An Energy Conservation Based High-efficiency Dimmable Multi-channel LED Driver April (Yang) Zhao and Wai Tung Ng The Edward S. Rogers Sr. Electrical and Computer Engineering Department, University of Toronto Toronto, Ontario M5S 3G4
[email protected] Abstract—This paper presents a high-efficiency multi-channel LED backlighting driver with local dimming capability. A sequential burst-mode channel multiplexed pulse width modulation dimming scheme is introduced to regulate and maintain uniform LED current. An energy conservation technique is utilized to handle load disconnect situation, which significantly improves the efficiency over the entire dimming range. The proposed approach eliminates the dedicated linear current regulator and sensing components in each LED channel as required by recently reported methods. This work is well suited for digital IC implementation for dynamic LED luminance adjustment. The performance of the proposed method was verified experimentally using a 4.5 to 14V boost converter. An overall efficiency of 50-90% was observed over a wide range of LED luminance.
In this paper, a digital control method that integrates a burst mode PWM channel multiplexing and a DC-DC boost LED driver with energy conservation technique is proposed. This method can effectively avoid the common drawbacks appeared in previously reported designs. It meets LED current regulation requirement and achieves high efficiency without increasing circuit complexity.
VIL_R
aux_dri
main_dri
Figure 1. System level diagram of the proposed LED driver with burst mode PWM multiplexing scheme.
II.
PROPOSED LED DRIVER SYSTEM OPERATION
The proposed system is as shown in Fig. 1. It consists of a main boost converter stage with an auxiliary switch connected in parallel with inductor. The auxiliary switch is introduced to realize inductor current freewheeling during dimming dead-time (i.e. when all the LEDs are disconnected). The load consists of three LED strings and
This work was supported in part by Chungnam Display R&D Cluster Center (CDRC), Korea, and NSERC, Natural Science and Engineering Research Council of Canada.
978-1-4577-0541-0/11/$26.00 ©2011 IEEE
Rdummy
However, the power conversion efficiency of this approach is strongly affected by the selection of the
Saux
INTRODUCTION
Light-emitting diodes (LEDs) are rapidly gaining acceptance as a reliable and energy efficient illumination source for LCD backlighting units. In particular, high contrast LCD panels require the ability to locally dim individual LED regions, depending on video content. Various circuit techniques utilizing current regulator (linear or switched mode) have been proposed for driving parallel connected LED strings. In comparison, linear regulator offers a low cost solution while switched mode converter offers higher operating efficiency. A straight forward approach is to employ a linear current regulator for each LED string. This offers precise current regulation, but efficiency decreases at higher output voltages. The combination of linear and switched mode regulators allows the advantages in both cost effectiveness and power conversion efficiency. In this case, the LED string voltage is typically powered by a switched mode voltage pre-regulator, and the LED string current is individually regulated by linear regulators. In order to optimize efficiency, the output of preregulator is often set at the lowest possible level to ensure proper operation of the linear regulators. Sensing techniques has to be used to achieve this goal. This idea was implemented in similar ways in previously reported selfadaptive drives [1], [2].
VIL_REF
I.
reference voltage. The constant off-time control scheme [3] and the more recently one-comparator counter based inductor current sampling method [4] require one converter to control each string of LEDs. This unavoidably increases system cost in multi-string LED applications.
2576
one dummy resistor, which are sequentially connected to the output based on the channel multiplexing PWM dimming cycle and load pre-regulation period (described below). The LED dimming scheme and closed-loop current control for this prototype is implemented digitally using an Altera DE2115 board. The same Verilog code can be used to synthesize the layout of a custom driver IC chip when it is ready for mass production. III.
FUNCTIONALITY DESCRIPTION
increasing the output voltage Vout without bound in an attempt to regulate the LED current, Iout. When M turns on, controller will see a very high Vsense and therefore have to reduce Vout by reducing duty cycle. Consequently, the control loop oscillates with each PWM dimming cycle, resulting in LED brightness flicker. Gacio et al. [5] used a very high dimming frequency (far beyond the closed-loop cut-off frequency) in order to meet the closed-loop compensation requirement. This technique suffers from unnecessary high switching loss and low contrast ratio associated with higher dimming delay portion in the dimming cycle.
A. Multi-channel Burst Mode PWM Sequential Dimming The luminance of LED is proportional to its forward bias current. A straightforward method of adjusting the LED brightness is current modulation as shown in Fig. 2(a), i.e. analog dimming. This method suffers from two main drawbacks. One is color spectrum shift with varying LED current. This can be detected by human eyes as a change of color. The second drawback is current control inaccuracy. The closed-loop output current control is achieved by sensing the voltage at output sense resistor Rsense. The control becomes very difficult at small current due to the low signalto-noise ratio of Vsense. Burst mode PWM dimming, shown in Fig. 2(b), has proven to be a better dimming approach which can effectively eliminate the color shift with varying current. It is realized by keeping the LED current constant (usually at nominal operating current specified on LED datasheet) and regularly disconnecting the LED strings depending on the dimming ratio. The PWM dimming signal is typically in the 100-400Hz range. The perceived LED brightness is proportional to the time averaged current.
Brightness v D × Iout
33.33%
d1
33.33%
d2
33.33%
d3 Dimming cycle
Figure 3. Timing diagram for burst mode PWM multiplexing scheme with equal dimming ratio for maximum brightness. d1 d2 d3
D1 D2 D3 Dload
d4
Dimming dead-time Dimming cycle
(a)
Dload
d4
Pre-regulation
where D is the duty ratio of the PWM dimming signal. swi_main Daux
swi_aux
Conservation
(b) Figure 4. Timing diagram for arbitrary dimming duty ratios d1, d2, and d3. (a) Constant current regulation using dummy load. (b) Constant current regulation using energy conservation scheme.
Figure 2. Analog dimming and burst mode PWM dimming.
However, the closed-loop voltage regulation in PWM mode dimming is difficult to realize. When the dimming MOSFET M turns off, the load is essentially disconnected from the converter. The sensed voltage Vsense becomes zero, and the closed-loop controller will try to compensate by
In the proposed boost converter LED driver, multiple LED strings are sequentially connected to the output for certain portion of dimming cycle and the dummy load is connected during PWM dimming dead-time (when no LED string is on). The perceived LED string brightness is determined by the time averaged load current, the number of LED strings and its PWM dimming duty ratio, D1-3. Fig. 3 shows the maximum brightness case, where no dummy load is used and each LED string is connected for 1/3 of the dimming cycle. The maximum brightness is design to be equivalent to 1/3 of the output current. The closed-loop constant current regulation is implemented by sensing the
2577
voltage on the output sense resistor, Rsense. Since there is always one LED string connected, the closed-loop controller is enabled for the entire dimming cycle. Fig. 4(a) shows an arbitrary LED brightness case where dummy load is connected during PWM dimming dead-time. Significant power is wasted with prolonged dead-time. B. Energy Conservation Scheme An energy conservation scheme is utilized to improve the conversion efficiency during PWM dimming dead-time. Fig. 4 shows the timing diagram for arbitrary dimming duty ratios with and without utilizing energy conservation scheme. Case (a) is with the constant current regulation achieved by using dummy load during the entire dimming dead-time. Case (b) is the proposed method where the dimming dead-time is divided into two periods, energy conservation duration and load pre-regulation duration. In the energy conservation period, the load is disconnected from the output and inductor current circulates through the auxiliary switch. During this time, no power is wasted except for resistive drop in the freewheeling path. The closed-loop controller is temporarily disabled and main switch is turned off. The pre-regulation period allows the DC-DC converter to have sufficient time to bring Vout back to regulation before the LEDs are reconnected. It is designed to compensate for any inductor current loss (due to parasitic resistance in the freewheeling path) and output charge leakage during the conservation period. This effectively eliminates brightness flickering and achieves good dimming accuracy.
increase in error and avoid the possibility of inducing large transients at the start of the pre-regulation period. D. Design Considerations An important design consideration is the current associated with the maximum required LED brightness in a particular application. In this work, a load multiplexing scheme is used to facilitate the closed-loop current regulation and to economize the LED backlighting unit design. The number of LED strings at the output is defined as current sharing factor, which is chosen as 3 in this case. It should be noted that the maximum brightness of each LED string is reduced proportionally with this current sharing factor compared with single LED string scenario. However, the luminance reduction can be compensated by choosing LEDs that can tolerate higher peak current (up by a factor of 3 in this case). The required maximum luminance can be obtained by averaging the current over each dimming cycle. For the selection of dummy load, a resistance matching the equivalent load of each LED string is suggested. This can greatly minimize the closed-loop compensator transient response time, and therefore effectively reduce the LED brightness flickering after the pre-regulation period. In this work, a 75 resistor is used as Rdummy to match the resistance of the four LEDs (part# OVS5MWBCR4) connected in series with a forward current of 180mA.
Since the dummy load is only connected for a short time before the first LED string (d1) turns on, significant power saving can be accomplished. Moreover, the power loss associated with the dummy load can be optimized by adjusting the pre-regulation duration. For example, a shorter pre-regulation duration gives better efficiency but requires faster closed-loop control scheme to ensure constant load current when first LED string turns on. Compared with PWM dynamic bus voltage regulation [6], the proposed approach no longer requires LED voltage sensing and reference current adjustment, thus greatly simplifies the LED driver design. C. Closed-loop Control Current programmed mode control (digital PI compensator) is used in the proposed LED driver for two reasons. One is due to its simple dynamics. The compensator lead network can be eliminated, resulting in less digital implementation cost. Second is due to the direct regulation of inductor current, where excessive increase in inductor current can be effectively avoided. During the energy conservation period, the closed-loop PI compensator is disabled, the latest duty cycle is stored and output errors are reset to 0. At the beginning of preregulation period where dummy resistor is connected, the closed-loop compensator will recall the stored duty cycle and output error as the starting state. In this way, the integral component of the PI compensator will not cause a sudden
Figure 5. The prototype PCB for the four independent LED driver, each with three output channels.
IV.
EXPERIMENTAL RESULT
The proposed energy conservation based multi-channel dimmable LED driving scheme was verified on a 4 × 3 LED backlighting unit prototype. Four independent drivers were implemented on a multi-layer PCB, and each driver was designed to have multi-channel (three in this case) outputs as shown in Fig. 5. In addition, a separate 4 × 3 LED backlighting unit with four LEDs per region (d1, d2 and d3) was also implemented, see Fig. 6. The power conversion efficiency tests were performed on one of the four multi-
2578
channel LED drivers. The functionality of the proposed LED driving scheme was also verified.
A. Conversion Efficiency The conversion efficiency test was conducted on one of the multi-channel LED driver. The proposed PWM dimming scheme was verified experimentally with two different preregulation times (1/8 and 1/16 of PWM dimming cycle, corresponding to 0.625ms and 0.3125ms at 200Hz). The conversion efficiency is measured for dimming ratio between 10 to 100%. The boost converter design parameters are as listed in Table 1. An overall efficiency of 50 to 90% was achieved over a wide range of LED luminance. As shown in Fig. 8, there is a substantial improvement when compared to the case where the dummy load resistor is constantly connected whenever the LEDs are off. This efficiency level also compares favorably with previously published result [5]. Also, the case with pre-regulation time equal to 1/16 of the dimming cycle gives better efficiency. Table 1: Boost Converter Design Parameters
Figure 6. LED back light unit PCB with four columns of LEDs. Each column consists of 3 regions. Each region consists of 4 LEDs.
A typical system level setup for the LED back lighting unit with local dimming capability is as shown in Fig. 7. The test setup consists of the LED drivers, the LED backlighting unit, the LCD panel (screen and LCD TFT), a LCD controller and a Video Processing Module inside the backlighting (BLU) Control Unit. The latter three components are not the focus of this work.
Parameter
Value
Switching frequency
781kHz
Main inductor, L
10 μH
Output Capacitor, C
100 μF
Input voltage
4.5V
Output voltage
14V
Output current
180mA
PWM dimming freq.
200Hz
Pre-regulation duration
1/8 (0.625ms), 1/16 (0.3125ms)
LED Driver Efficiency
100% 80% 60% 40% Dpre_reg = 1/16
20%
Dpre_reg = 1/8 with dummy load
0% 0%
Figure 7. A typical LED backlighting system with local dimming capability.
The luminance for each LED region in every column is adjusted by the PWM dimming signals applied to the corresponding LED driver. A Video Processing Module (VPM) was designed to analyze and calculate the luminance of each region for a given video frame. In the following testing, it is assumed that the luminance values for every region are constantly updated by the VPM.
20%
40% 60% Brightness Ratio
80%
100%
Figure 8. Measured LED driver efficiency measurements for preregulation times equal to1/16, 1/8 of PWM dimming period.
The load current waveform during the energy conservation and pre-regulation periods is as shown in Fig. 9. The load current waveform drops to zero during the energy conservation period. Current ringing only occurs at the beginning of the pre-regulation period. The load current remains constant during the duration of the three LED channels PWM dimming cycle. Fig. 10 shows the case with
2579
0% dimming ratio for all three LED channels with 1/16 preregulation time. Constant current at the end of pre-regulation period indicating that closed-loop controller has bring the output voltage back to nominal voltage, which can reduce the flickering when LED channel turns on. It should be noted that the pre-regulation time used in our testing is not yet optimized. It can be further shortened if the amount of charge leakage on the output capacitor and the parasitic resistance in the inductor current freewheeling path can be reduced. In addition, a higher closed-loop bandwidth for the digital PI compensator will also allow the use of a shorter pre-regulation time, resulting in optimal conversion efficiency.
V.
CONCLUSION
A high efficiency energy conservation based dimmable multi-channel LED backlighting unit driver is presented. The proposed dimming method completely eliminates the dedicated linear regulator and sensing components in each LED strings. It offers a low component count and high efficiency dimming solution. It is also very well suited for multi-string LED dynamic luminance adjustment. Future work will involve the migration to the implementation of a custom driver IC chip.
Figure 11. An LED backlighting unit testing scenario showing 1%, 10% and 100% luminance for each output of the four multi-channel LED drivers. Figure 9. Load current waveform during PWM dimming period, energy conservation and pre-regulation period.
ACKNOWLEDGMENT The authors would like to acknowledge Prof. Hyoung Gin Nam from Sun Moon University, Korea for facilitating this project. Special thanks to Yu Tang and Meng Long Tu for their assistance in the testing of the LED driver circuits. REFERENCES [1]
[2] [3]
[4] Figure 10. Load current waveform during PWM dimming period, energy conservation and pre-regulation period in the case of 0% LED luminance. [5]
B. LED Back-lighting Unit Dimming Testing Fig. 11 shows a testing scenario with 1%, 10% and 100% luminance for each channel of the LED driver unit. A light diffuser was placed in front of the LED backlighting unit when this picture was taken. Running patterns for various LED luminance values that simulate the effect of motion pictures were also successfully tested.
[6]
2580
Y. Hu and M. M. Jovanovic, “LED driver with self-adaptive drive voltage,” IEEE Trans. Power Electron., vol. 23, no. 6. pp. 3116-3125, Nov. 2008. L. Burgyan and F. Prinz, “High efficiency LED driver,” U.S. Patent 6690146, Feb. 10, 2004. W. H. Chang, D. Chen, H. S. Nien, and C. H. Chen, “A digital boost converter to drive white LEDs,” in Proc. IEEE Appl. Power Electron. Conf. (APEC), Feb. 2008, vol. 1, pp. 558-564. K. I. Huw and Y. T. Yau, “Applying one-comparator counter-based sampling to current sharing control of multi-channel LED strings,” in Proc. IEEE Appl. Power Electron. Conf. (APEC), Feb. 2010, pp. 737742. D. Gacio, J. M. Alonso, J. Garcia, L. Campa, M. Crespo and M. RicoSecades, “High frequency PWM dimming technique for high power factor converters in LED lighting,” in Proc. IEEE Appl. Power Electron. Conf. (APEC), Feb. 2010, pp. 743-749. M. Doshi and R. Zane, “Digital architecture for driving large LED arrays with dynamic bus voltage regulation and phase shifted PWM,” in Proc. IEEE Appl. Power Electron. Conf. (APEC), Feb. 2007, pp. 287 - 293.
