Get to know this life changing technology: 4 of 5
The USB PD specification allows for delivery of power up to 240W. When power is at these levels, there is always a risk that a defect in the cable or in a device may result in overheating or perhaps even fire. There is concern that the recent influx into the market of many uncertified products may further compromise safety with higher power levels. For these reasons, the USB PD specification includes provisions to minimize the risk of accidents caused by defects or product misrepresentations.
Potential Accident
Safety is an important issue for designers and users of USB charging. Connecting a Micro-B plug upside down, for example, has been known to result in socket damage and burnout. The advent of USB PD makes safety an even more important concern, owing to support for considerably higher voltages and currents.
Cable defects are another concern. Resting a heavy object on a cable, for example, may damage the internal insulation, causing a short-circuit current surge that can cause a rapid rise in internal temperature.
Other types of potential failures must also be considered. As we’ve seen, USB PD allows source and sink to negotiate to select a voltage over the range of 5V to 48V. It remains possible, however, that an error along the way may result in an unexpectedly high voltage applied to the VBUS, potentially leading to overheating or fire.
Yet another concern is the recent influx into the market of counterfeit products—many of which fail to meet the relevant safety standards. These devices may operate correctly at the conventional 5V, but create problems as voltages go higher. There are, in fact, reports that the use of such products has resulted in device damage, overheating, and fire. Because these devices misrepresent themselves as quality branded products, customers can often be misled into buying them.
Designers of USB PD devices must therefore take precautions against a wide range of hazards—including those that might result from socket or cable damage, or from connection to counterfeit components. The USB PD standard itself implements a variety of protections to counter these hazards. Devices that are truly USB PD compliant can therefore be trusted to operate at a high level of safety.
Negotiation Determines Voltage and Current
Let’s consider how USB PD charging works, assuming that the device to be charged is connected by USB Type-C® cable to an AC adapter. In particular, let’s see how this scenario successfully sidesteps some of the problems indicated above.
First, notice that there is no danger of causing damage by inserting the plug upside down, since USB Type-C plugs are interchangeable. So, USB Type-C connections eliminate this problem completely.
Figure 1: USB 3.1–compliant USB Type-C cable includes e-Marker that returns information about the cable’s specifications, its manufacturer, and more.
In our previous session, we presented a table showing combinations of USB PD power, voltage, and current. (Please refer to chapter (2) USB PD: The Technology 1, Table 1). The table shows that in cases where the voltage is 20V and the power is above 60W up to 100W, the current is limited by the cable specifications (see the marked “2” in Table 1). This compliance information and cable specification is retrieved from e-Marker built into the cable itself.
The source and sink enter the negotiation process to determine the appropriate power conditions. At the negotiation process, in accordance with the Power Rules—taking into account the source’s capability, the sink’s required voltage and current, and the cable. After connection, VBUS Voltage will be 0V until Source-to-Sink attach is detected. The VBUS Voltage will be 5V from the time a Source-to-Sink attach is detected until negotiation is completed. Once negotiation is completed, the voltage is at the agreed-upon level and USB PD power supply begins.
Protection Circuitry
Where the cable is USB PD-compliant and the equipment is working properly, the Power Rules will ensure that the charge parameters are set within the limits of permissible voltage and current. But things are not quite safe yet, as there is still a danger that a defect in the cable or a connector may lead to operational errors, device damage, or a potential fire hazard.
Under USB PD, problems might potentially arise with any of three parameters: voltage, current, and temperature. This is why the USB PD standard also provides for the capability to detect such problems—so that operation can be stopped, or conditions adjusted, before equipment damage or fire can arise. The USB PD specifications provide for overvoltage protection (OVP), overcurrent protection (OCP), and over-temperature protection (OTP); power supply must be reduced when any of these conditions are detected. Adherence to USB PD specifications, therefore, helps ensure that effective protection is in place.
Authentication (C-AUTH)
Certified USB logos on cables and AC adapters indicate that these products are certified by the USB-IF (USB Implementers Forum). Use of certified products ensures that OVP, OCP, and OTP are implemented at both the source and the sink. The cable’s e-Marker will also provide information about the cable’s specifications.
But there remains yet another potential hazard: the unwitting use of counterfeit products. Counterfeits may carry certified USB logos even though they are not in fact certified. Purchasers can not be sure that a product is legitimate simply by looking at the logos. And the ID information returned by a cable’s e-Marker may also be counterfeited—an inferior cable may store and return ID data copied directly from a high-quality model.
| C-AUTH Result | Sink's Behavior |
|---|---|
| Failed authentication | Request up to 500mA at 5V |
| Passed authentication; but not on whitelist | Request up to 500mA at 5V |
| Passed authentication; on whitelist | The device is authenticated and trusted. The sink can trust e-Marker information, etc. |
Therefore, the USB-IF has established USB Type-C Authentication (C-AUTH)—a means for authenticating devices and thereby protecting against data tampering and illicit use. The USB PD specification implements C-AUTH through use of a public key infrastructure (PKI) which is a time-proven approach in the Internet world. We will not go into details of PKI implementation here; C-AUTH proceeds that an initiator (laptops, tablets, etc.) reads out a certificate chain from a responder (cable, AC adapter, etc.), after that, the initiator verifies a certificate. Providing an external database (whitelist) containing the status of compliance tests allows the initiator to authenticate the responder and then check the whitelist to ensure that the responder is included in the list. This double checking process—authentication and whitelist—makes it possible to reliably identify devices and their compliance status.
In addition, C-AUTH can cover not only sources and sinks, but also cables. This makes it possible to build a more secure system.
Supplement: Other Charging Methods
All compliant USB charging method using USB Type-C connectors and cables are conventional USB battery charging, in accordance with USB Type-C and USB PD. Any other use, however, is not provided for under USB specifications. While numerous mobile phone vendors had adopted non-standard rapid charging methods, these are not permissible in USB Type-C implementations. And while early USB Type-C specifications allowed for the current to be increased while holding VBUS at 5V, this was no longer permitted—and devices using this approach were to be phased out. From the beginning, USB standards prohibited VBUS voltages in excess of 5V. It is only with the advent of USB PD that use of higher voltages has become permissible.
In our next session, we will introduce some of the Renesas products that currently implement these USB PD requirements.
Module List
- USB Power Delivery (1) Enhanced Convenience in USB Charging
- USB Power Delivery (2) The Technology 1 - Convenience and Safety
- USB Power Delivery (3) The Technology 2 - USB Type-C and Role Swap
- USB Power Delivery (4) USB PD Safety Implementation
- USB Power Delivery (5) Faster Development with Renesas Solutions