Optimizing Inverter Power Supply with Planar Transformers and NXP GD31xx Gate Drivers

Isolated gate drivers require an isolated power supply to generate the positive and negative voltage rails essential for driving IGBT, SiC or GaN power modules. Recent advancement in gate driver technology now enable more compact and efficient inverter designs by integrating advanced control features, e.g., for secondary side regulation.

Automotive traction inverters using SiC or IGBT power modules typically require a bipolar gate drive referenced to each power device's source, isolated from the low-voltage control circuit. These gate drivers are supplied by small isolated converters, typically operating from a low-voltage supply (12 V-24 V) and delivering low-power (1-2 W) bipolar voltages such as +15 V or +18 V and -5 V, for example.

Common converter topologies include LLC, flyback and push-pull. Push-pull converters often suffer from poor output voltage regulation, requiring pre-regulated input voltage, while LLC converters can be complex to design, especially for a wide input/load range of operation. The latest advancement in NXP's GD31xx family of gate drivers enables efficient power conversion with enhanced control mechanisms, supporting a broad range of magnetics and operating conditions. With an external discrete switching MOSFET, the converter can be designed for high input voltage operation (12 V, 24 V or 48 V for example.) or to supply multiple gate drivers in a semi-distributed or centralized architecture.

GD31xx Planar Transformer Demonstrator

The demonstration platform presented in this article features two gate drivers from NXP's GD31xx family, each controlling their own isolated high-voltage side supplies using onboard planar transformers. The gate drivers ensure closed-loop operation, providing accurate voltage regulation and fast load response. Post-regulation is achieved using an LDO for positive voltage and a shunt regulator for negative bias, ensuring noise-free gate drive and on-the-fly programmability (see Fig. 2). Typical voltage regulation accuracy is below 2%, with negative voltage regulation comparable to TL431-like shunt ICs, while minimizing power dissipation due to the closed-loop operation.

Transformer Design and Performance

The transformer was designed for 48 V nominal input (24 V - 65 V input range at full output power) and delivers +15/-4 V outputs at 1 W. This allows for the driving of approximately 2 µC of gate charge at 20 kHz. The peak efficiency reaches 74% (see Fig. 3), a reasonable value given the low output power level, where the gate driver's quiescent consumption significantly impacts overall efficiency.

Advantages of Planar Transformers

Why are planar transformers considered for such applications? A typical inverter consists of a vertically stacked configuration of a power module, a gate driver board, and a control board. Inverter density can be significantly increased by reducing the vertical spacing between these boards. Since auxiliary supply transformers are often the tallest components on a gate driver board, adopting planar transformers minimizes height and improves mechanical robustness.

Additionally, reducing component height enhances vibration immunity due to the lower center of mass. In this demonstration platform, the PCB solution measures just 11 x 19 mm and is less than 5 mm tall, including board thickness (see Fig. 4). Compared to conventional transformer designs, this resulted in a threefold height reduction and a 28% improvement in power density.

Design Considerations and Trade-offs

Transformer design optimization involves balancing leakage inductance, which impacts efficiency, and interwinding capacitance, which affects common-mode noise. For this design, a compromise was reached at L _leak = 5 % L _pri and C _ww = 4.3 pF by fine-tuning the PCB stack-up, leveraging varying dielectric thicknesses in this six-layer design. Hi-pot testing up to 3.5 kV confirms compatibility with the latest SiC and IGBT power modules.

Despite these benefits, planar transformers are not yet universally adopted. Some challenges must be considered when integrating these designs into mass production. While the raw material cost of a ferrite core set may be attractive compared to conventional discrete transformers, the cost of the mounting solution must be factored in. Assembly typically involves adhesives-sometimes reinforced with spring clips-to secure the core halves to the PCB. This step is critical to ensuring mechanical robustness and maintaining electrical properties, particularly the final air gap of the core.

Furthermore, achieving the required PCB tolerances and layer stack-up for proper transformer performance can drive up PCB manufacturing costs. In such cases, module-based planar transformers may be a more viable alternative, limiting PCB complexity to the transformer area rather than the entire board.

Overview of the benefits of Advanced Control features

Planar-based transformer designs for automotive inverter applications offer significant benefits, enabling designers to further push the boundaries of power converter density. With advanced control capabilities and high integration, NXP's GD31xx gate drivers provide a flexible solution that accommodates a wide range of transformer designs and power applications across various semiconductor technologies. This versatility allows engineers to develop scalable and reusable solutions across multiple platforms, optimizing efficiency and reliability in modern power systems.