seems you have a thermal issue..

Indeed... but it certainly was worse before the cap replacement.

The unit has been operating open for a few hours and the cooler temperature has resulted in better operation again.

L1 is very close to the output FETs, maybe it's getting too hot. I rekon it could be relocated, or replaced with a larger inductor.

 

I think you may have the wrong FET installed. It may be a misprint or faulty

 

Installed in Q1: FQP12N60C

Installed in D8, D8B: MBRF20150CT

(I mis-typed... there is just one FET and two rectifiers. )

 

I didn't replace C17; it is a 2.2uF polarized radial-lead cap.

Would a non-polarized, axial cap be suitable?

(If so, I can try that before placing an order with an online supplier.)


Edit: or perhaps a 4.7uF polarized radial?

Thanx.

 

That's a 600v 12A MOSFET with an Rds(on) of 0.65 Ohms. Seems like your Vdss is pretty high, which is why Rds(on) is high. What's the peak voltage you're seeing across it's drain and source connections?

 

Stick with the 2.2uF polarized.. A 20% difference isnt bad but 400% is real bad.

 

That's a 600v 12A MOSFET with an Rds(on) of 0.65 Ohms. Seems like your Vdss is pretty high, which is why Rds(on) is high. What's the peak voltage you're seeing across it's drain and source connections?

Using a DMM, I read 33V one direction, 28V the other. That's with the unit cold.

I'll check again when the unit is hot.

Thanx.

 

Well, the unit is getting warm and the DMM is going spastic, with peaks above 1800VAC(?)

Maybe I'm doing something wrong, but the DMM reads the mains as 120VAC.

Confused.

 

I also get different readings when:

Laptop is shut off: 900V, 300V
Laptop off, Battery removed: 200V, 92V
Laptop disconnected: 70V, 35V

?

 

Does your meter have a frequency meter?

 

Yes, my meter has a freq feature: 20KHz

 

See what the frequencies are at those voltages. If they are supa-high, you prolly have an oscillation problem.

 

The only thing that seems remotely stable is the mains. Everything else is hopping all over the place... the FET legs are jumping rapidly in the teenKs and over the 20K range.

 

Have you replaced ALL of your caps? I think you will find that your largest cap is probably severely limited, causing wild oscillations

Im also wondering if ZD1 zener is shorted. It may be allowing the oscillations also..

 

I did not get all the caps replaced. I lifted the big cap out of an old PC supply just to wing it, and C17 2.2uF cap will also have to be ordered.

C12 replaced by 330uF 600V (from PC)
C16 replaced by matching cap
C17 not replaced
C15, C15b replaced with matching caps
C14, C14b replaced with same as C15, C15b

image here:
http://forum.allaboutcircuits.com/attachment.php?attachmentid=18185&d=1270649784

Sorry if that info was not clear..

Thanx,

TD

 

U need to change the smaller and replace the bigger one with exact value, get low ESR caps.
Oh yes. I am talking about the primary side.
Still If sign of whining is there, check for the resistor value gone high in the PWM RC network

 

May take a few days to get parts. Genuine radio shops are locally extinct.


In the meantime, perhaps someone would be kind enough to explain how this thing functions...

Here's my sad attempt at a stab at a guess:

Incoming AC is rectified into ratty DC and passed through the big cap to suppress the really ugly stuff...?

That DC is passed to Q1, which is controlled by N1, to provide PWM-chopped DC to the big inductor/transformer... the combination of which function as buck converter...?

The lower-voltage output from the inductor/transformer is passed to the output stage(s) which provide more harmonic filtration and capacitance to supply the laptop's erratic and fussy input...?

I haz read a little on the interwebs... but it's mostly Greek to me.


I get the impression the input stage is not doing its cleaning-job properly, or at least we won't be able to tell until the caps are known to be good.


Thanks for the tips.