Philips ledtv backlight makes PSU reboot.

[Tony.T]

Everytime i connect the backlight connector to the PSU the backlight lights up very much and then PSU reboots.  i put a clampmeter around one of the led+ leads and can see 300mA consumption before it reboots.

Tested a powersupply with 24 volts over 32 backlight leds  (8columns *4 rows...wired as 2 pairs ... but connected together on the circuitboard on PSU)   and at 24 volts it stopped at 666mA.

WAAYY to much....

When i test each backligt led one by one it has 2.5V voltage drop and takes 30mA

ALL the leds are working and have same voltage drop and takes 30mA.


it does not make any sense to me....??!!! 

So when connected together in PSU...it seems like shortcut and stops PSU.  each led seems ok. When connectig led tester on the connector directly that is normally plugged into the PSU...same readings and every led is lit up.

Any ideas?

[inse]

As the forward voltage of the LEDs drops with temperature, they need to be driven with current control rather than voltage.
Or at least with a current limiting resistor.
Check the LED control circuit on the power supply board, additionally you can do a test with a series resistor.

[Tony.T]

Tested the PSU directly to the leds and turned the psu on by setting 3.2V to ps_on and backlight_on from other powersupply.  the leds turned on.

As soon I connect the motherbord to PSU and let the motherboard turn on the leds the PSU reboots.

Without leds connected.... the tv turns on and motherboard works and i get logo on display (without backlight). As soon i connect the backlight connector the psu starts to reboot.

it MUST be that the PSU goes into overcurrent protection when both motherboard and backlight is connected at the same time. 

i had to replace the mosfet on the PSU. also the fuse was blown.   wonder if something more is broken on PSU. ..  it can drive only motherboard....or only leds without shutting down...but not both at the same time...

PSU is TPV 715G8620-P01

can someone point out which part is involved in overcurrent protection?

[inse]

Check the electrolytics, maybe there is too much ripple somewhere

[bateau020]

Which mosfet did you replace? Q9101 (probably) or Q9105? (unlikely to be Q9403). Often other components blow around the mosfet, like U9101, various (zener) diodes and some of the high voltage caps.

edit: service manual attached, just in case you don't have it.

[Tony.T]

The mosfet Q9801 on the hot side was blown.  Nothing else.