Universal serial bus (USB), originally conceived for data uses, is the most commonly used power source for charging cellular phones and lower power portable devices, typically <7.5 W. USB Power Delivery (USB PD) extends this range to include higher power devices including tablets and laptops. While extremely useful for on-the-move consumers, the wider output voltage (5-20 Volts) and higher power requirements (up to 100 Watts) create special challenges for the power supplies operating from lower voltage sources. USB ports deriving power from automotive battery rails (12 V) or from portable battery banks (power banks) must be capable of creating a voltage higher or lower than the input voltage source.