The disclosed embodiments provide a charging system for a portable electronic device. The charging system includes a first bidirectional switching converter connected to a first power port of the portable electronic device, a low-voltage subsystem in the portable electronic device, and a high-voltage subsystem in the portable electronic device and a second bidirectional switching converter connected to a second power port of the portable electronic device, the low-voltage subsystem, and the high-voltage subsystem. The charging system also includes a control circuit that operates the first and second bidirectional switching converters to provide and receive power through the first and second power ports and convert an input voltage received through the first or second power port into a set of output voltages for charging an internal battery in the portable electronic device and powering the low-voltage subsystem and the high-voltage subsystem.