How can I isolate the output of a SMPS
Switched Mode Power Supplies (SMPS) take many forms.

Some provide isolation. Examples are the mains adapters (often called chargers) for laptops and phones. Also the power supply in a desktop PC provides an isolated output. This is needed to disconnect the output from mains voltage so that the output becomes safe to touch by humans without getting an electrical shock.

These isolated power supplies always use a transformer to transfer electrical energy into magnetic energy and then back into electrical energy. Transformers use separate windings to put the energy into the transformer and extract it out of the transformer again and that provides the isolation.

Here's an example of a 150 W SMPS:


Size of this module: 11.5 x 6.5 x 3.5 cm

The yellow/black component is the transformer.

A "traditional" mains (50/60 Hz) transformer is much larger, this one is also rated for 150 W:



But it is much heavier and larger: 10 x 8.2 x 8.2 cm. Also, this is just the transformer, there is no rectification and voltage regulation like the other module has.

Not all SMPS provide isolation, some just increase or decrease a DC voltage. An example is a USB adapter for use in a car. It converts the 12 V from the car battery into 5 V for USB. Using switching this can be done more efficiently with less energy lost and converted into heat. There is no transformer in such an SMPS and it also does not provide isolation as that is not needed.

It is not possible to simply "add" a transformer to an SMPS that isn't isolated. SMPSs output DC and transformers need AC to work. When present, the transformer is part of the design of the SMPS.

Nearly all SMPSs use high-frequency switching. This is done for efficiency reasons. Transformers and inductors (inductors are used in the non-isolated SMPSs) can be made smaller, more efficient and cheaper when operated at a high frequency (usually 50 kHz up to 1 MHz). Mains transformers which work directly at the mains frequency of 50 or 60 Hz need to be more bulky, heavy and therefore expensive compared to a transformer working at a much higher frequency.

Note that I write that some transformer are designed for use at a high frequency. That frequency (AC signal) itself has to be provided to the transformer. This is usually done using switching transistors and an oscillator circuit.