The world’s growing population and rising development create an ever-increasing demand for electricity, while environmental issues force us to do more with less energy. One important technical innovation that will help us deal with these issues is the use of gallium-nitride (GaN) in electronic power supplies.
GaN-based switching power transistors require a smaller size than silicon (Si) transistors for the same current capabilities, and they can operate at high voltages and high frequencies with better performance and lower losses. GaN is first being used in higher-performance power designs, since these push their operating frequencies and require precise switching characteristics. As the technology matures and prices drop, GaN’s benefits should start becoming available to even the smallest of systems.
For all its advantages, the effect of GaN is just beginning to be felt in power supply design. Differences in gallium-nitride and silicon technologies have compelled IC suppliers and system designers to proceed carefully, resolving complexities issue by issue. Design solutions are often presented in the components themselves—GaN switches, Si switch drivers, high-speed switching controllers and other components. System-level IC solutions that are appearing today can enhance power supply development with co-designed devices and multiple chips that are integrated in a modular package.
GaN’s role in power supplies:
Familiar electronic equipment usually operates from switched-mode power supplies (SMPSs), which convert alternating to direct current (AC to DC) and step line voltages of 110-120 V or 220-240 V down to 12-, 5-, 3.3-V and lower levels. SMPSs are also used for DC-to-DC conversion, higher voltages in inverters for renewable energy, and in automotive electronics, industrial equipment and other high-power systems.
In the general flow of an SMPS, first an input voltage, often AC at a high voltage and low frequency, is rectified to DC. A high-frequency power switch then converts the DC signal to a pulsed voltage waveform, which is next transformed to the required voltage and filtered for a steady output to the low-voltage system. As load requirements change, feedback from the output is used by a controller-regulator to supply variable pulse-width-modulated (PWM) signals to the power switch driver.
Silicon MOSFETs (metal-on-silicon field-effect transistors) have been traditionally used for power switches but are being steadily replaced by GaN FETs. Depending on system requirements, various design topologies with different numbers of power switches may be used.
The switch-driver combination is extremely sensitive, since it must perform precisely as commanded by the controller; otherwise, the power system becomes unstable. Moreover, even the slightest noise that is injected into the feedback loop may result in instability.
All grid-connected systems require isolation to protect the downstream equipment. Also, the high-frequency operation of the power conversion must not be allowed to create noise on the source power line. By allowing higher-frequency operation, GaN devices address the isolation requirement and the need to reduce electromagnetic interference, helping to reduce the size of the isolation transformer and input filter.
GaN’s advantages for SMPS designs
Compared with Si, GaN offers lower losses at higher voltages, and it also uses less energy to turn on and off. GaN switches are offered over a wide range of operating voltages, permitting designers to reduce the physical size of systems through efficient operation at higher switching frequencies over a large range of input and output voltages.
GaN offers a band gap energy nearly three times that of Si, so that a greater jump in voltage is required to change it from an insulator to a conductor. Theoretically, GaN should perform better at higher temperatures, since it can sustain more heat before it becomes conductive, potentially improving performance in high-temperature environments.
Silicon MOSFETs still have a sizeable cost advantage over GaN today, but over time the differential will be reduced, in part because the GaN material is layered over a Si substrate. IC manufacturers can leverage decades of Si processing experience as they build GaN power switches.
Products aid effective GaN design
Today, the design challenge lies in taking full advantage of GaN transistors’ potential while avoiding unwanted effects. To operate at higher frequencies, GaN FETs demand precise timing in the switch-driving signal, and they are also extremely sensitive to parasitic impedances due to packages, interconnect and outside sources.
Mature Si processes have enabled the integration of tunable Si-based switch drivers that can minimize parasitics and keep timing precise. These gate drivers integrate the components required for optimizing the performance of GaN switches, serving to reduce board space and helping simplify design. The devices also offer important protections for efficient switch operation.
But gate drivers by themselves can only offer so much optimization because even the shortest possible traces from the driver to the GaN switch give rise to delays. There is also a need to control the variability that results from the layout of the passive components that are important in coupling the devices.
It is unlikely that the driver and switch, based on different materials, will be integrated economically on a single die in the near future. However, modules that package the two devices and passive components offer several advantages. A driver-switch module shortens inter-chip connections, minimizes delay times and stray impedances, and can also reduce parasitic factors of a multi-chip design significantly, some by an order of magnitude or more. Additionally, a module significantly reduces the SMPS area and component count and lowers system manufacturing costs.
The controller-regulator is also important in a GaN system-level solution because it must respond to changes in output voltage in real time, operate at very high frequencies and meet precise pulse width requirements to minimize conduction losses. Fortunately, digital power controllers are available which can support these requirements.
Conclusion
Now that these enabling components are available, SMPS developers will be able to design higher-performance systems much faster. The success of these systems will push GaN into new application areas, both at the high-power, industrial end of the scale and at the low-power, mass-market end. Power supply applications will finally be able to fully leverage the benefits of GaN.
