EVLB001 Datasheet, PDF (29/35 Pages) ATMEL Corporation – Automatic microcontroller dimmable ballast

Conclusion
In practice, C1 may have to be larger depending on the amount of ripple allowed by C2
and to account for component tolerances, minimum voltage, and current in the regulator
diode. C1 must be a non-polarized type with a voltage rating to withstand the peak line
voltage including transients. A high quality film capacitor is recommended.
7.3
Appendix 3: PFC The function of the PFC boost regulator is to produce a regulated DC supply voltage
Basics
from a full wave rectified AC line voltage while maintaining a unity power factor load.
This means that the current drawn from the line must be sinusoidal and in phase with
the line voltage.
The ballast PFC circuit accomplishes this by means of a boost converter operating (See
Figure 7-3) at critical conduction so that the current waveform is triangular (See Figure
7-4).
Figure 7-3. PFC Boost Regulator
POWER
VOLTAGE
PFC BOOST REGULATOR
PFC Inductor
Ioff
Vbus
Vin
Ion = (Vin x t )/ L
PFC Switch
The boost switch ON time is maintained constant over each half cycle of the input volt-
age sinusoid. Therefore the peak current for each switching cycle is proportional to the
line voltage which is nearly constant during Ton. (Ipeak = Vin x Ton/L). Since the aver-
age value of a triangular waveform is half its peak value, the average current drawn is
also proportional to the line voltage.
Figure 7-4. Main voltage supply cutting
Main Supply
Voltage
Ipeak = Vin x Ton / L
Imean = Ipeak/2
Actual switching frequency
is higher than shown
PFC
DRIVING
7-27
7597B–AVR–10/07
ATAVRFBKIT / EVLB001 User Guide