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This paper presents a digital feedback-clamped synthetic ripple based hysteretic modulator for optimal battery charging. The proposed digital hysteretic modulator ensures a constant current (CC) charging for a low battery state of charge (SOC) and a constant voltage (CV) charging for battery SOC near 100 %. The modulator facilitates smooth and stable transition from CC to CV without the need of any extra switch or control loop. The proposed digital modulator generates carrier to the hysteretic comparator by generating piece-wise linear synthetic ripple using sensed converter voltages (switch node voltage and the output voltage), without the need of direct inductor current sensing and adding the output voltage to it. This facilities immediate (one switching cycle) fault detection incase of the main switch failure of the converter. A variable voltage feedback clamping circuit used in the proposed modulator makes the charging system feedback controlled in CC mode, in CV mode, and even during the transition from CC to CV. Experimental and simulation results verify the operation of the proposed charging system on 20 W prototype. The performance of the charger is verified on a 6 V-4.5 Ah lead acid battery as well.
Date of Conference: 25-28 Oct. 2012