Ballast Demonstrator User Guide

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7629A–AVR–04/06

the drive voltage appears across the inductor. A smaller voltage appears across the res-

onating capacitor and the lamps. However with 1 mH gapped inductance, there is

sufficient inductor current to power the filaments.

For lamp ignition, the frequency is reduced from 80 KHz to 40 KHz at 30 KHz/sec

towards resonance causing the lamp voltage to rise to about 340 Vpeak.

Ignition occurs at about 40 KHz for a 18W T8 lamp. The plasma established in the lamp

presents a resistive load across the resonating capacitor thereby reducing the voltage

across the capacitor and shifting the reactive power in the bridge circuit to resistive

power in the lamp.

A further reduction in frequency to 32 KHz at 30 KHz/sec establishes maximum bright-

ness as the resonant circuit now has a leading (capacitive) power factor causing more

voltage and current (approx. 360 Vpeak) across the capacitor and the lamp.

4.2.1.1

Single lamp

operation

Single lamp operation can be detected from the 380VDC bus current through the 1 Ohm

sense resistor. At preheat the current for one lamp is half that for two lamps. This cur-

rent is also used to sense open filament condition or lamp removed under power

condition. An abrupt change in the bus current is a good indicator of lamp condition that

does not require a high frequency response nor a minimal response due to reactive

currents.

Once single lamp condition is detected, the minimum run frequency is determined by

Ix380V = Single Lamp Power. If the single lamp condition occurs during "run" as noted

by a decrease in current of more than 20% from the preset level, increase the frequency

until the single lamp power conditions are met. If the current increases by more than

20% , assume the lamp has been replaced. Step Increase the frequency to 80 KHz to

restart the ignition process. This is necessary to preheat the new lamp filament to

ensure that the hot lamp will not ignite much sooner than the cold lamp exceeding the

balance transformer range.

Repeat ignition sequence. With one cold lamp in parallel with one hot lamp, it may be

necessary to restart several times to get both lamps to ignite.

The AT8xEB5114 internal amplifier has the gain preset in the program to 10. This scales

the lamp current input to a reasonable A/D resolution.

4.2.1.2

Lamp Number

Sequence

After Vbus = 380 V start preheat

Start half-bridge drive with 12.5 uS total period (80 KHz)

If I > 20 ma, then 2 lamps. If I < 20 ma assume a single lamp.

I < 10 ma assume an empty fixture = fault & shutdown.

4.2.1.3

Start Ignition

Sequence

1.

Sweep half-bridge frequency down at 30 KHz/sec

2.

Stop sweep at 40 KHz or 25 uS period (12.5 uS pulses for each ½ bridge FET)

3.

Check I > 100 ma (2 lamps) or > 30 ma (1 lamp) for proof of ignition

4.

Hold ignition frequency for 10 mS

5.

Measure the lamp voltage collapse for proof of ignition (P4.1/AIN1 < 200 mV)

6.

If the lamp voltage has not collapsed, increase frequency to 77 KHz for preheat

for 1 second. Repeat ignition sequence.

7.

Proceed to full power setting at 30 KHz/sec rate after ignition is detected.

4.2.1.4

Power Control

Calculate input power for both lamps = I x 380VDC.

Adjust freq. up (lower power) or down (higher power) at 30 KHz/sec rate.

Limit freq. to 32 KHz to 80 KHz range.