GaN reference designs set to play central role in power electronics applications

Yet there remains the challenge common to all new technology introductions: how to create an informed wealth of knowledge to enable designers to easily and quickly design in the new products without having to go through the headaches of being an early technology adopter. This issue is compounded with the introduction of GaN devices, as there have been a number of inflated claims for the technology, and real devices are only just becoming available for engineers to develop with. Because of this, GaN Systems – the Canadian manufacturer of GaN transistors – is keen to develop a 'GaN ecosystem' of suppliers and users to support designers and systems engineers and facilitate the easy adoption of GaN technology. The excitement about GaN stems from its material and electronic properties. GaN devices offer four key characteristics: high operating temperature; high current density; high speed switching; and low on-resistance. These characteristics stem from the properties of GaN, which, compared to silicon, offers ten times higher electrical breakdown characteristics, three times the band gap, and exceptional carrier mobility. GaN Systems uses a patented island structure to construct transistors. The switch is formed from a sea of source and drain islands with a common gate region running between them. This results in a reduction in transistor die area of up to four times. More, the semiconductor processing is simplified, thereby minimising manufacturing cost. This technology also allows breakdown voltages in excess of 1200V to be achieved. The current in each source island flows directly from the die through a copper post to the interconnect pattern on the surface to which the die is mounted. Each drain island is connected by a through-substrate-via to a common connection pad on the back of the die. Because there are no large current flows in the on chip metallisation, high currents can be switched without danger of electromigration. The construction also allows for flip-chip assembly of the die. This eliminates the need for bond wires which have significant inductance that contributes to high frequency switching transients and power loss in designs best operated at switching speeds of around 50V/ns. Initially produced using a GaN on SiC process, these devices are now migrating to GaN on silicon, promising further cost reductions. To demonstrate the performance of its devices, GaN Systems has worked with power module maker Arkansas Power Electronics International (APEI) to develop a high efficiency 2kW to 5kW boost converter with a switching frequency of up to 1MHz. The converter was tested under two operating voltage conditions. First, a 200V input was applied and a 400V output was established. The output power was then swept from 750W to 2kW. The system maintained an efficiency of greater than 98% over the entire operating range (see Fig 1). Since there is so little loss in the converter power stage, only passive air cooling is necessary. In another series of tests, a 300V input was applied and a 400V output was established. Note that this required an adjustment of the converter duty cycle due to the different conversion ratio. Here, the output power was swept from 2kW to 5kW. As shown in fig 1b, the system maintains an efficiency of more than 99% from 2kW to 4kW, then dips slightly to 98.5% efficiency at 5kW. The converter has an approximate mass of 487g, which equates to a gravimetric power density of approximately 10kW/kg. A major factor in the boost converter's high efficiency is the ultra fast switching speed of the GaN Power HEMT. The turn-on transition takes only 8.3ns and exhibits very little noise (overshoot and ringing). There is some slight oscillation on the gate-source voltage, but it is very well controlled. The turn-on slew rate is approximately 48V/ns, which is extremely high – even when compared to current SiC power devices. The turn-off transition takes approximately 3.7ns, yielding a turn-off voltage slew rate of approximately 107V/ns. In summary, the GaN power HEMT demonstrates very fast switching performance which translates into very low switching losses. This boost converter design, the first of its kind on a gallium nitride platform, not only demonstrates the effectiveness of the technology but also is a stepping stone to future automotive products, such as charging units for plug-in hybrid electric vehicles and all electric vehicles. The GaN Systems' demonstration model is suitable for incorporation into a charger of this type. Another reference design – developed by Converter Technologies and centred on GaN Systems' GS50610Q 650V gallium nitride power transistor in a thermally enhanced PQFN package – focuses on power factor correction. The design includes built in drive circuitry, which contributes to the simplicity of the converter design and its ability to run at high frequencies. In this 600W dual phase interleaved power factor correction reference design, the power stage magnetics have been sized for full power operation with a switching frequency of 200kHz per phase. This allows benchmarking of system performance at switching frequencies in excess of 500kHz. The higher frequency enabled by the gallium nitride based design enables the use of significantly smaller magnetics with lower losses and a reduced winding capacitance. System efficiency at 200kHz was found to be 97.5% and the power factor was in excess of 0.98. As well as reference designs and real converter demonstrations, GaN Systems will shortly have evaluation kits available, including a 400V boost converter demo board and a pair of the company's GS30603M 600V gallium nitride switches. Why gallium nitride? SiC and GaN devices can be switched at much higher frequencies than those based on silicon, leading to smaller transformers: increasing the switching frequency from 20kHz to 100kHz means five times less volume and weight. Similarly, the output filter components are reduced in size and weight. Diode conduction losses remain a problem for SiC devices and, in practice, both Si and SiC diodes are usually limited to 175°C operation and Si and SiC based power converter systems can require water cooling. The design of GaN Systems' diodes results in very low conduction losses because of the relatively low voltage drop and stability at high temperatures. Air cooling is possible and substantial weight and size reductions can be achieved. Girvan Patterson is GaN Systems' chief executive. For more information on both reference designs, go to www.gansystems.com/downloads.php.