EVLB001 Datasheet, PDF (12/35 Pages) ATMEL Corporation – Automatic microcontroller dimmable ballast

Ballast Demonstrator Operation
Limit pulse width to 25uS or as determined by the haversine peak voltage.
The adjustment of the PFC TON and TOFF is down as follows:
- The TOFF is automatically adjusted by hardware at each PFC inductor current zero
crossing detection,
- The TON is adjusted by software accordingly to the Vout measurement each time the
main supply voltage reach zero (Each half period of the main voltage supply)..
4.5 Lamp Operation
Description
T4 primary and C11 form a serial resonant circuit driven by the output half bridge. Since
the output is between 400V and 0V, DC isolation is provided by C14 to drive the lamp
circuit with AC. The lamp is conected in parallel of the resonating capacitor C11 (But
there is no current through the lamp). The lamp filaments are driven by windings on T1
secondaries to about 3Vrms so that the resonating inductor current provides the starting
lamp filament current.
Initially, the system is set up at 80KHz, a frequency well above resonance the frequency
then ramps down to 55 kHz for ignition. 80 kHz provides a lagging power factor where
most of the drive voltage appears across the inductor. A smaller voltage appears across
the resonating capacitor C11 and the lamps. However with 1 mH gapped inductance,
there is sufficient inductor current to heat the filaments.
For lamp ignition, the frequency is decreased from 80 kHz to 40 kHz with 30 kHz/sec
slope towards resonance causing the lamp voltage to rise to about 340V peak. 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.
4.5.1
Single Lamp
Operation
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.
Not implemented.
Single lamp operation can be detected from the 400VDC bus current through a 1 ohm
sense resistor sensed by the differential input PB3/PB4. The AT90PWM2B/216 differen-
tial 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*1/400V = 200mA*1 = 200mV. At preheat,
the current for one lamp is half of that for two lamps. This current is also used to sense
open filament condition or lamp removed under power condition. An abrupt change in
ATAVRFBKIT / EVLB001 User Guide
4-10
7597B–AVR–10/07