CCFT Inverter Selection

Most LCD displays use a Cold Cathode Fluorescent Tube (CCFT) for a backlight. This is essentially a miniature version of the tube used for overhead lighting. These tubes typically require a 600-1100 volt AC driving source. This voltage is generally supplied by a inverter from a DC source. It is very important to select an inverter with proper electrical characteristics. The two critical parameters are the initial strike voltage and the run-time current. The strike voltage is the maximum level of the minimum voltage required to assure the tube will always turn on. Once a tube begins to conduct, the voltage required to maintain the light drops dramatically. As a display ages, the strike voltage will generally go up. Sharp specifies a minimum strike voltage and you must insure that the inverter can meet or exceed this value. It doesn't matter if the value is exceeded because the tube will self-regulate and cut off the voltage as soon as it strikes.

Most inverters are designed as constant current devices that will adjust the voltage to supply the required current. The constant current level must match the display. If you supply a higher current than the rated value, the tube will operate, but the tube life will drop dramatically. Many inverter suppliers actually make inverters specified for a given display type which makes the selection process very easy.

Finally, you have to take care to operate an inverter within the total power range for which it is specified. Inverters are designed for short term excessive power output in order to satisfy the initial strike voltage requirement, but cannot deliver this power level on a continuing basis. For example, a 3 watt inverter will actually drive a 6 watt backlight, but the inverter will quickly burn out or shut down automatically via a self-contained thermal protection device. Depending on the amount of stress, these early failures may occur after several months of operation.