Why is it so difficult to have high switching frequencies at high wattage power converters..viz., at 1 MW - Frequency is 1 MHz, 1 KW - 100 KHz, 10 W - 1 MHz. What will be the issues if we go for 1 MW - 1 MHz?
The switching losses are related between the current and voltage on the switching device. Some loss occurs on every switching and if the switching occurs very frequent (depending on the switching frequency.) the total losses will be so much that heat sink will not be able to dissipate the heat from the device.
On the other hand, if the current you are switching is too much, and too fast, the leakage inductances will cause high voltage spikes which may damage the device (because of the di/dt). Another reason is the leakage capacitors and inductors in the circuit are always resonating around MHz range. If the switching frequency is around that range, the leakage inductors will act as a dominant factor (act as another load) in the circuit. These factors must always be considered when designing a high frequency power circuit.
If you go to the basics of losses calculation of any converter, you will find that switching losses of semiconductor switches are directly proportional to your switching frequency used in the converter. More the switching frequency more that switching losses. For MW scale converter it plays an important impact in terms of efficiency, thermal management etc. So it is one fact where we do not use higher frequency for higher power converter.
The other issue is that the slew rates needed for high frequency might be outside the capability of the semiconductor switches.