Sizing the PSU for USB Type-C Power Pass-Through

It may appear that some USB Type-C docking stations, hubs, and adaptors do not work in applications requiring USB Type-C Power Delivery Pass-Through. This is the case when an undersized Power Supply Unit (PSU) is used. All of the power (host and device) in the USB Type-C PD connection, even the device with the PD Pass-Though port, must be considered when sizing the source (PSU).

For this reason, a given PSU that worked just fine with the PC it came with does not appear to work when used on a PD Pass-Through port device. Many PC comes with a 45W Type-C PSU that will not work when connected through a PD device that itself reserves 15W for its functionality, leaving only 30W for the original PC. In this example, it was noted that some PC (Dell) will establish a PD contract, but pop-up warnings that an undersized PSU is being utilized and battery power will also be needed. Other PC (HP) will give similar pop-up warnings, and even establish a PD contract, but they will not actually accept any power from the PSU PD Pass-Through port and use will use their battery power exclusively.

For this example, a 60W power supply was needed with the device PD Pass-Through port. None of Targus' devices with PD Pass-Through appear to be affected since they do not reserve as much power as others tested.