GaN-based DC-DC Conversion Achieving High Reliability, Low EMI and Balanced System Performance



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DC-DC power conversion circuits improve the switching frequency consistently over the past five decades, pursuing better dynamic response and higher power density. To empower such a trend, silicon power transistors have been advancing continuously. However, they are approaching the theoretical limit of performance, slowing down the developing pace of power electronics. With superior switching characteristic, gallium nitride (GaN) high-electron-mobility transistor (HEMT) rapidly emerged, pushing the operating speed of DC-DC power circuits to a record high level. GaN technology is thus recognized as a promising candidate to enable the next-generation switching power conversion. However, it still faces formidable challenges before the industry-wide adoption, including unique reliability issues, considerable electromagnetic interference (EMI) emissions and intensified power design trade-offs. This dissertation delivers key innovations in power stage, gate driver and control scheme, intending to conquer these challenges. To improve the reliability of GaN power stage, an online condition monitoring is developed to prognose the current-collapse (or i-collapse) effect in GaN HEMT, sensing its dynamic onresistance as aging precursor. A gate leakage inspired junction temperature TJ sensor is integrated to determine the TJ of GaN HEMT, facilitating the calibration of temperature effect on the dynamic on-resistance. As the benefit, TJ-independent online condition monitoring is accomplished, significantly improving the monitoring accuracy. Further, to enhance the system longevity, a proactive temperature frequency scaling scheme is designed to modulate the operating speed according to the thermal stress and power conditions, thereby extending the GaN lifetime while minimizing the impact on the system performance of the converter. To reduce the conducted EMI noise, an adaptive strength gate driving scheme is developed for GaN HEMT. By modulating the driving strength at the start point of Miller Plateau during the switching transitions, it achieves an independent control of low di/dt and high dv/dt. Thus, the conducted EMI noise, mainly caused by di/dt, is reduced, while the switching power loss overhead is minimized. By such a means, the classic design trade-off between EMI noise and power efficiency is effectively balanced. To facilitate such an active control, an emulated Miller Plateau tracking scheme is proposed to identify the critical di/dt and dv/dt instants, which are susceptible to load current and power input voltage conditions. These proposed techniques are incorporated in a GaN-based buck converter for verification. Moreover, a continuous random spread-spectrum-modulation (C-RSSM) technique is utilized to scatter the EMI spectra evenly and continuously, attenuating the EMI further. For demonstration, the proposed C-RSSM scheme is applied to a GaN-based buck converter with peak current mode control. In the meantime, a one-cycle on-time rebalancing scheme is designed to overcome the crossover frequency limit existing in the conventional PWM control, thereby stabilizing the duty ratio under frequency modulation. Beneficially, the output jittering effect induced by RSSM is removed, balancing the trade-off between EMI and output regulation.



Gallium nitride, Transistors, Electromagnetic interference, Spread spectrum communications