Variable Frequency Average Current Mode Control for ZVS Symmetrical Dual-Buck H-Bridge All-GaN Inverter

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Abstract
The quasi-square-wave (QSW) zero-voltage-swit-ching (ZVS) symmetrical dual-Buck H-bridge (DBHB) all-GaN inverter has been proven to have high efficiency and high density in Google’s Little Box Challenge. This variable frequency QSW ZVS inverter is drawing more and more interests to the industry and academia. However, the AC current control with cycle-by-cycle variable frequency ZVS operation is a basic challenge for this inverter. The conventional control methods for this QSW ZVS inverter, such as the hysteresis current mode control and the variable on time control, require high-speed current sensing or zero-crossing-detection (ZCD) circuits, which are very sensitive to the noises. In this paper, a variable frequency average current mode control with a variable frequency pulse-width-modulation (PWM) is discussed and implemented for this QSW ZVS inverter to eliminate the high-speed current sensing or ZCD circuits. For any synchronous half-bridge type converter, the voltage conver-sion ratio is only dependent on the duty-cycle in either continuous-conduction-mode (CCM) or QSW ZVS mode. The frequency of the PWM triangular carrier is another degree of freedom. In this control method, the averaged inductor current is sensed and regulated with the average current mode feedback controller, and the duty-cycle is generated by the average current controller. The frequency of the PWM carrier is variable and directly calculated based on the duty-cycle signal, the sensed and filtered voltage and current signals. Since all the sensed signals for this current controller are low speed and filtered signals, this current control is highly immune to the noises. The reliability of ZVS for this inverter is improved. The complexity of the controller is also reduced. The detailed analysis of the QSW ZVS conditions for the full range operation of the symmetrical DBHB inverter are investigated in this paper. More negative currents can be introduced to reduce the switching frequency and increase the reliability of ZVS. A frequency limiter helps limit the frequency when the current is close to zero or under light load conditions. A frequency updating method with a constant and low interrupt frequency in digital controller is also discussed. Finally, to verify this control method, a 1kW all-GaN DBHB inverter prototype is developed and tested in this paper.
Funding Information
  • donated samples from Navitas Semiconductor

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