RLDED3258A-F another crap TV from RCA

[paulstef]

Two TVs with exactly the same fault.

The main inductor on the secondary gets extremely hot until it burns the solder pad and falls out of the PCB. It seems the OB3350CP LED controller doesn't work and the MOSFET stays on all the time.http://www.go-gddq.com/upload/2014-0...0611591515.pdf

At least that's the only explanation I have, it's in a boost configuration.

Recoiled the inductor and it worked for a minute then the inductor got to hot again. Mosfet burnt. Swapped the mosfet, nothing, swapped the OB3350, nothing.

Blue light turns on but no Volts for the LEDs.

The backlight LEDs are all good.

[budm]

Re: RLDED3258A-F another crap TV from RCA

"It seems the OB3350CP LED controller doesn't work and the MOSFET stays on all the time" That will be correct assumption if the GATE drive signal stuck high instead of switching on and off at high frequency.

[paulstef]

Re: RLDED3258A-F another crap TV from RCA

Yes. The circuit is so small I will give it a try and draw it. Maybe it becomes clearer then.

[budm]

Re: RLDED3258A-F another crap TV from RCA

So the resistor on the Source pin is still OK?
Most of the time they will just use the same circuit as shown in the application notes of the IC.

[paulstef]

Re: RLDED3258A-F another crap TV from RCA

So the resistor on the Source pin is still OK?
Most of the time they will just use the same circuit as shown in the application notes of the IC.

Yes, they are 3x 1206 packages in parallel. I guess they would support a couple of amps before burning.

I attached a circuit diagram for others who have the same problem. From here I have to check a few things but have to stop for now. Maybe I continue on sunday.

Not exactly sure why they would put in D18?. When the current rises above the diodes voltage drop it would limit the voltage on the FB input. Then the chip would "think" the current in the LEDs is lower than it actually is. Weird.

[paulstef]

Re: RLDED3258A-F another crap TV from RCA

The primary doesn't work either...

Not really necessary to know how it works but it would be great if I knew what the controller IC is. It says MaP. It's a SOT23-6.

[paulstef]

Re: RLDED3258A-F another crap TV from RCA

Well, I have two TVs , and on the other one primary is working and I got 12.3V on one secondary as predicted.

There are 24 LEDs in series in this TV and I get only 37V to them. I measured the minimum voltage to drive some current through them but forgot how much it was. It was more than 1.5V though that's for sure. Already put it back together.

Must be something on the secondary.

The controller should boost the voltage to drive the LEDs and then regulate the current through them if I understand correctly.

Mmh, after replacing main FET and controller it works, more or less. Voltage at the LEDs is 71V... BUT. the coil that burned heats up extremely fast. Turned the TV off when it started smelling... what the heck?

[budm]

Re: RLDED3258A-F another crap TV from RCA

24 LEDs x 3.3V = 79V, the boost converter should boost up the Voltage to drive the LED string.
Usually the Anode of the string will be connected to the B+ of the filter cap of the boosted out put, the Cathode is then connected to the Drain of the MOSFET, the Source of the MOSFET is connected to the current sensing resistor, another end of resistor is connected to the B+ return line. The MOSFET is driven with PWM signal.

[paulstef]

Re: RLDED3258A-F another crap TV from RCA

Thanks budm.

The secondary schematic is as per my post number 5.

The mean voltage on the two parallel feedback resistors (1.2 and 3R) gives me a mean current of 128mA through the LEDs. Does this make sense?

[paulstef]

Re: RLDED3258A-F another crap TV from RCA

I messed up:

On the resistors in series with the LEDs it's 150mV, therefore 175mA through all the LEDs.

The current sensing resistors on the source of the FET have a peak of 2.8V on them 500ns long at 212kHz. That would be 14Amps peak. Seems a lot to me. However it's a duty cycle of about 10%, so mean current is less than 1.4A... The inductor is 24 or 25 gauge.

[budm]

Re: RLDED3258A-F another crap TV from RCA

The current through the LED is high, usually more than 100 mA but they are pulsed otherwise they will burn up quickly.

[paulstef]

Re: RLDED3258A-F another crap TV from RCA

Well in this set with the controller OB3350 the LEDs are directly connected to the booster output voltage.

It looks like the inductor I recoiled has a too low inductance. If I get 14A in 500ns that would mean it's 100nH? That's way lower than the 47uH in the datasheet.

Even though I recoiled it with the same gauge and length...

[budm]

Re: RLDED3258A-F another crap TV from RCA

"LEDs are directly connected to the booster output voltage" That is correct, its the feed back on that current sensing resistor (with the Diode in parallel) is what is controlling the constant current for the LED.
The current sensing on the Source pin of the MOSFET for the boosted voltage is for protection if the too much current is being drawn through the MOSFET.
So when you rewind the inductor, did you keep the same turn count?

[paulstef]

Re: RLDED3258A-F another crap TV from RCA

Yes, turn count is the same. However the first time I replaced the inductor it worked for some time until it overheated. I thought I had it repaired so I noticed too late that it was overheating.

I just replaced it again with the coil I had prepared for the second TV and yes it works. The first coil when overheated was shorted.

So now I'm at the same point as before, everything seems to work except the inductor heats up quite quickly to 80C.

The LED current would be about 350mA continuous...

Think I need to measure the real current through the inductor, there is no reason for it to heat.

[paulstef]

Re: RLDED3258A-F another crap TV from RCA

I guess I call it a day for today.

I have about 1.2A through the inductor, 350mA through the LEDs, the rest through the MOSFET. Everything looks good, except for the heating.

I wonder if these inductors are just poorly designed and I should just use a heavier gauge. They don't saturate or anything it just seems that 1.2A for a AWG25 is too much.

Will try my idea tomorrow.

thanks budm again.

[budm]

Re: RLDED3258A-F another crap TV from RCA

"The LED current would be about 350mA continuous..." That is a lot of continuous current for the LEDs.

[paulstef]

Re: RLDED3258A-F another crap TV from RCA

I applied 1.2A DC to the inductor, it doesn't heat AT ALL. An ESL meter shows 386uH.

Therefore it must be the core that's heating not the wire, due to the switching (212kHz)

350mA through the LEDs seems a lot but is consistent with the calculation for the resistors in series with the LEDs. The datasheet says 300mV. That's what I have at those resistors (3R || 1R2).

However, 1.2A in the inductor vs 350mA in the LEDs looks like an awful bad efficiency to me. The 900mA difference can be measured at the FETs source resistors. Nothing strange there.

In short, the inductor seems to be ok. There is too much AC current through it heating the core IMO.

Kind of stuck.

[paulstef]

Re: RLDED3258A-F another crap TV from RCA

I found out that iron powder cores can indeed be permanentely damaged when overheating. The insulation between the particles is destroyed when it's overheated. Therefore eddy currents increase since the particles aren't separated anymore. It's like a pure iron ring then.

That's why it heats up.

The circuit is extremely forgiving when using other inductors, even fast satuarting ferrite cores.

I ended up using an input filter core of a PC power supply which is also bigger. I also lowered the LED current by using 2x 3.3R in parallel instead of 1.2R and 3R.

Looks good so far. No overheating. Beautiful current waveforms on the source of the FET.

[drussell]

Re: RLDED3258A-F another crap TV from RCA

Nice work!

I was thinking it was a core saturation problem but had never thought about the fact that a powdered core might be damaged by overtemperature. Nice find!

Thanks for the information!

[budm]

Re: RLDED3258A-F another crap TV from RCA

That is good news, I thought it was FERRITE CORE, I would try lowering to current through the LED as much as you can to extend the life, looks like they try to get lotS of lights by pushing it so hard, but then the warranty will run out when they quit working.