Hello everyone!
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)
My Zyxel ES1100 PoE router got smacked by a power surge. The router consists of a smps and the actual board that is responsible for the networking.
The networking board is still ok. I've tested it with another power supply. The reason why I'm trying to fix the original psu, is that my replacement psu is too big to fit inside the router case - and I thought it would be fun and educational to fix the original one
![Cheesy :D](https://www.eevblog.com/forum/Smileys/default/cheesy.gif)
The original psu is 48Vdc and 1.56A. Input voltage is 240Vac.
The power surge took out atleast the fuse, MOV, coil, resistor (R16) and obliterated the mosfet.
Here's what I've replaced so far:
- the bridge rectifier
- MOV and NTC
- all capacitors (except smd capacitors) on the primary side
- original mosfet SMK1260 with 11N60C3
- resistor R16
- original two coils with one made from slightly thicker wire. I took the replacement from computer psu.
I've also checked the diode D2 which is next to resistor R16, and it seems to be ok.
After replacing all the parts I turned the device on only to blow the fuse. After releasing the magic smoke, I realized that my replacement mosfet has a metal back, whereas the original has a plastic one. So the mosfets drain was connected straight to GND via the heatsink.
Removing the heatsink GND connection solved this problem and the psu turned on without blowing a fuse. But there was no voltage arriving to the primary side transformer windings (shown in red).
The PWM controller chip is OB2269. Pinout is as follows:
1. GND
2. FB
3. VIN
4. RI
5. RT
6. Sense
7. VDD
8. Gate
Voltage arriving at pin 3 (through R3, R4 and R5) is 120Vdc! Datasheet shows VIN max voltage should be 36V.
![WTF? :wtf:](https://www.eevblog.com/forum/Smileys/default/wtf2.gif)
The resistors R3-R5 all measure 0.324Mohm.
And to top it all off, while having my multimeter positive lead connected to rectifiers + side, and trying to measure gate voltage with the negative lead I've managed to blow the fuse - again
![Face Palm :palm:](https://www.eevblog.com/forum/Smileys/default/facepalm.gif)
And ideas what went wrong? I've always thought that one can measure voltages anyway round with a handheld multimeter. I'm 99% sure I didn't short anything out while doing the measurement
![Roll Eyes ::)](https://www.eevblog.com/forum/Smileys/default/rolleyes.gif)
So now I have a dead mosfet, resistors R13 and R14 shorted and probably the IC is toast as well
![Tweak :-/O](https://www.eevblog.com/forum/Smileys/default/smiliey_cal.gif)
Is it possible that the IC was toast in the first place and because of that couldn't drive the mosfet properly thus leaving it in some intermittent state? But that does not explain why there is 120Vdc arriving to the IC.
Anyway, thanks to those who made it through my wall-of-text. Even if this simple circuit beats me, I'm going to call myself a winner for learning how switched mode power supplies work - in theory
![Laughing :-DD](https://www.eevblog.com/forum/Smileys/default/smiley_laughing.gif)