AN4130 datasheet(8/41 Pages) STMICROELECTRONICS

8 / 41 page

Theory of operation

AN4130

8/41

Doc ID 023315 Rev 1

2.5

Using the HVLED815PF current limit for power factor

correction

2.5.1

Average current regulation

The HVLED815PF does an excellent job of regulating output current in a DC input flyback

supply. It calculates the peak current at which to shut off the driving FET by continuously

looking at the duty cycle. The error between desired duty cycle and actual duty cycle

appears as a current on the ILED pin - a capacitor on this pin integrates the error to zero

over time. Since the voltage on this pin, divided by 2, directly sets the current at which the

FET switch turns off, the output current is regulated.

In DC-input flyback power supplies very small capacitors are used on the ILED pin for quick

response to changing loads or input voltage. The capacitor on this pin can be much larger if

the load voltage does not change rapidly. LED current can be regulated more slowly,

averaging out the error over several cycles of input voltage. A 4.7 µF low-voltage ceramic

capacitor is used here.

The average LED current is kept constant even if the input voltage waveform is grossly

distorted, such as a rectified sinewave, as occurs in the PFC-flyback topology. The input

current waveform, however, is truly ugly. Note the magenta trace in the figure below.

Figure 5.

Current distortion with sinewave input

Where:

●

Yellow = line voltage

●

Magenta = line current.

The peak FET shut-off current remains at the same level throughout the AC half cycle, but

the duty cycle of the converter changes. (FET ON-time increases at lower input voltage - it

takes longer to reach the same current if the converter input voltage is lower). The resulting

input current waveform is VERY rich in harmonics (THD is in the range of 130%), though

power factor is actually pretty good.

2.5.2

Adding an AC component to the current regulator

If an AC signal is injected into the ILED pin, the instantaneous FET peak current can be

controlled, while the average output current (a DC level) remains regulated. The figure

below shows the injection of a small fraction of the line voltage into the bottom of the ILED

capacitor. The change in the input current waveform is dramatic. But it is best for only one

AN4130 Datasheet(PDF) 8 Page - STMicroelectronics