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Switching frequency selection

Switching frequency selection is actually a tradeoff, and follows the below guidelines:
  1. Lower frequency (Eg 30kHz) means bulkier magnetics and capacitors; Higher frequency (Eg 1Mhz)) means smaller parts, hence more compact PSU.
  2. Stay away from exact 150kHz as this is the low end of any EMI compliance; So, if your frequency happens to be exactly 150kHz, then your PSU will be a strong emitter; For many commercial low cost PSUs, 100 KHz has been used for many years, which is why many inductors and capacitors are specified at 100kHz.
  3. Higher frequency >/= 1MHz converters provide for better transient response. Obviously, the control IC should be capable of supporting. There are plenty of resonant converters available.
  4. Higher frequency results in higher switching losses; To control that, you will need faster switching FETs, Diodes, capacitors, magnetics and control ICs.
  5. Higher frequency MAY result in more broadband noise; its not always true, since noise can be controlled by good PCB layout and good magnetics designs.

Board power DC/DC converters are commonly built using 1MHz switchers.
Chassis power Telecom/Server PSUs seem to stay with 100-300KHz range.

Manufacturers are able to achieve exceptional density by virtue of High frequency resonant topologies, but they have to achieve high efficiencies too; Else, they will generate so much heat that they cannot meet UL/IEC safety requirements.
In some cases, they will leave the thermal problem to the user.  Usually, the first few paragraphs of any reference design discusses the tradeoffs.

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