Variable Frequency Multiplier Technique for High Efficiency

This paper presents a variable frequency multiplier technique (VFX) that enables design of converters for wide input and/or output voltage ranges while preserving high efficiency. The technique is applied to an LLC converter to demonstrate the effectiveness of this technique for converters having wide input voltage variation such as universal input power supplies. This technique compresses the effective operating range required of a resonant converter by switching the inverter and/or rectifier operation between processing energy at a fundamental frequency and one or more harmonic frequencies. The implemented converter operates over an input voltage range of 85 V to 340 V but the resonant tank and conversion ratio has only been designed for half this range and VFX mode of the inverter is used to enhance this to the full range. Experimental results from a 50 W converter show an efficiency of 94.9% to 96.6% across the entire input voltage range, demonstrating the advantage of using this technique in such applications. Introduction A trend in power electronics has been to strive for high power density and efficiency across over a wide operation range [1]. High power density can be achieved by switching the converter at a high frequency. At these high frequencies, resonant converters use soft switching (i.e. Zero Voltage Switching (ZVS) and Zero Current Switching (ZCS)) to reduce the overlap of current and voltage during switching transitions to achieve high efficiency [2], [3]. Although soft-switched resonant converters can achieve high efficiency at nominal operation, the efficiency tends to degrade considerably with variation in input voltage, output voltage and power level [4]. Resonant converters commonly use frequency control [2], [3] and/or phase shift control [5], [6] to compensate for variation in voltage and power levels. If switching frequency is increased to reduce power or gain of the converter, such as in Series Resonant Converters operated above resonance for ZVS, switching losses will increase. Also, with a wide frequency range the magnetics cannot be optimally designed. Furthermore, circulating currents may increase resulting in higher loss at light loads. With phase shift control operation over a wide range is likewise challenging. The two legs of the inverters are phase shifted with respect to each other and thus have asymmetrical current levels at the switching transitions. The inverter leg in which current is leading the voltage can lose ZCS and the leg in which current is lagging can lose ZVS. Other control techniques such as asymmetrical current mode control [7] and asymmetrical duty cycle PWM control [8] also have limitations such as loss of ZVS. In this paper, a Variable Frequency Multiplier (VFX) technique is introduced and its effectiveness is demonstrated for a universal input power supply. In the VFX technique, additional "frequency multiplier" modes of operation of the inverter and/or rectifier are used to provide additional sets of operating characteristics for the converter to achieve and maintain high performance across a wide operating range. Frequency multiplier circuits are often used in extreme high-frequency RF applications (e.g., where transistor f! is a concern), and are sometimes used in switched-mode inverters and power amplifiers (e.g., [9], [10]). While it has been proposed to employ frequency multipliers in dc-dc converters (e.g., [11], [12]), this is not usually done, as the output power of a frequency multiplier inverter is inherently low relative to the needed device ratings. However, here we propose use frequency multiplication as an additional operating mode of the inverter rectifier, for wide-range voltage and/or power conditions. In this context, frequency multiplication can be used to extend the efficient operating range of a converter and improve its performance across power and voltage. While the proposed VFX technique can be applied to the inverter and/or rectifier and for wide input and/or output range, here we demonstrate it for wide input voltage range using VFX of the inverter. Universal input power supplies need to operate over a wide input voltage range and it is a challenge to design resonant power converters for such wide range of operation. In this paper we demonstrate the VFX technique in the inverter of LLC converter to achieve efficient operation across an input voltage range from 85-340 V. 2014 IEEE Energy Conversion Congress and Exposition, Sept. 2014 (to appear).