Self Heating Backlights
One problem with Cold Cathode Fluorescent Tubes(CCFTs) is operation in cold temperatures. When a CCFT gets very cold, the strike voltages become excessive or it may not turn on at all. The classical solution to this problem is to use a heating structure with the CCFT. SHARP self-heating backlights are a much better solution.
Self-heating backlights are essentially inefficient CCFTs that can be overdriven with excess current. The excessive current is dissipated as heat which warms the tube. Since the heating is internal, the temperature rise is rapid and full brightness is achieved within two minutes (a requirement for the automotive industry).
The chart above shows that a conventional CCFT started at -20º C and overdriven with 9 mA of current would take 4 minutes to achieve just 40% of its rated brightness. Note that a self-heated lamp starting at the same temperature has almost the same performance as a conventional CCFT starting at room temperature.
Self-heating backlights require an external control to regulate the CCFT current. A block diagram of the closed loop control system is shown below. A thermistor is mounted on the CCFT to sense the tube temperature. This signal is fed to a look-up table to find the proper current settings for the inverter at a specified brightness. Conventional inverters without this control would just provide a single constant current to the lamp.
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