In this example, the sink requests 12V from the adapter-but the adapter has no obligation to support 12V, as this is not a normative voltage. The relevant power rule, for the example mentioned above, requires that compliant devices be designed to guarantee operation in cases where the source power (W) ≥ sink power (W). Note that in the discussion below, “source” denotes the power supplier, and “sink” denotes the power consumer. USB PD addresses this issue through the use of “power rules” that dictate how compliant power sources must behave. Since you don’t know the adapter’s output voltage capabilities, however, you can’t be sure it’s usable. Suppose, for example, you want to power a device that operates on 12V at 2A (24W), and you’ve found an AC adapter rated at 27W. The convenience should not be sacrificed even when supporting multiple voltage levels. Table 1: Normative Voltages and Currents (Checkmark indicates that the current varies according to the PD Power.) And while typical USB Type-C TM cable carries up to 3A current, USB Type-C TM cable which support USB PD is allowed to carry up to 5A-it means the maximum power is 100W. For this reason, USB PD adds support for 9V, 15V, and 20V (see Table 1), so as to work with a much wider range of devices. Higher voltages are needed to enable faster charging and to drive motorized appliances in particular. But many digital devices require more than this. Conventional USB implementations can only provide a 5V power supply.
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