A:Answer The USB-PD 3.0 spec includes more intelligence for controlling amperage and voltage while charging. When adjusting voltage or amperage it can move by as little as 20mV or 50mA steps, which is constantly negotiated every 10 seconds by the source.
To get the full 15 watts from a MagSafe charger, the USB-C adapter has to be able to provide 20 watts because there are losses. The new USB-PD 3.0 spec allows a 20 watt adapter to deliver 9 volts and 2.22 amps (19.98 watts).
Older chargers will not deliver. At best, the provide 5 volts and 3 amps (15 watts) meaning you get about 10-12 watts at the MagSafe charger.
Likewise, plugging this charger directly into the iPhone 12 will deliver 20 watts. Older chargers can only manage about 18 watts, so you will get slightly faster charging with this new charger.
