-24

Ballast Demonstrator User Guide

7629A–AVR–04/06

Figure 6-2. Positive Line Half Cycle

1.dV = 2Vpk-Vo-2V

D

2.dQ = CdV or C = dQ/dV

For example, to obtain 15 ma at 20 VDC from a 220 Vrms 50 Hz line:

1.dQ/dt = (15 millijoules/sec)/(50 cycles/sec) or 0.3 millijoules / cycle.

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

+220V x 1.4 – 20V- V

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.

6.2

Appendix 2: 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 6-3) at critical conduction so that the current waveform is triangular (See Figure

6-4).

Figure 6-3. PFC Boost Regulator

AC

C1

C2

“Positive” line half -cycle:

Vo

Ich2

PFC Inductor

PFC BOOST REGULATOR

POWER

VOLTAGE

Vin

Vbus

Ion = (Vin x t )/ L

Ioff

PFC Switch