PassMark USB Power Delivery Tester (PM125) - Charger not recognized

[dattatreyaroy]

Hello,

I am trying to test a 10W (5V/2A) USB-C charger (BC1.2 compliant) with the PM125 PassMark device. Unfortunately the device is not recognizing the charger at all. I have tried using different USB-C cables but to no avail. When I charge my phone using the same charger and a USB sniffer, the phone charges and the sniffer detects a BC1.2 protocol (sometimes it also detects a propreitary Apple/Samsung protocol). Also the charger does not have standard pull-up resistors on the CC lines (CC pins are floating) but the D+/- are shorted, thereby making it compliant to BC1.2 and non-compliant to USB type-C norms. Any idea why the PassMark tester system is not able to recognize the charger? Ideally even if the charger isn't USB type-C compliant the tester system should be able to fall back and detect a BC1.2 connection. Or am I missing something here?

Best regards,
Datta

[Nik (PassMark)]

Hello,

A Type-C port that is a power source/provider must output 0V until it detects a valid power sink/consumer device is attached, after which it then applies 5V on the VBUS line. The process to detect a valid power sink is attached involves the power source putting a voltage on the CC line, and so if the CC lines are left floating then this detection method would not work. Could you please let me know what triggers the USB-C charger to switch from an output of 0V to 5V. Is it always outputting 5V on VBUS even when nothing is connected?

[dattatreyaroy]

Hello,

Yes, so the secondary side of the transformer outputs 5V all the time, even if no load/sink is connected to the output (I have also attached the schematic)
I agree with the USB-C detection part, the charger is not USB-C compliant. But if the USB-C detection doesnt work, shouldn't the tester system revert back to BC1.2 detection, as if a USB-A to USB-C cable were connected without the CC line? That should theoretically work here as the D+/- lines are shorted (not shown in schematic).

[Nik (PassMark)]

Hello,

Thank you for the extra information about the charger! Based on this I highly recommend to make some modifications to the design of the charger, as it's likely to cause issues with other devices detecting the connection and is slightly dangerous. This is due to some other small details about how USB connectors work that is not obvious from the specifications:

The reason why 5V can only be output after detecting a power sink device is connected is to prevent the issue of 2 USB-C chargers being connected to each other. If 2 chargers both output 5V and connect to each other, the one with a lower voltage is going to have current flowing into it, which is bad for a power supply and can damage it. So why can USB-A ports always output 5V then? A USB-A port is always the host / power source, and it must connect to a USB-B port which is always a device / power sink. This is why you don't ever see USB-A to USB-A cables used (they exist but are dangerous and non-compliant).

Because USB-C uses the same connector on both sides unlike USB-A, the USB-C specifications had to add this extra signalling on the CC lines and having VBUS 0V to prevent this issue when people accidentally connect things wrong. For the case of USB-A to USB-C cables, the CC line pull-up resistor is inside the cable, and so the CC lines is not floating in this case either. This is why most Type-C devices rely on monitoring the CC line for connection and can still work with USB-A chargers.

To solve this problem the modification that would need to be made to the charger is to either include a pull-up resistor for the CC line to VBUS line, or switch to using a USB-A connector in the charger (USB-A connectors are cheaper and stronger, they just don't support higher voltages / currents). If you do want to test this charger with the PM125 however, you will need to add the pull-up resistor between the CC line and VBUS so that it complies with the USB-C standards and be detected.

[dattatreyaroy]

Hello,

Okay got it. Thank you for the detailed answer.