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AC Power Adapter Repaired

By on November 23, 2015
power adapter repair









This AC Adapter was brought to me with the complaint that it was dead. In order to open the adapter, we need to pierce the rubber bushes underneath, and remove the two screws hidden under it. On opening the adaptor, I noticed that the 47/450V cap was bulgy. When I checked the other components, I found that fuse was blown, .5 Ohms resistor was open that goes to the source of the mosfet 2SK2141, one IN4148 diode was open, the mosfet was dead short, ESR of 47/50V in the primary, 1/50V, two 1000/25 in the secondary were all out of the limits.

So, replaced all these components and checked the rest of the components thoroughly in the primary and secondary and did a dry solder patch up. I replaced all components from my stock except the mosfet. I was not having 2SK2141 made by NEC. This is a plastic encapsulated package without a visible heat zinc. Downloaded its data, which was as follows:

DESCRIPTION : The 2SK2141 is N-channel Power MOS Field Effect Transistor, Designed for high voltage switching applications. FEATURES : • Low On-state Resistance RDS(on) = 1.1 W  MAX. (VGS = 10 V, ID = 3.0 A) • LOW Ciss Ciss = 1150 pF TYP. • High Avalanche Capability Ratings • Isolated TO-220 (MP-45F) Package ABSOLUTE MAXIMUM RATINGS (TA = 25 °C) Drain to Source Voltage VDSS 600 V Gate to Source Voltage VGSS ±30 V Drain Current (DC) ID (DC) ±6.0 A Drain Current (pulse) ID (pulse)* ±24 A Total Power Dissipation (TC = 25 °C) PT1 35 W Total Power Dissipation (At = 25 °C) PT2 2.0 W Storage Temperature Tstg –55 to +150 °C Channel Temperature Tch 150 °C Single Avalanche Current IAS** 6.0 A Single Avalanche Energy EAS** 12 mJ *PW £ 10 ms, Duty Cycle £ 1% **Starting Tch = 25 °C, RG = 25 W, VGS = 20 V ® 0

Since I was not having this with me, I studied the data of all the mosfets I had with me and found that P6NK90ZFP was more or less suitable. Here is its data:

N-CHANNEL 900V – 1.75W – 5.8A TO-220/TO-220FP/D2PAK Zener-Protected SuperMESH™Power MOSFET, ABSOLUTE MAXIMUM RATINGS VDS Drain-source Voltage (VGS = 0) 900 V, VDGR Drain-gate Voltage (RGS = 20 kW) 900 V, VGS Gate- source Voltage ± 30 V, ID Drain Current (continuous) at TC = 25°C 5.8 5.8 (*) A, ID Drain Current (continuous) at TC = 100°C 3.65 3.65 (*) A, IDM () Drain Current (pulsed) 23.2 23.2 (*) A, PTOT Total Dissipation at TC = 25°C 140 30 W, Derating Factor 1.12 0.24 W/°C, VESD(G-S) Gate source ESD(HBM-C=100pF, R=1.5KW) 4000 V, Dv/DT (1) Peak Diode Recovery voltage slope 4.5 V/ns, Viso Insulation Withstand Voltage (DC) — 2500 V

So, I used it in its place. The key factors to be checked are the ampere, wattage, resistance and voltage. Here is the picture of the ACA with all its components replaced:

ac power adapter repaired

power adapter repair

Here is the picture of components replaced:

ac adapter repair

Since I was quite sure that there were no such defective component that could blow the fuse again (I had even checked the pins of 3842 PWM IC with my analogue multimeter for any short), I switched it on. There was no output. I checked for any wave form using my oscilloscope by placing the probe on top of the SMPS transformer, as suggested by Jestine Yong in his SMPS repair book. Could not find any. So replaced the IC and switched it on:

repairing power adapter

It worked. I had removed the wire from the secondary for easy maneuvering, marking colour of its leads on the PCB. Fit that back, replaced the heat Zincs on to the mosfet as well as double diode IC at the output. Fit the covers and tested it for a few minutes. It worked just fine! Since I had used an IC socket, after removing the PWM IC, I checked all  my stock of 3842 and 3843 with me and found all of them working fine. So, this ACA served as a test jig for me!

repairing ac adapter

The ACA is now ready for return to the customer.












This article was prepared for you by Parasuraman Subramanian from India. He is 65 years old and has more than 30 years’ experience in handling antiques equipment Valve Radio, Amps, Reel Tape Recorders and currently studying latest techs classes conduct by Kerala State Electronics Technicians’ Association. He was a BBA graduate, retired as MD of a USA company and presently working as Consultant Manager, Purchase & IT, in Irinjalakuda Cooperative Hospital.

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  1. Car

    November 23, 2015 at 10:22 pm

    Nice work! A lot of defective components but ... You have fix finally! Bye!

  2. Robert Calk

    November 23, 2015 at 10:30 pm

    Good job, Parasuraman. Thanks for the article.

  3. mahien

    November 24, 2015 at 12:00 am

    Dear parsuram sir
    well done
    i love india but never been there..
    Keep up the go0d work sir..

    Go0d evening
    sir jestine..

  4. Allan

    November 24, 2015 at 12:19 am

    I so much like this repair. Lots of testing and in the end a successful repair.
    Thanks for sharing.

  5. Yogesh Panchal

    November 24, 2015 at 12:42 am

    Good job sir,
    please keep sharing the experience.

  6. Paris Azis

    November 24, 2015 at 12:47 am

    Well done Parasuraman.Thank you for sharing this experience.

    Best Regards

  7. Hermann

    November 24, 2015 at 1:11 am

    Interesting read, as always!

    From my experience, when the MOSFET shorts its gate with source, it very likely kills the PWM chip, because the full 350+ volts will hit the chip with no chance of survival.

    • Paris Azis

      November 26, 2015 at 5:51 pm

      Hi Hermann

      You are right about the explanation you expressed (or at least I fully agree with it).
      A simple proof about the validity of this opinion as a true one is the fact that when a driver (and of course isolating) transformer is used between the P.W.M I.C and the power output semiconductors in order to trigger them, this IC rarely gets destroyed. Usually only the power semiconductors are found shorted out (along with the main fuse of course and perhaps the relevant resistors in their emitters or sources, if they are used, depending on the type of the semiconductor used).
      Furthermore, this is the usual situation related to small SMPSes, especially to those working in “current mode”, like this one of the present repair. In fact these I.Cs rarely escape their instant death when the device they drive fails…


  8. Albert van Bemmelen

    November 24, 2015 at 1:49 am

    Hi Parasuraman, well done! It must give a very nice feeling to have repaired such an important Switching Power Adapter. Repairing those new SMPS supplies is great fun! Anyway, the 3842 PWM controller chips can also be tested with the great working little tester that was published a while ago on one of Jestine's articles, or published in his e-book. That is also maybe a bit safer because that one works on a low transformer voltage. I believe it was only 13 V AC. Thanks for your article.

  9. Dennis Breda

    November 24, 2015 at 1:53 am

    Thank you for the thorough explanation. I work alone and am isolated from sharing knowledge. This is a great form.

  10. mabrouk

    November 24, 2015 at 3:19 am

    thanks a lot for sharing this good work

  11. Peter O

    November 24, 2015 at 7:23 am

    Surprising that so many components had failed.
    You persisted & gave yourself time to complete the repair.
    Generally I'd suggest that "replace with new" would probably be more economic.
    However this is not always easy either, & aquisition delays can be lengthy.
    I suspect many like me would like to know how long this repair took?

    • Parasuraman S

      November 24, 2015 at 1:41 pm

      Thanks for your support. It took just less than 30 minutes to complete the job. After reading and understanding Jestine Yong's SMPS repair book, in which he states that SMPS servicing is an altime challenge for any technician, I love such challenges and spare no effort to understand and rectify the root causes.

  12. reza

    November 25, 2015 at 3:56 am

    hi Parasuraman Subramanian
    thank u very much.

  13. Bhupendra Thakur

    November 25, 2015 at 10:20 pm

    Dear Parsuraman,Good work.Thanks for sharing the information.

  14. Agustin Lalu

    November 26, 2015 at 12:11 pm

    Very interesting and lots of information on how to find the right components for subtitute very well done sir thank you very much for sharing Godbless...


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