Pros and cons of Switch Mode Power Supplies (SMPS)
Description of operation
In a switch mode power supply system, input filtering helps to provide immunity from mains-borne noise while also reducing emission back into the mains supply, which is a greater concern with switch mode power supplies than with their linear counterparts. After filtering the AC line voltage is rectified before power factor correction is applied to reduce power losses together with current harmonics that can cause interference to other appliances connected to the same supply.
The DC voltage is then switched at a frequency much higher than that of the AC line, allowing it to be converted via the transformer to another voltage, with reduced core losses whilst using a much smaller and lighter transformer. Switching is controlled via the SMPS Controller, which typically generates a PWM signal, taking feedback from the power supply output.
Pros and cons compared to linear power supplies
- Pro: Typically much smaller and lighter
- Pro: More efficient than regulated linear power supplies
- Pro: Often lower cost due to much smaller transformers used
- Con: More complex
- Con: Can be a potential source of EMI and RFI*
- Con: Slower transient response times*
*Switch mode power supplies can be designed for certain specialist requirements, e.g. use with low noise circuits and inductive loads such as motors
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RFI is a big issue with switching power supplies (as is long term reliability) - I have seen very many (sometimes high end) switching power supplies with the bifilar choke linked out, and associated capacitors not fitted - many plug top supplies don't have any RFI filtering designed in at all...