Background and requirements

Introduction

USB-C, as well as supporting very fast data transfer, allows for power transmission using the USB-C Power Delivery (PD) protocol. This allows for up to 240W of power delivery at a voltage of 48V and at 5A. Lower voltages and power levels are also possible via negotiation between the power source and load. The widespread adoption of USB-C in the commercial world makes it a convenient option for power low to medium power projects. Such an implementation requires a USB-C trigger device, which communicates with the power source to set the desired voltage and current limit. This is the focus of this project.

USB-C Power Delivery

General Overview

The PD protocol supports standard voltages and currents such as 5V, 9V, 15V at 3A in Standard Power Range (SPR) mode, and 20V, 28V and 48V at 5A in Extended Power Range (EPR) mode. Newer power adapters also support a Programmable Power Supply (PPS) mode which allows the load to select a custom voltage within an allowable range in 100mV or 200mV increments, and even set the e-fuse current limit in 50mA increments.

The exact capabilities of each device vary and are communicated to the load in the form of Power Data Objects (PDOs). These contain information about the various output configurations the source supports. For example, on PDO could be: - 20V
- Max current 3A
This information is communicated to the load for each PDO, including adjustable modes via PPS, and the load then selects the desired output.

SPR, EPR and PPS

Standard Power Range (SPR) typically contains modes up to around 21V and up to 100W. Extended Power Range (EPR) was later added and can support up to 48V and 240W. While possible, it is not common to see 240W adapters, and typical adapters will go up to 140W (28V/5A).

Programmable Power Supply (PPS) mode is available in both SPR and EPR, and allows the user to set a custom voltage and e-fuse current limit within a range and in predefined increments depending on the mode (SPR or EPR).

Common PDOs

PDOs are usually listed on the adapter. The most common ones are: - 5V (varying current limits) - 9V @ 3A -- 27W - 15V @ 3A -- 45W - 20V @ 3A -- 60W - 20V @ 5A -- 100W - 28V @ 5A -- 140W

USB-C Cables

USB-C cables usually also have chips and communicate with the source to communicate their specifications. These include max currents and voltages. This will then inform the PDOs the load receives.

Project Requirements

Use cases

This trigger board is meant to provide quick and simple means of powering a wide range of projects. Hence, it should be designed with universability in mind. This is covered in the subsequent sections.

Power levels

As previously mentioned, maximum power levels available from typical adapters is 140W at 28V. Hence, designing for this max voltage level is safe bet and allows me to use readily available, small parts.

Geometry

The small components are important to reduce the overall size of the device. This trigger board is designed with many use cases in mind. Hence, it should be as small as possible in order to fit in as many projects as possible. A good universally compatible shape to aim for is relatively square. Additionally, mouting holes should be added so that the device can be attached to other boards with stand-offs.

This also means that passives will be chosen to be relatively small in size, while still easily hand solderable. For this project, the chosen package is 0603, with potential for smaller packages in later projects.

Cost

This board should be as cheap as possible in order to allow for the manufacturing of many of these boards for projects.

Complexity

The timeline for the design and manufacture of the board is tight, so it should be kept simple. This means using a PD controller IC and simple MCU.

Robustness

This project should be robust enough to handle high currents and demanding loads.