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Lamp driving circuit Disclosure Number: IPCOM000029411D
Publication Date: 2004-Jun-28
Document File: 4 page(s) / 477K

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Lamp driving circuit

The invention relates to a lamp driving circuit for a high pressure lamp or UHP lamp. The lamp driving circuit can be used in a light valve projector wherein a light source with a high brightness is required to provide a light beam to be modulated by the light valve. The light valve can be a digital mirror device, a liquid crystal on CMOS (LCOS) device or a transmissive LCD panel. A conventional lamp driving circuit is based upon two independent operating half-bridges consisting of semiconductor switches, for example, power FETs.

Fig. 1 discloses an example of a lamp driving circuit comprising two half-bridge circuits wherein the inventive circuit can be applied. In operation, the left half-bridge is operating as a down-converter, wherein the duty-cycle is determining the output voltage, which is the voltage present on capacitor Cdown. The right half-bridge is operating as an alternator, which takes care that an alternating current is flowing through the lamp. In case the lower alternator-FET is conducting, the voltage on capacitor Cdown is the voltage on the lamp. In case the upper alternator-FET is conducting, the lamp current will change direction and the voltage on the lamp is the supply-voltage minus the voltage on capacitor Cdown.

In normal operation, the down-converter half-bridge is switching at high frequency (>100kHz), while the alternator half-bridge is commutating at low frequency (often around 100Hz). Furthermore, the lamp driving circuit is provided with a microprocessor for controlling the lamp power and a device for measuring the lamp voltage and the lamp current. The microprocessor is arranged to determine, in a first commuting period, the power dissipated in the lamp by multiplying the measured values of the lamp voltage and the lamp current, respectively, and to determine a reference value (Rref) for a second, consecutive, commutation period in dependence of the calculated power and a predetermined nominal power. In the conventional lamp driving circuit, the lamp power control voltage is constant in each commutation-perio...