USB Power Delivery: The Technology 1 - Convenience and Safety

Get to know this life changing technology: 2 of 5

The USB PD specification enables up to 240W USB power supply, sufficient to drive large devices such as laptops and monitors. This specification increases convenience, with provisions included to ensure safety. Here we look at some of the technologies that realize these features.

New Capabilities

USB PD3.2 (below, “USB PD”) allows to realize up to 240W power delivery through a single cable. By this specification, it is possible to supply power over USB to multiple devices simultaneously, eliminating the need for a dedicated AC adapter for each device. It also provides two other important functions: FAST ROLE SWAP (FRS) allows PC's power role and USB Hub's power role to switch immediately; and PROGRAMMABLE POWER SUPPLY (PPS) and ADJUSTABLE VOLTAGE SUPPLY (AVS) makes possible for rapid charging without overheating. Below, we look at USB PD features, and explain the new capabilities.

Not Just 5V Anymore

Legacy USB can only provide a 5V power supply. But many digital devices require more than this. Higher voltages are required to enable faster charging and to drive a motor in particular. To share the USB AC adapter by a wider range of devices, USB PD defines normative voltages such as 9V, 15V, 20V in addition to 5V. Also, if products support Extended Power Range (EPR) expansion commands, it can use 28V, 36V, and 48V as shown in table 1. Sharing a USB AC adapter is possible if the devices are designed to work according to normative voltage.

And while standard USB Type-C^® cable carries up to 3A current, USB Type-C cable which supports 100W power delivery or EPR is allowed to carry up to 5A, —it means the maximum power is 240W.

¹ Checkmark indicates that the current varies according to the PD Power. Current value (A) calculated by PD power/target voltage. Maximum value is 3A.

² Current value (A) calculated by PD power/target voltage. The maximum value when using a 100W (5A) cable is 5A, but the maximum value when using a 3A (Standard) cable is limited to 3A.

³ Current value (A) calculated by PD power/target voltage. The maximum value when using cable that supports EPR command is 5A, but the maximum value when using a 3A (Standard) cable is limited to 3A @20V and even when using 100W (5A) cable, the maximum value is limited to 5A @20V.

"Power Rules" that realizes sharing of AC adapter

Suppose, for example, you have a device that works on 24W, and you’ve found a USB Type-C AC adapter rated at 27W. Since you don’t know the adapter’s output voltage capabilities, however, you can’t be sure it’s usable. USB PD addresses this issue through the use of “Power Rules” that dictate how compliant power sources must behave.

Note that in the discussion below, “Source” denotes the power supplier, and “Sink” denotes the power consumer.

In Power Rules, requires that compliant devices be designed to guarantee the operation in cases where the Source power (W) ≥ Sink power (W). This is the concept that if you can know in advance the power supply capacity of a Source, you can develop a Sink that works according to that power supply capacity. In the previous example, the Sink requires less power than the Source AC adapter, so it must be able to work correctly. Figure 1 is a seamless graph of Table 1.

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Source PDP Rating: PD power (W) Rating that the source can supply

Figure 1: Normative voltage vs Source PDP Rating

For example, a Source indicated as 50W can supply 5V, 9V, 15V or 20V as normative voltage, and can deliver 3A@5V, 3A@9V, 3A@15V, 2.5A@20V. If a Sink is mentioned as being able to work at less than 50W, it must be able to operate by at least one of these combinations.

AC adapters that comply with USB PD3.1 and earlier specifications require to support it as option if they need, for example, 12V output in addition to the normative voltage. On the other hand, AC adapters compliant with USB PD3.2 are now required to support SPR AVS, so products with a Source PDP larger than 27W in Figure 1 can provide voltage that is 9V or higher. So, it is possible to supply 12V output as a part of normative voltage range. However, even if you bundle an AC adapter that supports optional voltages or USB PD3.2, the sink must also be able to work at normative voltage (Not AVS) to share the AC adapter.

In order to draw a current exceeding 3A, it is necessary to use a 100W(5A) or EPR compatible cable with lower loss. 100W(5A) or EPR compatible cables are always implemented an identification component called an e-Marker. For Sources with source PDP more than 60W as shown in Figure 1, Source communicates with the e-Marker to check cable compatibility before initiating power delivery. If the cable cannot be identified as a 100W (5A) or EPR compatible cable, the maximum current will be limited to 3A, so even if the Source PDP is 100W to 240W, the supplied power will be limited to a maximum of 60W.

FRS (Fast Role Swap) that immediately swaps between “power provider” and “power consumer”

As mentioned in a previous session, Fast Role Swap (FRS) capability is a technology that is expected to greatly change the power supply by using USB. It allows the relationship between Source and Sink to be switched in a short period of time. Suppose, for example, a laptop PC being powered by a USB hub with an AC adapter, as shown in Figure 2. If the USB hub loses AC adapter in this state, the laptop can continue operating using the battery inside PC, but the hub itself will stop operating. At that time, If PC are communicating with another device via the hub, that communication will be incomplete. FRS is a mechanism that exchanges the roles of source and sink in a short period of time (within 150 μsec) and keeps the communication via the USB hub.

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Figure 2: Change in power feeding direction by FRS

Please refer to USB Power Delivery (3) The Technology 2 - USB Type-C and Role Swap about “Role Swap” in detail.

Programmable Power Supply (PPS) and Adjustable Voltage Supply (AVS) —Reduced Conversion Loss

As shown in Figure 3, a preferred way to charge a lithium-ion battery, for example, is to begin with constant current charging (maintaining a fixed current while gradually increasing the voltage) and then change to constant voltage charging (maintaining a fixed voltage while gradually reducing the current).

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Figure 3: Charging pattern of lithium-ion battery (changes in voltage and current)

When charging a lithium-ion battery with an AC adapter that outputs voltage level is fixed, the AC adapter cannot directly realize the charging pattern of the lithium-ion battery.

For example, if you connect a USB AC adapter that can constantly output the current required in constant current charging mode and the voltage level required in constant voltage mode to a lithium-ion battery charger, the charger converts the input power into voltage and current according to the charging pattern to charge the lithium-ion battery. In this case, during constant current charging mode, a large potential gap occurs between the voltage applied to the lithium-ion battery and the fixed voltage supplied by the AC adapter. In general, this gap is converted into heating, resulting in a reduction in charging efficiency.

PPS (Programmable Power Supply) is an optional feature of USB-PD that allows source to change the voltage (in 20mV steps) and current (in 50mA steps) in the range from 5V to 20V. If a Sink is connected to a PPS capable Source, it can request the Source to change voltage and current with small steps. As a result, the AC adapter can output the charging pattern with the voltage and current waveforms as shown in Figure 3, reducing heating and conversion loss.

On the other hand, AVS (Adjustable Voltage Supply) was standardized for a simpler variable voltage source, rather than something as complicated as PPS. In fact, AC adapters do not usually support constant current operation, which increases costs if support PPS. AVS has specifications that allow the output voltage to be changed in 100mV steps in the range from 9V to 48V.

Ensure safety through compliance with international standards (IEC 63002) and Authentication

USB PD also realizes communication between power supplies and devices as defined by the international standard (IEC 63002). Under international agreements, products procured by governments and some public companies are required to comply with international standards, and products that use USB PD can gain an advantageous position when it comes to procurement.

In addition, when the product is certified by USB-IF, the organization that develops the USB specifications, you can obtain permission to use the USB logo mark as a certified product. It will be able to appeal into market. On the other hand, just by looking at a product, you may not tell whether it is truly a certified product or not. Device-to-device authentication (USB Authentication /C-AUTH) was introduced as a mechanism to recognize certified products by system level.

In our next session, we will look at USB Type-C connection sequence and Role Swap to improve USB flexibility.

Module List

1. USB Power Delivery (1) Enhanced Convenience in USB Charging
2. USB Power Delivery (2) The Technology 1 - Convenience and Safe
3. USB Power Delivery (3) The Technology 2 - USB Type-C and Role Swap
4. USB Power Delivery (4) USB PD Safety Implementation
5. USB Power Delivery (5) Faster Development with Renesas Solutions