Virtual prototype and GaN HEMT based high frequency LLC converter design

: Gallium nitride (GaN) high-electron-mobility transistor (HEMT) with the merits of the fast switch speed, low on resistance and low switch loss is applicable to high-frequency LLC converters. However, voltage stress and ringing problems during the fast switching transient have retarded the promotion of switch speed and prolonged the design cycles. This study proposed an economic and time-saving virtual prototype based LLC converter design method, based on which the voltage stress and ringing problem can be analysed and suppressed by iteration design. The virtual prototype comprises of compact GaN HEMT device model and accurate circuit model. The correctness and effectiveness of the virtual-prototype-based design method is verified with well matching simulation and experiment waveforms.


Introduction
LLC converter can operate in soft switch mode which makes it very attractive to high switch frequency and high power density applications, such as power supply for data centres and communication stations [1]. The design method of LLC converter resonant parameters has already been thoroughly detailed in many papers [1,2]. However, the design difficulties of converter hardware are becoming more and more challenging as the switching speed is increasing, especially when the emerging gallium nitride (GaN) high-electron-mobility transistor (HEMT) are adopted [3,4].
The fast switch speed of GaN HEMT evokes high voltage stress and annoying ringing problems which has increased the instability and electromagnetic interference noise to the converter, especially for badly designed printed circuit board (PCB). Reusch and Strydom [5] has pointed out that the voltage stress and ringing come from the resonance between power loop inductance and power switch parasitic capacitance and has given some advances to minimise the power loop stray inductance by optimising the PCB layout and routing, but it has not given any quantitative analysis. Long et al. [6] proposed a comprehensive analysis of GaN turn-on overvoltage problem which is helpful to understand the effect that power loop stray inductance and switch speed have on the switching voltage stress; however, it was too complex for engineering application. For a long time, there has been an eager demand for a feasible method to direct the design of power converter hardware, which can help to determine whether the power loop stray inductance of the power converter is acceptable under an allowable maximum voltage stress. Therefore, this paper proposes a virtual-prototype-based LLC converter hardware iteration design method to facilitate the engineering design process.

Virtual prototype based design method
The circuit diagram of the LLC converter is shown in Fig. 1, in which Q 1 -Q 4 are GaN devices with the device type EPC2032, and inductance L r1 , capacitance C r and magnetic inductance L m make up the resonant tank. L ds shown in Fig. 1 are the stray inductance of the power loop and it is this inductance resonating with the parasitic capacitance of GaN switch that results the voltage stress and ringing problems.
The virtual prototype is a simulation system mainly comprising of the circuit parameters and device models, which can be used to find out the upper limit L ds(max) of the stray inductance under an acceptable maximum voltage stress by parameter sweep. The power loop stray inductance L ds can be controlled by deliberately designed PCB layout and wire routing. It must be pointed out that the power loop inductance is not a simple sum of all the stray inductances distributed along the power loop because of the existence of mutual inductances, while it can be accurately extracted by Ansys Q3D. The LLC converter hardware design can be realised by a virtual iteration method as shown in Fig. 2.
The up-to-date GaN device models are freely accessible from device manufacture's website. The output characteristics and parasitic capacitances data of GaN HEMT EPC2032 drawn from the device model and device datasheet are shown and compared in Fig. 3. From Fig. 3, it can be drawn that the device model is already accurate enough for virtual prototype simulation.

Experiment verification
The design parameters of the LLC converter are listed in Table 1. With the compact GaN HEMT model and design parameters, the simulation platform can be constructed to find out the upper limits of power loop stray inductance under different driving resistances and maximum voltage stress constraints. Based on the virtual prototype simulation, it can be found out that the power loop stray inductance should be controlled within 2 nH with 15% voltage safe margin warranted when 100 V voltage rating GaN HEMTs are adopted.
An optimised PCB layout and routing has been found as shown in Figs. 4b and c by iteration design, and the power loop stray inductance has been controlled to as low as 0.3 nH. It is not until the optimal PCB layout has been found that converter hardware will be manufactured, which can save a lot of time and cost. The experiment board used to verify the effectiveness of the simulation prototype is shown in Fig. 4a.
The experiment measured and simulated gate-source and drainsource voltage of the switch Q2 and resonant current of the  Fig. 1 are shown and compared in Fig. 5. The switch frequency of the LLC converter is as high as 1.3 MHz. From Fig. 5, it can be seen that the simulated waveforms are highly consistent with the experiment measured waveforms, which can effectively verify the correctness and effectiveness of the proposed virtual prototype based LLC converter design method.

Conclusion
This paper has proposed a virtual-prototype-based high-frequency LLC converter design method feasible for engineering application. The effectiveness of the proposed design method has been verified by experimental result. The proposed design method can facilitate the usage of GaN HEMT and promote the success probability of LLC hardware designs, which is meaningful to save a lot of design time and cost.

Acknowledgments
This work was supported in part by the National Natural Science Foundation of China under Grant no. 51777186.