Improving Current Regulation for Offline LED Driver Jianwen Shao STMicroelectronics Schaumburg, USA [email protected] Abstract—A buck converter is a very common choice for non-isolated offline LED applications, using peak current regulation. The problem for peak current regulation is that the average current of the LED string will vary with different numbers of LEDs. This paper presents two simple and cost effective ways to compensate the average current variation without losing the simplicity of peak current regulation. INTRODUCTION The simple buck converter with peak current regulation is shown in Figure 1. The peak current of the LEDs is regulated at a constant value. With the same value of peak current, the average current of the LEDs differs when the number of LEDs is different in the string [1] [2]. One of the solutions is to sense the average current of the LEDs in the high side, but it is costly to add such a current sensor. Reference [3] proposes to put a MOSFET in the high side so current sensing can be done in the low side. This increases the cost of the driver by adding high side gate driver. Reference [4] uses the integration of a sensing current to improve the accuracy, but it loses the instant current protection. In this paper, two simple solutions are presented to improve the average current regulation without changing the topology or significantly increasing the cost. inductor or the LED current. If the peak current value Vs is fixed, we can calculate the average current. LEDs I. II. IMPROVE CURRENT REGULATION BY MODULATING PEAK CURRENT For the Buck converter in Figure1, the controller can only control the peak current of the 978-1-4244-4783-1/10/$25.00 ©2010 IEEE L Vdc c o n t r o lle r Vs Rs Fig. 1 typical buck converter with peak current control Ipeak = Vs / Rs (1) 1 1− D * * Vled (2) 2 L*F 1 1− D Iave = Vs / Rs − * * Vled (3) 2 L*F Where Ipeak: peak current of the LEDs; Iave: average current of the LEDs; Vled: voltage drop of the LED string; Vs: internal reference value for current setting; Vdc: dc voltage; D: duty cycle 601 Iave = Ipeak − D=Vled/Vdc; frequency. L: inductance; F: switching L E D s We can see from equation (2) that the average current Iave will heavily depend on Vled. When the number of LEDs changes, the average current of the LEDs varies, even though the peak current of the LEDs is the same. The test result is shown in Figure 2. V d c c o n tr o lle r R 1 V s R 2 R s L ED averag e cu rren t (m A ) 400 Fig. 3: modification of the circuit by adding voltage feedback 390 380 370 360 350 Series2 Iave = 340 330 1 R2 R2 R2 1 1 − D * [Vs * (1 + ) − Vdc * + Vled * ] − * * Vled Rs R1 R1 R1 2 L * F 320 (6) 310 300 3 4 5 6 7 8 Iave = LED num ber R2 R2 R2 Vs 1 1− D * (1 + ) − Vdc * +( − * ) * Vled Rs R1 R1 * Rs R1 * Rs 2 L * F (7) Fig.2 Average LED current with different number of LEDs 1 1− D R2 − * R1* Rs 2 L * F of (7) as small as possible, and if Vdc is constant, then voltage drop of the LEDs will not have a significant impact on the average current. Figure 4 is the test result. If we can make the third term If the peak current value can be adjusted according to the LED voltage, then it is possible to keep the average LED current constant. There are numerous ways to implement the improvement. The following section gives two examples. 400 A. Improvement method 1: Modulate the peak current by dc voltage and LED voltage feedback LED average current(mA) 390 Figure3 shows the modification of the circuit. We can get equations as following. Vs = Ipeak * Rs * Ipeak = R1 R2 + (Vdc − Vled ) * R1 + R 2 R1 + R 2 R2 R2 R2 1 * [Vs * (1 + ) − Vdc * + Vled * ] Rs R1 R1 R1 380 370 dc and LED voltage feedback 360 350 w /o compensation 340 330 320 310 300 3 4 5 6 7 8 LED Num ber (4) (5) Fig.4: average current variation before and after the modification 602 Figure 5 shows the current waveform. The peak current is modulated according to the number of LEDs to improve the average current regulation. From Figure 6, we can have the following equation: Vs = Ipeak * Rs * R1 − Vled * R 2 R1+ R 2 R1+ R 2 Current of 8 LEDs string 100mA/div (8) Ipeak = 1 R2 R2 * [Vs * (1 + ) + Vled * ] Rs R1 R1 (9) Current of 3 LEDs string Iave = R2 R2 1 1 1− D * Vs * (1 + )+( − * ) * Vled Rs R1 R 1 * Rs 2 L*F (10) Again, if we select R1 and R2 so that R2 1 1− D − * is close to zero, we can get a R1 * Rs 2 L * F constant average current. Figure 7 shows the test results. Fig.5: current waveforms for different LED numbers B. Improvement method 2: Modulate the peak current by LED voltage feedback only If the dc voltage is not well regulated, then the above method will not be effective, since the Vdc term will affect the average current, too. In this case, only the LED voltage is used to modulate the peak current, as shown in Figure 6. A coupled inductor is used to sense the LED voltage drop. L ED averag e cu rren t(m A ) 400 390 380 370 360 w /o compensation 350 w ith LED voltage feedback 340 330 320 310 300 3 LEDs 4 5 6 7 8 LED Number Vdc Fig. 7: average current variation before and after the LED voltage feedback -Vled controller R1 Vs R2 Rs Fig.6: LED voltage feedback for peak current modulation The above two methods are very simple and are easy to implement for a typical PWM controller. 1 1− D *Vled can be kept From equation (3), if * 2 L*F constant by modulating switching frequency, it is possible to achieve the constant average current, too. 603 III. [1] CONCLUSION In offline LED applications, the simple fixed peak current regulation will cause an average LED current variation when the number of LEDs is different in the string. This paper presents two simple and low cost methods to correct the variation by modulating the peak current according to the dc voltage and the LED voltage drop. One method is good for applications where the dc voltage is well regulated; the other one is applicable when dc voltage is not well regulated. The test results prove the effectiveness of the proposed methods. [2] LED Drivers Tutorial, http://www.st.com/stonline/products/applic ations/blocks/lighting/lite017.shtml HV9910, datasheet from Supertex, www.suptertex.com [3] Odile Ronat, Peter Green, Scott Ragona, “Accurate current control to drive high power LED strings,” in IEEE 2006 Applied Power Electronics Conference, 2006, pp. 376-380. [4] Yuan Fang1, Siu-Hong Wong2, and Lawrence Hok- Sun Ling, “A Power Converter with Pulse-Level-Modulation Control for Driving High Brightness LEDs,’’ in IEEE 2009 Applied Power Electronics Conference, 2009, pp. 577-581. REFERENCES Appendix: schematic of test circuit: 604