What is a Switching Power Supply?
There are many types and designs of power supplies. This article will describe what is commonly known as a switching power supply, sometimes referred to as a switched mode power supply (SMPS), or 'switcher'.
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All electronic circuits require a voltage source in order to operate. In the United States, the most common voltage available to consumers from the electric utility is the 120V RMS nominal alternating current found at outlets throughout homes and businesses. In contrast, most electronic circuits in use today require a stable direct current source with a much lower voltage - usually 3 to 12 volts.
Virtually every electronic device that operates from the AC mains must include a circuit that will derive a different voltage from the 120 volts that the utility provides. This is accomplished by a type of circuit referred to as a power supply. Power supplies can be generally grouped into two classes of operation.
Linear power supply designs are quite common and familiar to most people. They use a 'brute force' method of power conversion and regulation. The AC mains voltage is fed directly to a transformer which converts the 120V AC to the highest voltage required by the circuit it is designed to supply. The lower voltage is then converted to DC via a bridge rectifier, and then regulated by a transistor operating within its linear range - thus being dubbed a 'linear' power supply.
The advantage of linear power supply designs is that they are simple and straightforward. Disadvantages include inefficiency (50% or less), large physical size and weight, and generation of heat.
Switching power supply designs are typically more complex than their linear counterparts. As a result, the SMPS is also more versatile. SMPS design allows for either AC or DC input, and can output either AC or DC voltages, depending on circuit topology.
In a popular AC to DC SMPS design, the incoming AC mains voltage is first converted to DC using a bridge rectifier. This DC voltage is then fed to a 'chopper' stage containing highly efficient MOSFET switching transistors. The gates of the transistors are fed by an oscillator providing a constant frequency, pulse width modulated (PWM) signal (usually in the range of 50 to 300KHz) in order to 'chop' the DC into a high frequency AC voltage. This AC voltage is fed into the primary winding of a high frequency transformer. The transformer secondary winding voltage is then rectified to DC for use in the circuits where it is required. Regulation of the output voltage is accomplished by a circuit that samples the output voltage, and signals the PWM oscillator to vary the ratio of transistor 'on' time to 'off' time to maintain a constant voltage output.
There are significant advantages of this design over the linear design. The high efficiency of the switching transistors (80 to 90%) means less energy is wasted as heat, and the high frequency of operation allows for a much smaller and lighter transformer to be used.
Should an AC input voltage not be available - such as in an automotive application - the designer need only remove the rectifier stage from the power supply input, and use a high frequency transformer with the appropriate turns ratio to provide the desired output voltage.
In summary, the principal advantages of the SMPS are its ultra high efficiency, along with a significant reduction in size and weight. In today's world where efficiency and portability have become more important than ever, the SMPS is quickly becoming the preferred design for power supply circuits.
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