Modern mobile phone chargers are much lighter and less bulky than they used to be. I assume that this is because they no longer use a transformer with a metal core to step the voltage down. So how do they work?
鈥� Today鈥檚 cellphone chargers use 鈥渟witch-mode鈥� power supplies, which convert the mains frequency from 50 or 60 hertz to a much higher frequency. Then they feed this through a transformer to reduce the voltage in the normal way. The high frequency allows the transformer to be much smaller than in the past, and to use a ferrite core rather than the old-style laminated steel core, making it much lighter than one designed to work at mains frequencies. These new power supplies have been made possible by transistors capable of operating directly at mains voltage, which do the switching to the higher frequency.
Perry Bebbington, Nuthall, Nottinghamshire, UK
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鈥� The size of a transformer is inversely proportional to the frequency of the electricity supplying it. The lightweight chargers raise the frequency before the current reaches the transformer, so they can be a small fraction of the size that would be necessary for a 50 or 60-hertz transformer. Modern transformers often run at between 50 and 200 kilohertz. The iron core is still needed, but it is made out of ferrite 鈥� essentially iron filings glued together. It is often torroidal (like a ring doughnut) and is only about 10 millimetres in diameter in a typical cellphone charger.
The mains input is first 鈥渞ectified鈥� 鈥� the alternating current is turned into direct current. A high-voltage switch turns this on and off to make an alternating current at a frequency in the range 50 to 200 kilohertz. This is fed to the transformer, which produces low-voltage alternating current that is rectified again to give the final low-voltage direct current required by the phone. The output voltage is regulated by a feedback loop that controls the high-voltage switch. Power supplies that work this way are called 鈥渟witch-mode鈥�.
This design can be reasonably efficient over a wide range of input voltages and often consumes very little power when there is no load. Switch-mode chargers are often rated for inputs of 100 to 240 volts, so they can be used worldwide. The power supplies of laptop and desktop computers are all switch-mode, which is why they are so light for their power rating. Switch-mode supplies are also becoming common in microwave ovens, which is why some old ones have one end that is really heavy whereas newer ones don鈥檛.
鈥淟aptop power supplies are all switch-mode, which is why they are light for their power rating鈥�
Malin Dixon, Nuneaton, Warwickshire, UK