STMicroelectronics L6563H 100 W TM PFC demonstration board EVL6563H-100W EVL6563H-100W Data Sheet

Doc ID 16261 Rev 3

15/33

The power stage gain of PFC pre-regulators varies with the square of the RMS input voltage 
as well as the crossover frequency fc of the overall open-loop gain because the gain has a 
single pole characteristic. This leads to large trade-offs in the design. For example, setting 
the gain of the error amplifier to get fc = 20 Hz at 264 Vac means having f

c

 

≈

 

4 Hz at 88 

Vac, resulting in sluggish control dynamics. Additionally, the slow control loop causes large 
transient current flow during rapid line or load changes that are limited by the dynamics of 
the multiplier output. This limit is considered when selecting the sense resistor to let the full 
load power pass under minimum line voltage conditions, with some margin. A fixed current 
limit allows excessive power input at high line, whereas a fixed power limit requires the 
current limit to vary inversely with the line voltage.

Voltage feed-forward can compensate for the gain variation with the line voltage and allow 
overcoming all of the above-mentioned issues. It consists of deriving a voltage proportional 
to the input RMS voltage, feeding this voltage into a squarer/divider circuit (1/V

2

 corrector) 

and providing the resulting signal to the multiplier that generates the current reference for 
the inner current control loop.

In this way, a change of the line voltage causes an inversely proportional change of the half 
sine amplitude at the output of the multiplier (if the line voltage doubles, the amplitude of the 
multiplier output is halved and vice versa) so that the current reference is adapted to the new 
operating conditions with (ideally) no need for invoking the slow dynamics of the error 
amplifier. Additionally, the loop gain is constant throughout the input voltage range, which 
improves significantly dynamic behavior at low line and simplifies loop design.

Actually, with other PFCs embedding the voltage feed-forward, deriving a voltage 
proportional to the RMS line voltage implies a form of integration, which has its own time 
constant. If it is too small, the voltage generated will be affected by a considerable amount of 

         

CH1: GD - pin #15

CH2: ZCD - pin #13

CH3: CS - pin #4 

CH4: L2 inductor current

         

CH1: GD - pin #15

CH2: ZCD - pin #13

CH3: CS - pin #4 

CH4: L2 inductor current