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Design and Control of High Power Wide Gain Range LLC Converters

Circuit and waveforms
Fig. 1. Circuit topology and waveforms of three-mode operations
  In LLC converters with regulation requirements, usually there are two constrains in the design procedure: First is to select appropriate magnetizing inductance Lm and deadtime td according to selected power switches to realize zero voltage switching (ZVS) at DCX operation (fsfo) while achieving a minimum device loss (or close to the minimum). The second constrain is to satisfy the gain requirement within a practical switching frequency range.
  However, in high-power wide-gain-range applications, there is hardly a solution to meet both constrains. Thus, multi control methods are combined to solve the problem. In general, the con-verter will operate in DCX mode when the input voltage is in the nominal voltage range to maxim-ize the peak efficiency and operate in Boost mode when the input voltage is lower than nominal values to boost gain and operate in PWM mode when the input voltage is higher than nominal values to reduce switching frequency and device losses.
  Fig. 1. shows the topology and key waveforms of three-mode operations. 3D gain curves and operating path are shown in Fig. 2.
3D gain curves
Fig. 2. 3D gain curves and operating path

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