tailieunhanh - composite 2012 Part 12

Tham khảo tài liệu 'composite 2012 part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Introduction to LED Backlight Driving Techniques for Liquid Crystal Display Panels 213 Under this circumstance the overlap is zero corresponding to the lowest brightness. Compared with the conventional dimming scheme it is apparently recognized that the load variation of the SRC is less with the proposed PSPWM dimming function. To further investigate the operating principle of the PSPWM dimming a more general case with N shunt LED arrays is discussed as follows. Figure 7 shows the waveforms of the N driving currents and the output current of the SRC. As stated earlier the duty cycle range of the dimming signal is from 1 N to 100 . In terms of the phase angle if a complete period is 360 the duty cycle range is from 360 N to 360 . Assuming that the dimming signal for the LED array 1 starts at 0 then the dimming signal for the k-th LED array would start at ộk 360o X k -1 N 14 If the duty cycle of each dimming signal is ộd then the average driving current of one LED array is I M XI 360o 15 avg d p where Ip is the amplitude of the driving current for each LED array. Therefore the average output current of the SRC is Io avg Md XIp X N 360o 16 Fig. 6. The PSPWM Dimming Method. It can be observed from Figure 7 that if the end of the dimming signal for LED array 1 is at ộd where ộd is between ộk and ộk 1 and k 1 then the output current of the SRC in the range of ộk to ộk 1 is . kIp for ộk ộ ộd z k -1 Ip for ộd ộ ộk 1 17 o 214 New Developments in Liquid Crystals This is also the SRCs output current in each duration from ộj to Ộj 1 where j 1 to N. Therefore the average output current of the SRC is now kIp x ộd - 360 N X k -1 k - 1 Ip x 3600 N X k-ộd .N z xIt 3600 360 N p 18 i g__1 _g j 4 1 f 2 B 4 kộd 4 k-i 155 k---------------------- 390 ------------------------H Fig. 7. Current Waveforms of N Shunt LED Arrays for the PSPWM dimming. A favored feature is that the load variation of the SRC is always within one step change of Ip no matter what the .