Debug current/voltage measurement

A project log for PocketPD - USB-C Portable Bench Power Supply

Leveraging the Programmable Power Supply (PPS) of USB PD 3.0 and 3.1 to make an ultra-compact bench power supply

centylabCentyLab 02/27/2024 at 04:580 Comments

The prototype version of PocketPD utilizes a sense resistor of 10mOhm to measure the current reading of the load. Here is the configuration for V.Prototype:

The power goes directly from USB-C VBUS, and passes through the sense resistor, thru the PMOS output switch to the banana jack terminals.

Issue: VBUS reading is higher than VOUT, more than expected. There must be more voltage drop somewhere.

Current (A)VBUS voltage (V)Vdrop across PMOS (V)Calculated Ron

It was a bit surprising that Ron is consistent across multiple input voltages as Vgs and Ron are dependent on each other. Further investigation is needed

Conclusion: The resistance of PMOS (68mOhm) is higher than expected (18mOhm) which causes the discrepancy between the OLED measured and the Banana jack output. 


But !... The current reading is now calibrated. We are getting the correct amperage report through the Rsense resistor!