Dual SmartRectifier and DirectFET® Chipset Overcomes Package Source Inductance Effects and Provides Accurate Sensing for Synchronous Rectification in DC-DC Resonant Converters

Abstract

The off-line-power-supply market is increasingly demanding greater power efficiency and, even more often, multiple output voltages. While the general market trend and government regulations have been driving AC-DC-converterefficiency requirements, the second trend is a function of applications' power requirements as is the case in typical consumer-electronic equipment such as LCD TVs.This paper describes how an innovative chipset achieves output synchronous rectification in LLC-resonant convertersamongst the most common topologies. Multiple secondary windings can easily generate several output voltages, but this approach increases the circuit complexity for efficient synchronous-rectification control. The proposed approach consists of an SOIC-8-packaged dual SmartRectifierTM controller coupled with DirectFETO transistors. I. LLC SERIES RESONANT CONVERTER WITH MULTIPLE OUTPUT VOLTAGESThe LLC-resonant converter offers significant advantages that make it an attractive topology compared to PWM converters in front-end DC-bus conversion in medium-and high-power AC-DC power supplies. These advantages include soft-switching independent of load, lower peak currents, greater power density, lower electrical stress on its power devices, and lower EMI emissions. Among the different resonant topologies, the LLC resonant converter has been the most attractive topology for front-end DC bus conversion [1]. It utilizes the magnetizing inductance of the transformer to construct a complex resonant tank with buck/boost transfer characteristics in the soft-switching region [2].Typical implementations of these converters use diodes for secondary rectification due to the complexity, cost, and Vout 2 Cres Figure 1: multi-output series resonant LLC converter with Schottky Rectifiers.unsatisfactory performance of existing discrete and integrated alternatives ( Figure 1). Additionally, these converters must operate in burst mode during light-load conditions to meet the stringent standby power constraints from various energycompliance agencies such as Energy Star and the CEC. With few exceptions [3,4], there have been few research efforts for implementing output synchronous rectification in resonant converters; existing controller ICs for synchronous rectification are based on PLL control and rely on synchronizing signals from the primary-side to anticipate the secondary device turn-off transitions. Apart from the fact that these control techniques are not suitable for synchronous rectification in resonant converters, these ICs cannot operate under burst-mode conditions. Further, the complex operating modes of resonant converters allow neither self-driven gatedrive techniques nor the use of primary gate signals to drive the secondary-side MOSFETs [4].

Dual edge tracking control for synchronous rectification (SR) of LLC resonant converter

“…Therefore, secondary winding voltage cannot be used to achieve self-driven synchronous rectification. Recently, extensive studies have been carried out to generate proper gate drive signal for SR MOSFET of LLC resonant converter [5][6][7][8][9][10][11][12][13][14][15]. In general, the SR driving method can be classified into three methods; voltage based, current based and control based methods.…”

“…Therefore, special care should be taken in PCB layout when discrete MOSFETs are required for SR [5]. Special package or integration of MOSFET and SR controller into one package is also required to minimize the effect of the stray inductance [6]. To compensate the induced voltage across the stray inductance, a compensation network composed of capacitor and active switch was proposed in [7].…”

“…The voltage based method determines the turn-on and off instants based on the instantaneous SR drain-to-source voltage [5][6][7] as illustrated in Figure 1. Once the body diode of SR starts conducting, drain-to-source voltage drops below the turn-on threshold triggering the turn-on of SR. Then, SR current resonates and the SR current is proportional to drainto-source voltage.…”