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Developing AC/DC adaptors for USB Type-C power

Posted: 10 Mar 2016     Print Version  Bookmark and Share

Keywords:USB Type-C  AC/DC adaptors  Power Delivery  flyback topology  TL431 

Now that major players such as Apple and Google have paved the way, 2016 promises to see widespread adoption of the USB Type-C (Type-C) standard. There is little doubt that consumers will be pleased with the smaller connectors, faster speed and reversible connections (no more fumbling around for the correct orientation to insert the cable). However, USB Power Delivery (PD) represents a major evolution, if not a revolution, for AC/DC adaptors.

Type-C PD allows for up to 3A of output current versus only 2.1A available on the legacy USB. There are also provisions for up to 5A of output current with the use of a special cable. This higher output current provides a modest increase in output power with a 5V output. However, Type-C PD also supports adjustable output voltages of up to 20V, meaning that up to 60W can be provided down a standard Type-C cable (100W with the special cable). Long term, Type-C could be used to power much more than our mobile devices. Monitors, lamps and other small appliances might also be powered by Type-C.

Figure 1: Compared to legacy USB (left), USB Type-C (right) connectors are smaller, non-polarized and have the same connector on both ends of the cable.

From the power supply perspective, what does this all mean? The first challenge is how to support an adjustable output voltage. In general, the flyback topology will continue to dominate in adaptors, because of its low cost and tolerance to wide input and output variations. The typical approach is to design power components for the highest output voltage and load current. At these conditions, components will see the highest thermal, voltage, and current stresses. With 3A or more of output current, most adaptors will now favour synchronous rectifiers, as opposed to output diodes to reduce losses and control component temperatures.

While power components are selected at the highest output voltage, be careful to ensure that everything operates normally when the output voltage is at its lowest setting. The biggest challenge here is keeping the primary controller biased. Usually, this means a larger number of turns on the bias winding of the transformer. A clamping circuit likely needs to be added to prevent over-voltage conditions on the controller when the output voltage is set to 20V. The synchronous rectifier and associated driver also must be capable of operation when the output is only 5V.

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