Switching power supply fundamentals

The efficiency of a switching mode power supply can be much higher than that of a linear power supply — especially near full load. Switching mode power supply efficiencies can exceed 85% as compared to 50%-60% for typical linear supplies.
By Jack Smith, Senior Editor, Plant Engineering magazine June 12, 2005

The efficiency of a switching mode power supply can be much higher than that of a linear power supply — especially near full load. Switching mode power supply efficiencies can exceed 85% as compared to 50%-60% for typical linear supplies.

A linear power supply uses a power transformer to drop the original 120 Vac down to something more manageable for the rest of the supply. But power transformers used in linear supplies are bulky and heavy, which tends to make their associated equipment bulky and heavy. Furthermore, the lower the frequency to be transformed, the more iron is needed for the transformer to work efficiently.

Switching mode power supplies eliminate this problem. Electronic components are less expensive than copper and iron. A power supply that is more than twice as complex as a linear supply may still be cheaper to build if it can use a smaller transformer.

Most switching supplies begin with an extremely crude linear power supply. Then an oscillator is connected to the linear front end to create a new ac voltage, though at a much higher frequency. The oscillator rapidly switches the crude supply on and off — the result of which simulates an ac voltage. Because the ac is artificially created, we can choose its frequency. If we choose a high enough frequency, only a very small amount of iron is needed in the transformer, as opposed to the much larger amount in the bulky and heavy power transformer used in linear supplies.

Because of its emphasis on efficiency, the switching power supply design minimizes the use of components such as resistors that create power losses. The switching supply design uses components that minimize losses such as switching regulators, capacitors, inductors, and transformers that are much smaller than bulky power transformers.

A switching power supply provides an improved means of regulation. All power supplies have the capability to provide slightly more power than is actually needed — the circuits that sense the output voltage actually discard the excess current as dissipated heat, which also wastes power. But a switching supply can do this more intelligently by varying the duty cycle of the ac voltage it creates.

The frequency at which a switching supply oscillates can easily be selected during its design. The duty cycle can be determined also. Not only is the duty cycle variable, it can be controlled to regulate the voltage and current produced by the power supply.

The switching regulator is a circuit that operates in a closed-loop mode to regulate the power supply output. If the output voltage is sensed as being too high, then the “off” time can be made slightly longer. If the output voltage is too low, the “on” time is adjusted instead. The duty cycle of a power supply is always expressed as a percentage, with 50% meaning on for exactly half the time. The advantage is that the switch dissipates very little power in either of these two states.

Power conversion is accomplished with minimal power loss, which means higher efficiency.