Control of Solid-State Lamps Using a Multiphase Pulsewidth Modulation Technique

This paper describes a multiphase pulsewidth modulation (MPWM) technique used for controlling the luminance of a solid-state lamp (SSL). For a SSL consisting of multiple high-brightness LEDs (HBLEDs) that are electrically connected as a series-parallel network, the desired output luminance is obtained by modulating the average current through each parallel HBLED string. In conventional pulsewidth modulation (PWM) and pulse code modulation techniques, the average HBLED string current is controlled by simultaneously varying the "ON" time duration of each current regulator circuit, resulting in large current transients and pulsating light output. The proposed MPWM technique operates by uniformly time-shifting the individual ON/OFF control signal pulses, thus avoiding large current transients. When compared to conventional PWM dimming, MPWM dimming reduces the risk of visible flicker and lowers the magnitude of audible noise. The reduced magnitude of output current transients results in lower electromagnetic interference and enables optimization of the size of passive components. In this paper, the operation of the MPWM technique is explained in detail. Experimental results are presented for a 27 W boost converter powering 64 HBLEDs with digital MPWM and digital voltage loop implemented using a Virtex-4 field programmable gate arrays.