Linear vs. switching power supplies Not always an easy choice(2)

　　The typical linear supply has an efficiency of about 20% to 40%. This figure pales in comparison to the efficiency of a typical switching supply which comes in at 60% to 80% and can reach 90% in some cases.

　　There are also major differences in size and weight. The switching supply is much smaller and lighter, largely because of the smaller transformer, discrete semiconductors, and passive components. For example, a 250-W linear power supply would require 600 in3 (a little under 9,000 cm3) of mounting space and would weigh 26 lb. (about 12 kg). A comparable ac/dc switching power supply could occupy one-tenth that volume and weigh just 2 lb. (0.9 kg).

　　The size of the switching supply is also a function of its switching frequency, ranging between a few hundred kilohertz for larger supplies to as high as a few megahertz for those supplying less power. Operation at high frequencies permits the use of smaller passives and reduces the overall footprint although overall efficiency will drop because of higher switching losses.

　　Why would an engineer even consider a linear supply with its lower efficiency, larger size and higher weight? Here, noise on the dc output and transient response come into play.

　　The linear supply is a continuous-function unit with no discrete time clocking or switching action. The linear supply itself does not generate any EMI or RFI. As a result, its output is virtually free of any noise and ripple. Any noise at the load arises outside the supply itself from pickup in the power wiring between the supply and load. Chokes and other filter components as well as careful cable routing can attenuate this noise.

　　In contrast, the switching supply is inherently a source of noise. Its output contains a fundamental at its clock frequency as well as numerous harmonics. Typical noise levels are on the order of hundreds of microvolts to tens of millivolts. These levels are unacceptable for many applications where the output voltage is at single-digit levels or the load is sensitive to supply rail noise.

　　Switching-based noise can be filtered to some extent but is difficult to eliminate entirely. In addition to generating noise on the output cables, the supply also radiates noise which can induce unexpected and frustrating problems elsewhere in the system. While filtering can attenuate the output noise to acceptable levels, the problem of radiated noise is much more difficult to manage.

　　Further, the frequency of the switching-induced noise may interfere with other clocked signals, resulting in beat frequencies and other interference. To head off such difficulties, the switching supply’s clock frequency may need to be synchronized with the system clock.

　　In addition, there are increasingly stringent regulatory limits on the amount of noise a power supply can generate in different frequency bands, both as a function of power supply wattage and global location. Some switching supplies meet the regulatory mandates by using spread spectrum clocking to spread the noise energy across a wide band. This way, the noise does not exceed allowed limits at the clock frequency or its harmonics. While this technique works in the “legal” sense to meet mandatory standards, the supply noise can still affect internal system circuitry.

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