GaN – 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 wireless power transmission and RF envelope tracking. This article will examine this rapidly evolving trend of conversion from power MOSFETs to gallium nitride transistors, but first sharing a few lessons we learned designing higher frequency transistors.
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 wireless power transmission and RF envelope tracking. This article will examine this rapidly evolving trend of conversion from power MOSFETs to gallium nitride transistors, but by first sharing a few lessons we learned designing higher frequency transistors.
What have we learned?
A. Influence of Package on Performance
Understanding the importance of parasitics on performance, the semiconductor industry has improved the packages over time. As power device technology evolves, improved device packaging must be also developed. To look at the impact of packaging on performance a 1 MHZ, 20 A, 12-1.2 V point of load (POL) converter with an EPC2015 eGaN FET is evaluated in various packages. Device loss breakdown for each package is shown in figure 1.
Figure 1: Device loss breakdown by package type.
For an ultra-fast eGaN FET in a SO-8 package, only 18% of the switching losses are the result of the die, while 82% of the losses are caused by the parasitics inductance of the package.
Evaluating an eGaN FET in a LFPAK package, an improvement to the SO-8 , 73% of the loss is still contributed by the package because the large common source inductance (CSI) limits the speed of the device.
The DirectFET, designed to minimize CSI, can improve performance over its predecessors and reduce the package related losses to 47% of the total loss. To fully utilize the improvements offered by gallium nitride a better package is required.
Figure 2: eGaN FET LGA connections layout.
For the gallium nitride eGaN FET, a higher voltage lateral power device, all of the connections are contained on the same side of the die as shown in figure 2. This allows for the die to be mounted directly to the PCB, minimizing the total parasitics to the internal busing and external solder bumps. To further decrease parasitics, the drain and source connections are arranged in an interleaved linear grid array, providing multiple parallel connections to the PCB from the die.
The result of the improved packaging is a significant reduction in package related losses, with only 18% of the loss being contributed by the package and 82% from the die.
As gallium nitride transistors move higher in switching frequency, the package becomes even more critical, positioning the eGaN FET to enable operating frequencies not possible with traditional MOSFETs by providing improved device figure of merit combined with an unmatched low parasitic package.
B. Circuit Layout Is Critical to Lower Loop Inductance for Higher Efficiency, Reduced Overshoot
With the significant reduction in package related losses, the layout of the eGaN FETs becomes critical to high frequency performance. From the efficiency curve obtained from experimental prototypes, this can be seen (figure 3). By testing layouts with similar common source inductance but different loop inductances, the impact of layout on efficiency was compared for a buck converter using eGaN FETs at 1MHz.
Figure 3: Layout impact on efficiency; Vin=12 V, Vout =1.2 V, Fs=1 MHz, L=150nH.
An increase in the power loop inductance from around 0.4nH to 2.9nH results in an additional 40% of loss, decreasing achievable efficiency by over 4%. With the superior packaging advantage of the eGaN FET, a large improvement in performance can be obtained when compared to the traditional MOSFET technology.
Figure 4: (a) voltage overshoot for 1.0nH=9.45V=70% overshoot, (b) voltage overshoot for 0.4nH=2.7V=30% overshoot.
With the increased switching speeds of eGaN FETs, which are provided by reduced figure of merit and packaging, the parasitics must be minimized not only to improve efficiency, but also to reduce device overshoot. For an eGaN FET design with a high frequency loop inductance of 1nH, a small value for a traditional MOSFET layout, a 70% overshoot is seen, limiting the voltage operation of the converter. By reducing the parasitics, overshoot can also be reduced. For a PCB layout with a loop inductance of around 0.4nH, there is only a 30% overshoot, thus increasing voltage capability and reducing EMI.
C. Smaller Die Size Needed for Higher Frequency
If a device is too large for a given application, the switching losses due to output capacitance may limit the performance at higher frequencies. In order to achieve higher frequencies, die size optimization is needed.
Figure 5: Die size impact on efficiency.
EPC Gen 3 EPC8000 Family of Products Blurs the Line Between Power and RF Transistors
Cutting new ground for power transistors, EPC’s third generation devices have switching transition speeds in the sub nano-second range, making them capable of hard switching applications above 10MHz. Even beyond the 10MHz for which they were designed, these products exhibit very good small signal RF performance with high gain well into the low GHz range, making them a competitive choice for RF applications.
Power systems and RF designers now have access to high performance GaN power transistors capable of amplification into the low GHz range, enabling innovative designs not achievable with silicon.
eGaN FETs Optimized for High Frequency Applications
Hard switching power conversion at 10 MHz and above requires both high-speed eGaN FETs and a circuit that supports low common source inductance and power loop inductance. The ultra high-speed capabilities and improved device pinout of the Gen 3 EPC8000 family of gallium nitride transistors enable this class of converters in applications such as envelope tracking and wireless power transmission. These eGaN FETs can achieve switching transition speeds in the sub-nano seconds range, and the gate drive loop and drain-source power path are designed for ultra low inductance.
Figure 6: EPC8007 driven by LM5113 gate driver.
Figure 6 is an oscillogram of a 40 V rated eGaN FET switching in about 500 picoseconds. This part was driven by an LM5113 from Texas Instruments that was driving the gate of the eGaN FET at 1.2 nanoseconds. Simply put, the eGaN FET is switching on and off in half the time of the gate driver applied!
Figure 7: Gain vs. frequency for EPC8002.
We can also look at this high-speed capability in terms of gain vs. frequency. In figure 7, it can be seen that this same device has a gain of 21.9 dB at 1GHz. This is ideal for applications such as envelope tracking and other high bandwidth power amplifiers.
In every power conversion application, eGaN FETs improve performance compared with the best power MOSFETs available.
Emerging Applications Benefitting from eGaN FET Higher Performance
Figure 8: Efficiency of wireless power voltage mode class-D operation.
A. Wireless Power
On the global landscape, one of the most exciting applications to emerge in the past few years is wireless power transmission. In the next few years we will be able to eliminate the electrical outlets on the walls and simply transmit the power wirelessly and efficiently. These systems use eGaN FETs because of their ability to operate at high frequency, high voltage, and high power. Figure 8 gives the efficiency of a wireless power voltage mode class-D operation.
B. Envelope Tracking
Figure 9 shows the forecasted growth rate of wireless communications and the various peak-to-average power ratio (PAPR) of various RF amplifiers.
Figure 9: (top) Forecasted growth rate of wireless communications; (bottom) PAPR of various RF amplifiers.
As the bandwidth and spectral efficiency (bits per second per Hertz) of wireless communication have increased, so have the PAPR (peak to average power ratio) of these RF amplifiers. These increases in bandwidth have had a serious impact on the efficiency of these amplifiers, which has actually gone down – with improvements in materials and technology.
Figure 10: Hybrid linear amplifier and multiphase buck converter.
A variety of different approaches have been investigated to achieve this. One such method shown in figure 10 would be implemented through the use of a hybrid linear-amplifier and multi-phase buck converter as, where the buck converter supplies only the high-power, but lower -frequency transient components. Alternative methods employing boost converters or Class-S amplifiers have also been demonstrated. Regardless of the implementation, gallium nitride is being seen an as enabling technology for both ET converters and wide bandwidth RFPA designs.
Figure 11: Efficiency of amplifier using EPC8005 FETs; Vin=42 V, Vout=20 V.
The efficiencies of an amplifier using EPC8005 eGaN FETs operating at operating at five and 10 MHz are shown in figure 11.
GaN Into the Future
Today EPC offers a range of products from 40 V up to 200 V. They range from 4 milliohms to 300 milliohms and can operate efficiently in the multi-megahertz range. Going forward EPC is working is progressing in four areas:
- In the next several months, EPC plans to introduce 600 V enhancement mode products in the popular 5 mm x 6 mm PQFN package. The products will be ideal for applications such as power factor correction, off-line power supplies, and solar micro-inverters.
- EPC is sampling customers on much higher frequency Gen 3 products designed for RF, Envelope Tracking, and Wireless Power applications
- EPC is in development of monolithic devices that integrate half-bridge functions as well as driver functions on the same chip as the power devices. Look for those in 2014.
- Our technology progresses on all fronts. Today all EPC eGaN FETs are on a generation 2 technology. We will migrate to a Gen 4 technology in 2014 with about a factor of two in improvement of the figure of merits.
Figure 12: GaN into the future.
Summary
- Packaging and circuit layout are critical to get the most out of the efficiency improvements from GaN technology.
- Smaller EPC Gen 3 eGaN FETs with switching transition speeds in the sub nano-second range have been optimized for wireless power and envelope tracking systems and enable significant performance improvements over Gen 2.
- Efficient power conversion can now be done at frequencies well above 10 MHz.
- Even at lower frequencies you can always improve efficiency with eGaN FETs!
