An estimator of luminous flux for enhanced control of high brightness LEDs

Usually, PWM operation is selected to drive High Brightness LEDs, because the dimming behavior is more linear than in DC operation. Nevertheless, to obtain an enhanced operation of the device (full output light control), the luminous flux should be measured. This paper proposes a control method based on an estimator of the luminous flux emitted by the LED. Firstly, the LEDs are characterized. Based on these characterization, an estimator of the flux emitted by the LED is defined. The estimator provides this flux value from only two temperature values (the case temperature and the ambient temperature). The estimator is defined and validated in steady state as well as in the transient stage. The flux estimator is then validated with actual measurements. Once the estimator is defined and validated, the electronic driver to supply the LEDs as well as the digital control scheme are presented. Finally, a built prototype is presented, and the experimental results are shown. Conclusions of this work show that with the presented estimator, both the steady state and the transient luminous flux can be accurately estimated. Thus, the output light control of the LEDs can be accomplished by sensing temperature rather than luminous flux. The final output characteristic of the system shows linearity between the output flux and the reference value, with a DC operation of the LEDs.