S. Ghafoor
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The global transition toward sustainable and electrified energy systems is fundamentally transforming electrical power grids. Increasing penetration of renewable energy sources, electrification of transport and heating, and widespread deployment of power-electronics-based interfaces are driving the evolution of today’s AC grids toward future hybrid AC/DC architectures. While this transformation enhances flexibility and controllability, it also introduces fast switching transients, non-sinusoidal waveforms, and steep voltage gradients that impose novel electrical stresses on existing grid assets. Conventional high-voltage test equipment, designed for sinusoidal AC, DC, and impulse testing, is no longer sufficient to realistically replicate these in-service conditions. This creates a clear need for compact, flexible, and high-voltage Arbitrary Waveform Generator (AWG) capable of emulating future grid stresses. Power-electronics-based AWGs using modular multilevel converter (MMC) architecture offer a promising solution due to its scalability and waveform flexibility. However, practical implementation of MMC-based high-voltage test sources is often limited by excessive system complexity, large numbers of low-voltage submodules, more points of failure, and high cost. Increasing the voltage capability of a single MMC submodule therefore emerges as a key enabler for reducing system complexity, footprint, and cost while maintaining high performance.
Within this context, this PhD thesis addresses the fundamental challenge of realizing high-voltage and high-speed switching, using commercially available low-voltage wide-bandgap semiconductor devices. The work investigates series connection of SiC MOSFETs and GaN HEMTs as a cost-effective and scalable approach to enhance voltage-blocking capability.
The thesis establishes a comprehensive understanding of voltage imbalance mechanisms in series-connected devices, identifying gate-drive signal mismatch as the dominant contributor to dynamic voltage imbalance, while also revealing the critical and often overlooked influence of measurement-probe-induced parasitics on voltage distribution across series connected devices. Based on these insights, a transformer-coupled, gate-current-synchronized driving approach is identified as the most effective voltage-balancing technique. To overcome the inherent frequency and duty-cycle limitations of conventional transformer-based drivers, a novel programmable dual-transformer gate driving architecture is developed. This approach decouples switching control from transformer constraints, enabling flexible, microcontroller-compatible arbitrary waveform generation while maintaining nearly uniform voltage sharing across series-connected SiC MOSFETs. Experimental validation demonstrates stable operation at kilovolt levels with nearly even voltage balance.
The work further extends and modifies the proposed new series-connection and gate-current synchronization concepts to ultrafast GaN HEMTs, addressing the challenges posed by nanosecond-scale switching. The developed open-loop, dual transformer gate driving strategy is shown to be well suited for GaN devices, and systematic optimization of transformer and excitation-stage parameters enables balanced voltage sharing at kilovolt levels while preserving the intrinsic speed advantages of GaN technology, confirmed by experimental validation on a hardware prototype under high-voltage, high-dv/dt switching conditions.
Overall, this thesis provides a simple, scalable, and experimentally proven high-voltage switching solution that enables low-voltage wide-bandgap devices to be used in medium-voltage systems. The proposed high-voltage switch has the potential to significantly reduce the complexity of MMC-based arbitrary waveform generator, enabling compact, cost-effective high voltage testing system capable of emulating realistic electrical stresses of renewables-rich hybrid power grid.
...
Within this context, this PhD thesis addresses the fundamental challenge of realizing high-voltage and high-speed switching, using commercially available low-voltage wide-bandgap semiconductor devices. The work investigates series connection of SiC MOSFETs and GaN HEMTs as a cost-effective and scalable approach to enhance voltage-blocking capability.
The thesis establishes a comprehensive understanding of voltage imbalance mechanisms in series-connected devices, identifying gate-drive signal mismatch as the dominant contributor to dynamic voltage imbalance, while also revealing the critical and often overlooked influence of measurement-probe-induced parasitics on voltage distribution across series connected devices. Based on these insights, a transformer-coupled, gate-current-synchronized driving approach is identified as the most effective voltage-balancing technique. To overcome the inherent frequency and duty-cycle limitations of conventional transformer-based drivers, a novel programmable dual-transformer gate driving architecture is developed. This approach decouples switching control from transformer constraints, enabling flexible, microcontroller-compatible arbitrary waveform generation while maintaining nearly uniform voltage sharing across series-connected SiC MOSFETs. Experimental validation demonstrates stable operation at kilovolt levels with nearly even voltage balance.
The work further extends and modifies the proposed new series-connection and gate-current synchronization concepts to ultrafast GaN HEMTs, addressing the challenges posed by nanosecond-scale switching. The developed open-loop, dual transformer gate driving strategy is shown to be well suited for GaN devices, and systematic optimization of transformer and excitation-stage parameters enables balanced voltage sharing at kilovolt levels while preserving the intrinsic speed advantages of GaN technology, confirmed by experimental validation on a hardware prototype under high-voltage, high-dv/dt switching conditions.
Overall, this thesis provides a simple, scalable, and experimentally proven high-voltage switching solution that enables low-voltage wide-bandgap devices to be used in medium-voltage systems. The proposed high-voltage switch has the potential to significantly reduce the complexity of MMC-based arbitrary waveform generator, enabling compact, cost-effective high voltage testing system capable of emulating realistic electrical stresses of renewables-rich hybrid power grid.
...
The global transition toward sustainable and electrified energy systems is fundamentally transforming electrical power grids. Increasing penetration of renewable energy sources, electrification of transport and heating, and widespread deployment of power-electronics-based interfaces are driving the evolution of today’s AC grids toward future hybrid AC/DC architectures. While this transformation enhances flexibility and controllability, it also introduces fast switching transients, non-sinusoidal waveforms, and steep voltage gradients that impose novel electrical stresses on existing grid assets. Conventional high-voltage test equipment, designed for sinusoidal AC, DC, and impulse testing, is no longer sufficient to realistically replicate these in-service conditions. This creates a clear need for compact, flexible, and high-voltage Arbitrary Waveform Generator (AWG) capable of emulating future grid stresses. Power-electronics-based AWGs using modular multilevel converter (MMC) architecture offer a promising solution due to its scalability and waveform flexibility. However, practical implementation of MMC-based high-voltage test sources is often limited by excessive system complexity, large numbers of low-voltage submodules, more points of failure, and high cost. Increasing the voltage capability of a single MMC submodule therefore emerges as a key enabler for reducing system complexity, footprint, and cost while maintaining high performance.
Within this context, this PhD thesis addresses the fundamental challenge of realizing high-voltage and high-speed switching, using commercially available low-voltage wide-bandgap semiconductor devices. The work investigates series connection of SiC MOSFETs and GaN HEMTs as a cost-effective and scalable approach to enhance voltage-blocking capability.
The thesis establishes a comprehensive understanding of voltage imbalance mechanisms in series-connected devices, identifying gate-drive signal mismatch as the dominant contributor to dynamic voltage imbalance, while also revealing the critical and often overlooked influence of measurement-probe-induced parasitics on voltage distribution across series connected devices. Based on these insights, a transformer-coupled, gate-current-synchronized driving approach is identified as the most effective voltage-balancing technique. To overcome the inherent frequency and duty-cycle limitations of conventional transformer-based drivers, a novel programmable dual-transformer gate driving architecture is developed. This approach decouples switching control from transformer constraints, enabling flexible, microcontroller-compatible arbitrary waveform generation while maintaining nearly uniform voltage sharing across series-connected SiC MOSFETs. Experimental validation demonstrates stable operation at kilovolt levels with nearly even voltage balance.
The work further extends and modifies the proposed new series-connection and gate-current synchronization concepts to ultrafast GaN HEMTs, addressing the challenges posed by nanosecond-scale switching. The developed open-loop, dual transformer gate driving strategy is shown to be well suited for GaN devices, and systematic optimization of transformer and excitation-stage parameters enables balanced voltage sharing at kilovolt levels while preserving the intrinsic speed advantages of GaN technology, confirmed by experimental validation on a hardware prototype under high-voltage, high-dv/dt switching conditions.
Overall, this thesis provides a simple, scalable, and experimentally proven high-voltage switching solution that enables low-voltage wide-bandgap devices to be used in medium-voltage systems. The proposed high-voltage switch has the potential to significantly reduce the complexity of MMC-based arbitrary waveform generator, enabling compact, cost-effective high voltage testing system capable of emulating realistic electrical stresses of renewables-rich hybrid power grid.
Within this context, this PhD thesis addresses the fundamental challenge of realizing high-voltage and high-speed switching, using commercially available low-voltage wide-bandgap semiconductor devices. The work investigates series connection of SiC MOSFETs and GaN HEMTs as a cost-effective and scalable approach to enhance voltage-blocking capability.
The thesis establishes a comprehensive understanding of voltage imbalance mechanisms in series-connected devices, identifying gate-drive signal mismatch as the dominant contributor to dynamic voltage imbalance, while also revealing the critical and often overlooked influence of measurement-probe-induced parasitics on voltage distribution across series connected devices. Based on these insights, a transformer-coupled, gate-current-synchronized driving approach is identified as the most effective voltage-balancing technique. To overcome the inherent frequency and duty-cycle limitations of conventional transformer-based drivers, a novel programmable dual-transformer gate driving architecture is developed. This approach decouples switching control from transformer constraints, enabling flexible, microcontroller-compatible arbitrary waveform generation while maintaining nearly uniform voltage sharing across series-connected SiC MOSFETs. Experimental validation demonstrates stable operation at kilovolt levels with nearly even voltage balance.
The work further extends and modifies the proposed new series-connection and gate-current synchronization concepts to ultrafast GaN HEMTs, addressing the challenges posed by nanosecond-scale switching. The developed open-loop, dual transformer gate driving strategy is shown to be well suited for GaN devices, and systematic optimization of transformer and excitation-stage parameters enables balanced voltage sharing at kilovolt levels while preserving the intrinsic speed advantages of GaN technology, confirmed by experimental validation on a hardware prototype under high-voltage, high-dv/dt switching conditions.
Overall, this thesis provides a simple, scalable, and experimentally proven high-voltage switching solution that enables low-voltage wide-bandgap devices to be used in medium-voltage systems. The proposed high-voltage switch has the potential to significantly reduce the complexity of MMC-based arbitrary waveform generator, enabling compact, cost-effective high voltage testing system capable of emulating realistic electrical stresses of renewables-rich hybrid power grid.
Journal article
(2024)
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W. Zhao, S. Ghafoor, Gijs Willem Lagerweij, Gert Rietveld, P.T.M. Vaessen, M. Ghaffarian Niasar
This paper comprehensively reviews several techniques that address the static and dynamic voltage balancing of series-connected MOSFETs. The effectiveness of these techniques was validated through simulations and experiments. Dynamic voltage-balancing techniques include gate signal delay adjustment methods, passive snubbers, passive clamping circuits, and hybrid solutions. Based on the experimental results, the advantages and disadvantages of each technique are investigated. Combining the gate-balancing core method with an RC snubber, which has proven both technically and commercially attractive, provides a robust solution. If the components are sorted and binned, voltage-balancing techniques may not be necessary, further enhancing the commercial viability of series-connected MOSFETs. An investigation of gate driver topologies yields one crucial conclusion: magnetically isolated gate drivers offer a simple and cost-effective solution for high-frequency (HF) applications (2.5–50 kHz) above 8 kV with an increased number of series devices. Below 8 kV, it is advantageous to move the isolation barrier from the gate drive IC to an optocoupler and isolated supply, allowing for a simple design with commercially available components.
...
This paper comprehensively reviews several techniques that address the static and dynamic voltage balancing of series-connected MOSFETs. The effectiveness of these techniques was validated through simulations and experiments. Dynamic voltage-balancing techniques include gate signal delay adjustment methods, passive snubbers, passive clamping circuits, and hybrid solutions. Based on the experimental results, the advantages and disadvantages of each technique are investigated. Combining the gate-balancing core method with an RC snubber, which has proven both technically and commercially attractive, provides a robust solution. If the components are sorted and binned, voltage-balancing techniques may not be necessary, further enhancing the commercial viability of series-connected MOSFETs. An investigation of gate driver topologies yields one crucial conclusion: magnetically isolated gate drivers offer a simple and cost-effective solution for high-frequency (HF) applications (2.5–50 kHz) above 8 kV with an increased number of series devices. Below 8 kV, it is advantageous to move the isolation barrier from the gate drive IC to an optocoupler and isolated supply, allowing for a simple design with commercially available components.
Journal article
(2024)
-
Sohrab Ghafoor, Mahesh Kulkarni, Reza Mirzadarani, Peter Vaessen, Mohamad Ghaffarian Niasar
To enhance the voltage-handling capability of a switch, the series connection of switching devices is a cost-effective method that preserves many advantages of mature low-voltage devices. Dynamic voltage imbalance and electrical isolation for the devices at the high voltage (HV) side are two important challenges associated with series connection topology. Transformer-coupled gate drivers are excellent for providing both dynamic voltage balance and high galvanic isolation. However, they can only provide the switching function at the transformer pulse frequency. To generate complex waveforms of future power-electronics-dominated grids, a switch with user-defined turn-on/off timing is required for testing grid assets under high-voltage conditions. This article presents a simple, cost-effective open-loop gate driver that overcomes this limitation by introducing two sets of complementary pulse transformers to initialize programmable frequency and duty cycle. Successful experimental verification of the series-connected SiC mosfets prototype is performed at 3.2 kV at various frequencies and duty cycles. The article also demonstrates that the measurement probes placed across series-connected mosfets significantly affect the voltage distribution and validate a compensation mechanism.
...
To enhance the voltage-handling capability of a switch, the series connection of switching devices is a cost-effective method that preserves many advantages of mature low-voltage devices. Dynamic voltage imbalance and electrical isolation for the devices at the high voltage (HV) side are two important challenges associated with series connection topology. Transformer-coupled gate drivers are excellent for providing both dynamic voltage balance and high galvanic isolation. However, they can only provide the switching function at the transformer pulse frequency. To generate complex waveforms of future power-electronics-dominated grids, a switch with user-defined turn-on/off timing is required for testing grid assets under high-voltage conditions. This article presents a simple, cost-effective open-loop gate driver that overcomes this limitation by introducing two sets of complementary pulse transformers to initialize programmable frequency and duty cycle. Successful experimental verification of the series-connected SiC mosfets prototype is performed at 3.2 kV at various frequencies and duty cycles. The article also demonstrates that the measurement probes placed across series-connected mosfets significantly affect the voltage distribution and validate a compensation mechanism.