Anatomy of Switching Power Supplies

Introduction

Power supplies used on the PC are based on a technology called “switching mode” and thus are also known as SMPS, Switching Mode Power Supplies (DC-DC converter is another nickname for switching mode power supplies). In this tutorial we will explain you how switching power supplies work and we will provide a journey into the PC power supply showing you its main components and what they do.

We have already published a Power Supply Tutorial, where we dealt with form factors, how to calculate the power supply nominal power rating and also explained the basic power supply specs. In the present tutorial we go a step further, explaining what is inside the box, what are the power supply main components, how to identify them and what they do.

There are two basic power supply designs: linear and switching.

Linear power supplies work by getting the 127 V or 220 V from the power grid and lowering it to a lower value (e.g., 12 V) using a transformer. This lower voltage is still AC. Then rectification is done by a set of diodes, transforming this AC voltage into pulsating voltage (number 3 on Figures 1 and 2). The next step is filtering, which is done by an electrolytic capacitor, transforming this pulsating voltage into almost DC (number 4 on Figures 1 and 2). The DC obtained after the capacitor oscillates a little bit (this oscillation is called ripple), so a voltage regulating stage is necessary, done by a zener diode or by a voltage regulator integrated circuit. After this stage the output is true DC voltage (number 5 on Figures 1 and 2).

Linear Power SupplyFigure 1: Block diagram for a standard linear power supply design.

Linear Power Supply WaveformsFigure 2: Waveforms found on a linear power supply.

Although linear power supplies work very well for several low-power applications – cordless phones and video games consoles are two applications that come in mind –, when high power is needed, linear power supplies can be literally very big for the task.

The size of the transformer and the capacitance (and thus the size) of the electrolytic capacitor are inversely proportional to the frequency of the input AC voltage: the lower the AC voltage frequency, the bigger the size of those components and vice-versa. Since linear power supplies still use the 60 Hz (or 50 Hz, depending on the country) frequency from the power grid – which is a very low frequency –, the transformer and the capacitor are very big.

Also, the higher the current (i.e., the power) demanded by the circuit fed by the power supply, the bigger the transformer is.

Building a linear power supply for the PC would be insane, since it would be very big and very heavy. The solution was to use the high-frequency switching approach.

On high-frequency switching power supplies, the input voltage has its frequency increased before going into the transformer (50-60 kHz are typical values). With input voltage frequency increased, the transformer and the electrolytic capacitor can be very small. This is the kind of power supply used on the PC and several other electronic equipments, like VCRs. Keep in mind that “switching” is a short for “high-frequency switching,” having nothing to do whether the power supply has an on/off switch or not…

The power supply used on the PC uses an even better approach: it is a closed loop system. The circuit that controls the switching transistor gets feedback from the power supply outputs, increasing or decreasing the duty cycle of the voltage applied to the transformer according to the PC consumption (this approach is called PWM, Pulse Width Modulation). So the power supply readjusts itself depending on the consumption of the device connected to it. When your PC isn’t consuming a lot of power, the power supply readjusts itself to deliver less current, making the transformer and all other components to dissipate less power – i.e., less heat is generated.

On linear power supplies, the power supply is set to deliver its maximum power, even if the circuit that is connected to it isn’t pulling a lot of current. The result is that all components are working at their full capacity, even if it isn’t necessary. The result is the generation of a greater heat.

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Author: Gabriel Torres

Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.

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