Introduction:
The buck derived forward converter is one of thenwst popular switchnwde topologies, second only tothe infanwusflyback converter. High input to outputstep down or up in voltage is easily achieved byusing the appropriate transformer turns ratio.Galvanic isolation is frequently added between thesupply and load ..grounds" for increased safety andprotection or to supply power to an isolated load.F or these and other reasons, the forward converterhas become a cost effective solution to many powermanagement needs. With its continuing proliferation,new opportunities emerge for nwre efficientoperation, higher switching frequencies, reducedEMl/RFl and extended duty cycle operation. Thispaper presents an innovative technique to properlyclamp and reset the forward converter's maintransformer while achieving low loss, zero voltagetransitions of the power switch under wide dutycycle variations without the excessive voltage stressotherwise seen.verter utilizes a much lower peak current than it'sflyback counterpart. This is advantageous with lowvoltage inputs, where even at 50 Watts, the peakprimary current reaches tens of amps.The conventional forward converter uses a singlepower switch, a definite advantage over the twoswitches used in the half bridge or four required bya full bridge converter. Also, the forward's switchis "low" side referenced for simplified interface tothe PWM controller, although a two transistorvariety was also common and similar to one half ofa full bridge. But by-and-large, the single switchtype is the most predominant in use today. Mostoff-line designs operate over a universal 85 V AC to265 V AC input range, although some 110!220 V ACmodels still incorporate jumpers to accommodateboth inputs. Forwards are extremely popular in DCto DC conversion, especially in Telecommunicationsand distributed DC bus applications. Manyutilize peak current mode control and limit the dutycycle to 50% maximum, although the recent trendhas been towards "stretching" this to 70% to efficientlyaccommodate a wider input range.General ApplicationsForward converters offer a cost effective solutionto fill the void created between the low powerflyback converter and the more complex high powerbridge types. Typically, this span covers mostapplications between 125 and 1000 Watts, but theexact level depends on a number of variablesincluding input voltage and its range. Using continuousinductor current operation, the forward con-Conventional Forward Converters:First generation forward converter designs werelimited to operate below a 50% maximum dutycycle to insure proper reset of the main transformer,preventing potential saturation problems. A separate"reset" winding was incorporated in the maintransformer to provide a convenient route for


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