How does USB PD determine power levels? Negotiation mechanisms and design pitfalls

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Design Interface Parade Technologies

What is USB PD, and why does it negotiate power?

USB Power Delivery (USB PD) is a standard for dynamically controlling voltage and current in USB Type-C connections to supply optimal power to each device. Traditional USB power supply was very simple; the voltage was basically fixed at 5V, and power supply began immediately upon connection. However, recent devices have vastly different power requirements. For example, smartphones and laptops require significantly different amounts of power, and 5V may not be sufficient in some cases.

USB PD was introduced to solve this problem. USB PD allows the use of multiple voltage levels, such as 9V, 15V, and 20V in addition to 5V, making it possible to handle higher power. However, when there are multiple voltage and current options like this, it is necessary to decide in advance which power to use. In USB PD, the power source presents information on the power it can supply, and the device (sink) requests the power it needs, so that power is supplied under conditions agreed upon by both parties. This exchange is called "negotiation".

In other words, USB PD doesn't automatically power on when connected; rather, the optimal power level is determined by agreement between the devices.

Why does USB PD start at 5V?

With USB PD, power is determined by negotiation, but it doesn't immediately supply high voltage (9V or 20V) the moment it's connected. Power always starts at 5V. So why is it designed this way?

■ Reason ①: To ensure secure connection for all devices
USB Type-C is characterized by the fact that various devices can be connected using the same connector. However, not all connected devices necessarily support USB PD (Power Delivery). If a high voltage is output from the start, power will be supplied to devices that are only designed for 5V, which could cause malfunction or damage. Therefore, it is designed to start supplying power at 5V, which is safe for all devices.

■ Reason ②: Because you connect without knowing the other party's capabilities.
Immediately after connecting, it's not yet clear how much power the other device can handle. USB PD exchanges the following information through communication:
Power supply side (Source): What voltage and current can be supplied?
Device side (Sink): What power does it require?
However, if a high voltage is applied before this exchange takes place, it cannot be safely controlled. Therefore, the procedure is to first connect at 5V and then increase the voltage through subsequent negotiation.

USB PD Negotiation Process

So far, we've learned that USB PD is a "mechanism that determines power through negotiation," and that it starts at 5V. But how exactly does this "negotiation" take place?

The operation of USB PD can be broadly understood as being divided into two parts: "processing immediately after connection (Type-C stage)" and "processing to determine power (USB PD stage)".

Connection and 5V power supply (at the Type-C stage)

With USB Type-C, the connection status and role are determined immediately after the connector is connected. Specifically, the following processes take place:

・Connection detection (Attach)
- Determining the connection direction (CC1 / CC2)
- Determining the power source and the sink.
- Start of 5V power supply

At this stage, USB PD negotiation has not yet begun; the purpose is simply to establish the initial state necessary for a secure connection.

Negotiations to determine power levels (USB PD stage)

Once the connection and 5V power supply are complete, negotiation via USB PD begins. USB PD exchanges power-related information via communication on the CC line to determine the final voltage and current. Importantly, the voltage is not determined unilaterally, but rather gradually through communication between the Source and Sink.

In USB PD negotiation,
　
• Source indicates the available power supply.
The sink selects the necessary power from among them.
- The voltage will switch after an agreement is reached.

The power is determined through this process. After Accept, the Source side performs a voltage switching process (Power Transition), and PS_RDY indicates the completion of this process.

Why doesn't the voltage increase with USB PD?

A common issue with USB PD is that the voltage doesn't rise above 5V. This behavior is easily understood when you consider the negotiation process described earlier. In USB PD, the voltage is determined as a result of negotiation, so if the process stops midway, it won't proceed any further and will remain at 5V.

The important thing is not to think that "USB PD is not working," but rather to look at "how far the process has progressed." For example, by looking at each stage of the negotiation, the troubleshooting can be organized as follows:

Negotiations have not even begun.
No electricity request has been made.
- The conditions do not match and an agreement has not been reached.
- The voltage switch did not activate after the agreement was reached.
- The transition to the final state is not complete.

The key point here is that "if 5V is being output, the connection is functioning correctly." USB PD operation consists of two stages: "connection and 5V power supply → voltage determination through negotiation." Therefore, "remaining at 5V" can be seen as the negotiation process being stopped midway.

Therefore, when troubleshooting USB PD issues, it's important to focus on "where it's stopping" rather than "whether it's working or not."

Common pitfalls in design

As we've seen, USB PD problems occur during the negotiation process. Therefore, it's crucial to understand in advance where problems are likely to occur during the design phase. The main points where people often stumble in USB PD design can be summarized into the following four:

■ First: CC Line Design
In USB Type-C, the CC line is used for connection detection and initiating communication. If there is a problem here, the negotiation itself will not start.

■ Second: PDO settings
If there is a mismatch between the power requirements set by the Source and the power requirements selected by the Sink, the negotiation will not be successful in the first place. Many cases where "the voltage does not rise" stem from this issue.

■ Third: PD communication (negotiation) itself
Since USB PD relies on communication over the CC line, messages may not be exchanged correctly if there are problems with signal quality or timing.

■ Fourth point: Cable constraints
USB Type-C cables have current capacity limitations; for example, a 3A-rated cable cannot supply more power if more is required.

In short, we can summarize that "USB PD problems stem from one of the following: circuitry, settings, communication, or cable."

Summary: The key to USB PD design is knowing "where to stop".

USB PD may seem complex at first glance, but it's actually quite simple. It works in the following sequence: connection → 5V power supply → negotiation → voltage determination, and any problems are simply occurring somewhere along this process.

Therefore, the important thing is not to think that "USB PD is not working," but rather to look at "how far it has progressed" and isolate the problem. Having this perspective makes designing and debugging USB PD much easier.