New 65W USB-C Reference Design Keeps Its Cool

The adoption of USB-C as a global charging standard is gathering pace, particularly in the EU where it has been made mandatory for many applications. Most manufacturers do not wish to make two separate designs for their products, so if they have any sales in Europe at all, it makes sense for them to implement only USB-C charging. This has led to the rapid growth of the standard around the world, which is only set to increase with Apple expected to announce that it too will include USB-C fast charging in its latest iPhone. The standard is now so popular and convenient that designers want to use it whenever possible, even in products that are out of scope for the legislation.

That rise in demand has created a new market for third-party USB-C chargers. These chargers are made by manufacturers, many of whom would like to stand out from the crowd by adding their own innovation with even faster charging, charging multiple products at the same time or running cooler. In addition to the normal plug-in chargers we use at the moment for our phones and other mobile devices, manufacturers would also like to design products with integrated USB-C charging, such as wall sockets and other consumer devices.

To assist those third-party manufacturers make highly efficient and differentiated designs, power device suppliers have created reference designs to showcase the abilities of their products. Which, in turn, has led almost to a competition to see who can provide the highest efficiency and the most power through the small USB-C connector. Pulsiv is the latest company to stake a claim for the highest efficiency design at the USB-C 65W level with its PSV-RDAD-65USB reference design. Unusually, Pulsiv do not make GaN transistors themselves (in this design, they used devices from Innoscience). The company’s innovation is in the design of the circuit and the tightest control of GaN device switching. I covered the company’s underlying technology in an earlier TechTalk blog here. In short, the charging and discharging of the storage capacitor has been developed to provide accurate power factor correction (PFC), which removes the need for a PFC inductor and allows the storage capacitor to be fitted in parallel with the power supply. The high level of control is achieved through the use of the company’s OSMIUM microcontroller running a dedicated algorithm.

The new 65W USB-C design is the Pulsiv’s first complete power supply design, as up until now it had only launched a front-end solution, allowing designers to develop the back-end to suit their own application. The algorithm in the PSV-RDAD-65USB reference design senses the AC line voltage and frequency, and adjusts the storage capacitor charging time to ensure the circuit draws no line current at the AC zero voltage crossing. This enables a simple half-active bridge implementation to increase efficiency, especially at low line conditions. MOSFETs in the lower half of the AC to DC bridge are carefully controlled in combination with high-side diodes. An industry standard quasi-resonant flyback converter topology has been used for the design.

Dr Tim Moore, Chief Product Officer explained, “We chose a half-active bridge for the additional 0.7 to 1% efficiency. Normally you would be a bit nervous, particularly on the low line, of using a half-active bridge as the currents that pass through can be pretty beefy and can destroy the transistor. We actually think this is the only 65W USB-C design available that uses a half-active bridge”.

OSMIUM technology generates a HVDC output that can vary between the peak AC input and 150V to drive the quasi-resonant flyback at maximum efficiency. A wider voltage range reduces primary side inductance to allow the use of an EQ20 transformer, which was developed together with magnetics experts, Frenetic. The smaller transformer leads to a 20% size reduction and 50% efficiency improvement over the RM8 core that is normally used in other designs.

The PSV-RDAD-65USB reference design shows a significant improvement in thermal performance and reduces critical component temperatures by more than 30% over other designs. At full load, the flyback transformer reaches 33.9°C at 230VAC and 30.3°C at 265V above an ambient temperature of 26.1°C. This will enable 65W fast charging in space constrained environments and/or heat sensitive applications such as in-wall plug sockets.

Moore thinks the lower temperature will be a critical factor in adoption of USB-C in those types of applications. He expands by saying, “Other manufacturers talk a lot about efficiency percentages, which is great from an engineering perspective. However, you have got to concentrate on what the end user wants and understands. If I turn around and say, our power supply has a peak efficiency of 96%, what does that mean to the end user? They want to know how hot the socket gets when using full power, and how long does it last in the wall. If it gets too hot, it is not going to last a great deal of time”.

The next step for the company is to develop a 240W design to cater for newer applications that will come onto the market in the near future, for example, electric lawnmowers. Moore expects that as more people become used to the convenience of USB-C charging, manufacturers will expand the number of products that ‘cut the cord’. It is a market that hasn’t really been accounted for by analysts and one that has the potential to grow very large in the medium term.

www.pulsiv.com