Counter question: what at all do you know about SMPS ?

And BTW: Which type of SMPS are you considering ?

And which type of 'noise' ? - ripple ? EM emissions ? or ???

I don't have in depth knowledge about smps ,but I have an idea about it`s working and various topologies .I was reading an article on it and I came across that in general high frequency switching causes em noise which I want to know ,that how it is generated.how it can be suppressed.

OK - so we'll start at the basic level:

ANY switching generates disturbances. Due to changes of voltage and current, and due to parasitic effects of the devices involved. This is true for frequencies from (near) DC up to "high frequencies".

Technologies to suppress these disturbances start with controlling the slopes (voltage, current) during switching. The slope control has its limits - you cannot switch slower than the frequency required. (THUS the reference to "high frequencies".) AND it is generating so-called "switching losses" that also limit the applicability of slope control.

Beyond that everything is about "suppressing" (or attenuating) what's left in terms of disturbances: "burn off" disturbance energy in ferrite chokes or resistors, or try to buffer the transitions by means of coils and capacitors.

None of this is easy to achieve, there are always tradeoffs between reducing the disturbances and achieving the performance requirements. Quite an interesting field to work in.

Hope, this helps to clarify things a bit.

Much of the noise is generated as a direct consequence of the switching process. In its open position, an ideal switch provides infinite resistance to the flow of electrical current. In its closed position, the ideal switch offers zero resistance, allowing current flow with negligible drop in voltage. An ideal switch would control the conduction of high-speed pulsed waveforms without adding transient events or causing voltage spikes. Unfortunately, real switches do not change states instantaneously but require transition periods known as rise and fall times. These transition periods tend to impose distortion on the harmonic components of high-frequency SMPS waveforms. The switching process produces voltage spikes resulting in EMI and RFI. The voltage spikes are caused by short-duration charging and discharging of parasitic capacitances in the power-supply circuitry.Magnetic components like transformer, inductor also will play important role in producing EMI.

Noise suppression can be done by optimizing switching frequency, by placing electrostatic shields between a transformer’s primary and secondary winding in order to minimize emissions from primary-winding voltage spikes being transmitted to the secondary winding. A Faraday shield is one type of structure that is commonly placed between a transformer’s primary and secondary winding to reduce EMI. It usually consists of a turn  ( should not be a closed loop) of thin copper foil around the primary coil that is attached to the circuit or system ground plane. This shield prevents high-frequency current coupling from the primary to the secondary winding. Coupling of these unwanted currents normally occurs as a result of inter winding capacitance. Finally placement of components is also will be one main area to consider in the design.