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The effect of a current-sensing resistor used in current-mode control of pulsewidth modulator (PWM) converters on required MOSFET aspect ratio, the ratio of channel width to channel length generally known as W/L, is studied. The results can also be applied to the effect of source parasitic resistance in a power MOSFET. These resistances require a significant increase in aspect ratio to achieve a desired maximum current capability. Experimental results are presented verifying the validity of the theoretical results.