When I take a look into most PC power supplyie (also some high power ones=800W) I don't really see transformers which seem big enough to handle the required power. Looking around the internet, a transformer (e.g. 230V to 24V) with 100W of power is about the size and weight of these ATX power supplies with lots of times higher power.

I am trying to design my own simple mains power supply but don't understand how commercial power supplies get away with such small transformers while I seem to need a really big one(if I trust online ratings)

I suppose these ATX power supplies use PWM/are switching power supplies and not linear ones.

Can someone explain to me why they only need some very small transformers?

  • \$\begingroup\$ See also How does a cell-phone charger work. \$\endgroup\$
    – Transistor
    Jun 10, 2018 at 19:56
  • \$\begingroup\$ The cores are very small because very little energy is stored in these cores, because of the very fast switching frequency. \$\endgroup\$ Jun 13, 2018 at 4:10

2 Answers 2


The fundamental reason why switching power supplies can use a much smaller transformer is because they use a frequency that is typically a thousand times higher than 50 or 60 Hz.

This means that the primary inductance (normally around 10 henries for a conventional AC transformer) can be reduced to circa 10 mH. It then follows that the core only needs to be wound with a fraction of the number of turns and it makes the core a lot smaller.


You should consider the difference that also exists in small power supplies to provide DC power to small appliances and devices. These power supplies, often called wall warts or power bricks, used to almost exclusively use transformers to step the 220V or 110V AC mains voltage down to something closer to what was required for the load. These units were heavy and bulky compared to the typical AC to DC adapters that are in typical use today. For example compare to the very small size of a typical smart phone charger that you have for your phone.

The difference is that the modern chargers, just like the ATX power supply although on a much smaller scale, eliminate the iron transformer and lots of copper wire in favor of using a high frequency switching technology. The high frequencies used permit energy conversion with much less transformer inductance and thus the transformer cores are very much smaller by comparison. In addition these small cores use materials that respond to the high frequencies much better than iron as a result they are much lighter weight as well.

The switching power supplies are more efficient and thus generate less heat. That can translate to a much higher power conversion capability in a given volume for the same or less temperature rise.

My first computer back in the dim past was a Cromemco Z2 rack mount S100 chassis. It's power supply looked like the picture below. That heavy duty transformer in the center was about the same volume as todays ATX type power supply. As you can guess that supply just rectified the transformer secondaries and then filtered the DC voltages with those large capacitors to ~9V and ~+/-18V. Further regulation to produce clean +5V and +12V/-12V had to be done on each plug in S100 circuit board.

The original Z2 specs claimed that the supply would provide 30A on the 9V output, 15A on the each of the +/-18V outputs. That corresponds to an estimate of about 800W of raw power for the computer.

enter image description here

Compare that to the typical ATX power supply for a gaming PC that may be rated at 800W.

enter image description here

  • \$\begingroup\$ Those are some seriously gigantic capacitors! Also, should probably point out that laptops do still use large, bulky power bricks - it's just that the brick is on the charging cord, rather than the laptop itself. \$\endgroup\$
    – Vikki
    Jun 20, 2018 at 22:11
  • \$\begingroup\$ @Sean - If you have a 3 or more year old laptop with a docking station then yes; it will have a huge power brick. Most of the newest laptops have much slimmer power supplies. \$\endgroup\$ Jun 21, 2018 at 0:17

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