How DC voltage has an effect onTransformer in SMPS

Helloo ALL,

I am trying to understand SMPS and its different topologies...

But during my Study on Flyback and Forward SMPS,I came across a thing i couldnot understand..So if anyone can help me out plz....
Questions are

1. We connect a transformer after the rectification for control the output. But as far as i Know Transformer creates Magnetic flux for varing currrent(AC current) and since we are passing a DC current (as this current is constant and doesnot vary) through this Transformer,How will it grow that mutual inductance and will supply voltage to transformer.

2. Except Isolation is there any other reason to use this transformer in first place..

I would like it if anyone can solve my problem...

Thanks and Regards

Nikhil

Hi Nikhil,
Even though the voltage is rectified it may not be filtered and still varying with time thus allowing the transformer to work. Could you post the schematic?

Hey Mate,

I am asking in General...As in case of SMPS, The rectified and filtered DC voltage is given to the transformer in the common circuits I m going through in Internet... But how will this DC voltage have an effect on transformer when it is not varying....

Thanks...

Hi Nikhil,
Please post the circuits that you find so everybody has a better idea what you are talking about. A DC voltage on an inductor does not produce an varying magnetic field and hence cannot transfer any voltage on the secondary. Maybe the circuits you are looking at have some detail that you are not able to understand. It would be a good idea to post it here so that more people can look at it and share their thoughts.

Regards

I wonder if your getting mixed up between a transformer and an inductor, some inductors look like a transformer, and commonly you will see an inductor on the rectified dc output of a switch mode supply.
If this is the case then the inductor is there to reduce the amount of ac ripple on the output caused by the switching process and the fact that the flyback transformer as allready mentioned cannot produce a steady DC.
The inductor does 'see' some ac, its there to try and remove it.
The main considerations for the inductor are that it doesnt saturate during operation (in most cases) and that it doesnt present too high a dc resistance, there are all sorts of other considerations but these are a couple of the important ones.

In the case of flyback supplies, you shouldn't think of the "transformers" as actually working like transformers, but rather like coupled inductors. The difference is that a transformer normally does not store energy in its core (or at least it's not desireable to do so), the energy comes in the primary and out the secondary at the same time. In a flyback supply this is not the case. First current rises in the primary, but is not delivered to the secondary until the primary switch opens. That means the energy was stored in the core during the switch on time. That's what's called "flyback" action, which is different from normal forward transformer action.

The rectified and filtered DC voltage is given to the transformer.

Click to expand...

What do you exactly mean with "given to the transformer". Connecting a DC voltage to both terminals of a winding creates a short circuit.

Yes, you are right. No transformer, even flyback transformer can transfer power from primary to secondary in pure DC current. What u have seen is, ac current is rectified to pure dc -> then this dc current is chopped at very high frequency (like 50Khz to couple of hundred Khz) with help of PWM circuit to generate pulsing dc. Then this pulsing feed to the flyback transformer. Thus the power transfered to secondary.

Main purpose of a transformer is power conversion. Example 220v to 12v. Also there are lots of use of different kind of transformers. u may google to find out all those purpose.

There is a one more way to use DC-signal to control transformer or power supply
It´s called magnetic amplification. (mag amp)

In general : magnetic amplifier

Magnetic amplifier - Wikipedia, the free encyclopedia

**broken link removed**

YouTube - Homemade magnetic amplifiers made from common materials.

Magnetic Amplifier in SMPS :

http://focus.ti.com/lit/ml/slup129/slup129.pdf

Regards KAK

Also if you see the flyback or forward transformer it is a reactive element(primary inductance or secondary).So it will not
dissipate energy or power(ideally).

Aso the DC voltage given to switching transformer is chopped one.

so a dc-dc power conversion can be made much efficient.

dr pepper said:

I wonder if your getting mixed up between a transformer and an inductor, some inductors look like a transformer, and commonly you will see an inductor on the rectified dc output of a switch mode supply.
If this is the case then the inductor is there to reduce the amount of ac ripple on the output caused by the switching process and the fact that the flyback transformer as allready mentioned cannot produce a steady DC.
The inductor does 'see' some ac, its there to try and remove it.
The main considerations for the inductor are that it doesnt saturate during operation (in most cases) and that it doesnt present too high a dc resistance, there are all sorts of other considerations but these are a couple of the important ones.

Click to expand...

What do you exactly mean with given to the transformer. Connecting a DC voltage to both terminals of a winding creates a short circuit.