First, a review of a "linear" power supply. A linear supply or regulator produces an output voltage which is less than the input voltage. This is done by dropping the difference voltage across a pass transistor, and generating heat. A linear regulator with a 5 volt 1 amp output, and a 12 volt input dissipates 7 volts (12 minus 5) times 1 amp or 7 watts, in addition to power used to drive the internal circuitry. In this example, the efficiency upper limit is 5/12 or 0.417 (41.7%). When the input voltage is the AC mains, 115 or 230 volts RMS, 50 or 60 Hz, a transformer is needed to provide electrical isolation, per safety agency requirements (UL, CSA, VDE, etc.). Since the transformer core flux has a frequency of 50 or 60 Hz, the core must be quite large (and heavy) in order to prevent magnetic saturation. After the transformer, there is a full wave rectifier and a bank of filter capacitors, followed by the linear regulator circuit. The linear regulator must have an input greater than the output. At minimum line, 85, 90, or maybe 100 volts RMS, the rectified and filtered voltage at the regulators input must be above the desired output voltage. At nominal line of 117 volts, the voltage at the input of the regulator is quite a bit above the minimum needed. This extra voltage multiplied by the current is equal to the additional power that must be dissipated. At high line, 127 or 132 volts RMS, more heat is generated.
A switch mode power supply, herein called SMPS, regulates by switching the transistor between saturation (fully on), and cutoff (fully off). When the transistor turns on, energy is delivered to an inductor, and in some cases to the output capacitor and load. When the transistor turns off, the inductor's stored energy is delivered to the output filter cap and load. The transistor is operating either at full current and minimum voltage, or full voltage and minimum current, which results in low amounts of wasted power. Efficiencies of SMPS's nowadays are 80 to 95% and even higher in some cases.Typical switching frequencies are anywhere from 25 kHz to over 1 MHz, with 50 kHz to 250 kHz the range I usually use when designing an SMPS. In an offline SMPS (ac mains), the transformer needed for isolation is operating with a core flux in the frequency range mentioned above, much, much higher than 50 or 60 Hz. This results in a core much smaller and lighter. Also, since much less heat is generated due to high efficiency, smaller parts and smaller heat sinks (sometimes the pc board alone can act as a sufficient heat sink) can be used.
Another advantage to an SMPS is that the input voltage does not have to be above the output. While a linear regulator can only step down, a switcher can step up or down. Some circuits (buck-boost, SEPIC, isolated flyback) can do both, delivering a steady output voltage from an input less than or greater. A common boost circuit can be found in a photoflash unit which converts a 3 or 6 volt input (2 or 4 1.5 volt "AA" cells) to 400-500 volts to power the flash tube. A linear regulator cannot boost.
The down side to an SMPS is that it is very noisy compared to linear. Also, the cost is generally higher, but the cost differential for an SMPS vs. linear is not as large as it used to be. Substantially more parts are needed for an SMPS.
This is only a brief overview, not an exhaustive analysis, but I hope it helps. Best regards.
Claude
