How to choose between a linear and a switching regulator?

Which one is better? | 2020-06-30, 22:49:00

You might wonder how to differentiate between two regulators, whom, let's say, are suitable to do the same job, are both available and at similar price, but one of them is a 'linear regulator' while the other one is a 'switching regulator' (in all their name variations, synchronous pump, asynchronous, buck, boost, inverting etc).

What are linear regulators? How do linear regulators produce work?

Linear regulators are the simplest to use, the most robust, in less need of external components and well established on the market.

Let's take for example a 78xx series datasheet. The 78xx, 79xx, regulator series is the most common one, along with LM117/317/1086.

Examining the schematic of it, 

we observe it's made from the most basic components, transistors, resistors, capacitors, and zener diodes.

It operates on a simple principle it cannot disobey: volts are reduced to heat.

Provided an input voltage greater than the output, the output voltage is maintained stable by simply powering a transistor network to allow/amplify the passage through or either to dissipate the extra without passing through.

Advantages

-if it's a fixed output type, it barely requires to external capacitors;

-if not, two external resistors to set the voltage;

-no logic signals to operate it;

-no significant noise is generated;

-continuous operation without external intervention.

Disadvantages

-might require an external heatsink to dissipate the heat since the bigger the difference there is between input and output, more it has to lose as heat;

-low efficiency.

How do switching regulators produce work?

Let's examine an LM2596 regulator, a mature "simple switcher" (async), very common, that comes in a TO package alike the 7xxx series.

Here we learn that the feedback network of transistors and zeners can be replaced with a switch (still a transistor) and a clock (oscillating latch).

What if, instead of waiting for voltage drops or surges to activate a transistor network, we could simply count x^n times necessary to allow energy passage (open a gate) so we never have to lose or to gain anything? We obtain a switching regulator!

This regulator has the capability to slice the necessary quantity of energy into small blocks/chunks (pulses) so that it mimics a linear output by generating a pulsating current, frequency-filtered with an inductor and rectified with an external capacitor.

In this way, most of the energy lost by dissipation is the heat generated by turning the switch on/off.

Modern switching regulators replace the transistors with gates, that is, FETs, which reduce the heat losses to a minimum, being up to 9x% efficient.

Advantages

-heat losses reduced to a minimum due to replacing passive dissipating components with active transistors;

-less heat allows discarding the heatsink, which means lower costs and less PCB area occupied.

-a higher efficiency reduces the load on the power source, leaving more power available for more justified power-hungry resources (logic ICs, electromechanical components etc);

-increased portability and size-reduction for the circuit;

-might have input/output logic pins for status and control.

Disadvantages

-the running clock, the oscillator, shall introduce noise;

-moreover, most of the switching regulators use an external inductor, which further increases the noise in the circuit;

-requires more external components than linear regulators (an inductor, resistors, capacitors, probably at least a diode/zener).

What's the difference between buck, boost, asynchronous and synchronous switching regulators?

A buck regulator is step-down regulator, one that has the output lower than the input.

A boost regulator is a step-up regulator, one that provides a higher output than the input.

A buck-boost regulator can do both.

charge-pump is a switching regulator without an inductor, which relies on capacitor discharge.

An asynchronous regulator has one output switch and it requires an external zener diode for a current return path, which shall incur heat losses through it, as well as some output feedback hysteresis.

synchronous regulator has two output switches, one for the high, and the other for the low of the switching cycle. This increases the conversion's efficiency, with a small increase in heat and complexity. See this document for a more thorough explanation.

An inverting regulator can be either linear or switched and it changes the polarity of the input voltage, i.e. analog to digital operations.


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