During the financial crisis 13 years ago and amid record high oil prices, the emphasis on switching to renewable energy and hybrid electric vehicles (EVs) was palpable. Fast forward to today, and it seems the pipe dream ideas from a decade ago are starting to come to fruition. There is greater emphasis among large companies, governments and private citizens on finding renewable solutions to climate challenges. EVs are one of the many solutions that will help the world’s major polluters reduce their emissions.
Some of the major challenges in greater adoption of EVs have included the need for more supportive infrastructure, greater vehicle range, in-vehicle power management and faster charging systems. Among these, vehicle charging systems probably receive the least focus, yet they are a critical enabler of greater EV adoption. Gallium nitride (GaN) metal-oxide-semiconductor field-effect transistors (MOSFETs) are just one of the important components needed in these systems thanks to their greater thermal handling, high power delivery, high mobility and other advantages. By including these advanced components in novel multiphase power converter designs, engineers can develop faster charging solutions that are less encumbered by thermal constraints.
Why GaN MOSFETs on SiC?
Power systems engineers should already be familiar with the role of FETs in switching converters. GaN MOSFETs provide several advantages in many systems, ranging from DC power systems to high-frequency RF systems:
- Their high mobility and wide bandgap (3.4 eV) of GaN ensures lower conduction losses (lower R-ON during operation)
- They can be operated at higher temperatures than Si devices, allowing GaN MOSFETs to be used in higher power applications
- They have lower input (gate) capacitances than comparable Si devices, thus they can be fabricated as smaller devices as needed
Although GaN can be deposited on Si, GaN devices deposited on SiC will be taking advantage of the higher thermal conductivity of SiC to dissipate heat from the device. This means GaN-on-SiC power MOSFETs can maintain lower losses when used in power-demanding applications, including in DC-DC converters for EV charging. These devices hold promise for use in DC-DC conversion for Level 3 charging stations using unique power conversion system designs.
(Learn more about power MOSFETs at Globalspec.com)
GaN MOSFETs in multiphase converters
A critical enabler of fast EV charging is the topology of the DC-DC converter in the charging unit. Earlier Level 1 and Level 2 EV charging systems used an AC-DC converter to provide charging. Today’s Level 3 systems provide fast EV charging at rates reaching 20 miles of range per minute. Level 3 EV charging systems typically use a multiphase buck, boost or buck-boost topology to provide high power delivery to a battery bank and charge management system at an EV charging station.
These power regulator designs make use of multiple interleaved power stages to enable high power factor in a smaller package than a comparable buck converter. A multiphase buck converter power stage can also be implemented with galvanically isolated designs (e.g., flyback or LLC resonant converter) to provide some measure of user safety. A traditional multiphase DC-DC converter is the Vienna buck topology, which is implemented using a half-bridge LLC resonant regulation stage on the output side of the system.
In this system, an input AC voltage is rectified and filtered, which is then fed into the converter section. The switching elements are driven at different phases; note that the above system could be extended to any arbitrary number of phases, although three or four is typical in a multiphase converter. The inductor selection in the two full-bridge driving stages provides the gain required for resonant power conversion through the transformer. Finally, the output power is stabilized to a DC output through a capacitor bank; this could also be implemented as a low-pass differential pi filter.
If the FETs in this system can be run at higher frequencies, and with deeper modulation into the ON state, there will be two primary benefits:
- Switching noise (ripple) on the output power will be lower.
- A physically smaller inductor can be used to provide a lower profile design while meeting a specific ripple target on the output.
By implementing the system in an N-phase multiphase topology, the apparent switching frequency will be a multiple of 2N of the PWM frequency in the switching elements. In other words, the above topology mimics a single-phase switching converter operating at higher frequencies, which provides lower ripple on the output power.
Using GaN MOSFETs provides one way to enhance this type of system. By using GaN MOSFETs in these designs, the switching converter section can be run at an even higher switching PWM frequency and faster edge rate because charging/discharging modulation is not limited by inherent capacitances in the device. With higher PWM frequencies typically comes a faster edge rate, allowing the design to be more deeply modulated between ON and OFF states, giving a lower R-ON value. The design then dissipates less power as heat, and that heat can be more easily dissipated into the PCB substrate and enclosure.
Challenges in multiphase buck converter design
Academic and industrial research into these power system designs is ongoing, with many articles focusing specifically on the use and limits of GaN MOSFETs in interleaved power systems such as the topology shown above. Power systems innovators that want to build highly power efficient regulator designs can look to GaN-Si or GaN-SiC as material platforms of choice. The use of GaN MOSFETs as switching elements for power delivery is not limited to automotive. Envelope tracking RF power systems are one area that requires efficient high power conversion, and GaN MOSFETs are used in these systems for the same reason they are used in EV charging.
Some of the challenges in these systems surround the implementation of a control strategy for power delivery, which requires phase sensing in a feedback loop back to the input stage of the design. Such a strategy is already used for precise current regulation in high-current DC-DC converters, such as LLC resonant converters running in half-bridge or full-bridge topology. Implementing a control strategy requires routing directly back to a controller IC; this is often done with optical coupling when galvanic isolation is required. Embedded developers can implement novel control strategies in an MCU, in an FPGA, or with a combination of off-the-shelf controllers and specialty logic.
