viewsonic 32in lcd tv power supply broken

[alain28]

[alain28]

DARFON
4H.B0280.001 /C2

Caution: for security mains isolation transformer is used.

For the 24v output, the nominal current is 6A.(max 7A)
In order to reduce thermal dissipation it is used a Synchronous Rectification with 2 Mosfet in //.
The controler is N3856VA (V is for SO package)
http://www.niko-sem.com/download/N3856VA,PA%20DATA%20SHEET(rev1).pdf

PWM controller is
NPC1377
http://www.onsemi.com/pub/Collateral/NCP1377-D.PDF
I think B version (to be confirmed)

Switching VMOS transistor is 11N80-C3

For bench test(dummy load) the min current is
5V 0,4A I set to 1,2A (2* 2*2 Ohms)
12v 0,04A I set to 0,6A (2* 10 Ohms)
24v 0,01A I set to 1,2A (2* 10 Ohms) reduced to 0,7A (35 Ohms)

I will continue investigation later, a problem have occured and I had to replace 11N80 (shorted) and npc1377 (consumption increased abnormaly)

The main power is 5V, normaly always ON.
12v also but switched by a transistor (detected by the 24v)
Power On should be biased via 2,2 KOhms with 5v.

AM

[alain28]

I am waiting NCP1377B.
For N3856VA if trouble I will replace the rectifier by Schottky diode.
(seems not easy to find or expensive like NCP1377B)
I am also waiting 11N80 transistor (Infineon)


The LCD TV is IISonic II3210J
_________________
AM

France

[alain28]

The fix goes slowly....
I get the NCP1377B and the Mosfet transistor 11N80 (Infineon).
Unfortunately an error on the order and no Schottky diodes...
The synchronous rectifier with 2 Mosfets(IRF3415) was removed. as the N3856VA (niko-semiconductor)
I add 2 diodes 1N5404 in serie on the current sense limitor in order to avoid an overvoltage on the PWM controller if the power transistor blow out.(seems always in CC...)
I find a fast rectifier diode (BYX71-350, old part), I think better to find 150v Schottky diodes.
The power supply is ok but the 24v is only loaded @ 0,7A. (nom is 6 A, max 7 A) The diode dissipation should be 0,5 to 1W
I try again to install the N3856VA but nothing on the output.(seems not overheated)
I add the 2 Mosfets and the fuse on the board blow out (T4A) and the 11n80 is in CC.
I come back to the diode BYX71-350 , the power supply is again good.
I will order Schottky diodes and try to find a N3856VA.
_________________
AM

France

[alain28]

The TV is operative.

The Synchronous Rectifier on the 24v is replaced by 2 * MBR20200CT
(200v, 2*10A)
THe PCB is designed for this in fact. (2 small shorts to add)
Ok @ 6A the heatsink is very hot but in the TV the power requested should be the half.

I will made a Web page with pictures.

Still nothing for the N3856VA but I think investigate only for future.
The TV will remain like this.
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AM

France

[vtech]

.... have to say; kudos on your persistance

[alain28]

sure....
for you
http://pagesperso-orange.fr/alain.mionnet/enii3210j.htm

I got some components by
http://www.farnell.com/
In france.

NCP1377B is a problem to get, as N3856VA.

I hope work on N3856VA later.
_________________
AM

France

[alain28]

I have still the LCD TV.... no time to watch.
I should power-on 2 times...
I will have N3856V but I have received recently an e-mail from an american who have a ViewSonic n3250w 32" LCD
with the same symptom of the power supply.
The reference is also Darfon 4H.B0280.001 /C2
It seems a trouble on N3856V.
I think that the best for him for the moment is to have Schottky rectifier diodes.
_________________
AM

France

[barnesmr1]

Does anyone know if the same power supply can be used for the N3251w model? As far as i can tell the only difference between the 51 and the 50 is the tuner.

my power supply is the same model, similar construction as the links posted above, but with /D after the model number.

[alain28]

I don't know but
/C2 is informaton on the revision of the board
/D follows /C
Letter should be the PCB status and number the part list status.
The changes are the for improvements, last rev should be the best.
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AM

France