AC Power Adapter Repaired
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:
Here is the picture of components replaced:
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:
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!
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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