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 <shb7@cornell.edu> 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 <abraham.manu@gmail.com> 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
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