Laptop power adapter repair.

[JonathanAnon]

Hi guys,

I blew up my own laptop charger somehow... did hear anything actually blowing, but I opened it up and the fuse, bridge rectifier and secondary schottky diode are all blown.... I only got the laptop as a "test" machine, so it's pretty old and requires 15v / 4amp.

I have a few old 19V /3.42amp chargers that I can take apart and use for bits.. So I'm gonna take the brige rectifier and schottky diodes from the 19v charger and replace the blown ones in the 15v / 4 amp charger... Now this sounds simplistic, but is there any reason why it wont work...There are only two things I'm worried about.

1. There is obviously a difference in the currents, so this is probably the most serious issue.. but the laptop should only be drawing 4 amps on full white screen, with all fans, wifi, everything running, which is not normally the case. So I should be pulling 4 amps through the components, probably closer to the 3.42amp they were designed for.

2. The secondary diodes are designed to rectify 19V DC from the high frequency AC from the switch mode transformer.. Again there is the issue with the current above, but (big disclaimer) if you ignore that then it should be basically rectifying 15v instead of 19v. I mean they are just diodes after all.. But is there a risk that the 19v adapter's dual diodes may not be "fast switching enough" for the 15v adapter.. Is it a big gamble to just say that the oscillator circuits in both adapters work at roughly the same high frequency.

I have a feeling I'm gonna blow something up with this one.. ;-) I will video it and stand back, like the guys watching the Trinity test.

thanks,
J

[midibob]

Re: Laptop power adapter repair.

Did you check the FET too?
Whenever I work on switch mode PSU's I always put a low wattage light bulb in series with the mains, saves blowing up all the new components until you locate the cause.

Good luck
Midibob

[JonathanAnon]

Re: Laptop power adapter repair.

Hey bob, FET is checking out good... When I first opened it, there were shorts on both primary and secondary.. I took out the SMPT to verify this. Checked the FET first on the primary side, but everything was coming up shorted.. Took out the BR and the primary side short seems to disappear.. There certainly could be other parts blown, but nothing immediately visible, other than the fuse as well.

I have my own little bulb tester
http://omg.wthax.org/6pnnW2.jpg

[midibob]

Re: Laptop power adapter repair.

Your bulb tester is exactly the same as mine LOL!
Great minds think alike.

Midibob

[JonathanAnon]

Re: Laptop power adapter repair.

Your bulb tester is exactly the same as mine LOL!
Great minds think alike.Midibob

hah!.. I read books where the author suggests soldering a bulb across the main 240v fuse holder, but I got thinking about it and there were a number of reasons I think my ( /your ;-) ) implementation is better:
1. You can change the bulb (and thus the current limit) easily.
2. It is far safer for the repair technician. No risk of temporarily soldered wires breaking of / crossing and blowing up. It also put more distance between the technician and the board in case something does blow up.. not that it ever has.
3. It is safer for the board, as you get to leave the board intact. And similar to number 2, if a temporarily soldered wire breaks or crosses it can do further damage to the board..

I actually thought it was irresponsible of those books to suggest toying with the main fuse... particularly given it was a tutorial book.

Re: the adapter. I'm gonna throw in those two components today and try it with my bulb tester.

[ben7]

Re: Laptop power adapter repair.

I guess it's possible that the driver IC for the MOSFET could be broken though.

You theoretically could have a problem with the newer schottky diode, but I bet it will probably be better than the old one, depending on how old the old adapter really is.

P.S. Pictures?!

[JonathanAnon]

Re: Laptop power adapter repair.

Okay...

I changed in the schottky dual diodes and the bridge rectifier.. Plugged in and the bulb came on.. turns out the main FET was shorted on two pins. I obviously didnt check it properly... I threw another FET in, and tested again with the bulb in series, and the bulb stays off... The filter cap had 320V across it when I checked... But the temporary FET I put in is not compatible I think.. I'm gonna look and see if I have another similar FET in old PSUs...

Hi Ben, I would have added a few pics but it's a laptop charger, so the bits are all on top of each other, I dont think it would contribute anything to my original question..

[midibob]

Re: Laptop power adapter repair.

Glad you isolated the FET. I usually find the FET has gone but on replacement nothing. You may find there's a zener (20v ish) feeding from a metal oxide resistor (if its good quality). The zener usually goes s/cct and the resistor sometimes o/cct.
I'd also check the cap in the oscillator section, usually 1uF or less, as these normally fail blowing up everything else.
Just guessing here of course as it could be a completely different design.

Midibob

[JonathanAnon]

Re: Laptop power adapter repair.

bahhhhhhhhhh

I'm now getting 320v across the main filter cap.. but getting nothing on the dual diodes on the secondary side.. I dont think the oscillation circuit is working. I'd love to have an oscilloscope or some high frequency AC detection for testing the pulses out of the PWM.. All I can do for the moment is check the power to the PWM chip.. but I'd be pretty confident there's something wrong between the main filter cap and the primary side of the transformer..

[ben7]

Re: Laptop power adapter repair.

bahhhhhhhhhh

I'm now getting 320v across the main filter cap.. but getting nothing on the dual diodes on the secondary side.. I dont think the oscillation circuit is working. I'd love to have an oscilloscope or some high frequency AC detection for testing the pulses out of the PWM.. All I can do for the moment is check the power to the PWM chip.. but I'd be pretty confident there's something wrong between the main filter cap and the primary side of the transformer..

AKA the pwm chip is dead ... the MOSFET shorted and took out the pwm chip (or its supporting component(s))

[JonathanAnon]

Re: Laptop power adapter repair.

AKA the pwm chip is dead ... the MOSFET shorted and took out the pwm chip (or its supporting component(s))

The PWM chip may or may not be working. It's not showing up faulty with the multimeter for the moment anyway.. I found a resistor open along the line to pin 5, and I've no idea how to find out it's value... Think I'm gonna have to bail on this one...

Image attached.

http://omg.wthax.org/6nbygX.jpg

[ben7]

Re: Laptop power adapter repair.

The PWM chip may or may not be working. It's not showing up faulty with the multimeter for the moment anyway.. I found a resistor open along the line to pin 5, and I've no idea how to find out it's value... Think I'm gonna have to bail on this one...

Image attached.

http://omg.wthax.org/6nbygX.jpg

It could likely be connected to the gate of the mosfet. Try to see if any of its pins connect to either the mosfet gate, or the output pin of the pwm chip.

[JonathanAnon]

Re: Laptop power adapter repair.

It could likely be connected to the gate of the mosfet. Try to see if any of its pins connect to either the mosfet gate, or the output pin of the pwm chip.

I think you're right on that one, Ben... The PWM chip is a DAP8A PBJW .. I cant find an exact pinout for the IC but the DAP8A family appear to be similar to this..



And pin 5 is the pin that drives the MOSFET..

[ben7]

Re: Laptop power adapter repair.

I think you're right on that one, Ben... The PWM chip is a DAP8A PBJW .. I cant find an exact pinout for the IC but the DAP8A family appear to be similar to this..



And pin 5 is the pin that drives the MOSFET..

SMD parts like ICs and transistors use codes occasionally. There is no 'DAP8A' chip. BUT, there is an 'NCP1203' chip which is marked with the device code 'DAP8A'

And, the datasheet pinout for the NCP1203 lists pin 5 as the output for the mosfet. The diagram you attached actually looks exactly like the one in the NCP1203 datasheet.

You can try replacing that resistor with one of, say, 4.7 ohms.

Of course, I would test for shorts to ground on each leg of the burnt resistor - that shorted mosfet could have dumped power through the resistor and into the pwm chip, and/or a zener diode (used for protecting the mosfet gate), either of which could be shorted. If you are lucky, then just the resistor could be the problem.

Cheers,
-Ben

[budm]

Re: Laptop power adapter repair.

The MOSFET probably had Drain to Gate shorted out so it took out resistors and Diode in the Gate drive circuit including the PIN 5 output circuit inside the IC. You should also check and see if the MOSFET has a very low Ohm resistor connected to the Source pin, see if that resistor is open or not.

[JonathanAnon]

Re: Laptop power adapter repair.

You can try replacing that resistor with one of, say, 4.7 ohms.

how do you deduce that, Ben? the gate triggers the source/drain on and off to generate the AC signal for the primary side of the transformer.. What's the purpose of a 4.7ohm resistor between source and drain.. used in conjunction with a transistor for O/C protection? I'll see if I can track one down..

The MOSFET probably had Drain to Gate shorted out so it took out resistors and Diode in the Gate drive circuit including the PIN 5 output circuit inside the IC.

yeah it seems to make sense alright.

[ben7]

Re: Laptop power adapter repair.

how do you deduce that, Ben? the gate triggers the source/drain on and off to generate the AC signal for the primary side of the transformer.. What's the purpose of a 4.7ohm resistor between source and drain.. used in conjunction with a transistor for O/C protection? I'll see if I can track one down..




yeah it seems to make sense alright.

It is used to limit the current, as far as I know. The mosfet gate is like a capacitor, so it can draw a very high peak current when charging up. Take a look at smps circuits on the web, and you will see that most have that resistor there. Usually anywhere from 10 to 0.5 ohms is used. 4.7 ohms seems to be a good value for testing.