A New, Simple, “universal”, Low Cost LED Driver and Controller Akram M. Fayaz ∗ , Charif Karimi ∗∗ , Daniel Sadarnac ∗∗ ∗ Control Department ∗∗ Energy Department SUPELEC - Systems Sciences E3S Plateau de Moulon 3 rue Joliot-Curie 91192 Gif sur Yvette cedex - France EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 1
Problem statement and outline Problem statement: Propose a driver and controller able to control the brightness (the current through) of a large class of LEDs (different characteristics) regardless of the source voltage level (higher or lower than that required for a given LED). Outline: Existing approaches Our approach: - Choice of the converter - Modeling - Control - Simulation and experimental results Conclusion EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 2
Existing approaches: By the first converter I LED By the second converter en t Two converters are used to control the brightness of the LED, A first converter provides a constant current (nominal current of the LED). A second converter chaps this constant current to provide the current giving the required brightness. Drawbacks: The structure is complex and the number of components is high. EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 3
Our approache: The LED is not used in optimal conditions in the sense that the chapping of the current through the LED, even to a few kHz, can affect its life-time. Our approach: A unique converter directly provides the desired current feeding the LED. I LED By a unique converter t EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 4
Choice of the converter L 1 C I e I L + I 2 L s V c + L 2 C s V e V T r V s R shm R shL Figure: Sepic converter with coupled inductors. The SEPIC has been chosen because: It can lower and raise the battery voltage. Constraints on the voltage across the capacitor in series are weaker. As the output voltage is not reversed, the implementation of detection of current through the LED (for the control purposes) requires fewer components. EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 5
Modeling Goal Modeling and control were realized according to the control goal: Regulate precisely the LED current and, impose a very fast current-mode internal loop, on the magnetizing current I m = I e + I 2 . The controller comprises two loops: Clock S V s , ref V I m , ref V s + − Controller − H ( s ) Q + V s , mes R V I m , mes I mes Gain and filter Inner-loop Gain and filter EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 6
Modeling Goal 1 An inner-loop, which a very fast current-mode control on the magnetizing current. The latter is also the transistor current. This control loop allows: The protection of the transistor and the reliability of the system as a whole. A faster dynamic, a simplified model and thus the simplification the synthesis of the outer-loop control law. 2 An outer-loop: It is added because the current mode control does not allow a precise regulation of the LED current. EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 7
Modeling Computation of the average model Assumptions: The input voltage V e is considered as a constant. The two inductors L 1 and L 2 are fully coupled ⇒ V C = V e L 1 = L 2 = L m The variations of the magnetizing current’s ripples are neglected. The LED is equivalent to a constant voltage source in series with a resistance: V 0 + R L I L , R L = LED’s internal resistance + R shL . I e (1 − D )( V e + Vs ) L 1 V s = V 0 + R L I L V e (1 − D ) I m (1 − D ) I m R L L 2 V e C s V 0 I L I 2 EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 8
Modeling Computation of the average model From the circuit: dI L � (1 − D ) I m = R L C s dt + I L DV e − (1 − D )( V 0 + R L I L ) = L m dI m dt For small variations around ( I L 0 , I m 0 , α 0 , V e 0 , V s 0 ), after linearization and Laplace transform: H ( s ) = i L ( s ) i m ( s ) = G 1 − τ n s 1 + τ d s , (1 − α 0 ) with G = , τ n = L m I m 0 R L C S V e , τ d = (1 − α 0)2 RLIm 0 (1 − α 0)2 RLIm 0 1+ 1+ Ve Ve And after computation and adjustment of the parameters: H ( s ) = 0 . 681 − 5 . 4 × 10 − 6 s 1 + 31 × 10 − 6 s EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 9
Control Internal-loop control Goal: Provide a very fast internal loop and protect the transistor and system as a whole. Functioning: The clock and the RS Flip-Flop allow the control of the transistor at a constant chapping frequency: The rising edge of the clock signal triggers (closes) the transistor. The blockage of the transistor is caused by the “substractor”: Whenever the measured current exceeds the reference current, the Flip-Flop opens the transistor. EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 10
Control External-loop control The choice of the controller imposed by the following requirements: Simplicity in the sense that the controller should admit the lowest possible number of tunable parameter ⇒ lowest number of electric components (lower cost). Robustness with respect to LED’s parameters’ variations. This makes the controller “universal”, in the sense that it works for a large class of LEDs with suitable performances. No steady-state error : In steady state, the “precise required current” must be provided to the LED. ⇓ The PI controller has been chosen. The PI parameters are computed by pole placement EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 11
Controller Performance robustness assessment by the step simulation Step Response Step Response Step Response varying τ n varying τ d varying G 1.2 1.2 1.4 1.2 1 1 1 0.8 0.8 0.8 0.6 0.6 Amplitude Amplitude Amplitude 0.6 τ n τ d G 0.4 0.4 5 τ n 5 τ d 5 G 0.4 G / 5 τ n / 5 τ d / 5 0.2 0.2 0.2 0 0 0 −0.2 −0.2 −0.2 −0.4 −0.4 −0.4 0 2 4 0 2 4 0 2 4 Time (sec) Time (sec) Time (sec) −3 −4 −3 x 10 x 10 x 10 Figure: Parameters individually modified by 500%. EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 12
Controller Stability robustness assessment using the nyquist diagrams Nyquist Diagram varying τ d Nyquist Diagram Nyquist Diagram varying G varying τ n 40 15 50 40 30 τ d τ d G 10 5 G 5 τ d 5 τ d 30 G / 5 τ d / 5 τ d / 5 20 20 5 Imaginary Axis 10 Imaginary Axis Imaginary Axis 10 0 0 0 −10 −10 −5 −20 −20 −30 −10 −30 −40 −40 −15 −50 −4 −2 0 −1 −0.5 0 −4 −2 0 2 Real Axis Real Axis Real Axis Figure: Parameters individually modified by 500%. EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 13
Controller Simulation (using LTspice) and experimental results: The parameters of the controller have been computed using the parameters of this LED: V 0 = 18 V and R L = 1Ω. 900mA 2.0V 850mA 1.9V 800mA 1.8V 750mA 1.7V 700mA 1.6V 650mA 1.5V 600mA 1.4V 650mA 1.3V 600mA 1.2V 550mA 1.1V 1.0V 500mA 450mA 0.9V 0.6ms 0.8ms 1.0ms 1.2ms 1.4ms 1.6ms 1.8ms 2.0ms 2.2ms 2.4ms Figure: Simulation result (with LTSpice) for the LED considered to buil the Figure: Experimental result transfer function, for the LED considered to V 0 = 18 V and R L = 1Ω. build the transfer function, V 0 = 18 V and R L = 1Ω. EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 14
Controller Simulation (using LTspice) and experimental results: The same controller with another LED: V 0 = 11 V and R L = 3Ω. 2.1V 960mA 2.0V 900mA 1.9V 840mA 1.8V 780mA 1.7V 720mA 1.6V 660mA 1.5V 600mA 1.4V 540mA 480mA 1.3V 1.2V 420mA 1.1V 360mA 300mA 1.0V 240mA 0.9V 0.6ms 0.8ms 1.0ms 1.2ms 1.4ms 1.6ms 1.8ms 2.0ms 2.2ms 2.4ms Figure: Simulation result for another LED with Figure: Experimental result V 0 = 11 V and R L = 3Ω. for another LED with V 0 = 11 V and R L = 3Ω. EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 15
CONCLUSION This work has shown that: The commonly used two converters based architecture can successfully be replaced by a unique converter and the current through the LED can be directly regulated. The combination of a very fast current mode control and the coupling of the two inductors in the Sepic, provide a fast dynamic and much simpler model. The fast current-mode with combined simple PI allows achieving robust stability and robust performances for different LEDs. EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 16
Thank you ! EPEC 2012 A New, Simple, “universal”, Low Cost LED Driver and Controller 17
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