What Are Switch-Mode Power Supplies?

Linear power supplies, once quite common, have now been mostly replaced by switch-mode power supplies (SMPS). The liner power supplies typically had a dissipative regulator – a voltage control element – usually a transistor that dissipated power equal to the difference between the unregulated input voltage and the fixed output voltage times the current flowing through it. The dissipative element prevented the linear power supplies from reaching high efficiencies.

On the other hand, the switching regulator in a switch-mode power supply behaves more like a continuously variable power converter. That allows the difference of the input and output voltages to affect the efficiency of the switch-mode power supply only marginally. Therefore, the switching regulator acts as a non-dissipative regulator, since the regulating device always operates either in a cut-off mode or in saturation.

Typically, the SMPS chops the input DC supply at a high frequency using an active device such as a power MOSFET or BJT and feeds the chopped voltage to the converter transformer. As the chopping frequency is high, the transformer is made of a ferrite core that can handle such high frequencies. Another advantage in keeping the operating frequency high is that the size of the magnetics decreases. The output of the converter transformer is rectified and filtered before being useful for the load. A part of the output voltage is fed back to the regulating/drive circuitry of the switching element to achieve regulation.

An SMPS usually has an oscillator that switches the control element on and off. When switched on, the control element pumps energy into the primary of the converter transformer. As the switching element switches off, the magnetic field associated with the energy in the converter transformer creates a secondary voltage in the output winding of the transformer. This voltage is rectified, filtered and fed to the load.

The frequency of switching, or the duty cycle of the oscillator is varied to control the energy fed into the converter transformer and consequently the output power delivered. An SMPS operates at a high efficiency since only the energy necessary to maintain the load current is pumped in, leading to minimal power dissipation.

The higher frequency of operation of an SMPS, typically in KHz/MHz, drastically reduces the physically massive power transformer (hallmark of a linear power supply) and the corresponding power line magnetics meant for filtering. That reduces the overall size of the power supply and this is evident from the tiny wall-wart power supplies available for, say charging smartphones.

SMPS are designed for specific applications. They are available in different topologies such as DC to DC converters, forward converters, fly-back converters and self-oscillating fly-back converters. Although the principle of operation remains the same for all, the manner in which the switching operation works is the main difference between the various topologies.

Usually, SMPS employ a method called the pulse-width modulation or PWM to control the average value of the output voltage. The area under the output waveform defines the average voltage of the repetitive pulse waveform. As load increases, the output voltage tends to fall. On sensing this, the feedback/control circuit modifies the PWM to increase the voltage to the required level.