An energy storage device includes buck and boost switches that charge and discharge a charge storage element (e.g. a supercapacitor or ultracapacitor) over a broad voltage range. Integrated charge limiting prevents the storage element from sinking a potentially damaging level of current. Pulse-width modulated control signals are combined with higher-frequency signals to allow the resulting modulated control signals to pass through relatively small, inexpensive isolation transformers. Pairs of control signals may be combined to produce composite control signals that turn power switches on and off, or may be used to separately turn the power switches on and off.