Hi Sean, One thing that I still fail to understand in there is that the Push-Pull mo= del is the most effecient one, AFAIU. But according to the previous link with t= he flyback DC-DC converter it mentions and effeciency of 90% which is not achievable by a push-pull (~86%) model even... Any idea, how that's justified ? Regards, Manu On Fri, Feb 22, 2013 at 6:27 PM, Sean Breheny wrote: > HI Manu, > > The design shown at the link you provided appears to be a flyback > design. You are correct that in flyback converters, the turns ratio > does not define the input voltage to output voltage ratio. Flybacks > work by building-up a large magnetic field in the core of a > "transformer" by putting current through the primary and then suddenly > shutting off the current in the primary. The result is an inductive > "kick" or "spike" because of V=3DL * di/dt. This spike occurs across > both the primary and secondary since they are magnetically-coupled. > The turns ratio determines the ratio of the *spike* voltage but not > the absolute magnitude of the spike - which is instead determined by > the inductance of the windings, the current which was built-up before > being shut off, and the impedance of the load on the output. > > I put transformer in quotes above because usually transformers are > designed so that the coupling between primary and secondary is so > tight that the current flow in each of them causes nearly equal and > opposite magnetic flux so the total magnetic field in the core is > minimized. Flyback transformers are in some sense not true > transformers because they are designed to allow a significant magnetic > field to build up in the core during each cycle. This results in a > larger amount of leakage inductance than in a normal transformer, so > they are perhaps more properly called coupled-inductors. > > For SMPS designs based on true transformers (like half-bridge, > full-bridge, and forward converter topologies), the voltage ratio is > indeed nearly equal to the turns ratio. > > Sean > > > On Fri, Feb 22, 2013 at 7:37 AM, Manu Abraham wr= ote: >> Came across this site: >> http://www.danyk.wz.cz/dcdc_en.html >> It seems the turns ratio doesn't apply at all here ?? >> >> V1/V2 =3D N1/N2 >> V1=3D325V >> V2=3D12V >> V1/V2=3D27.08 >> N1=3D80 >> N2=3D6 >> N1/N2=3D13.3 >> >> In many of the SMPS designs that I am finding that the turns ratio do no= t match >> the voltage ratio ? >> >> Regards, >> Manu >> -- >> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .