Continuously improving the energy efficiency of equipment is a never-ending goal. The unique high thermal conductivity, high breakdown field strength, high saturation electron drift rate and other characteristics of the third-generation wide bandgap (WBG) semiconductor materials can not only meet the requirements of modern Electronic technology for high temperature, high power, high voltage, high frequency and resistance to The latest requirements such as radiation can also reduce the energy loss by more than 50%, and the maximum volume of the device can be reduced by nearly 75%. Wide-bandgap semiconductor materials represented by gallium nitride (GaN) and silicon carbide (SiC) have begun to challenge the dominance of silicon (Si) materials in the RF and power fields.
The forbidden band width of SiC is 3eV, and SiC-based MOSFETs are very suitable for high breakdown, high power and other applications operating at high frequencies. Compared to Si, parameters such as RDS(on) vary less with temperature, allowing wider operating boundaries in the design, providing additional performance.
GaN has a higher band gap (about 3.4 eV) and higher electron mobility than SiC. Its breakdown field strength is 10 times that of Si, the electron mobility is 2 times that of Si, the output charge and gate charge are both 10 times lower than those of silicon, and the reverse recovery charge is almost zero, which is the key to high-frequency operation. Arguably, GaN is the most suitable technology in modern resonant topologies.
Figure 1: GaN is especially suitable for high frequency switching applications, while SiC is more suitable for high temperature applications
Image credit: Infineon
The fast “fire” GaN charger market
In theory, GaN has greater technical advantages than traditional silicon MOSFETs, and more and more “players” have joined. As prices continue to drop, GaN devices are emerging as strong contenders in the power market. Although the existing GaN market is small compared to the $32.8 billion silicon power device market, according to market research firm Yole Developpment, GaN devices have begun to penetrate many applications. If it is the most eye-catching or the most popular application in the market, it is none other than GaN chargers.
Figure 2: Market research firm YoleDeveloppment predicts that the application of GaN technology in the field of fast charging and wireless charging is likely to quickly detonate the GaN power device market
Image credit: Yole Developmentpment
In February this year, Xiaomi launched a 65W GaN charger with a cross-sectional area only slightly larger than a one-yuan coin and a weight of about 82 grams. Its physical size is much smaller than existing chargers, making it very portable. But its charging speed is super fast, and it only takes 45 minutes to charge a large 4500mAh battery from 0 to full capacity.
In fact, there is a reason for the popularity of the GaN charger market. Presumably many consumers have had such a painful experience. Every time they travel, they must carry multiple chargers and adapters for mobile phones, tablets and laptops. The limited luggage space is occupied by these “must haves”. How to solve this problem? The standard of unified charger and adapter should be the most thorough solution, and it seems that the operability is not high at present. The only realistic approach is to increase the power density and minimize the size of these chargers and adapters.
Small size and fast charging are the biggest advantages of GaN chargers. Once they are available, the market quickly accepts this product. According to statistics, at CES 2020 at the beginning of this year, GaN chargers can be seen everywhere, and there is a tendency to spark a prairie fire.
Figure 3: Commercial application of GaN devices
Image source: Xiaomi
Design Example of a GaN Charger
Behind the rapid commercialization of every new technology, there will always be many technological “promoters”, and GaN chargers are no exception. As the market heat continues to heat up, various GaN manufacturers are also stepping up to introduce new solutions to meet market needs. Below we introduce a few typical design examples for your reference.
GaN charger solution based on Infineon CoolGaN?
Infineon is the only company in the industry to master all power semiconductor technologies such as Si, SiC and GaN, and its CoolGaN™ 600 V enhancement mode HEMT (High Electron Mobility Transistor) is specially optimized for fast turn-on and High energy efficiency and high power density can be achieved in Mode Power Supplies (SMPS) with the highest figure of merit (FOM) of all 600V devices currently on the market. CoolGaN™’s output charge and gate charge are both 10 times lower than those of silicon, and the reverse recovery charge is almost zero, which is the key to high frequency operation. The device has very little output capacitance, provides excellent dynamic performance in reverse conduction, and greatly increases operating frequency, thereby increasing power density by reducing the overall size of passive components.
In GaN chargers, in order to achieve a breakthrough in power density, the typical power topology used in such systems is currently the flyback power conversion topology. Infineon CoolGaN? products use this topology, which has a great breakthrough in power density, up to 20W/in3 (maximum output power is 65W).
The asymmetric flyback power conversion topology recommended by Infineon in its GaN charger white paper is shown in Figure 4, which utilizes the magnetizing current and the ZCS (Zero Current Switching) of the synchronous rectifier switches to form the ZVS (Zero Voltage Switching) of the primary side half-bridge switch), laying the foundation for achieving the highest conversion efficiency.
Figure 4: Schematic diagram of synchronous rectification asymmetric PWM flyback
Image credit: Infineon
Based on the above topology, Infineon has developed a 65W prototype using 500v/140mΩ MOSFET, which supports USB-PD, the output voltage ranges from 5V/3A to 20V/3.25A, and the operating frequency ranges from 100kHz to 220kHz. The prototype has a maximum efficiency of 94.8% and a minimum full-load efficiency of 93% at Vin=90V.
Navitas GaNFast? Power Chip
Through the dismantling of some geeks, we know that the GaNFast gallium nitride power chips NV6115 and NV6117 launched by Navitas are also the key components in the Xiaomi 65W GaN charger mentioned above. A breakthrough has been achieved in power density. The high-frequency switch formed in conjunction with ON Semiconductor’s NCP51530 (700 V high and low-side driver) and Texas Instruments’ UCC28780 (flyback controller) greatly reduces the size of the transformer in the charger.
Navitas’ NV6115 and NV6117 are 650 V GaNFast power ICs optimized for high frequency soft switching topologies. Extends the functions of traditional topologies such as flyback, half-bridge, and resonant to simplify the design of high-frequency, high-energy-density switch-mode power supplies.
Figure 5: Schematic diagram of typical application of NV6115
Image credit: Navitas
According to statistics, there are about 50 mobile fast charging products and platforms using GaNFast on the market, including wall chargers that are being mass-produced by top OEMs, with power ranging from 24W to 300W. Samsung, Oppo, Xiaomi and Verizon are all Navitas customers. Chargers with GaNFast are USB-C capable, providing enough power not only to quickly charge laptops, but also to charge smartphones, tablets, headsets, and VR headsets and all other mobile devices quickly and safely.
Indispensable support element
To design a complete and efficient GaN charger, in addition to the above-mentioned GaN devices, some peripheral components that play a key supporting role are also indispensable.
As can be seen from Figure 4, there is an electrolytic capacitor in the center of the schematic, which is a resonant capacitor here. In an asymmetric flyback converter, energy storage and energy transfer from primary to secondary are shared between the resonant capacitor and the transformer. Therefore, the size of the transformer can be greatly reduced. As mentioned in Infineon’s CoolGaN?GaN charger solution, high-quality capacitors are also key components in the design of GaN chargers, such as chip capacitors used in charger output filtering. Small-sized capacitors are very helpful The size of the charger is reduced.
This view of Infineon is in line with market trends seen by KEMET’s technical experts. In a recent video interview with “Big Coffee in DK”, KEMET’s technical experts revealed that two KEMET polymer tantalum capacitors were used in the 65W GaN charger launched by Xiaomi and used in parallel for output filtering.
?Click here to view the video interview
Compared with the solution of using polymer aluminum electrolytic capacitors for output filtering in traditional chargers, the most prominent advantage of using polymer tantalum capacitors is that the volume is smaller – it is estimated that it can be reduced by 75%, which is also required by GaN chargers. product characteristics are consistent. There are also some charger solutions that use high-capacity chip ceramic capacitors in parallel for output filtering, but as the voltage increases, the ceramic capacitors will show a certain DC bias characteristic, which affects the final effect. Due to the use of multiple capacitors in parallel, the PCB footprint and overall system cost will also be increased.
Compared with traditional manganese dioxide tantalum capacitors and other types of capacitors, other advantages of polymer tantalum capacitors include: higher withstand voltage, improved derating characteristics, safer, stable capacitance, etc. It has become an important technical option in the design of GaN chargers.
Of course, in addition to the output capacitors of GaN chargers, some safety capacitors are also used at the input end, and KEMENT also provides corresponding products. KEMET summarizes some capacitor products that can play a role in GaN chargers, including:
T521 Series High Voltage Polymer Capacitors
A750 Polymer Aluminum Electrolytic Capacitors
CAS safety chip ceramic capacitors and R46 series X class safety capacitors
X7R Chip Ceramic Capacitors
The product details related to the above GaN chargers can be obtained on the Digikey website, or you can place an order directly on the website.
At present, power applications such as fast charging of mobile phones are the main application markets for GaN power devices. Before entering this field, many are small and medium-sized enterprises, and the market influence and attention are slightly insufficient. But when smartphone makers such as Xiaomi began to bring their GaN-based charging products to the market, the spring of GaN charging products also arrived. Are you also ready to participate in this hot market? I hope these new technologies and solutions introduced in this article can help you in your next technical decision.