Within power-conversion electronics, a general rule of thumb is often applied: It’s not possible to achieve high power density through single-stage, high ratio conversion at frequencies above 10 MHz while simultaneously achieving high energy conversion efficiency without paying a large cost premium through complexity, such as in the use of resonant topologies.
This is due to the generally slow, highly variable, and switching characteristics of the power devices used in the power-conversion stage. From the advent of rectification through linear regulation, the adoption of switch-mode power supplies and resonant converters, power-conversion topologies, and the control circuitry that realize these architectures were developed to exploit the inherent capabilities and avoid the deficiencies of the available power switch technology. When these power semiconductor technologies vary incrementally, advances in the architectures, drivers, and control circuits drive radical advances in power-conversion electronics, such as multiphase or sine-wave conversion.
However, on the rare occasion when the nature of the power switch itself changes radically, it’s the nature of the power switch that drives radical, potentially revolutionary, changes in power electronics.
Thirty years ago, International Rectifier launched the first commercially viable silicon MOSFETs, trademarked HEXFET, for the rapid adoption of switch-mode power supplies (SMPS) over then-dominant linear supplies employing bipolar transistors. From planar HEXFETs to TrenchFETs and superjunction FETs, silicon MOSFETs have since made some two orders of magnitude improvement in figures of merit (FOMs) to effectively serve a variety of markets (see "Figure Of Merit: A Refresher Course").
Meeting these new requirements and challenges requires novel materials and transistor structures. To that end, scientists and engineers at IR developed a gallium-nitride-based (GaN) power-device technology platform that they claim will deliver a 10 times more cost-effective performance over existing silicon devices. More than five years of device R&D resulted in a proprietary GaN-on-silicon epitaxial process and device design and fabrication platform for power conversion.
GaN-on-Si based HEMTs
The basic-current GaN-on-Si structure is a high electron mobility transistor (HEMT), based on the presence of a two-dimensional electron gas (2DEG) spontaneously formed by the intimacy of a thin layer of aluminium gallium nitride (AlGaN) on a high-quality GaN surface (Fig. 1). In addition, a cost-competitive device fabrication process was achieved using standard silicon CMOS manufacturing facilities.
The GaN technology platform, dubbed GaNpowIR, takes full advantage of the intrinsic features and capabilities of these new GaN-on-Si-based power HEMTs. It also includes the development of complementary gate-driver circuitry and advanced packaging solutions, as well as controller ICs and novel circuit topologies.
Compared to SiC and silicon devices, an inherent blend of high-conduction electron density, high electron mobility, and higher bandgap provides GaN-based HEMT devices with a reduction in device specific on-resistance (RDS(ON)) for a given reverse hold-off voltage capability. This is illustrated in the calculated material limit curves for (non-highly compensated) unipolar devices (Fig. 2).
Shown for reference are measured results for FETs using the three materials, as well as for highly compensated superjunction (SJ) and bipolar (IGBT) device structures in silicon. Figure 2 also illustrates results from the early-stage development of the GaNpowIR technology platform at IR (IR GaN).
Since GaN-based power devices achieve a combination of low gate capacitance and low on-resistance, they permit much higher frequency-efficient switching converters than possible with silicon MOSFETs. Results based on device modeling, extrapolated from early measured results, indicate that R(on)*Qg FOM for first-generation GaNpowIR HEMTs, to be introduced this year, is 33% lower than that of state-of-the-art silicon MOSFETs. In fact, ongoing engineering efforts are expected to provide further significant improvements in the next few years.
Figure 3 shows that R(on)*Qg for GaNpowIR devices is expected to be as low as 13 mΩ-nC by 2011. Then by 2014, it’s expected to be less than 5 mΩ-nC, which is an order-of-magnitude improvement over state-of-the-art silicon-based devices available this year.
Likewise, Figure 4 depicts the expected effect of the power switch’s improved R(on)*Qsw FOM on the size and efficiency of a dc-dc converter, including the output filter. Current state-of-the-art, multi-phase, silicon-based solutions perform 12- to 1.2-V conversion efficiently up to about 2 MHz per phase.
The GaNpowIR technology platform is expected to enable efficient high ratio power conversion to greater than 50 MHz per phase in the near future. As can be seen, the improvements in the power-switch FOM enable a corresponding increase in operating frequency and a corresponding decrease in converter size, without a reduction in power-conversion efficiency.
The frequency shown in Figure 4 is chosen to provide a constant conversion efficiency of 85%. When the frequency is high enough (20 to 60 MHz), there’s a dramatic reduction in external components as well as the unwanted distance between the converter and the load, which reduces parasitic related power losses. Figure 4 reflects these power savings.
Consequently, the dominant power-conversion application FOM will be better using GaN-based solutions. Due to improvements in the device RQ FOM, together with improved packaging and drive technologies, an order-of-magnitude improvement is expected for this FOM over the first five years of the GaNpowIR platform’s commercial introduction.
Several prototypes were built to demonstrate the distinct advantages of the new GaN-on-Si-based power devices. One such prototype is a low-voltage point-of-load (POL) converter. Designed for a 12-V input to 1.2-V output conversion at 10-A load current, this GaN-based (POL) converter operates at 5 MHz to deliver efficiency that is comparable to a commercially available silicon solution running at 1 MHz, but at less than one-third the size. Both solutions integrate the controller/driver IC and output inductor within the power stage package.