Monolithically Integrated Drivers for Eco-friendly LED System-on-a-Chip Applications Lisong Li, Yuan Gao, Philip K.T. Mok, Kei May Lau, Salahuddin Raju, C. Patrick Yue and Johnny K.O. Sin Department of Electronic and Computer Engineering, The Hong Kong University of Science of Technology Clear Water Bay, Kowloon, Hong Kong Email:
[email protected] Abstract—Monolithically integrated Light-Emitting Diode (LED) drivers greatly reduce the cost as well as enhance the reliability of LED system due to their reduction of bulky and expensive off-chip components and assembling cost. However, integrating all the passive and active components of an LED driver on chip is very challenging. This paper discusses current issues of designing a monolithically integrated LED driver and presents some considerations to these problems.
I.
INTRODUCTION
LED is considered to be the next generation of lighting source owing to its many preferable characteristics, including superb efficacy as high as 200 lm/W, 100,000 hours of lifetime, improved physical robustness and environment-friendly property. However, the diode nature of LED results in mandatory driving circuit to properly regulate its current and brightness. In a LED system, the driver usually takes up considerable part of the whole cost. Current LED drivers need some bulky and expensive off-chip components, which increase both the sizes and costs of LED driver. Meanwhile, it is time consuming to choose off-chip components and interconnect them with the IC. Integrating these off-chip components will reduce the sizes and costs of LED products and push LED to replace traditional incandescent and fluorescent light bulbs. Moreover, since LED is built of semiconductor material, it is possible to integrate all the power electronics, active devices and passive components to a selfcontained LED system-on-a-single-chip (LED SoC) [1]. Fig. 1 shows the architecture of the proposed LED SoC. II.
DESIGN CONSIDERATIONS
Fig. 2 shows a typical two-stage offline LED driver. The first stage serves to regulate the power factor (PF) and total harmonic distortion (THD) of the system in order to comply with Energy Star Program [2] and IEC Standards [3]. The second stage is a dc-dc converter to control the current of the LED. We illustrate this stage with an inverting buck converter, which is suitable for high voltage step down applications. Considerations of how to minimize different components of LED driver such that integration is feasible will be discussed in the following paragraphs. A. Storage Capacitor Bulky energy storage capacitor Cstorage between 1st stage and nd 2 stage is usually necessary for filtering the 50Hz/60Hz low frequency AC power ripple. The capacitor will complete a charge-discharge procedure for each half AC line cycle. The storage energy for each time is defined as ∆E, while the total energy transferred to output in the same period defined as E. Hence, for given LED output power Pout and AC line frequency fac, Cstorage can be expressed as The authors acknowledge the support of a grant (T23-612/12-R) from the Research Grants Council (RGC) of the Hong Kong Special Administrative Region Government under the Theme-based Research Scheme.
Cstorage =
Pout ΔE ⋅ 2 ⋅ Vavg ⋅ ΔV ⋅ f ac E
(1)
where Vavg and ∆V represent average and ripple voltage on the storage capacitor, respectively. The normalized storage energy ∆E/E, as shown in Fig. 3, determined by the difference between input power and output power, is approximate 0.32 for those perfect LED drivers with unity PF and constant output current. Take a typical 10W, 220V 50Hz AC powered LED driver as an example, and assume Vavg is 350V, which is a reasonable value for a boost PFC stage, and 5% voltage ripple on the storage capacitor. Based on (1), therefore, a 5.2µF, 400V voltage rating capacitor is needed. Electrolytic capacitor (Ecap) and film capacitor are two candidates suitable for this voltage and capacitance range. However, each of these capacitors has its own issue. E-cap, though widely employed in current commercial LED drivers, is not an adorable choice because of its reliability issue. The typical lifetime of E-cap is at least 10 times shorter than LED lighting source itself. Film capacitor can perform better in longer period than E-cap, but with several times larger physical size. Therefore, to realize highly integrated system, the challenging but important work is to reduce the storage capacitance. The storage capacitor size can be reduced by increasing the ripple voltage [4]. If the voltage ripple of above example is increased to 20%, the capacitance then will decrease to 1.3µF, only one fourth of before. Nevertheless, it should be noted that the capacitor volume is not proportional to its value since the rating voltage also increases. Besides, high voltage stress will be introduced with this method both for first and second stages should also be considered. Another possible method is to reduce the storage energy by reshaping the input power and/or output power [4], [5]. The cost is non-unity PF and/or low frequency current ripple at output. A preferable strategy for our case is to meet minimum requirements for PF and output current ripple and push the capacitor size smaller. Fig. 3 shows one of these approaches called input harmonic current injection. The input current as well as input power are shaped with help of 3rd and 5th harmonics, resulting in 44% and 62% reduction of required storage energy for two standard lines (PF=0.9 and PF=0.7) set by [2]. B. Power Transistor In majority part of existing offline LED drivers, the controller is integrated while other parts of the converter including the power MOSFET and power inductor are yet offchip. With the advance of the IC technology, many foundries have developed high-voltage process which enables the integration of power MOSFET. Current advanced integrated
power MOSFET could sustain as high as several hundreds of volts and several amps. Besides MOSFET, GaN HEMT is also an excellent candidate for power transistor and exhibits many superior characteristics. Recent work [6] has achieved the integration of LED and GaN HEMT on the same substrate. C. Power Inductor Integrating the inductor for power applications on chip is historically a problem. Recent literatures [7]-[8] have reported some advanced monolithic integrated inductors aimed at power applications with high inductance density of 200nH/mm2 while maintaining high Q as summarized in Fig. 4. However, these inductors are in μH range while typical power inductor used in offline applications are in hundreds of μH range. Therefore, in order to exploit the newly developed integrated power inductor, the value of inductor needs to be reduced to μH range. Taking inverting buck converter as an example, the inductor value needed is given as (V − VLED ) D L = DD (2) ΔI L ⋅ f For given input and LED voltages, the numerator of (2) is fixed. In order to reduce L, one possible way is to increase the switching frequency. A simple calculation could be made to estimate the frequency range applicable for integrated inductors. Assuming VDD=310V, VLED=50V, ΔIL=200mA and L=5μH, then switching frequency f=41MHz. Switching at such high frequency and voltage, the power loss due to charging and discharging the drain capacitance of MOSFET is extremely large. The power loss can be expressed as P = CV2f. Assuming the MOSFET has an output capacitance of 100pF and taking the above calculated number into (2), the power dissipation is as high as 394W. One way to solve this problem is zero voltage switching (ZVS), which drives the drain voltage to zero before the MOSFET turns on. In this way, the overlapping of rising drain-source current and falling drain voltage will not occur and the this switching loss can be eliminated [9]. However, the control methodology of this kind of converter is different from normal dc-dc converter, since resonance is involved and thus pulse-width modulation (PWM) control cannot be adopted. Another potential approach for reduction of inductor value is to push converter working at deep Discontinuous Conduction Mode (DCM). Plenty of switching loss will be saved compared to those high frequency LED drivers. However, in this case, the peak current stress suffered by power switch, power diode and inductor is significantly increased compared to Continuous Conduction Mode (CCM), resulting in more constraint requirements for these components. In addition, bigger capacitor is also needed at output for filtering the current ripple. D. Controller Controller is the last but not least concern for fully integrated LED driver. With smaller capacitors and inductors, precise timing control of the controller is critical and the controller may fail to regulate and may degrade the driver efficiency because of insufficient speed for controller and power switch driver, especially for those with integrated high voltage power MOSFET. Advanced technology or more complicated circuits may be necessary for better regulation and efficiency of fully integrated LED driver.
III.
CONCLUSION
We present some considerations of designing monolithically integrated LED drivers. Some calculations are made to testify our viewpoints. Great reduction of storage capacitance could be achieved by voltage ripple increasing and/or power reshaping with reasonable PF and LED current ripple. The dcdc converter needs to either work in high frequency with soft switching or work in DCM to reduce the inductance so as to finally integrate the driver on chip. REFERENCES [1] K.M. Lau, H.W. Choi, S.-W. R. Lee, P.K.T. Mok, J.K.O. Sin, C.P. Yue, and W.-H. Ki, “Cost-effective and Eco-friendly LED System-on-a-Chip,” China Solid State Lighting, Beijing, China, Nov 2013. [2] “Program requirements for solid-state lighting luminaires: eligibility criteria—version 1.1, Revised, ENERGY STAR,” Washington, DC. [3] “Electromagnetic compatibility (EMC)—part 3: limits-section 2: limits for harmonic current emissions (equipment input current < 16A per phase), ” IEC1000-3-2, 1995. [4] L. Gu, X. Ruan, M. Xu and K. Yao, “Means of Eliminating Electrolytic Capacitor in AC/DC Power Supplies for LED Lightings,” IEEE Trans. Power Electronics, vol. 24, pp. 1399–1408, May 2009.:. [5] Q. Hu and R. Zane, “Minimizing Required Energy Storage in Off-Line LED Drivers Based on Series-Input Converter Modules,” IEEE Trans. Power Electronics, vol 26, pp. 2887–2895, Oct 2011.:. [6] Z.J. Liu, T. Huang, J. Ma, C. Liu and K.M. Lau, “Monolithic Integration of AlGaN/GaN HEMT on LED by MOCVD,” IEEE Electron Device Letters, Vol. 35, No. 3, Mar 2014. [7] R. Wu and J.K.O. Sin, “A Novel Silicon-Embedded Coreless Inductor for High-Frequency Power Management Applications”, IEEE Electron Device Letters, Vol. 32, No. 1, pp. 60-62, Jan. 2011. [8] R. Wu, J.K.O. Sin and C.P. Yue, “High-Q Backside Silicon-Embedded Inductor for Power Applications in μH and MHz Range,” IEEE Trans. Electron Devices, vol. 60, pp. 339–345, Jan 2013. [9] S. Bandyopadhyay, B. Neidorff, D. Freeman, and A.P. Chandrakasan, “90.6% efficient 11MHz 22W LED driver using GaN FETs and burstmode controller with 0.96 power factor,” IEEE Int’l Solid-State Circuits Conf. San Francisco, CA, USA, Feb 2013, pp. 368–369.
Fig.1: LED System-on-a-Chip architecture [1]
PFC
DC-DC
Controller
Fig. 3: Storage energy reduction with harmonic injection Fig. 2: Two-stage off-line LED driver
Fig. 4: Measured peak quality factor (Q) and inductance density (LDensity) of the leading technologies for inductor integration on silicon
