Effectiveness of FRFET® MOSFETs for the CCFL inverters in LCD TVs

For the backlighting of most LCD panels such as TVs and monitors today, CCFL (cold cathode fluorescent lamp) is widely used because it can provide both low cost manufacturing and high quality color gamut. To illuminate the CCFL, half-bridge or full-bridge resonant inverters are necessary. Although resonant inverters can work under the ZVS condition when the PFM (pulse frequency modulation) method is applied, most major LCD manufacturers adopt the PWM (pulse width modulation) method to control their LCD’s CCFL inverter because not only the color gamut might be varied but also its stability will be unstable when the inverter is operated by the PFM method due to its incremental resistance according to the varying the switching frequency. In the cases where CCFL inverter works with PWM method, the hard switching operation is inevitable and the additional diodes must be required because the reverse recovery behavior of the MOSFET’s body diode is inferior to FRD (fast recovery diode), which causes a large amount of switching loss and it will even make the system failure due to the shoot-through current. In this paper, a newly improved FRFET MOSFET with superior reverse recovery characteristics is introduced and the experimental results for applying it to 42” LCD TV are presented. From these results, the validity of the new FRFET has been verified. The reverse recovery time is shorter than 40 ns and irr is also much smaller than the typical power MOSFETs, which are sufficient to prevent the MOSFET’s failures without the external diodes in CCFL BLU (back light unit) system.