AN2747 datasheet(7/44 Pages) STMICROELECTRONICS

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AN2747

Lamp description

7/44

Figure 6.

Instantaneous lamp power with sinusoidal and square waveforms

For ideal square waveforms, the instant power delivered to the lamp is constant and not

modulated by any frequency. This driving plus the low frequency driving allows obtaining a

system that is immune to the problem of acoustic resonance.

For this reason HID lamp applications using square wave current techniques are the best

solution.

1.1

Lamp phases

1.1.1

Ignition phase

The ignition voltage for HID lamps is higher with respect to the mains voltage. The peak

voltage value to initiate the discharge is very high. Typically this is about 3 kV ÷ 5 kV.

The voltage level at which an HID lamp ignites is called "ignition voltage". This voltage level

is referred to the lamp when it is cold but this value increases with lamp temperature.

To ensure immediate HID lamp restart, called “hot restrike”, very high re-ignition peaks are

necessary (about 20 kV).

Standard luminaries are not designed for hot restrike voltages. For this reason in order to

obtain hot lamp re-ignition, several minutes must elapse for the lamp to cool. The restart

time depends on lamp temperature.

1.1.2

Warm-up

When the discharge has been initiated by the starting gas and the lamp still does not burn

properly, a warm-up time is required.

During the warm-up time the gas temperature increases, increasing the light output. The

lamp voltage starts approximately from a quarter of Vlamp and increases up to Vlamp.

Instantaneous Power

Voltage

Current

Instantaneous Power

Voltage

Current

AN2747 Scheda tecnica(PDF) 7 Page - STMicroelectronics