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ATAVRFBKIT / EVLD001 User Guide

7-35

7597A–AVR–02/06

2.Over 1 cycle, the coupling capacitor (C1) will charge from –220V x 1.4 to

+220V x 1.4 – 20V- V

dV = 2*Vpk-Vo-2V

dV ~= 600V.

3. The required C1 ~ 0.3 millijoules/600V or 0.5 uF

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

Basics

The function of the PFC boost regulator is to produce a regulated DC supply voltage

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

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. (Ipk = Vin x Ton/L). Since the average

value of a triangular waveform is ½ its peak value, the average current drawn is also

proportional to the line voltage.

Figure 7-4. Main voltage supply cutting

PFC Inductor

PFC BOOST REGULATOR

POWER

VOLTAGE

Vin

Vbus

Ion = (Vin x t )/ L

Ioff

PFC Switch

PFC

DRIVING

Main Supply

Voltage

Ipeak = Vin x Ton / L

Imean = Ipeak/2

Actual switching frequency

is higher than shown

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