AN2747 datasheet(31/44 Pages) STMICROELECTRONICS

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AN2747

Lamp data

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Lamp data

The lamp data are given below and each is valid for the corresponding operating phase.

●

Ignition phase: The ignition voltage in case of a cold lamp is about 4-5 kV. The ignition

voltage increases with lamp temperature. The ignition voltage in case of a hot re-strike

can reach 25 kV. The circuit is not designed to supply this very high voltage pulse.

●

Warm-up phase: During this phase a high warm-up current must be supplied (30%

higher than nominal current) to prevent lamp turnoff. The lamp voltage increases

gradually starting from a quarter of nominal lamp voltage up to the nominal value. The

warm-up time is about 2 minutes. For a 250 W metal halide lamp a current of 3.2 amps

is applied.

●

Burn phase: The lamp is designed to be driven with a low frequency square wave AC

current to avoid acoustic resonance of the electric arc. Acoustic resonance occurs

approximately in the frequency domain: 1 kHz - 1 MHz. Some frequency ranges free of

acoustic resonance exist. The commutating frequency of the full bridge should be

limited to the domain: 50 Hz - 10 kHz to avoid any risk of acoustic resonance. In this

application the commutating frequency of 160 Hz has been chosen. This frequency has

been chosen in order to avoid a flickering effect. For frequencies higher than 130 Hz

eye sensitivity is practically zero. In Figure 31 a diagram of eye sensitivity is shown.

Figure 31.

Eye sensitivity versus frequency

The nominal lamp voltage is approximately 100 V and the nominal lamp power is 250 W.

The differential resistance of the lamp is small and negative. To obtain a stable operating

point, impedance in series with the lamp is needed.

AN2747 Datasheet(PDF) 31 Page - STMicroelectronics