actually I've many questions :) I've opened many mobile chargers and I didn't found the usual (bulky) transformer . I found only a small transformer and I think it called smps transformer.1- I can't understand how bulky transformers can be replaced by small ones! what is the idea ?2- are smps transformers connected directly to 220 volt like the bulky transformers ? if no, what is the design of the circuit that makes small transformers are able to be connected to 220 volt ?3- do smps transformers make a voltage drop like the bulky transformers ? or they have another job ?4- some smps transformers have 4 pins and the other have 5 or 6 pins, I know it should have 2 pins for input and 2 pins for output, why some of therm have more than 4 pins ?Thanks in advance, I hope i'm not bothering you because of my many questions :)
You are looking at a switching power supply where the small transformer helps bring down the voltage and isolates the output (we hope) from the 220. The output of a charger is very small and even smaller when at 220 so a large transformer is not needed and the rest of the electronics regulates the output voltage and perhaps limits the current as the device gets charged.
Notice the diodes on the board connected to the 220 vac pins. The ac voltage is converted to dc, high voltage dc. There is a small high frequency transformer on the right of the board. A high voltage transistor switches the high voltage dc through the transformer (chopping the dc basically makes the dc an ac voltage). The secondary of the transformer is low voltage ac, which is rectified to dc. There is a feedback circuit so that an increased load causes more current through the high voltage side to maintain a constant output dc voltage. It is possible the extra transformer windings are due different methods of driving the transformer, ie, push pull like some audio circuits. Other configurations include forward and flyback designs all requiring different winding connections. If the power supply requires several output voltages, additional windings are required for the secondary windings. The reason the transformer is small and light for the power transferred across it is that the frequency is so high, meaning less metal core is needed to transfer magnetic energy. At 50 Hz, a massive core is required to store magnetic energy for a relatively long time versus at 30,000 Hz, energy doesn't need to be stored for long at all.
Take a look at Farady's laws of induction. You will see that the voltage induced in a conductor is dependant mainly on the strength of the magnetic field and the speed at which the magnetic field changes, in the case of a transformer, that is the supply frequency. So, if we take a transformer and run it at a much higher frequency we can expect a given transformer to produce much more power, or we could make it smaller as it requires fewer turns on both the primary and secondary, this is what happens in a switching PSU. John m has explained the rest.
The higher the switching frequency, the smaller you can make the transformer without the metal core saturating.
This was an electronic switching power supply that was no more need the 50c/s big and large transformer. The small transformer is a high frequency transformer that working on about 30Kc. Mostly it also has a step down winding output about 12V at 30kc. Do not hook 220V 50c/s to it !
