Dear Mr.Kamarajugadda

To answer your question, it should be noticed that every power device used in converter has a specific maximum operating junction temperature(Tvjmax) which not allowed to exceed.

As temperature raise is caused by power losses of the device and thermal resistance is almost steady in one fixed design, then max allowed power losses can be determined by  Pmax = (Tj-Ta) / Rthja .

Answer coming:

1. power losses is composed by conductive losses and switching losses

2. increase switching frequency will lead to higher switching losses

3. reduce power rating(load current) could reduce both conductive and swithing losses

therefore there is a trade off between fsw and load current in Pmax to ensure the Tvj not exceeded.

dear Kamaraju,

Though electronics is not my area, let me explain simply.

Making and breaking a current of milli amps may be done at a very faster rate. But imagine the same with a few hundreds of amps. For each making and breaking, it requires some time for the other parameters like heat dissipation etc to recover. Right? And that puts a capping on the frequency. 

It is simple to understand....

High switching will lead to high switching losses....

So, as we increase the switching frequency, it also increases the losses of the system...So we need to keep higher power rating....

Now if you looked carefully at the figure, you will see that during switching transient period, both the current and voltage are not zero, thereby we have losses. Now increase the switching frequency, and these losses will be more relevant! Remember that the switching characteristics above are simplified to give intuitive understanding. Again, the switching transients vary depending on the switch itself.   Regards, Al-Motasem

Nagamalleswara,

There are three types of losses that take place in power electronics. On-state losses, off-state losses, and switching losses. Your question is about the last.

When we increase the switching frequency, there will be more losses in the switching period itself. The details of how this happen will heavily depend on what type of switch we are talking about (IGBT, MOSFET, Transistor, etc), but you may think about it like this:

Consider a transistor that is completely off, when we turn it on, it will not be on instantly as we usually assume it in simulations, but rather, there will be a short time in which the current will raise from zero up to specific value. Simultaneously, the voltage will decrease from certain value (because the switch was like open-circuit) to zero. Waveforms that explain this are shown below

Check the figure. Power loss is directly proportional to switching frequency.