There are ways to approach the "switchmode" - look back at the "Schematic", you'll see a MOSFET that pulses the line voltage appearing on the Transformer.
It uses an ordinary transformer, but it sends power thru it differently
If you notice, Europeans use 50Hz, while US uses 60Hz - but did you know that you can increase the effective power A transformer can pass by increasing the frequency? 80Hz or even 120Hz? You can lessen the power thru the primary because the Secondary can couple to the Primary more efficiently at higher frequencies - this principle works, but is a bane to tame it's harmonics.
If you take a typical transformer - if you use a really high-impedance meter, you'll see that both the PRIMARY and SECONDARY will try to rise and MATCH each other due to the nature of the coil coupling. It's when you place a load across the Secondary, does the power present in it's winds drop. They can use this proximity effect - as a means to determine the load - noting the Primary appearance and the Secondary appearance, once the Secondary starts to drop in voltage (power) the load sense circuit then increases the length or duration of pules into the PRIMARY - they it determines the amount of the sine wave that goes thru the primary wind, or the SECONDARY if they equip it as such...they can take some current from a capacitor placed as a tap, along with a resistor - that drains off current stored on the plates and can be "Captured" to power a load sense. To utilize this they put it thru a bridge rectifier, to either power the load sense circuit that looks forward into the output to sense a drop in voltage across the output terminals or provide a secondary power supply to power the switchmode circuit and the MOSFET which then the Switchmode sends pulses to the GATE of the MOSFET - for a length of time - as the demand increases - the pulses turn into more of a rapid on to off - then nearly fully on as a duration of the duty cycle becomes more towards 100% on - read this as the Primary or Secondary is sending power thru it's winds to capture and use - as the load or level of work rises.
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So I showed you a means to make this work thru a battery, the Schematic shows it as Vt - but the concept of a pulse train at a higher and higher frequency of pulses - RAISES the coupling effect and hence the level of power transfer into the Secondary from the Primary in the transformer - it increases it's efficiency.
Maybe this is more than you need to know but it' helps explain the noise they produce.
