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Ballast Demonstrator User Guide

7629A–AVR–04/06

3.1.3

PFC Boost

Regulator

The PFC (Power Factor Correcting) boost regulator circuit is used to convert the recti-

fied input line voltage to a 380V DC supply while maintaining a sinusoidal average input

current in phase with the input voltage. The microcontroller accomplishes this by switch-

ing the PFC FET with ON times that are constant over a haversine period and by

maintaining nearly critical conduction conditions. Since the current in the PFC inductor

is nearly triangular and its peaks are proportional to the input haversine voltage, the

average current is proportional to the input waveform. Therefore, the power factor is

maintained near unity.

3.1.4

PFC Magnetics

Without going into the derivations of the formulas used, the transformer design is as

follows:

L = [(1.4 * 90VAC) * (20 uS)] / 3.6A peak = 700 uH

A 3.6 Apk maximum FET current is 1.8 A approximately divided by the ON/OFF ratio.

The ON time has been discussed earlier and the OFF time maximum will occur at high

line condition at the peak of the haversine. A 16 mm core was chosen for the recom-

mended power density at 200 mT and 50 KHz.

3.1.5

Lamp Drive

The microcontroller sends rectangular pulses to the half-bridge driver (IXD611). The

IXD611 contains high side and low side FET drivers and floating high side supply cir-

cuitry to produce high side gate drive. (See more detailed description of the IXD611 to

follow) The pulses from the microcontroller are non-overlapping and 180 degrees out of

phase. A deadband time between HBRIDGE HI and HBRIDGE LO pulses insures that

both drivers are never on at the same time. The lamp drive is constant in duty cycle. The

power to the lamps is controlled by varying the frequency of the drive signals. The

IXD611 drives two FETs (IXTP3N50P) in a half-bridge configuration.

The output of the half-bridge is AC coupled by C11 to the lamps through a resonating

transformer and capacitor (T4 and C12). Additional windings on T4 supply filament cur-

rent to the lamps. Balance transformer T3 forces the current to be shared equally by the

two lamps. The lamp currents are conducted to circuit common through a 1 Ohm resis-

tor which is used to sense the lamp current so that lamp power may be controlled by the

microcontroller.

3.1.6

Control

The ballast is controlled by microcontroller U3. U3 is an Atmel AT8xEB5114 with an

80C51 core and specialized circuitry for controlling the ballast. Included are two PWM

units that are used for controlling the PFC drive and the half-bridge drive with deadtime.

An internal analog to digital converter converts input signals so the processor can moni-

tor and control the ballast.

The AT8xEB5114 pin connections for ballast control and scale factors for analog inputs

are as follows:

• P4.0/AIN0 VBus monitor input (VBus = AIN0 x 201)

• P4.1/AIN1 Rectified Lamp Voltage Sense (Vlamp = AIN1 x 294)

• P4.2/AIN2 Lamp AC Voltage (VAC ~= AIN2 x 446)

• P4.3/AIN3 Lamp Current (Amplify by 10) (Ilamp = AIN3/1Ohm)

• P3.3/AIN4 Haversine Voltage input (Vhaversine = AIN4 x 201)

• P3.4/AIN5 Temperature sensor (Vtemp = 1.1V @ 25C || .264V @ 85C)

• P3.6 NC (No Connection)

• P3.5/W1M0 PFC Drive

• P3.2/INT0 Current Zero Crossing Detect (Interrupt)

• P3.1/W0M1 Half Bridge high side drive