Reconfigurable and fault tolerant digital phase shifted modulator for luminance control of LED light sources

Abstract

This paper presents a digital phase shifted modulator (PSM) for luminance control of high brightness light emitting diode (HB-LED) based light sources. The design of the modulator is optimized to meet the growing demand for flexible, fault tolerant and hardware efficient dimming capability for LED lamps with a large number of series LED strings. Depending on the LED lamp specifications, the modulator can be configured to generate an arbitrary number of time synchronous as well as uniformly phase shifted output pulses. In case of LED string failure, two different online fault recovery techniques are introduced. After a fault event the modulator maintains the total LED lamp luminance by scaling the duty cycle of the control outputs, {c <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</inf> } or the forward current through the operating LED strings, I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ref</inf> . The desired resolution (dimming ratio) is achieved by performing Σ-Δ modulation of the input command and using a single high resolution DPWM module. Experimental results are presented for a system with 8 strings of series LEDs and an input command of 10-bit resolution.