The nice thing about on/off control is that it is unconditionally stable for both continuous or non-continuous mode of operation. Z. Cochrane

thankyou , though i beg for reconsideration, since it is common knowledge in SMPS theory that continuous mode, current mode converters with duty cycles greater than 50% suffer from the Right Half Plane Zero problem as well as Subharmonic Oscillations. I am wondering how the Powerint on/off control IC's get round the Subharmonic Oscillation problem and the problem of the Right Half Plane Zero? Any thoughts greatly appreciated For Right Half Plane Zero.....please see page 21 of http://focus.ti.com/lit/ml/slup113a/slup113a.pdf and http://www.eng-tips.com/viewthread.cfm?qid=229945&page=1 For subharmonic oscillation, please see page 5 of the above, and also application note 19 from Linear.com (page 72) http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1031,C1061,P1266,D4176

There are a couple of things to remember. First of all, >50% duty will only occur at input (bulk) voltages below the Vor (output voltage times turns ratio). In a typical converter, this will be only below 70Vac. Second, if you go through the numbers, the RHP appears at a frequency well above the crossover frequency such that the phase lag it adds will have very little influence.