A New, Simple, universal, Low Cost LED Driver and Controller Akram - - PowerPoint PPT Presentation

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

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Existing approaches:

en ILED t By the first converter By the second converter

Two converters are used to control the brightness of the LED, A first converter provides a constant current (nominal current

• f 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.

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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.

ILED t By a unique converter

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Choice of the converter

Ve Vc L1 L2 I2 Vs Cs Ls Ie IL + + Rshm VTr RshL C

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.

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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 Im = Ie + I2. The controller comprises two loops:

Vs,ref Vs,mes Controller VIm,ref VIm,mes H(s) S R Q + + − − Vs Gain and filter Gain and filter Clock Imes Inner-loop

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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.

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Modeling

Computation of the average model

Assumptions: The input voltage Ve is considered as a constant. The two inductors L1 and L2 are fully coupled ⇒ VC = Ve L1 = L2 = Lm The variations of the magnetizing current’s ripples are neglected. The LED is equivalent to a constant voltage source in series with a resistance: V0 + RLIL, RL = LED’s internal resistance + RshL.

Ve Ve Vs = V0 + RLIL L1 L2 I2 V0 Ie IL Cs (1 − D)(Ve + Vs) (1 − D)Im (1 − D)Im RL

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Modeling

Computation of the average model

From the circuit:

• (1 − D)Im = RLCs

dIL dt + IL

DVe − (1 − D)(V0 + RLIL) = Lm dIm

dt

For small variations around (IL0, Im0, α0, Ve0, Vs0), after linearization and Laplace transform: H(s) = iL(s) im(s) = G 1 − τ ns 1 + τ ds , with G =

(1−α0) 1+

(1−α0)2RLIm0 Ve

, τ n = LmIm0

Ve , τ d = RLCS 1+

(1−α0)2RLIm0 Ve

And after computation and adjustment of the parameters: H(s) = 0.681 − 5.4 × 10−6s 1 + 31 × 10−6s

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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

• f 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.

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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

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Controller

Performance robustness assessment by the step simulation

2 4 x 10

−3

−0.4 −0.2 0.2 0.4 0.6 0.8 1 1.2 2 4 x 10

−4

−0.4 −0.2 0.2 0.4 0.6 0.8 1 1.2 2 4 x 10

−3

−0.4 −0.2 0.2 0.4 0.6 0.8 1 1.2 1.4

Step Response Step Response Step Response varying G varying τ n varying τ d Amplitude Amplitude Amplitude Time (sec) Time (sec) Time (sec) G 5G G/5 τ n 5τ n τ n/5 τ d 5τ d τ d/5

Figure: Parameters individually modified by 500%.

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Controller

Stability robustness assessment using the nyquist diagrams

−4 −2 −40 −30 −20 −10 10 20 30 40 −1 −0.5 −15 −10 −5 5 10 15 −4 −2 2 −50 −40 −30 −20 −10 10 20 30 40 50

Nyquist Diagram Nyquist Diagram Nyquist Diagram Real Axis Real Axis Real Axis Imaginary Axis Imaginary Axis Imaginary Axis varying G varying τ n varying τ d G 5G G/5 τ d τ d 5τ d 5τ d τ d/5 τ d/5

Figure: Parameters individually modified by 500%.

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Controller

Simulation (using LTspice) and experimental results:

The parameters of the controller have been computed using the parameters of this LED: V0 = 18V and RL = 1Ω.

2.0V 1.9V 1.8V 1.7V 1.6V 1.5V 1.4V 1.3V 1.2V 1.1V 1.0V 0.9V 900mA 850mA 800mA 750mA 700mA 650mA 600mA 650mA 600mA 550mA 500mA 450mA 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 transfer function, V0 = 18V and RL = 1Ω. Figure: Experimental result for the LED considered to build the transfer function, V0 = 18V and RL = 1Ω.

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Controller

Simulation (using LTspice) and experimental results:

The same controller with another LED: V0 = 11V and RL = 3Ω.

960mA 900mA 840mA 780mA 720mA 660mA 600mA 540mA 480mA 420mA 360mA 300mA 0.6ms 0.8ms 1.0ms 1.2ms 1.4ms 1.6ms 1.8ms 2.0ms 2.2ms 2.4ms 240mA 2.0V 1.9V 1.8V 1.7V 1.6V 1.5V 1.4V 1.3V 1.2V 1.1V 1.0V 0.9V 2.1V

Figure: Simulation result for another LED with V0 = 11V and RL = 3Ω. Figure: Experimental result for another LED with V0 = 11V and RL = 3Ω.

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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.

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Thank you !

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