SMPS - lost VCC to IC's

Thread Starter

roadey_carl

Joined Jun 5, 2009
132
Hi Folks,

I've been learning about switch mode power supplies but for some reason I can't grasp how powering the IC's used to switch the mosfets is achieved. I'm working on a power supply for an LG TV which has lost power to the VCC of its 3 IC's so I'm having a little trouble tracking the problem down.

Could you please take a look at the schematic and guide me through what I'm doing wrong and explain the concept of how the IC's are powered. I know I'm missing the nail by a mile and I'm frustrated I can't grasp it.

Thanks for all your help
 

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ISB123

Joined May 21, 2014
1,238
They are generally powered through smaller sized transformer that should be situated somewhere close to the main high power transformer.
 

Thread Starter

roadey_carl

Joined Jun 5, 2009
132
But how can the small transformer function if the IC that switches it is powered via the secondary winding? What tells the mosfets to switch?
 

Dodgydave

Joined Jun 22, 2012
8,797
U500 is the smps chip, it gets its start up voltage on pin 8 , then when its self oscillating pin 6 is its supply, it regulates the output voltage on pin 2, via the opto-coupler, pin 5 is the pwm output to the mosfet.
 
Last edited:

hp1729

Joined Nov 23, 2015
2,304
Hi Folks,

I've been learning about switch mode power supplies but for some reason I can't grasp how powering the IC's used to switch the mosfets is achieved. I'm working on a power supply for an LG TV which has lost power to the VCC of its 3 IC's so I'm having a little trouble tracking the problem down.

Could you please take a look at the schematic and guide me through what I'm doing wrong and explain the concept of how the IC's are powered. I know I'm missing the nail by a mile and I'm frustrated I can't grasp it.

Thanks for all your help
Q504 (lower left quadrant) looks like it is the source of VCC that power such things.
 

Thread Starter

roadey_carl

Joined Jun 5, 2009
132
Thanks for all your help so far...

Q505, Q103, Q112 were faulty... I've replaced them and U500 U102 both have 13volt at VCC to GND.
U600 has no voltage at VCC to GND.

there is 5.1v on standby, secondary side of transformer... no 12, 16 or 24 volt though.

Any ideas?
 

Dodgydave

Joined Jun 22, 2012
8,797
Q103 powers the Vcc1 line, that powers the U600 chip pin8,. Check the b,c,e terminals of that transistor, and the Vcb control, U600 chip controls 24, and 16v pwm transformer.
 

SLK001

Joined Nov 29, 2011
1,543
Some LG TVs use a boosted voltage. Check for bad diodes in the input section. The voltage should be in the 300V range (not 160V).
 

Thread Starter

roadey_carl

Joined Jun 5, 2009
132
I think I'm going to have to call it a day on this one... I've been on it for weeks.

I got to the point where I had 5.1 standby then when I went back to check what Dodgydave said, Q502 had shorted for no reason ... luckily I was running through an isolated transformer and a 50watt lamp so it took most the load.
I've tested all the zener diode's, transistors, mosfets and opto couple's. I've replaced Q505, Q503, Q103, Q112, Q504... again!

now I only have 8v at U500, 0v at U102, 0v at U600.

I think I'm done!!! Thanks for all your help, it's much appreciated!

Carl
 
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