Switching Power Supplies-Overview

EMI filter: to prevent the conducted EMI into the 50Hz AC input

Input rectifier + filter: to convert 50Hz AC voltage to unregulated DC voltage

Switching converter: to convert DC voltage to high-frequency (HF) AC voltage

HF power transformer: to step-up or step-down the HF AC voltage and to be used for electrical isolation

Output rectifier + filter: to convert HF AC voltage to DC voltage

Error amplifier: to compare the output voltage V_o to a reference voltage V_o,ref and produce an output voltage

PWM controller: to control its output pulse-width according to the output voltage of the error amplifier

HF signal transformer: to be used for electrical isolation

Gate/base drive circuit: to drive the power switches in the switching converter

Note:

1. The power supply output voltage V_o is regulated by means of the feedback control circuit.
2. The HF signal transformer can be replaced by an optocoupler.
3. The power processor is a cascading of AC-DC, DC-AC and ACDC converters with a HF transformer for electrical isolation.

Advantages:

1. The switching devices in the switching converter operate as a switch (either completely on or completely off). Therefore, only switching losses are involved. This results the conversion efficiency is in 70 - 90% range. Furthermore, a switching element operating in switch-mode has a much larger power-handling capacity compared to its linear mode (refer to SOA).
2. Since a high-frequency isolation transformer is used, the size and weight of switching supplies can be significantly reduced.

Disadvantages:
Switching supply circuits are more complex
Switching supply circuits produce EMI due to high-frequency switching.
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