GaN — Still Crushing Silicon One Application at a Time
Enhancement-mode gallium nitride transistors have been commercially available for over four years and have infiltrated many applications previously monopolized by the aging silicon power MOSFET. There are many benefits derived from the latest generation eGaN® FETs in new emerging applications such as highly resonant wireless power transfer, RF envelope tracking, and class-D audio. This article will examine the rapidly evolving trend of conversion from power MOSFETs to gallium nitride transistors in these new applications.
Enhancement-mode gallium nitride transistors have been commercially available for over four years and have infiltrated many applications previously monopolized by the aging silicon power MOSFET. There are many benefits derived from the latest generation eGaN® FETs in new emerging applications such as highly resonant wireless power transfer, RF envelope tracking, and class-D audio. This article will examine the rapidly evolving trend of conversion from power MOSFETs to gallium nitride transistors in these new applications.
Wireless Power
On the global landscape, one of the most exciting applications to emerge in the past few years is wireless power transfer. Soon we will be able to eliminate the electrical outlets on the walls and simply transfer the power wirelessly and efficiently. These systems use eGaN FETs because of the FET’s ability to operate at high frequency, high voltage, and high power. Several topologies for driving the highly resonant transmission coil have been examined in the literature – most notably voltage mode class-D (VMCD) and class-E configurations. Recent studies have shown that a new topology, ZVS-VMCD has overall superior characteristics at 6.78 MHz (The standard adopted by the A4WP consortium). Efficiency, low cost, as well as stability under widely-varying load conditions makes this topology the likely winner in an application that will have a major global impact on the utility of our battery operated systems. Figure 1 compares the efficiency of four topologies using eGaN FETs; VMCD, current mode class-D (CMCD), single ended class-E (SECE), and zero voltage switching voltage mode class-D (ZVS-VMCD). Notable from figure 1 is the corresponding increase in output power with increase in efficiency. Figure 2 compares the power dissipation of the eGaN FETs (per FET) in each of these topologies as a function of DC load resistance. Only the ZVS-VMCD system with eGaN FETs is able to operate across the wide range of load veriations expected in a wireless charging system and without the need for a heatsink.
Figure 1: Efficiency comparison between various topologies used for wireless power transmission.
Figure 2: Power loss per FET as a function of DC load resistance for various topologies used for wireless power transmission. Power dissipation below 400 mW does not need heatsinking.
Envelope Tracking
As the bandwidth and spectral efficiency (bits per second per Hertz) of wireless communication has increased, so has the PAPR (peak to average power ratio) of these RF power amplifiers (RFPA). These increases in bandwidth have had a serious negative impact on the efficiency of these amplifiers even with improvements in materials and technology.
A variety of different approaches have been investigated to reverse this. One such method, shown in figure 3, is implemented through the use of a hybrid linear-amplifier and multi-phase buck converter. The buck converter supplies only the high-power, lower -frequency transient components. Alternative methods employing boost converters or class-S amplifiers have also been demonstrated. Regardless of the implementation, gallium nitride is seen an as enabling technology for both envelope tracking converters and wide bandwidth RFPA designs due to the high frequency, voltage, and current-handling capabilities of these transistors.
Figure 3: Hybrid linear amplifier and multiphase buck converter.
To demonstrate the capability of eGaN FETs for envelope tracking applications experimentally, a 10 MHz buck converter operating at a fixed input and output voltage of 42 V and 20 V respectively was constructed. The circuit consists of two eGaN FETs in a half-bridge configuration with high frequency supply capacitors, together with an LM5113 GaN driver and external logic circuitry to adjust dead-time. Efficiencies over 93% are achieved by the amplifier using EPC8005 eGaN FETs operating at 5 MHz and almost 90% at 10 MHz, as shown in figure 4 .
Figure 4: Efficiency of amplifier using EPC8005 FETs; VIN=42 V, VOUT=20 V.
Class-D Audio Amplifiers
The quality of sound reproduced by an audio amplifier, measured by critical performance parameters such as Total Harmonic Distortion plus Noise (THD+N), Damping Factor (DF), and T-IMD (Inter-modulation Distortion), is influenced by the characteristics of the switching transistors used. Class-D audio amplifiers typically use power MOSFETs, however, lower conduction losses, faster switching speed, and zero reverse recovery losses provided by enhancement-mode GaN (eGaN) FETs enable a significant increase in the sonic quality, and higher efficiency that can eliminate heatsinks. The result is a system with better sound quality in a smaller form factor that can be built at a lower cost.
To demonstrate the advantages of eGaN FETs in class-D applications, a Bridge-tied Load (BTL) output class-D amplifier demonstration system was built and tested . Each channel was designed to deliver 150 W into 8 ohms, or 250 W into 4 ohms load, with less than 0.1% THD, when powered from a ±27 V nominal power supply.
Faster switching speeds, shorter dead-times, and the absence of the diode recovery enable very low THD+N as shown in figure 5, while minimizing T-IMD and the EMI emissions from the amplifier.
Figure 5: THD+N of 0.003% at 8 Ω, 1 kHz, with the eGaN FET class-D amplifier
In addition, the losses in the eGaN FETs were so small that heatsinks could be completely eliminated, while delivering 150 W into an 8-ohm speaker (or 250 W into a 4-ohm speaker). Figure 6 shows the efficiency of the EPC2016 eGaN FET-based class-D power converter.
Figure 6: eGaN FET class-D amplifier efficiency — 4 Ω and 8 Ω speakers.
Summary
In addition to replacing the aging silicon power MOSFET in traditional power conversion applications such as DC-DC power supplies , gallium nitride-based transistors such as eGaN FETs from Efficient Power Conversion are growing in entirely new applications such as envelope tracking and wireless power. eGaN FETs are also enabling enhanced audio quality with greater efficiency in class-D audio amplifiers. This is just the start. Gallium nitride is a relatively new technology and will continuously improve in absolute performance at a rate reminiscent of Moore’s Law.
M. A. de Rooij, J. T. Strydom, “eGaN® FET- Silicon Shoot-Out Vol. 9: Wireless Power Converters,” Power Electronics Technology, pp. 22 – 27, July 2012.
W. Chen, et al., “A 25.6 W 13.56 MHz Wireless Power Transfer System with a 94% Efficiency GaN Class-E Power Amplifier,” IEEE MTT-S International Microwave Symposium Digest (MTT), pg. 1 – 3, June 2012.
To be presented: M. de Rooij and Peter Cheng, “Improving Wireless Energy Transfer Performance with eGaN® FET-based Converter,” Electronica Asia, Shanghai, March 18, 2014.
To be presented: J. Strydom, D. Reusch, “Design and Evaluation of a 10 MHz Gallium Nitride Based 42 V DC-DC Converter,” Applied Power Electronics Conference, APEC 2014, March 16-20, 2014.
Efficient Power Conversion, “EPC9106 – Demo Circuit,” data sheet, http://epc-co.com/epc/Products/DemoBoards/EPC9106.aspx
Lidow, A., J. Strydom, M. de Rooij, Y. Ma, GaN Transistors for Efficient Power Conversion, 1st ed. El Segundo: Power Conversion Press, 2012.
