CAPACITIVE LOAD DRIVER

摘要

An efficient power supply for driving a capacitive load which can be tuned electronically or otherwise to resonate at different frequencies, and which preferably can automatically compensate for ageing and temperature characteristics of the load when used in a phase-locked loop embodiment. The driver consists of a parallel resonant LC oscillator circuit (14) that uses the load (7) as the capacitance part of the oscillator, and a variable inductor (8) as the inductance part of the oscillator. The electronically tunable oscillator can form a voltage controlled oscillator which can be included within a phase locked loop system (4, 5, 6) to automatically lock itself to a known frequency irrespective of capacitive load. Part of the phase locked loop control voltage can be fed into the voltage output control stage (3) to compensate for the reduction of light output due to the ageing characteristics of an electroluminescent lamp. This is done by gradually increasing the output voltage as the EL capacitance falls so as to keep the light output constant. In addition changes in capacitance due to variations in ambient temperature can be compensated for, and the frequency hence colour of the emitted light automatically kept constant. The output voltage to the load (7) is isolated and fully floating, and can be referenced to a fixed voltage (usually ground) to prevent excessive voltage excursions. The driver exhibits inherent soft start and soft shutdown characteristics to eliminate high voltage spikes on power up or power down. The inductance and capacitance of the parallel resonant circuit acts as a filter to remove any high frequency content.