How is SMPS output of transformer stable regardless of its input

[abdulbadii]

How is a decent SMPS (its circuit) that can ensure transformer output being stable or nearly even regardless of the transformer unstable input?

brief yet clear explaining a roughly outlined circuit functions phases

[tunk]

I think diodegonewild@youtube has done some videos about how SMPSs work.

[Zero999]

How is a decent SMPS (its circuit) that can ensure transformer output being stable or nearly even regardless of the transformer unstable input?

brief yet clear explaining a roughly outlined circuit functions phases

The transformer is driven by PWM. A TL431+opto-coupler is used to sense when the voltage on the secondary has reached the required level and the duty cycle of the PWM is reduced, or increased accordingly.

[coromonadalix]

An smps / buck boost dc-dc converter  is supposed to have  what we call a feedback line / sense,  it must lower the output voltage if it tend to goes up and vice versa  to obtain what we call regulation / voltage stabilization

Same for linear psu's too

You see a lot of tl431 devices in them, normaly they are used for a voltage reference, but ive seen some starnge tricks with them

And normally  for the voltage / sense feedback a good psu have an optocoupler  to isolate this line  to get in the end an isolated psu between the input and output

Buck boost or buck down  dc-dc  for most of the time are not isolated between input and output  , and  i try to avoid them,  when they fail, the damages are more extensive ..

[jonpaul]

feedback

[ejeffrey]

How is a decent SMPS (its circuit) that can ensure transformer output being stable or nearly even regardless of the transformer unstable input?

brief yet clear explaining a roughly outlined circuit functions phases

Your question isn't exactly clear.  Are you asking how a transformer used in an SMPS can have an output set by a regulator rather than the input voltage * turns ratio?

Technically it can't.  Regulation of this kind requires energy storage and at least according to modern taxonomy transformers don't store energy.  If you look at a forward converter, you have an output inductor after the transformer, which is necessary for it's operation.  A forward converter is basically a standard buck converter with a transformer between the switches and the LC filter.  The transformer provides isolation and impedance transformation but is not responsible for the voltage regulation.  If you look at the output voltage at the transformer windings it will just be the input voltage * turns ratio with the duty cycle varying as the feedback loop adjusts to regulate the output voltage

You will see the same if you look at most of the other isolated SMPS topologies.  They use a transformer to do the bulk of the voltage transformation via the turns ratio and an inductor for regulation.

The main exception is the flyback converter.  A flyback converter does not use a separate output inductor.  Instead it does store energy in the transformer. While that component is called a "flyback transformer" it is actually considered a coupled inductor and the core is built with a gap just like a regular power inductor.

[CaptDon]

The answer is in almost every datasheet of SMPS controller I.C.'s as well as every online design guide for building SMPS's as well as many of the application notes for the transformer magnetic properties supplied by the transformer core manufacturers.

[magic]

brief yet clear

Flyback converters, for example, employ half-wave rectification, conducting alternately on the primary and the secondary, so input and output pulse amplitudes can be varied independently of each other.