The control IC for piezoceramic transformer (PZT) based LCD backlight inverter is presented. The novel control scheme controlling only the switch on-time, while fixing the switch off-time is developed for regulating the PZT's output power. In order to accommodate to this control scheme, the control IC implements the voltage-controlled oscillator having variable on-time and fixed off-time output waveform. The controller, in steady-state, always drives the PZT with zero voltage switching, so achieving very good efficiency. The free-running frequency of oscillator with duty ratio of 50 is tuned to the resonant frequency of PZT with one external capacitor and resistor, which also determine the fixed switch off-time of the controller. The minimum and maximum duty ratios are limited for start-up and stability, respectively. The soft starting function is automatically achieved by the new loop compensation scheme. The controller also implements the open lamp protection. No the protection for the short-circuited load is need by the PZT's characteristic for its load impedance. The backlight inverter system with our control IC can be implemented with much less external components, which considerably saves both cost and PCB size. The experimental results using discrete components had been shown the validity of our control scheme. The obtained power efficiency was 87.
展开▼
