Help understanding how a switch mode power supply transformer works with DC

[AstutePauciloquent]

We all know that transformers convert electricity on the principle of electromagnetic induction which requires a constantly changing magnetic field. To induce voltage in a secondary coil

So can some one explain why, when AC mains enters a computer power supply, (basic explanation) it goes through a EMI filter, then in a bridge rectifier, then into large smoothing capacitors. This effectually turning the current into unipolar pulsating voltage. This then goes into the main power transformer. But transformers require AC current to work, but it gets converted to DC before it enters the transformer.

Please help me to understand what I am missing here.

[timeandfrequency]

Hello AstutePauciloquent,

You will find a functional diagram of a vanilla SMPS here :
https://talema.com/wp-content/uploads/blog/smps/basic-smps-circuit-diagram.jpg

It is correct that the AC mains voltage is converted into DC in the first stages of the (computer) power supply.
But just before the transformer (which, as you underscored, only works with AC), there's ALWAYS an additionnal functional block : the chopper.
In the diagram it is named 'high frequency switch'. And thanks to this switch, the transformer is fed with a kind of square wave.
So the transformer sees a variable voltage on its' primary winding. And this works pretty well in millions of switching power supplies.

[soldar]

In a "normal" forward transformer the currents in primary and secondary always balance out so power is transferred directly and instantly from primary to secondary.

That is not how a flyback transformer works. This one stores energy from the primary as magnetic flux and then the primary shuts off and the energy is dumped into the secondary.

Totally different modes.

[AstutePauciloquent]

Hello AstutePauciloquent,

You will find a functional diagram of a vanilla SMPS here :
https://talema.com/wp-content/uploads/blog/smps/basic-smps-circuit-diagram.jpg

It is correct that the AC mains voltage is converted into DC in the first stages of the (computer) power supply.
But just before the transformer (which, as you underscored, only works with AC), there's ALWAYS an additionnal functional block : the chopper.
In the diagram it is named 'high frequency switch'. And thanks to this switch, the transformer is fed with a kind of square wave.
So the transformer sees a variable voltage on its' primary winding. And this works pretty well in millions of switching power supplies.

Thank you so much for this explanation, I kind of had an idea that it might be something to do with the PWM controller and switching Mosfets, but I didn't understand that the switching mosfets actually re-create a square wave AC current for the transformer. I feel so happy to gain this knowledge. I guess this is why they call it a switch mode power supply.

[PGPG]

actually re-create a square wave AC current for the transformer

're-create' is not the right words here as at input you have 50Hz. After bridge rectifier it is 100Hz and filtered to be DC. But then the main trafo works at something like 100kHz or more. The higher frequency the lower energy have to be transferred from input to output in one cycle so proportionally lower energy have to be stored in magnetic field so transformator can be smaller.

[AstutePauciloquent]

actually re-create a square wave AC current for the transformer

're-create' is not the right words here as at input you have 50Hz. After bridge rectifier it is 100Hz and filtered to be DC. But then the main trafo works at something like 100kHz or more. The higher frequency the lower energy have to be transferred from input to output in one cycle so proportionally lower energy have to be stored in magnetic field so transformator can be smaller.

Thanks for the correction

[timeandfrequency]

I guess this is why they call it a switch mode power supply.

Yes sir.
And as soldar stated above, several kind of SPS (Switching Power Supply) exist (it is sometimes called 'topologies') and those can use different 'modes'.

Many SMPS (M = Mains Mode) for computers use the 'flyback' topology (1).
Today, an additionnal stage is often added between the input rectifier and the switch : it is called the 'PFC' (Power Factor Corrector). It becomes compulsory because it limits the pulse currents drawn from the mains.

There are also more basic SPS topologies that do no use transformers, but only inductors (usually a part with only two connexion wires). If you want to dig into that subject, you may google for the words 'buck' or 'boost'.


(1) You may find interesting to follow this thread where several members try to provide some help in order to repair a badly damaged flyback. But it's not an easy task.

[Sorama]

Ain’t smps switched MODE power supply…

[timeandfrequency]

Ain’t smps switched MODE power supply…

Corrected in the above post, thanks.