By GRL Team on September 14, 2021

Introduction to PD 3.1 – the Latest USB-IF Power Delivery Specification

Granite River Labs (GRL)
By Cindy Chang – Test Engineer, GRL (Taipei)

Following on from the Power Delivery 3.0 standard, the USB-IF introduced its latest Power Delivery 3.1 Specification (USB Power Delivery Specification Revision 3.1, Version 1.0) in May 2021. New features include EPR (Extended Power Range), which was added to expand the maximum wattage of the PD 3.0 power supply from 100W to 240W (48V 5A).

The power supply requirements and behaviors are defined in the document, which reviews the PD 3.1 Spec and the new EPR function.


PD 3.1 Power Supply Specification

The PD 3.1 Spec includes the power supply specification previously defined in PD 3.0, as part of its SPR (Standard Power Range). In addition, an optional EPR function was added, which means that products with a maximum power supply wattage of 100W~240W must meet new requirements (See Table 1 below).

In the PD 3.1 Spec, the power supply specification previously defined in PD 3.0 is included in the SPR (Standard Power Range). You can refer to Table 3 in the GRL technical article entitled “Origin and Specification of Power Delivery” for the SPR specification, called SPR PDO and SPR APDO in the PD 3.1 Spec. Its power supply specification and upper limit of 100W remain unchanged.

Please refer to Table 3 in GRL’s technical article entitled the “Origin and Specification of Power Delivery” for detailed information on the SPR specification (known as SPR PDO and SPR APDO in PD 3.1 Spec), including its power supply specification and upper limit of 100W, which remains unchanged.


In the current specification, EPR PDO includes:

  1. Fixed PDO: Constant voltage output. In EPR mode, Fixed PDO refers to the specification with a voltage >20V, including 28V, 36V and 48V.
  2. AVS (Adjustable Voltage Supply) APDO: In EPR mode, the voltage output can be adjusted within a certain range, from a minimum of 15V to a maximum of 28V, 36V and 48V, depending on the wattage (Table 2).
    AVS is similar to PPS in terms of function, but the difference is that AVS does not support Current Limit operation, and the output voltage is adjusted in a unit step of 100mV (20mV for PPS).

圖片1-5-768x247-1Table 1. Specification Supporting EPR Product Power Supply (Refer to PD 3.1 Spec)

圖片2-3-2Table 2. AVS Voltage Range (Source : PD 3.1 Spec)


The following two examples are offered to help readers understand Table 2 (above).

  1. For the maximum output of 140W, the following conditions must be met:
  • SPR Fixed PDO: 5V@3A~5A 、9V@3A~5A、15@3A~5A 、20V@5A
  • EPR Fixed PDO: 28V@5A
  • AVS APDO 15V~28V@140W


  1. For an output of 144W, the following conditions must be met (see the wattage range in the second column of Table 1):
  • SPR Fixed PDO: 5V@3A~5A、9V@3A~5A、15@3A~5A、20V@5A
  • EPR Fixed PDO: 28V@5A、36V@4A
  • AVS APDO: 15V~36V@144W
    * The notation of AVS APDO is different from SPR Fixed PDO or EPR Fixed PDO. This is because the AVS operating current is limited by the wattage and varies with the current operating voltage. Since the current is not a fixed value, the wattage should be referred to instead of the current. See Table 3( below), where the maximum current is expressed in wattage.

圖片3-4-1Table 3. AVS APDO format (Source: PD 3.1 Spec)


In the 144W example, the current conditions of AVS APDO under the operating voltage of 28.8V~36V can be easily understood by referring to Figure 1 (below). The blue area in the figure represents the AVS operable range, which can be divided into two zones:

  1. Zone A, marked in yellow, can operate within the voltage range of 5A and under, but not exceeding the maximum wattage (See the first description in Table 1 - 15V~28.8V@5A).
  2. Zone B, marked in green, is subject to the wattage limit, and the operating current depends on the voltage, so it is expressed by the formula - 28.8V~36V@ (144/AVS voltage) A.

圖片4-6-1024x525-1Figure 1. Diagram of AVS Power Supply Mode


It is worth noting that the areas marked N/A in Table 1 are strictly not supported. Specifically, products below 140W do not support 36V and 48V, and products below 180W do not support 48V.

Another product design is a Shared Capacity Charger, which means that the power supply wattage on the product is shared. When part of the wattage has been used, the wattage available to the remaining ports is the total wattage minus the portion used. Subject to allocation and the actual available wattage at this time, this is called Equivalent PDP Rating. Refer to the table below (Table 4) for setting conditions.

Table 4. Conditions Supporting EPR product in Shared Capacity Power Supply Mode (Refer to PD 3.1 Spec)


Take the following product design as an example: The maximum total output wattage is 250W, and when the 2 ports are used alone they can support up to 160W respectively:

  • If used independently, the specification is (5V@3A~5A, 9V@3A~5A, 15@3A~5A, 20V@5A, 28V@5A, 36V@4.44A, 15V~36V@160W).
  • When one port is used 100W, the Equivalent PDP Rating of the other port is 150W, the power supply conditions are as follows: 5V@3A~5A, 9V@3A~5A, 15@3A~5A, 20V@5A, 28V@5A, 36V@4.16A, and 15V~36V@150W.



The power supply capability is displayed in Source_Capabilities, and the same concept is imported into EPR mode. EPR_Source_Capabilities has been added to PD 3.1 Spec to support and display the specifications of EPR power supply products.

As shown in the following diagram taken from the PD 3.1 Spec, the first 7 groups of Data Objects are filled with SPR PDO content, which must be the same as those in Source_Capabilities. If the SPR PDO has less than 7 groups, then 0 shall be used.

EPR PDO content is entered from Group 8 to Group 13, in sequence from low Fixed PDO voltage to high Fixed PDO voltage, followed by a group of AVS APDO.

圖片6-3-1Figure 2. EPR_Source_Capabilities format (Source: PD 3.1 Spec)


EPR mode process

Before entering EPR mode for power supply, Source/Sink needs to establish an Explicit PD Contract. During this, both parties confirm whether they support EPR mode in the Source Capabilities and Request Messages respectively. This is used as a reference to check the capabilities of both parties before entering EPR mode.

Before entering EPR mode, a communication and check process is required. The steps are as follows:

  • Enter EPR Mode
  1. Sink sends EPR_Mode message, where the Data Object is set as Enter, indicating that communication in EPR mode is required. The EPR_Mode message depends on the content (see Figure 3), which indicates different purposes.
  2. Source checks and confirms that both parties support EPR mode, and the current state is able to support EPR mode power supply. It sends an EPR_Mode message set to Enter Acknowledged, indicating that the current state of Source allows entry to EPR mode
  3. In products with wires, Source must confirm that the wire used can support EPR mode. It also sends a Discover ID Request to confirm that the wire can support a maximum voltage of 50V and a maximum current of 5A.
  4. If all of the above are OK, Source will send an EPR_Mode message set to Enter Succeeded in the Data Object to Sink, and then successfully enter EPR mode and move to the next step, which is “PD Negotiation.”


Figure 3. EPR Mode DO Message (Source: PD 3.1 Spec)

Figure 4. EPR Mode Data Objects (EPRMDO) (Source: PD 3.1 Spec)

PD Negotiate in EPR Mode

  1. Source sends EPR_Source_Capabilities to announce the power supply capability in EPR mode
  2. Sink selects PDO according to requirements, fills in the EPR_Request and sends it to Source
  3. When Source confirms that the requirements can be met, it will send back an Accept signal and, after adjusting the power supply state, transmit PS_RDY to complete this communication

In EPR mode, Source detects the CC state. If it is idle for too long, Source will initiate a Hard Reset. This results in an interruption of EPR mode, so Sink needs to transmit an EPR_KeepAlive message at specified intervals to maintain EPR mode. When Source receives this message, it responds to GoodCRC and EPR_KeepAlive_Ack and resets the timing.


Exit EPR Mode

Source/Sink may want to return to SPR mode due to various factors. However, before this, the voltage must first be lowered to a constant of at least 20V, which can be achieved by the following two methods:

  1. Source sends EPR_Source_Capabilities for re-communication, with the notification that EPR PDO is not included
  2. Sink sends EPR_Request and makes clear in the content that only SPR PDO is required. i.e. that EPR PDO is not included

               After either of these two actions are completed, the voltage should drop to 20V or lower. At this time, either Source or Sink can initiate EPR_ Mode and set the Data Object in the message to Exit, indicating that it will exit EPR mode. When either party sends this message, Source needs to send Source Capabilities within the tFirstSourceCap parameter time to re-establish the PD Contract and return to SPR mode.

Figure 5. Example of the EPR Mode Process (GoodCRC in the middle omitted)


Type-C cable and Connector 更新

The "Universal Serial Bus Type-C Cable and Connector Specification" has been updated to Version 2.1, with the goal of adding EPR functions at all supported speeds.


Table 5. Cable Categories (Source: Type-C Cable and Connector Spec)


EPR Cable

  • EPR cables must contain an E-Marker to clarify this capability.
  • The E-Marker must be set to EPR Mode Capable and provide notification that it supports 50V and 5A.
  • The minimum working voltage of EPR cables must be at least 53.65V.


According to some experiments, the Vbus pin can become damaged under the following conditions:

  • Source: When the current load is suddenly removed, and the voltage changes rapidly.
  • Sink: When the Vbus pin at the receiving end is in a high voltage state for a long time.
  • Cable:
  1. If the Vbus continues to oscillate (i.e within microseconds).
  2. When the current load is suddenly removed within 0.1-1 microseconds, leading to a sudden drop in IR voltage.



In recent years, the USB-IF has continuously developed and launched new schemes to improve PD. The extension of the specification through EPR mode enables this technology to be more widely used in a wide variety of products. However, due to the provision of higher wattage charging modes, there are more restrictions on the specification in EPR mode than the previous PD3.0.

The voltage selection allowed by EPR mode is different than in the past. For example, there is less flexible space for selective support, and products must be enabled in operational EPR mode. These changes are aimed at increasing safety and improving compatibility between products while extending functionality. However, applying this capability to future products will require careful evaluation.



  • USB Type-C® Cable and Connector Specification Revision 2.0
  • USB Power Delivery Specification Revision 1.0 Version 1.2
  • USB Power Delivery Specification Revision 2.0 Version 1.3
  • USB Power Delivery Specification Revision 3.0 Version 2.0
  • USB Power Delivery Specification Revision 3.1 Version 1.0


Cindy Chang – Test Engineer, GRL (Taipei)

A graduate of National Cheng Kung University’s Department of Materials, Cindy Chang has more than three years of Power Delivery testing experience, and is familiar with test specifications for Thunderbolt PD, USB-IF PD Compliance and Qualcomm® Quick Charge™ (QC). Currently, she is mainly responsible for PD testing at GRL (Taipei) and helping customers solve PD problems to successfully obtain certification.


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Published by GRL Team September 14, 2021