help with mosfets and switching regulator!

[philliesfan30m]

i took the board out and started testing some things but i need some help. i tested 5 n-channel mosfets on the board 1 was 2sk2655 and 2 of them were k3911 and 2 were k2698 here is what i got i don't know if i am doing it correctly i see a million different ways people check mosfets but this is the way i did it maybe i am wrong but i am trying to figure out if they are good or bad i think 1 might be bad but here are the readings.in diode mode

2sk2655 readings

-lead +lead
g-d OL
g-s 170
d-s 521

+lead -lead
g-d OL
g-s 1783
d-s OL

k3911 readings

-lead +lead
g-d OL both
g-s 1530 both
d-s 474 both

+lead -lead
g-d OL both
g-s 1690 both
d-s OL both

k2698 readings

-lead +lead
g-d OL both
g-s 780 both
d-s 510 both

+lead -lead
g-d OL for 1 mosfet and other reads 760 and climbs up
g-s OL both
d-s OL both

the 1 that climbs up is the 1 i am guessing is bad and they were all tested in circuit. should i take the 1 that climbs up out of the board and recheck? or do they all need to be taken out?

part#str-f6167 it is a OFF-LINE QUASI-RESONANT FLYBACK SWITCHING REGULATOR made by sanken but anyways is it possible to check them and if so how is it done? i checked it in diode mode and here is what i got. it has 5 pins so i'm gonna show all the checks i did pin 1 being on the left with it attached to the heatsink pins 2 and 4 furthest away from heatsink.

str-f6167 readings

- pin# + pin#

1-3 OL
1-5 OL
3-5 OL
1-2 220
1-4 OL
3-2 520
3-4 OL
5-2 797
5-4 OL
2-4 OL

+ pin# - pin#

1-3 785
1-5 1050
3-5 OL
1-2 220
1-4 860
3-2 OL
3-4 OL
5-2 OL
5-4 OL
2-4 601


i hope someone can understand what i did and give me an answer to my questions. thanks in advance

[philliesfan30m]

Re: help with mosfets and switching regulator!

any help?

[philliesfan30m]

Re: help with mosfets and switching regulator!

the closest datasheet i got to the switching regulator shows that pin 1 is over current and feedback pin 2 is source pin 3 drain pin 4 supply and pin 5 is ground so any advice on how to check it to see if it is working properly?

[philliesfan30m]

Re: help with mosfets and switching regulator!

can someone help me out please?

[chrisopher]

Re: help with mosfets and switching regulator!

look at the datasheet for the fet. assume a new fet is perfectly working and doesn't need testing. you should always replace fets like-for-like. If you cannot find one that is exact, look at the datasheets and compare all of the specs. It's like putting an exhaust from my mate's Corsa onto my Civic...

the DRAIN to SOURCE resistance should be 0L
but the SOURCE to DRAIN resistance usually will be pretty low (fets have a diode from src to drain)

this is assuming n-channel enhancement fets. That should help you out.

[chrisopher]

Re: help with mosfets and switching regulator!

ps, as far as testing chips go, you COULD look at the switching waveform (The GATE of the fet) on an oscillosope

Connect a digital load (or multiple low ohm power resistors) and increase the current draw from low to high. The switching waveform _should_ be a low % duty cycle at light loading, and up to 95% for almost max load.

Likely story is you won't have this equipment. I doubt it's the chip, but they cost like $2 so you aren't loosing anything if you replace the chip...

Post a schematic and pictures and i'll do my best to help you .

[philliesfan30m]

Re: help with mosfets and switching regulator!

thanks for the reply i understand it now from how you explained it. you are right i don't have an oscilloscope i am just learning eventually i will get 1 though. i can't find a schematic for the board just like i can't even find a datasheet for most of the parts on the power supply. but i don't think they are my problem. it has to do with a plasma tv power supply that i am not getting any voltage at the vsus and vadr but i am getting at all the other connectors on the power supply.

[chrisopher]

Re: help with mosfets and switching regulator!

if you can get a high-res picture of the top and bottom of the board it would help.

also a good idea would be to draw out a schematic of the whole power supply. or at least the outputs that do not work,

[philliesfan30m]

Re: help with mosfets and switching regulator!

https://www.badcaps.net/forum/showth...pioneer+plasma

this is the thread i have open i have some pics there but not of the bottom of the board but i will try to get some good pics of both the top and bottom