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Power supply modules improve efficiency by ‘valley switching’

Minimal extra component count adds new operating mode

25th May 2007 - Fairchild (www.fairchildsemi.com) has launched the Green FPS (Fairchild Power Switch) e-Series for flyback power supply designs that reduce standby power to very low levels. Fairchild designed the products for supplies of 25W and under, targeting consumer electronics products. in quasi-resonant mode, they use a technique called valley switching, and the company says that overall conversion efficiency improves by 1%, and levels of EMI by up to 5 dB, compared to conventional hard-switch converter topologies. FPS devices combine an avalanche-rated SenseFET, a current mode pulse width modulation (PWM) IC and various protection functions. Utilising advanced burst mode operation, the devices meet stand-by power regulations by reducing standby power consumption to below 0.2W at no load conditions (below 1W at 0.5W load). When you design with the devices, you can constrain operation to be within a narrow frequency range, maintaining valley switching, in which the MOSFETs turn on with minimum drain voltage and exhibit inherent frequency modulation resulting in the EMI noise reduction. The internal circuitry looks for a “valley” in the waveform to select he optimum moment at which to switch – however, the FETs are capable of conventional “hot” switching. The extra components to implement this mode are four passive components and a diode. Valley switching operation also enables these converters to perform soft switching, therefore increasing power conversion efficiency by 1%. According to Fairchild’s Alfred Hesener, Marketing Director Europe, a supply that uses the Green FPS can recoup the price premium for the BoM for the topology in two years, simply on the reduction in cost of electricity consumed in standby. Five devices in the family are all rated at 650V max, with output power at European power line voltages spanning 10 to 25W.