www.epc-co.com IN-VEHICLE POWER 25 www.power-mag.com Issue 4 2023 Power Electronics Europe 23 x 22mm for both outputs with an inductor height of just 3mm. The GaN FET chosen for this design is the EPC2055 40V, 3mΩ max RDS(on), 6.6nC QG, 0.7nC QGD, and 1.3nC QOSS, in a small 2.5 x 1.5mm footprint. It can deliver up to 29A continuous current and 161A peak current. It also exhibits very small switching losses at 2MHz switching frequency. Controller selection Conventional MOSFET analogue controller ICs are not fully compatible with GaN FETs because of the specific driving needs. For example, there is no over-voltage management for the bootstrap supply in the event where the lower FET reverse conducts, and high negative voltage spikes on the switch node during dead time can lead to unpredictable timing behaviour. As a result, digital controllers such as DSPs have been used for GaN FET-based designs but require additional support ICs such as a current sense amplifier, housekeeping power supply and a GaN FET-compatible gate driver. This approach adds to the overall bill of materials (BoM) and increases design complexity. In contrast, the LTC7890 controller from Analog Devices is a 100V, low quiescent current, dual two-phase synchronous step-down controller with dedicated driver feature for GaN FETs. The small 6.0 x 6.0mm QFN package is fully optimised to drive GaN FETs. The LTC7890 integrates a half-bridge driver and smart bootstrap diode and offers optimised near-zero deadtime or programmable deadtime and programmable switching frequencies up to 3 MHz. It also has low standby power consumption. The integration of GaNcompatible analogue controllers with integrated gate drivers eliminates the need for additional support ICs, simplifying designs and reducing the overall BoM. Optimising layout and thermal performance One of the design challenges in leveraging GaN FETs lies in minimising parasitic inductances in both the gate and power loops. Power loop inductances, as well as gate loop inductances, impact switching behaviour and efficiency. The internal vertical loop layout is a technique that helps minimise switching losses and ringing. High- and low-side gate loop inductances must also be minimised to prevent gate over-voltage and ensure smooth operation. Thermal management is another critical aspect of GaN FET design. Despite their efficiency, the compact size of nonpackaged GaN FETs necessitates careful attention to heat dissipation. In the absence of a heatsink, maximising the number of vias beneath the GaN FETs becomes crucial. This approach enhances thermal heat-spreading from the FETs into the PCB’s copper layers, ultimately improving overall thermal performance. Design validation The efficiency of the EPC9160 surpasses 93% for 5V output and 24V input. High switching frequency of 2MHz was chosen based on EMI requirements in automotive applications. It further demonstrates the fast switching speed of GaN FETs together with the smaller size of the inductor and capacitors. The EPC9160 power stage fits within an area of just 506mm2 (W = 23mm, L = 22mm). In a comparative analysis of systems built with GaN FETs vs systems built with Si MOSFETS, both operated at 2MHz and 10A, the GaN-based solution exhibited nearly 5% higher peak efficiency and 2W lower power loss at 12V input and nearly 7% higher peak efficiency and 3.1W lower power loss at 24V input (Figure 1). The superior efficiency and reduced power loss of GaN-based solutions also translates to cooler operation, as evidenced by a 10°C lower hotspot temperature than the silicon board in a thermal performance comparison (Figure 2). The vias underneath the FETs and the six-layer PCB with 2oz copper thickness help in reducing the temperature of the FETs by utilising heat spreading of the PCB copper layers. In summary, the automotive industry is undergoing a transformative phase, and the demand for advanced in-vehicle infotainment systems that provide better connectivity, improved vehicle safety, and enhanced in-vehicle user experiences is on the rise. GaN-based power solutions that incorporate analogue controllers offer a path forward, allowing for smaller, more efficient, and cost-effective designs that meet the demands of these sophisticated systems. www.epc-co.com Figure 2: Thermal performance comparison of GaN FET vs Si MOSFET www.power-mag.com
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