IXDN0036 datasheet(13/41 Pages) IXYS

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Ballast Demonstrator Operation

4-12

ATAVRFBKIT / EVLD001 User Guide

7597A–AVR–02/06

Initially, the lamp is started at a frequency well above resonance at 80KHz before ramp-

ing down to 55KHz for ignition. 80KHz provides a lagging power factor where most of

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 heat 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 340V peak. Ignition occurs

at about 40KHz 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 32KHz at 30KHz/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.

Dimming is accomplished by raising the drive frequency towards 100 KHz. The lower

lamp (capacitor) voltage caused by changing from a leading to a lagging (inductive)

power factor and the resulting drop in lamp current causes lamp dimming. The visual

perception of brightness is logarithmic with applied power and must be taken into

account in the control method scheme.

4.4.1

Single Lamp

Operation

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

sense resistor sensed by the differential input PB3/PB4. The AT90PWMx differential

amplifier has the gain preset in the source code at 10. This scales the 200mV for two

lamps to a reasonable A/D resolution. PB4 requires low pass filtering. Through the 1

ohm sense resistor R28, V = I*R = 80 Watts*1/380V = 210mA*1 = 210mV. At preheat,

the current for one lamp is half that for two lamps. This current 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 fre-

quency response or a minimal response due to reactive currents.

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

lamp current PB4 < 100mV. If the single lamp condition occurs while running, as noted

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

until the PB4 = 90mV. If the PB4 increases to 120mV, assume the lamp has been

replaced. Increase the frequency to 80KHz to restart the ignition process. This is neces-

sary to preheat the new lamp filament to ensure that the hot lamp will not ignite any

sooner than the cold lamp, exceeding the balance transformer range. Start the 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.

Note that the lamp and resonant circuit use a common return ground separate from the

rest of the circuit. The ballast demonstrator uses active power feedback of the sense

voltage vs. drive frequency to meet power objectives. Also note that the differential

amplifier is connected across the current sense resistor R28 to ensure a Kelvin connec-

tion. Layout of the amplifier + and – is critical for fast noise free loop response.

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