Panasonic Plasma TV TX-P50X50B - Shorted power supply

[Kairi]

Hi there,

My latest electronics project involves bringing back to live a 50 inch Panasonic plasma TV. When I first acquired it, I was told the power supply board was bad but the previous owners did not get this fixed as they couldn't afford it.

The model of this power supply board is N0AE6JK00006 and unfortunately I do not have a schematic.

Anyways, I checked the fuse which was turned out to have blown. I replaced the fuse but this blew as soon as I plugged it into the wall. At this point, I knew there was a short someone on the board and after some detective work with a multimeter, I traced the short to two power mosfets Q207 and Q208. They were shorting across all three legs no matter how I read them on the meter.

Thinking I found the cause, I ordered the exact replacement mosfets, soldered the pair on and replaced the fuse. About two seconds after I switched it on, the fuse blew again! I immediately checked the mosfets and to my worse nightmare... they were blown, in mere two seconds of receiving power!!!

I removed the board from the TV and checked everything I can on the hot side with my multimeter. All the resistors, diodes and the bridge were fine. I even lifted one leg from the board to check. I do not have a capacitor tester yet to check the capacitors however I did test them for dead shorts of which there were none.

Aside from what I think could be faulty capacitors, I'm worried that the mosfet gate drivers might be damaged. The gate connects to a series of SMD resistors before a small transformer before some more resistors and finally to an IC chip which I believe (not certain) is the gate driver IC. The chip has the following text printed on it...


25600
a351
19M

I am looking for any helpful hints which will help me repair the board and id I do need to replace the IC, where could I source this?

I have attached some pictures of the circuit board. The first one is the component side, an exact board on a parts website. The second picture is the underside of the board beneath the mosfets showing the area nearby and the what I think is the mosfet driver IC. The mosfets themself has been pulled from the board.

Thanks in advanced.

Top side
http://assets.shopjimmy.com/media/ca...0006-top_1.jpg

Bottom side
http://img855.*************/img855/5513/amth.jpg

[PlasmaRon]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

If the gate drive is driving the FETS on all the time, they will probably blow. There's usually a reason that the FETs are blown. Did you happen to ohms check the gate drive output with the FETs disconnected?

[Kairi]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

I checked both sets of SMD resistors and the diode and they seemed fine. I don't how how to check the actual driver though.

On an interesting note, the resistors seem to be connected in parallel and in series and the fact that it was arrange this way meant that some current could bypass the diode defeating the point of one being present? This would put the drain voltage into the drive circuitry???

[tom66]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

New TV! Wow. Seen a lot of Panasonic plasmas dying young now...
Anyway, I would ALWAYS, as a matter of precaution, replace the driver chip when replacing FETs.
I would also utilise the lightbulb trick as a diagnostic procedure and to prevent further blown devices: https://www.badcaps.net/forum/showthread.php?t=32748

[tw2005]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

Ucc25600

UK RS online

Might pay to check out the IGBTs in the SS section of that PSU as well on that heatsink next to the control buttons just in case there's more issues

[Kairi]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

I also found IR25600. Is this the same as UCC25600? UCC25600 doesn't mention anything about controlling mosfets whilst IR25600 does.

Here is a higher quality picture of the original chip on the board. I notice it was a U symbol or logo, does this mean anything?



I have checked those fets on the heatsink next to the switches and they seem fine... no shorts on any of them. I think some are voltage regulators but I couldn't test voltages as the thing wouldn't power on the begin with.

[tw2005]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

I'm going with the TI UCC25600, your device is clearly not an Internatinal Rectifier product by the markings, the datasheet shows you the format of the case marking, pin outs are different.

The TI datasheet talks about gate drive and applications like PSU. Pin 6 is gnd on the IR pin 3 is gnd.

You could test pin 6 to gnd to confirm . The images I've seen so far of the UCC markings seem to match yours.

That's my opinion, see what others think.

[Kairi]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

I assume that the negative leg of the bridge rectifier could be used as the ground. In which case, I tested pin 6 on the chip using a multimeter between pin 6 and the negative leg on the bridge rectifier and I can confirm that it has full continuity. Between pin 3 and the negative leg, the meter reads 069 on the diode setting.

[tw2005]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

I assume that the negative leg of the bridge rectifier could be used as the ground. In which case, I tested pin 6 on the chip using a multimeter between pin 6 and the negative leg on the bridge rectifier and I can confirm that it has full continuity. Between pin 3 and the negative leg, the meter reads 069 on the diode setting.

Sure , so I'm fairly confident the UCC is correct although I've never touched something like that. There may be other manufacturers of the same device but you get a hit on ebay for it with pics and it looks the same to me , I use RS here from time to time and the price looks ok and at least you can be sure of the authenticity of the part. There delivery is quick here providing it's in stock and in country, it says it's in stock next day delivery but you'd have a better idea who to go to in the Uk

unfortunately it looks like panasonic have gone cheap on their last plasma designs, very weird the combined PSU /SS design, makes for an expensive repair if just the SS section failed unless you can component level repair it .

As far as i know they're ceasing production on plasma.

[Kairi]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

My ESR meter has arrived so I tested all the electrolytic capacitors. They all passed except for two which has a high ESR value compared to others.

C203 - Rated at 100uf, it reads 93.86 uf with an ESR of 1.4 ohms
CE317 - Rated at 10uf, it reads 9.6 uf with an ESR of 2.1ohms

I understand C203 is the main filter capacitor and to my knowledge it needs to have low ESR. With this cap reading over 1 ohm and the other one at over 2 ohms, shall I earmark these for replacement?

[tom66]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

If you get the part#s of that 100uF capacitor you can look up what the ESR should be.

[Kairi]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

I wans't able to find a datasheet for the cap, however I got a list of capacitor types from Aishi's website. The Filter Cap is a HF series which is a long life cap suited for high ripple current. No mention about the ESR though.

[tom66]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

I would replace it to be certain then. 1 ohm is a little high for one like that.

[Kairi]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

Update...

Today I replaced the gate driver IC chip, two brand new mosfets and the fuse having purchased them from RS Components (Thanks TW2005, it was the correct one). Soldering the chip was tricky as the pins were very small but I got there in the end after several tries. I didn't replace the capacitor as they didn't have it in stock and the one listed in their catalogue had a equally high ESR as the original Aishi cap.

I connected it to my dim bulb tester and observed some strange results of which I will show you on a video I made. Basically the light flashes instead of starting bright and then glow... at least that's what I've learnt! I have tried a 150W, 100W and a 60W bulb; the TV is rated at 200W. The two power mosfets and the heatsink gets pretty hot. I've tested them and they are fine at this point in time.

I also measured the voltages at several places and my multimeter registers a fluctuating voltage on a majority of circuits.

The main capacitor C203 (Rated 450v) has between 289V - 390V
Standy 5v has a STEADY 4.96V
F15V has between 5.24V - 15.51V
15Vc has between 1.53V - 12.22V
15Vs has between 5.49V - 12.89V
Vda has 0V
Jumper 64 "Main_On" has between 0V - 2.9V

Judging from these values, it seems like that the power supply is turning on and off. I understand that switching power supplies are meant to do this but surely not that slow? How would you interpret these results

The video is here... https://www.youtube.com/watch?v=QEmttQgkK9A

[tom66]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

This is normal. The bulb, say it's a 100W, will at best allow 50W into a matched load. Realistically, it will allow less because the load is not matched.

Basically what this means is the TV is trying to power up but draws too much power (for the bulb), the bulb lights when this happens (so the voltage across it increases), the AC input the TV sees collapses and the TV stops working -- this is normal! It cycles because as soon as the TV turns off suddenly the AC has returned and it gives it another go.

With other TVs effects vary from getting a dim image on the screen to cycling off when the sound or pic brightness is turned up. Some plasmas will work but the image will be extremely dim or have a very pink tinge.

At this point you need to take a chance and plug the set in directly -- though since you have eliminated the most likely shorts, it will most likely be fine. Some people use e.g. 300W light bulbs in series with TVs too, which can work. But they are expensive and hard to get hold of.

[Kairi]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

Not willing to plug the TV directly into the mains, I made another dim bulb tester and wired them in series. It means that the power goes through two light bulb before reaching the TV. What I wanted to do was to create a higher wattage "bulb" by chaining the two, like 100W + 60W = 160W or 150W + 15W = 165W. I noticed that the bulb flashes in a longer sequence like as it did with a 60 watt bulb and that the brightness of the two bulbs varied, even if I use two 60W bulbs.

With the 150W and 15W bulb wired in series, the bulbs flashed constantly. I also noticed that the voltage dropped to about 100-120V on the DC side of the bridge and about 100V on the AC side. The main cap only held about 200V.

I'm in the UK so the line voltage is 230V AC.

What's happening here?

[tom66]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

Placing light bulbs in series does not create a high power light bulb. In fact the total power is reduced; look up series circuits. To do what you want you will need to put them in parallel.

Since the big cap voltage is hitting 390V the PFC is working.
All the main voltages are present.
Only additional check is Vsus and Vda, though, they may not power up until the set has got stable power.

So I'd plug it directly in.

[Kairi]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

Placing light bulbs in series does not create a high power light bulb. In fact the total power is reduced; look up series circuits. To do what you want you will need to put them in parallel.

Oh yes of course!!! Basic secondary school physics! Series circuit means less voltage and same current, whilst parallel circuit means same voltage and shared current. Duh, why didn't I think of that?

[Th3_uN1Qu3]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

So I'd plug it directly in.

Another vote here. Should start up fine now.

Generally, PSUs with APFC don't play well with series light bulbs. The behavior you see is normal. If something was wrong, the bulb would remain lit steadily at full brightness.

[Kairi]

Re: Panasonic Plasma TV TX-P50X50B - Shorted power supply

The fets and fuse have blown again upon plugging it in!!!!

Edit: replaced the fuse only and the light bulb stays bright.

ok I need to get to the bottom of this... What would cause the fets to blow? Which will in turn cause the transformer to draw so much current, blowing the fuse.