Hello, up until recently I have never questioned as to why a lot of commerical switching power supplies have little transformers. I have always used a transformer that exceeds the voltage/amperage output of my switching power supply's output to make sure enough power is avaible, but in doing so, the resulting transformer is bulky and massive. Take for example the transformers located within power supply units in a computer, they are relatively small, actually very small, to what they put out in comparision with just a bought transformer from say RadioShack. Is there a source for these small transformers that are cheap and available to the public? After thinking it over, I have come to the conclusion that there may just not be a transformer stepping down the voltage but instead, the 120/240v is just rectified and stepped down in someway other than bulky transformers. Of course there is the reason for isolation between mains and the actual supply via a 1:1 transformer, but the windings on a switching power supply's transformer is as thick as some of RadioShack's ones. Even if somehow those transformers are used in isolation, how would the stepping down occur, and what component is used to handle that much voltage and amperage? Thank you.
Most of the small transformers or inductors used in switching power supplies are custom made for the circuit. If you are building your own switching supply, then you can buy a few standard sizes/ratios from electronic suppliers. However, for a one-off project you may have to build your own. You can buy the 'core' of the transformer/inductor and you then wind enamelled wire around it. Selecting the appropriate type of core material, size or core, type of wire and number of turns can be difficult - you'll need to calculate the inductance values required, current handling and you need to know all the parameters of the core material. The reason that the transformers in your PC power supply are so small is because they operate at high frequency - this allows them to be smaller and to be made from ferrite instead of iron, which makes them more efficient. All the power in a typical PSU will go through 1 small transformer. Inside your PC PSU, the 220 V AC is rectified (if you use 110V, it is doubled and rectified) and smoothed. Then an electronic circuit switches it at high frequency and feeds the switched power into the transformer - essentially the power is converted into AC for the transformer. At the output of the transformer the power is rectified again and smoothed. Electronics on the output monitor the voltages and send signals to the high voltage switches (increasing or decreasing the power delivered to the transformer) so that the voltages are regulated. These signals are usually sent through a 2nd transformer to keep the power supply isolated.
Thank you for the much needed information. Edit: Would this be a viable option for constructing a transformer?
These toroidal transformers are meant for non-switching (50-60Hz) power supplies. The advantage of these kits is that you have to wind your own secundary windings and therefore be able to obtain custom secondary voltages. CD
That's fine for 50/60Hz, but at SMPS frequencies (20-40kHz), eddy currents would overheat the core much like how an induction furnace works to melt steel. In ferrite powder cores the metallic particles are partly insulated from each other so eddy current losses are very low. You can buy the ferrite cores in various shapes, some suitable for winding your own transformers.
