SPC and the Various Stages of a PSU

All PSUs that power every PC system today utilize Switching Power Conversion (SPC). The principle of SPC is quite simple, energy is drawn from the power grid, then it's chopped up with a high frequency in smaller energy packets and finally it's transferred with the help of components like capacitors and inductors. In the end all energy packets are merged and after some rectification processes energy is flowing smoothly from the output. So in PSUs we have as input 100–240V AC wall power (AC voltage and frequency differs from region to region) and several outputs that supply regulated DC (Direct Current), which are of course, always the same regardless the country/region. An interesting fact is that as the switching frequency increases the size of the energy packets is getting smaller; thereby the size of components (inductors and capacitors) that store and transfer those packets is also reduced. Finally, a PSU that utilizes SPC is called Switching Mode Power Supply or SMPS.

The two major advantages of an SMPS compared to a Linear Power Supply that uses a totally different design are drastically reduced size and weight and higher efficiency that can easily exceed 90%. On the other hand, the most significant drawbacks of an SMPS are its complexity and the production of Electromagnetic / Radio Frequency Interference (EMI/RFI) that makes necessary the use of an EMI filter (some of you may also know it as transient filtering stage because its role is twofold) and RFI shielding.


The figure above shows the block diagram of an SMPS, there are seven main components that turn AC wall power into several DC voltages used by the components in your computer.

• EMI/Transient Filter: Suppress incoming and out coming EMI/RFI and protects from voltage spikes
• Bridge Rectifier: Rectifies the AC power stream to DC
• APFC: Controls the current supplied to the PSU so that the current waveform is proportional to the mains voltage waveform
• Main Switches: Chop the DC signal to very small energy packets, with high frequency
• Transformer: Isolates primary from secondary side and converts (steps down) the voltage
• Output Rectifiers & Filters: Generate the DC outputs and filter them
• Protection Circuits: Shut down the PSU when something goes wrong
• PWM Controller: Adjusts the duty cycle of the main switches, in order to keep steady output voltage under all loads
• Isolator: Isolates the voltage feedback that comes from the DC outputs and heads to the PWM controller

The part of the SMPS before the power transformer is called “primary” side and the part after it “secondary” side.

In the following pages we will analyze all these individual stages that compose a power supply, in more detail.