65W quasi-resonant GaN charger reference design

August 07, 2020 //By Nick Flaherty
65W quasi-resonant GaN charger reference design
A 58cc quasi-resonant 65W GaN charger reference design from GaN Systems has a power density of 18.5W per cubic inch.

GaN Systems has launched a reference design for a high power density 65W QR (quasi-resonant) GaN charger for the consumer electronics market, including mobile phone and laptop computer applications.

The reference design supports USB power delivery PD3.0, PPS, QC, QC4.0+ and BC1.2 and consists of an operating charger and design documentation, providing a complete and simple to implement solution that assists customers in accelerating product development, roll out, and commercialization. 

The design, based on gallium nitride (GaN) provides an output of for USB-C 5V3A, 9V3A/12V3A/15V3A/20V3.25A from a 90-265V, ~ 50/60Hz input.

The 58cc design has a power density of 18.5 W/in3 and includes OVP, OCP, SCP and open loop protection. It uses the GS-065-011-1-L 650V 150mΩ GaN transistor

The Quasi-resonant design with synchronous rectification is a Discontinuous Current Mode (DCM) flyback topology with a valley switching turn-on to minimize the switching turn-on loss. It is also known as a variable frequency Flyback and is largely used in low power switch mode power supply SMPS applications such as charger, adapter and auxiliary supply.

This has the advantage of lower turn-on loss in the valley, so the losses due to the discharging of parasitic capacitor are significantly reduced. This makes the QR design efficiency higher especially high line input compare to a conventional fixed frequency flyback. There also less conducted EMI as the ripple voltage appearing across the bulk capacitor, means the switching frequency of QR Flyback is modulated at twice the mains frequency. This causes the spectrum to be spread over the wide frequency band rather than a single fixed frequency.

All of this leads to lower system cost with only one high voltage transistor on the primary side which reduces drive and transistor compared to other topologies such as LLC or Active Clamp Forward (ACF). However, it is still effectively a DCM flyback converter, so peak and RMS current remains higher compared to Continuous Current Mode (CCM) design.

The reference design makes it

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