FEATURE INTERCONNECTION Could the USB interface be truly universal?


Yong Ang, strategic marketing director at ON Semiconductorlooks at ways of meeting the challenges of USB PD through more efficient, integrated synchronous rectifier controllers


W


e are rapidly approaching a time when the USB interface could be


truly universal; not only able to deliver power and data through the same cable, but to do it in a way that makes almost all other forms of physical interconnection between consumer devices unnecessary, at least technically if not commercially. The specification for USB 3.x is perhaps


more ambitious than any other standard that has preceded it and with ambition come challenges. In this case they fall into two areas, maintaining signal integrity for the high-speed (up to 40Gbit/s) data, and delivering more power (as much as 100W) while providing a way for two connected devices to arbitrate about the sourcing and sinking of that power. These challenges are compounded by the fact that the Type-C connector is also universal, in that it can be inserted either way up (or down, come to that). This will create demand for new solutions that provide flexible power, high-speed data line and ESD protection, high-speed re-drivers and controllers that support USB 3.1 Gen1/2 speeds and power delivery. Many of these devices can be inserted into the cable itself to create so-called Active Cables, meaning manufacturers will also need access to packaged solutions housed in sub- millimetre packages with extremely low (less than 1pF) capacitance, such as those now offered by ON Semiconductor. Data aside, power is going to be a


significant aspect of the Type-C landscape, as it allows any two devices to decide, between them, if one is going to provide power to the other and at what level. The scenario often cited is a fully charged mobile phone providing power to a battery-depleted laptop, while also allowing the laptop to view the mobile phone as a mass storage device. In the near-term, the expectations are


that most of the high-end laptop power adapters manufactured over the next two years will be able to adapt their output based on the demand presented by the device, using the Power Delivery (PD) features of the USB3.1 specification. A key element of a PD-enabled power


adapter will be its ability to vary its output, as the specification allows for up to 100W of power (5V at 20A) to be delivered over the cable. However as this


14 JUNE 2018 | ELECTRONICS


isn’t a fixed output, the adapter needs to implement a new kind of power conversion and regulation, with extreme efficiency in both the primary and secondary side regulation stages.


Figure 1:


A classic Flyback based fixed output voltage power adapter using diode rectification


delay is particularly critical in a Flyback based converter topology designed to operate in Continuous Conduction Mode (CCM), as will be the case with USB PD power adapters. This is because, in CCM, the current flowing through the secondary side of the transformer never reaches zero before the FET on the primary side turns on; a delay in turning off the secondary side FET could lead to shoot-through, effectively a short-circuit between primary and secondary sides, exposing the power components to potentially damagingly high current. In a bid to further improve efficiency,


designers are turning to Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs), which are able to switch even faster than MOSFETs. However, the drive requirements for GaN HEMTs are different from MOSFETs and so the Synchronous Rectification controllers need to support this. As well as designing the controller to


In an effort to improve the general


efficiency of power converters, the industry in general has adopted a synchronous rectifier topology in secondary side regulation. In practice, this involves replacing the diode (operating as a solid state switch, as shown in Figure 1) with a much more efficient FET (also operating as a solid state switch, Figure 2), to rectify the AC voltage from the secondary side of the Flyback power transformer to a stable DC voltage at the adapter’s output terminal. At low output voltages the voltage drop across the diode becomes significant in overall efficiency, using a Schottky diode reduces this drop from around 0.7V to 0.3V, however replacing the diode with a FET with low on-resistance can further improve efficiency. To tackle the inherent complexity, power semiconductor manufacturers now offer Synchronous Rectifier Controllers, that drive the FET, turning it on and off at the right times. The controller will introduce some delay


in this respect, and the ideal would be to achieve zero delay. Although physics will not allow zero delay manufacturers are striving to achieve the shortest turn-on and turn-off delay possible. The turn-off


Figure 2:


A typical USB PD power adapter with Synchronous


Rectification design using a FET as the output switch


switch quickly, an effective way of minimising switching delays is to implement an efficient way of sensing the voltage drop across the transistor, which can be achieved using additional external components (which can itself introduce delays due to the resistance and capacitance of the components) or to use Direct Sensing (DS), by which the controller can directly monitor the load current as a function of the voltage drop across the transistor. In order to implement DS, however, the controller needs to be designed to withstand the high voltages that can appear across the transistor. A third consideration when selecting a


SR controller is the turn-on delay. When the load demands more power the primary side controller will supply it, however the secondary side also needs to respond to this demand by turning the transistor on as quickly as possible, to avoid the load current flowing through the body diode of the transistor rather than its channel. Portability and power-sharing are


coming together under the latest iteration of USB, which will see new and simpler ways of connecting and powering portable devices.


ON Semiconductor www.onsemi.com T: 01628 244326


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