How Voltage Regulators Work

voltage regulator

Voltage regulators are a common feature in many circuits to ensure that a constant, stable voltage is supplied to sensitive electronics. How they operate is typical of many analog circuits, the judicious and elegant use of feedback to adjust the output to a desired level.

Voltage Regulator Overview

When a steady, reliable voltage is needed, voltage regulators are the go-to components. Voltage regulators take an input voltage and create a regulated output voltage regardless of the input voltage at either a fixed voltage level or and adjustable voltage level (by selecting the right external components).

This automatic regulation of the output voltage level is handled by various feedback techniques, some as simple as a zener diode while others include complex feedback topologies that can improve performance, reliability, efficiency, and add other features like boosting output voltage above the input voltage to the voltage regulator.

How Linear Voltage Regulators Work

Maintaining a fixed voltage with an unknown and potentially noisy (or worse) input requires a feedback signal to know what adjustments need to be made. Linear regulators use a power transistor (either BJT or MOSFET depending on the component used) as a variable resistor that behave like the first half of a voltage divider network. The output of the voltage divider is used as feedback to drive the power transistor appropriately to maintain a constant output voltage. Unfortunately, since the transistor behaves like a resistor it wastes lots of energy by converting it to heat, often lots of heat.

Since the total power converted to heat is equal to the voltage drop between the input voltage and the output voltage times the current supplied, the power dissipated can often be very high and demand good heatsinks.

An alternate form of linear regulator is a shunt regulator, such as a Zener diode. Rather than act as a variable series resistance as the typical linear regulator does, a shunt regulator provides a path to ground for excess voltage (and current) to flow through.

Unfortunately, this type of regulator is often even less efficient than a typical series linear regulator and is only practical when very little power is needed and supplied.

How Switching Voltage Regulators Work

A switching voltage regulator works on an entirely different principal than linear voltage regulators. Rather than acting as a voltage or current sink to provide a constant output, a switching regulator stores energy at a defined level and uses feedback to ensure that the charge level is maintained with minimal voltage ripple. This technique allows the switching regulator to be much more efficient that the linear regulator by turning a transistor fully on (with minimal resistance) only when the energy storage circuit needs a burst of energy. This reduces the total power wasted in the system to the resistance of the transistor during the switching as it transitions from conducting (very low resistance) to non-conducting (very high resistance) and other small circuit losses.

The faster a switching regulator switches, the less energy storage capacity it needs to maintain the desired output voltage which means smaller components can be used. However, the cost of faster switching is a loss in efficiency as more time is spent transitioning between the conducting and non-conduction states which means more power is lost due to resistive heating.

Another side effect of faster switching is the increase in electronic noise generated by the switching regulator. By using different switching techniques, a switching regulator can step down the input voltage (buck topology), step up the voltage (boost topology), or both step down or step up the voltage (buck-boost) as needed maintain the desired output voltage which make switching regulators a great choice for many battery powered applications since the switching regulator can step up or boost the input voltage from the battery as the battery discharges. This allows the electronics to continue to function well beyond the point at which the battery could directly supply the right voltage for the circuit to work.