AQ: Switching frequency selection
Switching frequency selection is actually a tradeoff, and follows the below guidelines:
- Lower frequency (Eg 30kHz) means bulkier magnetics and capacitors; Higher frequency (Eg 1Mhz)) means smaller parts, hence more compact PSU.
- 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.
- 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.
- Higher frequency results in higher switching losses; To control that, you will need faster switching FETs, Diodes, capacitors, magnetics and control ICs.
- 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.
