Bi-Level Driving Technique

 
 

From the prior discussion on efficiency droop, one would have expected that, when an LED is driven by time-varying current, its optical power and efficiency will depend on the nature of the current waveform being used. Fig. 3 shows the simulated optical power and efficiency curves for three driving waveforms (DC, full wave rectified AC, PWM). For the same average LED current, PWM gives rise to the poorest optical power efficiency since the LED is driven with a high peak current during the “on” time and more power is lost due to the droop effect. Despite this drawback, PWM is widely used today for driving LEDs in LCD backlight or large-area display panels as dimming linearity is important in these applications.





Fig. 3: Simulated optical power and efficiency curves for three driving waveforms


The bi-level driving technique was developed to improve the optical efficiency of LEDs driven by conventional PWM while maintaining its dimming linearity. Fig. 4 illustrates how the bi-level driving technique is conceptualized by considering the droop-induced “concave” optical power curve of LEDs. Here Curve 1 and 2 represents the optical power emitted by an LED driven by DC and PWM (pulsating between IH and 0), respectively, where under the same average LED current denoted by If, the optical power produced by PWM is lower compared to DC by (AB). Now consider if the peak current of PWM is lowered from IH to Iα during ton, and the zero current is raised to I during (Tton) without changing the duty cycle D so that the average LED current is kept the same as before (note 1), it can be seen that the optical power produced by this new PWM-like waveform is now increased from B to C.



Fig. 4: Conceptualization of the bi-level driving technique



This new waveform is known as the bi-level current as it pulsates between two non-zero current levels. It is interesting to note that if Iα and Iβ are continuously varied according to the steps described above, eventually A will be reached when Iα = Iβ. In other words, the bi-level driving technique has inherited the features of both DC and PWM and thus with this current waveform the entire two-dimensional area bounded by Curve 1 and 2 is now “accessible” by an appropriate selection of Iα, Iβ and D. Fig. 5 shows the measured illuminance, illuminance efficacy and color deviation of a LUXEON K2 white LED sample for various driving waveforms. Here 10:1 and 10:2 denotes Iα : Iβ = 1000 : 100 mA and Iα : Iβ = 1000 : 200 mA, respectively. It can be seen that a bi-level driven LED consistently performs better than a PWM-driven one and the gain in optical efficiency improves as Iβ is increased.



Fig. 5: Measured illuminance, illuminance efficacy and color deviation of a LUXEON K2 white LED for various driving waveforms.




Fig. 6: Comparison between bi-level and conventional PWM current.




Fig. 7: Implementation of bi-level driving technique on (a) Philips 19” LCD display, and (b) Samsung 23” XL2370 LCD display.


Note 1: For this condition to hold, I should be set to Iβ = (IfD.Iα)/(1 – D). Further, it should be self-evident that the same average LED current If can also be obtained by keeping IH at it original value and decreasing the duty cycle D. This effectively reduces the contribution of the droop-affected point, IH, and improves the overall optical efficiency of the LED.




 

The birth of “bi-level” driving technique


Address

  1. Electronics and Communications Research Lab (EF502), Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong



Links


Department of EIE

at HK PolyU


Research Group

at EIE Dept.


HK PolyU




Recognition!

In 2009 and 2013, the bi-level driving technique and the video wall driving design had both won the Gold Medal in the International Exhibition of Inventions, Geneva, Switzerland. It has also been patented in the U.S. in 2010* and has immediately attracted interests from the industry.


*Patent

K. H. Loo, W. K. Lun, Y. M. Lai, S. C. Tan, and Chi K. Tse, “A Driving Method for Improving Luminous Efficacy of a Light-Emitting Diode,” US Non-Provisional Patent Application No. 12/366,304, Publication No. US 2010/0194300 A1, published on 5 August 2010


Publicatons

  1. 1.X. Lv, K.H. Loo, Y.M. Lai and C.K. Tse, "Energy-saving driver design for full-color large-area LED display panel systems," IEEE Transactions on Industrial Electronics, vol. 61, no. 9, pp. 4665-4673, September 2014.

  2. 2.K.H. Loo, Y.M. Lai, S.C. Tan and C.K. Tse, "On the Color Stability of Phosphor-Converted White LEDs Under DC, PWM, and Bi-Level Drive," IEEE Transactions on Power Electronics, vol. 27, no. 2, pp. 974-984, February 2012.

  3. 3.K.H. Loo, Y.M. Lai, S.C. Tan and C.K. Tse, "Stationary and Adaptive Color-Shift Reduction Methods Based on Bi-Level Driving Technique for Phosphor-Converted White-LEDs," IEEE Transactions on Power Electronics, vol. 26, no. 7, pp. 1943-1953, July 2011.

  4. 4.S. C. Tan, “General n-Level Driving Approach for Improving Electrical-to-Optical Energy-Conversion Efficiency of Fast-Response Saturable Lighting Devices,” IEEE Transactions on Industrial Electronics, vol. 57, no. 4, pp. 1342-1353, April 2010.

  5. 5.W-K. Lun, K.H. Loo, S.C. Tan, Y.M. Lai and C.K. Tse, "Bi-level Current Driving Technique for LEDs," IEEE Transactions on Power Electronics, vol. 24, no. 12, pp. 2920-2932, December 2009.

  6. 6.K.H. Loo, W-K. Lun, S.C. Tan, Y.M. Lai and C.K. Tse, "On Driving Techniques for LEDs: Towards a Generalized Methodology," IEEE Transactions on Power Electronics, vol. 24, no. 12, pp. 2967-2976, December 2009.