62 November/December 2023 | E-Mobility Engineering drive at high switching speeds. The design has a range of microwave signals, but getting power engineers to design these boards is impossible. “For higher power, there is a more limited repertoire of topologies for motors, and they were mostly using silicon. We have solved the problems with EMC and thermals that allow engineers to put together building blocks with GaN that will be more efficient and power-dense. “GaN Systems produces the highest performing 650 V power GaN devices, which allows us to achieve the highest efficiency when combined with our technology. The better the efficiency of power usage, the greater the range of the EV. “Our approach is to switch as fast as possible rather than slow the devices down. There is no parasitic diode so they switch like an RF device. The 3 x 2 mm die is switching 8 kW but there are no thermal conduits in the package to get the heat out, so the junction temperature goes up and it has to be de-rated,” Gwynne says. As soon as you go up in frequency, above 100 kHz, the thermal issues increase and the on-resistance goes up. The QPT design sees 20 W losses at 2 MHz compared to 100-130 W with other drivers and enables motors to be driven at up to 99.7% efficiency at peak load with hardly any decrease in efficiency at lower loads. “This is a colossal problem, and we had to come up with nine patents to get the thermal energy out without creating the RF parasitics from all the metal that couples to the rest of the circuit,” Gwynne says. “So you need microwave expertise up to the GHz range because of the switching edge speeds, and if you don’t properly manage that with picohenry inductance-level interconnect and correct impedance matching it basically goes bang. “This takes months of EM analysis of designs, which is outside the expertise of the power engineers. We do use exotic materials such as aluminium nitride [AlN] but the key is as you add that metal you need to do the GHz analysis to work out the coupling, so you need complex geometry that is developed iteratively.” The first-generation modules measure 30 x 30 x 18 mm, and are a discrete solution as a proof of concept. A custom 650 V GaN transistor is being developed in parallel, but in the meantime the designs are using transistors from GaN Systems. QPT has developed reference designs and RF filters for the I/O, and will be providing transistor modules and I/O modules. The modules will be an embedded die multi-level PCB with all the routing and drive circuitry and interconnect. “We are switching in 1.5 and 2 ns on a 540 V bus, which is a high dV/dt, and there are no isolated gate drivers that can handle this, so we developed our own to switch kilowatts at this speed,” Gwynne says. “We are using silicon germanium (SiGe) for a high speed controller and a GaN pre-driver from EPC, and in the first ASIC we will have all that integrated into one SiGe part and integrated into layers of the PCB.” The second-generation module being developed in parallel will measure 5 x 15 x 15 mm. “The modules are built together on a plate, and there’s a Faraday cage on the top so nothing is conducted or radiated, so we can pre-qualify the system, making it easier to qualify afterwards,” Gwynne says. “The beautiful thing for us is that because we can operate in the 650 V regime we can do motors up to 500 kW for EVs. “The Gen1 module is designed to work with pulse width modulation (see page 34) so it can be used with existing controller chips. These operate at up to 2 MHz with the TI C2000, but the drive technology works at up to 20- 30 MHz so you can reduce the size of the converter tremendously,” he says. Higher voltages Toyoda Gosei and Powdec in Japan have developed a high-performance horizontal GaN power device for a 24 kW power converter. The Polarisation Super Junction GaN power transistor has a different structure to the HEMT devices, and has a breakdown voltage of over 1500 V and a switching speed of 1 ms in an 800 V, 30 A module. There are also opportunities to build A GaN chip with integrated driver (Courtesy of Fraunhofer)
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