Hi, the power supply of my samsung TV doesn't work anymore and I'd like to understand the way it works.
I don't get how the SPC7011F gets a stable output voltage as I find a positive feedback.
Can someone tell me what's wrong with the following rationale ?
Here it is:
Vo voltage is controlled via the FB pin.
It is said that when the rising voltage ramp on non inverting terminal of the PWM comparator becomes greater
than voltage on inverting terminal of PWM comparator, Q1 mosfet isn't conducting anymore.
Thus the output capacitor gets loaded through D1 diode
Over a cycle, the output capacitor can only charge when Q1 is not conducting, and the less Q1 is conducting, the more the voltage Vo increases.
If Vo increases, the voltage Vfb on the FB pin increases.
When Vfb increases, the output of ERRAMP decreases, Vfb being on the inverting terminal of the ERRAMP amplifier.
And when the ERRAMP output voltage decreases, then the PWM comparator output will go high earlier in the cycle, so mosfet Q1 will be off earlier in the cycle and the output capacitor will charge earlier and therefore for longer duration in the cycle, which will increase Vo again.
This seems like positive feedback ...
Thanks for your help,
Cédric

 

Over a cycle, the output capacitor can only charge when Q1 is not conducting, and the less Q1 is conducting, the more the voltage Vo increases.

This is a "boost" power supply". The output voltage is about 400V while the input is lower. When Q1 is on power is stored in L1. When Q1 is off the voltage jumps up, pushed through D1 into Cout.
This is a "discontinues" power supply. What that means is that for each cycle (about 100khz) the current in L1 starts at 0 and ramps to some amount when Q1 is on. When Q2 is off the current ramps down to zero. At zero current the inductor will fall back off D1 and hand there with no current for the "dead time". (There is a chance this supply is running "continuous" mode where L1 current never drops back to zero, but the PFC power supplies I designed for Samsung were discontinues.)

The duty cycle never goes to 100%.

The input voltage is no DC. Most power supplies have a very large C1 that holds the voltage for several cycles of the power line. In this case C1 is so small that the input voltage is AC as in full wave rectified AC. The voltage is 0 when the power line is at zero crossing.

PFC is complicated and not like most PWMs. Ask more questions if you want.

 

Hi,
thanks for your answer
I had skipped the fact that the inductance voltage adds to the input voltage when the mosfet gets off
It gets clearer now !