There are also devices and methods to convert between power systems designed for single and three-phase operation. The standard power voltage and frequency varies from country to country and sometimes within a country. In North America and northern South America it is usually 120 volt, 60 hertz, but in Europe, Asia, Africa and many other parts of the world, it is usually 230 volt, 50 Hz. Aircraft often use 400 Hz power internally, so 50 Hz or 60 Hz to 400 Hz frequency conversion is needed for use in the ground power unit used to power the airplane while it is on the ground. Conversely, internal 400 Hz internal power may be converted to 50 Hz or 60 Hz for convenience power outlets available to passengers during flight. Certain specialized circuits can also be considered power converters, such as the flyback transformer subsystem powering a CRT, generating high voltage at approximately 15 kHz. Consumer electronics usually include an AC adapter to convert mains-voltage AC current to low-voltage DC suitable for consumption by microchips. Consumer voltage converters are used when travelling between countries that use ~120 V versus ~240 V AC mains power.
Why use transformers in power converters
Transformers are used in power converters to incorporate:
The secondary circuit is floating, when you touch the secondary circuit, you merely drag its potential to your body potential or the earth potential. There will be no current flowing through your body. That's why you can use your cellphone safely when it is being charged, even if your cellphone has a metal shell and it is connected to the secondary circuit. Operating at high frequency and supplying low power, power converters have much smaller transformers as compared with those of fundamental frequency, high power applications. Usually, in power systems, transformers transmit power simultaneously, no charge! The current in the primary winding of a transformer plays two roles:
It sets up the mutual flux in accordance with Ampere's law.
It balances the demagnetizing effect of the load current in the secondary winding.
Flyback converter's transformer works differently, like an inductor. In each cycle, flyback converter's transformer first gets charged then releases its energy to the load. Accordingly, flyback converter's transformer air gap has two functions. It not only determines inductance, but also stores energy. For flyback converter, the transformer gap can have the function of energy transmission through cycles of charging and discharging. The core's relative permeability can be > 1,000, even > 10,000. While the air gap features much lower permeability, accordingly has higher energy density.
