AN3372

Lamp operating point

Doc ID 018625 Rev 1

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Lamp operating point

For correct operation in each phase different electrical values (voltage and current) must be

applied. The chosen values for each phase are reported below.

Ignition phase

The ignition voltage, in case of a cold lamp, is about 3-5 kV. The ignition voltage increases

with increasing lamp temperature. The ignition voltage in case of a hot re-strike can reach

25 kV.

The circuit is not designed for hot re-strike.

Warm-up phase

During this phase a high warm-up current must be supplied (30% higher than nominal

current) to prevent the lamp extinguishing. The lamp voltage increases gradually starting

from a quarter to nominal lamp voltage up to the nominal value. The warm-up time is about

2 minutes.

For the specified lamp, 70 W metal halide lamp, a current of 1 Arms was 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 range free of acoustic resonance exists.

The commutating frequency of the half bridge should be limited to the domain 50 Hz - 1 kHz

to avoid any risk on acoustic resonance. In this application the commutating frequency has

been chosen in 160 Hz.

The nominal lamp voltage during burn phase is approximately 100 V and the nominal lamp

power is 70 W.

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

point, inductive impedance in series with the lamp is needed.