LLC Converter with Planar Matrix Transformer for High-Current-High-Power Applications

Due to this rapid growth, the rising need to address this surging energy demand has prompted industry leaders such as Google, Facebook, Cisco, and IBM to explore innovative alternatives in data centre and their power architecture. Figure 1 depicts the current architecture that makes use of a 12V bus.

Figure 1: Data architecture setup used today

As mentioned above, traditional designs, reliant on a 12V bus, suffer from excessive i2R losses and multiple energy conversion stages, resulting in diminished system efficiency. To combat these inefficiencies, a shift towards higher voltage distribution buses, notably 48V and 400V, is underway, exemplified by initiatives like the International Electronics Manufacturing Initiative (iNEMI) standardization project for DC/DC converters.

However, improvements in converter technology, especially for LLC resonant converters, are required to achieve better efficiency and power density. These converters utilize modern materials such as wide bandgap devices and magnetic structures to enhance performance, paving the way for future data centre power systems.

Integration of a Planar Matrix Transformer

For applications like computer servers needing low-voltage, high-current outputs, LLC converter design faces several difficulties in current sharing along with high di/dt AC currents concentrated at termination points causing significant termination losses. Issues like leakage inductances, winding losses, weight, and cost, still prove to be a liability in terms of operation. On the other hand, planar PCB winding transformers can be advantageous in terms of automatic manufacturing and high-power density. They use matrix transformers which ensure even distribution of secondary currents to SRs, addressing design challenges in a practical manner, especially when integrated into a structured LLC converter with a four-core design. The below figure 2 shows how a regular LLC converter is integrated with a matrix transformer.

Figure 2: LLC converters with an integrated matrix transformer

The matrix transformer utilizes four sets of UI cores, increasing core loss compared to a single ER-core setup. To mitigate this, primary winding modifications enable flux cancellation, reducing core size and loss. Integrating SRs and capacitors into the secondary winding minimizes AC termination losses and doubles the number of cores and SRs. This not only optimizes the design but also results in an efficiency rate of 97.1%, maintaining a power density of 700W/in³. However, challenges persist, such as complexity in manufacturing due to multiple core usage and the need for further efficiency improvements to replace existing DC/DC converters operating at lower frequencies.

Proposed Architecture for the Planar Matrix Transformer with PCB Windings

In designing a DC/DC converter, the main goal is to step down 380V to 12V with 67A output, equivalent to 800W. Four elemental transformers with a turn ratio of 4:1:1 was used for efficiency and a four-layer PCB winding structure was utilized, with the primary winding having two layers and each layer having two turns. A switching frequency of 1MHz was chosen, and round core pillars were preferred over rectangular ones for reduced winding losses. This change resulted in a shorter current path for the secondary winding, which further enhanced the efficiency. Such optimizations were highly crucial for a high-frequency matrix transformer design for LLC converters as they balance the power density and efficiency considerations.

Two unique matrix transformer structures have been proposed to address the challenge of multiple cores and enhance efficiency. In Structure 1, two magnetic cores are integrated into one, effectively reducing the complexity associated with multiple cores.

Figure 3: Flux distribution for structure 1

Meanwhile, Structure 2 further optimizes flux distribution by rotating one core and integrating them, resulting in a significant reduction in flux density within the magnetic plates. This reduction in flux density translates to lower core losses, particularly advantageous for high-frequency ferrite materials, where core loss is a critical factor affecting overall efficiency. Simulation results indicate that Structure 2 reduces core losses by approximately 40% compared to Structure 1, making it a superior choice in terms of efficiency and robustness, as it is less sensitive to tolerance variations between elemental transformers.

Figure 4: Flux distribution for structure 2

The winding arrangement in Structure 2 also contributes to its superiority, with a more evenly distributed flux pattern leading to reduced core losses and the coupling of the four elemental transformers in Structure 2 enhances robustness and reliability. The proposed design integrates secondary windings with components like Schottky rectifiers (SRs) and output capacitors to mitigate AC termination losses effectively. By utilizing a four-layer PCB winding implementation, the design simplifies construction while reducing inter-winding capacitance and overall PCB costs. Further enhancements to Structure 2 have been proposed to minimize core losses without sacrificing power density, ensuring optimal performance in LLC converter applications.

Conclusion

An experiment was conducted to study matrix transformers for high-output current LLC converters. A design was proposed, integrating four elemental transformers into one core, with windings on a simple four-layer PCB. This design reduces core loss by over half compared to state-of-the-art techniques, thanks to flux cancellation and reduced flux density. Additionally, it integrates output capacitors and SRs into the secondary winding, minimizing leakage and termination loss.

With GaN devices, the proposed design enables higher switching frequencies, enhancing efficiency, power density, and manufacturability. A 1MHz 380V/12V 800W LLC converter prototype using the proposed structure achieved 97.6% peak efficiency and a power density of 900W/inch³.