How to Pick Reliable Power Supplies? (1)

　　A few basic parameters can be useful for selecting power supplies that must work dependably in specific application scenarios.

　　Electronic devices require a reliable source that can supply power at all times. But power supplies are available in a wide range of voltage and current ratings. Engineers tasked with selecting an appropriate power supply must resolve numerous questions to determine which supply suits an application. A good place to start with this analysis is to look at two main power supply design topologies: switching and linear regulated.

　　Switching power supplies regulate the output voltage using a high-frequency switching technique that employs pulse-width modulation and feedback. Power passes from the input to the output through a switch which is actuated until the desired voltage is reached. When the output voltage reaches the predetermined value, the switch element is turned off, and no power is consumed. Generally speaking, switching power supplies are smaller, lighter, and more efficient than linear power supplies. For example, a 250-W linear power supply would take up a volume of 600 in3 and weigh 26 lb while a comparable ac-dc switching supply would require 60 in3 and weigh 2 lb.

　　Switching supplies can either step-up or step-down the input voltage to get the desired output voltage. Applications where a switching power supply may be preferable include those where small size, low weight, and high energy efficiency are important. They include high power/high current uses, portable equipment, control systems, dc motors, aviation and marine applications, network equipment, electrolysis and waste treatment operations.

　　Linear-regulated power supplies earned their name from use of linear (non-switching) techniques to regulate the supply voltage output. A linear-regulated power supply generates an output voltage by first converting the high-voltage ac into lower-voltage ac via a transformer, then converting the transformer output into an unregulated dc voltage via a rectifier and capacitor filters. An error amplifier compares the reference to the output and the resulting signal is used to ensure the output remains on the required voltage. The closed-loop design ensures the supply output stays at the nominal voltage despite changes in supply line or load values.

　　Linear power supplies typically step-down an input voltage to produce a lower output voltage. Though it is larger, heavier, and not as efficient as a comparable switching supply, the linear unit has beneficial attributes that the switcher supply cannot match. For example, the linear supply has no discrete time clocking or switching action. Consequently, the output is virtually noise and ripple free. Typical applications of linear regulated power supplies include telecommunications equipment, medical equipment, laboratory test equipment, low-noise amplifiers, advanced signal processing and data acquisition systems (sensors, multiplexers, A/D converters and sample and hold circuits), and precision measurement devices.

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