Power electronics range from the simplest to the most complex subsystems in modern products. This fact is not surprising because the applications also range from simple to complex. At the simplest, a power supply can be a big zener diode, such as those that find use in submarine cable-repeater pods.
These pods need the ultimate in reliability, and a resistor-plus-diode approach is the simplest and hence most reliable. The zener dissipates a significant amount of heat, but the temperature of the ocean floor easily removes that heat. Slightly up the complexity ladder are linear regulators—popular and useful parts.

A linear regulator operates like a valve. It resists the current in the circuit to ensure that voltage stays constant. Remember that the word "transistor" derives from the combination of the words "transconductance" and "varistor" (Reference 1). The transistor in a linear regulator pinches off the current to control voltage—hence, it provides transconductance. In this operation, it serves as a variable resistance, or varistor. Conventional linear regulators have NPN-pass elements. Low-dropout regulators use PNP transistors.

A more complex regulator is the charge pump. It uses several transistors as switches, not linear elements. These switches transfer charge to a capacitor and then change the connections so that the capacitor doubles or inverts the original voltage you impress upon it.

 You realize a large jump in complexity when you move to switching regulators. These circuits have high-frequency magnetics, a control loop, and at least one transistor acting as a switch...

All of these power circuits convert one dc voltage to another dc voltage. A significant number of designs use transformers to change ac voltage or circuits that convert ac to dc for subsequent dc-to-dc conversion. One of the most elegant ac-to-dc-conversion circuits, the PFC (power-factor-correction) circuit, uses a boost-converter topology to ensure that the input current to a converter is proportional to the input voltage, unlike the sharp spikes of input current that occur in conventional ac-to-dc circuits.

A new term in the power world is "digital power" (Reference 2). It can mean anything from simply being able to use a digital input to shut down the regulator, to having digital communication to the chip for monitoring and control of the analog-PWM process, to having a DSP close the loop and directly control the pass element with a PWM signal.

The overriding problem with linear regulators is heat. Because the regulator has both significant voltage and significant current running though the pass transistor, it dissipates a lot of power. Most linear regulators have a thermal shutdown, which may save the part from destruction, but it also makes the circuit unusable if the shutdown happens in any operating situation.


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