Recently, three master’s students from the ZJU-PMIC team successfully passed their thesis defenses. The two students who defended this time achieved innovative results in areas including high-temperature dynamic resistance evaluation of GaN devices and series-connected silicon carbide power modules.

Thesis Title: Evaluation and Analysis of High-Temperature Dynamic Resistance of GaN Devices Based on Multiple Double-Pulse Tests

Defendant: Jiang Xu (Master’s Student)

Supervisor: Wu Xinke

Research Summary: Gallium nitride high electron mobility transistors (GaN HEMTs) have broad application prospects in high-frequency, high-efficiency power electronics, but their dynamic on-resistance degradation seriously affects high-temperature reliability and efficiency. Existing evaluation methods are mostly limited to room temperature and simple operating conditions, making it difficult to reflect actual complex operating states. This paper builds a high-temperature dynamic resistance test platform with controllable junction temperature, adopts a multiple double-pulse test method, and further simulates complex operating modes such as intermittent operation of the device by adjusting the voltage stress time between adjacent double pulses (tstress), thereby achieving dynamic resistance evaluation of GaN devices under specific operating conditions and specific junction temperatures.

The study comparatively evaluates the high-temperature dynamic behavior of two typical devices—Hybrid Drain Gate Injection Transistor (HD-GIT) and Enhancement-mode p-GaN gate (E-Mode p-GaN)—and, from the perspective of semiconductor physics, deeply reveals the internal mechanisms of dynamic resistance degradation and behavioral differences in GaN devices, providing experimental evidence and theoretical reference for GaN device structure optimization and engineering selection.

Thesis Title: Research on Overcurrent Protection and Junction Temperature Detection Technology for Series-Connected Silicon Carbide Power Modules

Defendant: Hou Chunyao (Master’s Student)

Supervisor: Shao Shuai

Research Summary:

Directly connecting low-voltage SiC MOSFETs in series is an important technical route for building high-power-density medium-voltage power electronic equipment. For high-power applications, multiple SiC MOSFET chips usually need to be integrated and packaged in series-parallel configurations to form high-voltage, high-current SiC power modules. However, such modules have a large number of internal chips and complex structures, and simultaneously face issues such as chip parallel current sharing, device series voltage balancing, and thermal coupling, which can easily cause electro-thermal instability. Therefore, building comprehensive electrical stress protection and thermal state evaluation is a prerequisite for ensuring their reliable operation. Focusing on the above issues, this paper takes high-voltage, high-current SiC MOSFET series power modules as the research object and conducts research on overcurrent protection and junction temperature detection technology.

To address the demand of series-connected SiC modules for fast overcurrent detection and synchronous fault signal distribution, ZJU-PMIC proposes an overcurrent protection technology based on an integrated Tunnel Magnetoresistance (TMR) current sensor and optical isolation synchronous distribution. The full link response time of this technology—”measurement–decision–distribution”—is 81.6 ns, and the synchronization deviation among branches is less than 230 ps, verifying the effectiveness of this technology in overcurrent protection of SiC MOSFET series modules. To address the difficulty of directly measuring the internal chip junction temperature of SiC MOSFET series modules, a junction temperature detection method based on body diode forward voltage drop Vsd is proposed. Through back-to-back experiments on single devices and series-connected device modules, the measured junction temperature deviation is controlled within 2 ℃, proving the accuracy and applicability of this method in junction temperature detection of series-connected SiC power modules.

Thesis Title: Research on CLLC On-Board Charging Power Supply Based on Multiple Phase-Shift Control

Defendant: Du Wenqi (Master’s Student)

Supervisor: Zhang Junming

Research Summary:

As new energy vehicles gradually become a key force driving the green and low-carbon transformation of the transportation sector, the on-board charger (OBC), as a core energy replenishment component, needs to achieve high efficiency and high power density operation over a wide output voltage range. The CLLC resonant converter is the mainstream topology for high-voltage isolated bidirectional DC/DC in on-board chargers, but under traditional frequency modulation (PFM), it suffers from problems such as an excessively wide frequency regulation range, large circulating energy, and limited step-up gain. To this end, this research takes a 6.6 kW on-board charger as the research object and studies the multiple phase-shift control technology, time-domain model, multi-degree-of-freedom control parameter optimization, system-level control optimization, and experimental prototype development of the CLLC converter.

A variable-frequency multiple phase-shift control strategy is proposed, and time-domain modeling of the CLLC converter under multiple phase-shift control is carried out, obtaining high-precision steady-state time-domain models under three gain modes of the converter. A loss-optimized design method for resonant tank hardware parameters and control variables is constructed, and a simplified control method of “switching frequency power closed-loop + phase-shift angle one-dimensional lookup table” is proposed. Combined with circuit simulation, the advantages of this control strategy in narrowing the operating frequency range, suppressing circulating energy, and reducing device current stress are demonstrated. To address engineering problems arising in the practical application of multiple phase-shift control, methods such as low-frequency alternating switching of leading and lagging bridge arms, a pulse control strategy based on hiccup-density modulation, and a sampling correction strategy based on load feedforward are designed. A 6.6 kW liquid-cooled OBC prototype was developed. The magnetically integrated CLLC achieves zero-voltage switching (ZVS) for all switches and low circulating energy over a wide output voltage range of 230 V–480 V, with a peak efficiency of 97.8%, verifying the effectiveness of the proposed multiple phase-shift control method.