POWER ELECTRONICS HANDBOOK
Linear vs. switching power supplies: Not always an easy choice There are some uses that demand the low
ALEX K ARAPETIAN | ACOPIAN TECHNICAL CO.
noise and fast response only available through traditional linear supplies.
ASK
almost any engineer about linear power supplies and the likely instinctive reaction will be, “Sorry, I can’t use them — they’re too inefficient.” Any possibility of using a linear supply usually ends right there; it is as though you’re asking them to go back to vacuum-tube AM radios. Still, a good engineer knows that it’s wise to not make decisions based on assumptions and clichés, but rather on honest assessments of priorities and alternatives. This maxim also applies when deciding between a switching or linear power supply. Like many engineering decisions, this one depends on the specifics of the application, the features and functions it needs, priorities, and acceptable tradeoffs. First a quick review of the basics. A linear power supply first converts the high-voltage ac from the line into lower-voltage ac using a transformer. It then converts the low-voltage ac into an unregulated dc voltage via a rectifier and capacitor filters. An error amplifier with a voltage reference as one input and the output dc as the other controls a series-connected pass element. The error amplifier compares the reference to the output and regulates the output voltage by dropping excess voltage in the pass element, hence the designation “linear” supply. This closed-loop design ensures the supply output stays at the nominal voltage despite changes in supply line or load values. The downside of this arrangement is that the pass element is always in its active region. It dissipates power regardless of the power it is also delivering to the load; this is the major source of inefficiency in a linear supply.
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DESIGN WORLD — EE NETWORK
Acopian — Power Electronics HB 02-19.indd 16
2 • 2019
Now consider a line-powered switching supply. Here, the line ac is usually first converted to unregulated dc (again, via a rectifier and filter). Then the supply regulates that dc voltage down to the desired voltage. There are many topology variations of the switching concept, but all have a similar underlying principle. Again, an error amplifier compares the regulator output value to an internal reference, but here the pass element is rapidly switched on and off with a pulse-width modulation or pulse-frequency modulation scheme. The output pulses are filtered to form low-ripple dc and the resulting waveform becomes the dc voltage output. Because the pass element is either completely on or off, it is always in a mode where its dissipation is minimal. The main losses arise from the series onresistance when the pass element conducts and switching losses as it transitions between on and off states.
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2/19/19 3:29 PM
