Inrush Current Limit or Extreme Startup Response for LLC Converters Using Average Geometric Control

摘要

LLC resonant converters suffer from a startup inrush current that may push power switches beyond the safe operating area. The conventional method of limiting the startup inrush current is to adopt the frequency decrement method; however, doing so, results in the overdesign of magnetic components and the gate driver circuit, sluggish startup response, and circulating current in light and no-load conditions. To tackle these problems, this paper proposes a nonlinear controller called the average geometric controller (AGC), offering low-cost implementation requirements. The proposed controller uses the advantages of a new LLC average large-signal model, in order to analyze the large-signal nature of the converter. Studying the average nature of LLC converters enables the use of low-bandwidth sensors, and lower sampling rates, while improving the system performance. In addition to limiting the startup inrush current without employing a very high switching frequency, the proposed AGC provides an extreme dynamic startup response for LLC converters, with near zero voltage overshoot. In order to validate the theoretical analysis, experimental results of an LLC converter are presented using the proposed controller. Comparative experimental analysis is performed with a conventional method for limiting inrush current. Moreover, the accuracy of the average large-signal model is experimentally validated.