as well as a reverse p-n junction diode; this affects both the gate and the D-S path capacitively. This stuff is generations newer than anything I ever worked on; trying to wrap my brain around these interactions literally makes my brain hurt.
![Banging Head |O](https://www.eevblog.com/forum/Smileys/default/bangheadonwall.gif)
It feels like the same kind of voodoo as that 500MHz nuvistor oscillator we all know & love.
![Tongue :P](https://www.eevblog.com/forum/Smileys/default/tongue.gif)
I suspect you are both right; I think the PME261 was chosen BECAUSE of its high ESR in this circuit. I can see they had access to the same Y2 series polys; they're used elsewhere on the board. I was just looking for a better part since I'm already knee-deep in the hoopla.
*sigh* This is going to be another case like that bastard NVRAM; just do your best and live with it until it dies. Even if I just put the same exact PME261 back in it, may very well still outlive ME.
Under any circumstances, I've contrived a bodge that gets the old RIFA back in circuit mostly safely so I could put the scope back to standby duty instead of the repair queue; no, I'm not going to post pics of that crime.
Just know that I've admitted my guilt here and now; if Karma catches up with me, I can greet Murphy with a clear conscience. ![Evil >:D](https://www.eevblog.com/forum/Smileys/default/evil.gif)
mnem
![Popcorn :popcorn:](https://www.eevblog.com/forum/Smileys/default/EatingPopcorn.jpg)