USB Power Delivery: asking for 20 V
After attach, a PD charger lists the voltages it can supply (Source_Capabilities) over the CC wire. The sink requests one, the charger accepts and ramps VBUS. Higher voltage carries the same power with less current, so less is lost in the cable.
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How it works
Charger offers: #1 5 V 3 A, #2 9 V 3 A, #3 15 V 3 A, #4 20 V 3.25 A.
Trigger requests 20 V → waiting for attach.
The same 0 W at 5 V would need 0 A; at 0.0 V it needs 0 A. Cable loss goes as I², so much less at the higher voltage.
Try it on the bench
- Select the Trigger and request 12 V: this 65 W charger has no 12 V PDO, so it offers 9 V and the trigger refuses to switch its output on.
- Change the charger to 20 W (5/9/12 V): now 12 V works.
- Unplug the C → C cable and use a USB charger with a USB-A → C cable: no CC wire, no PD, 5 V only.
Parts on the bench
In the course
Zen’s Electronics Design Mastery, module “Capstone — Connected Tracker”.