Power supplies: linear or switch-mode?

Every electronic device needs a DC supply to work properly, with voltage and current values that vary depending on the behavior of each one, which means circuits must be used to obtain the required DC voltage. We have two main energy sources within our reach: the mains supply and standalone sources (such as batteries or accumulators), the former being the best option: 220-volt alternating current at 50 Hz in our country. Even so, we are not spared a supply with sporadic voltage drops or surges that have a significant effect on our equipment.

Until relatively recently, some people chose their amplifier according to its weight. Linear power supplies are based on a transformer, a heavy electronic component (and, according to tradition, the heavier the better). But it is also true that for some time now switch-mode power supplies, much lighter and more efficient, have been growing in popularity, alongside "digital" amplifiers (to which we devote an entire article in this issue). But if they are so perfect, why are there manufacturers, especially at the high end, who prefer to use heavy traditional power supplies?

Although there are European directives that plan to phase out traditional power supplies because of their high level of electromagnetic pollution and low efficiency, few manufacturers yet use switch-mode supplies. For the time being they are already fairly common in entry-level components and complete systems whose appeal lies in their looks, thanks in part to the compact size of the switch-mode solution.

The power supply, regardless of its electronic design, is the first element needed in any piece of electronic equipment. We rarely give it the importance it deserves, analyzing instead the stages that come after it, such as the audio and video decoders and converters in DVD-Video players, or the amplification stages of an A/V receiver. But the first, essential step is the conversion of 220-volt AC into a new DC voltage, whose voltage and current values will be determined by the technical requirements of each electronic solution (a supply for a player is not the same as a supply for an amplifier).

In fact, the power supply can be broadly divided into five parts: the transformation stage, the rectification stage, the filtering stage, the regulation stage and the voltage-dividing stage. Some entry-level devices, or those with lower electrical requirements, usually omit the regulation stage.



Stages of a power supply



The first task of a power supply is to transform the AC voltage of the public grid to the value required by the device's electronic circuits. This change is normally "downward", although it is possible to increase the input value. Another important function of this transformation stage is to isolate the device's electronics from the mains supply.

The next step is to convert the alternating current from the output of the transformation stage into direct current. Unfortunately, this direct current is not entirely linear, so a filtering stage is required, with the aim of obtaining direct current at the required voltage that is as flat or linear as possible. The final result will depend to a large extent on the quality of the filter used, which is one of the critical points of any power supply.

The next stage (which, as we have already mentioned, some products do not include) further smooths out the small fluctuations that may remain after filtering, keeping the voltage constant within a very tight margin and trying to prevent it from changing even when sporadic variations appear in the nominal voltage of the primary supply.

A piece of electronic equipment is made up of different electronic circuits, each responsible for its own tasks. Each circuit needs its own voltage, which is where the final dividing stage comes into play.



The transformer



When we talk about power supplies, the first thing that usually comes to mind is the typical transformer. This electronic device is responsible for transforming (hence its name) a voltage of a defined value into another voltage of a different value. To do so, its design relies on the properties of magnetic fields. A transformer is a magnetic circuit, made of stacked laminations of ferromagnetic material, around which two coils are wound. The first coil, connected to the mains, creates a magnetic flux which, as it passes through the second coil, is converted back into electrical voltage. The right combination of turns and ferromagnetic material achieves the desired voltage. The vast majority of electronic circuits make do with voltages of 24 volts, with current being a decisive factor in amplifiers. Some of you will have heard of toroidal transformers. These cylindrical transformers do not, in principle, differ in their electrical and magnetic properties from any other transformer, the only difference being their core, which is ring-shaped (toroidal). Although this type of transformer is usually used in high-end amplifiers (which are commonly heavy), its advantages include better efficiency thanks to lower flux leakage, and therefore lower weight and smaller size. Its drawback is the tendency of its metal laminations to vibrate, something whose solution requires better design, that is, higher cost and a higher final price.

Even so, all transformers and their respective electronic circuits (especially the diode bridges in the rectification stage) add losses or voltage drops, so only good design and the right choice of components will produce a clean, constant DC output. These components also include the capacitors, which are directly responsible for the rectification stage. The greater the capacitance and the better the holding time they allow, the more linear and pure the DC voltage generated will be.

Finally, the designer of a power supply must choose between regulators using Zener diodes, transistors (or a combination of both) or integrated circuits, different options with varying costs and more or less precise results.

So far we have briefly reviewed the anatomy of a basic power supply.

The main drawbacks of this power supply design, known as linear, are its weight, bulk and heat emission. We can verify this by holding our hand near an amplifier that has been running for several minutes: it will be hot. Energy is neither created nor destroyed, only transformed. Therefore, this heat is clear evidence that a significant part of the current drawn from the mains is wasted as heat; in other words, the efficiency of this type of linear supply is low. Another serious problem with linear power supplies is that they add distortion to the main electrical grid, a series of unwanted pulses that can have a significant effect on other electronic equipment. We have all witnessed how an old television, a refrigerator or some other appliance added noise to our listening in the form of an unwanted click.

Even so, these linear power supplies have their advantages, such as lower operating noise and, since they contain relatively little electronics, a lower incidence of problems, even from disturbances created by electromagnetic radiation (including that of the transformer itself, if it is poorly shielded).



Switch-mode power supplies



To solve the problem of weight and efficiency, NASA developed the first switch-mode power supplies in the 1940s, achieving a much more efficient and, importantly for them, lighter solution (something essential when every kilo of weight represents a high cost in space travel). A switch-mode power supply bases its design on the controlled transfer of energy between the primary and secondary circuits of a transformer. This revolution brought about a drastic change within the reach of very few, given its high cost and limited development. Today, switch-mode power supplies are very much present in our homes, thanks to their greater stability, safety and efficiency, at the same price as a linear supply. Laptops, many compact devices and the vast majority of televisions of the last 10 years use this type of power supply design.

Even so, a directive had to be drawn up in 2001 (the EN60555-2 standard) requiring a power factor correction (PFC) circuit for all types of power supply, with the clear intention of improving the quality of the electricity supply, that is, preventing interference and other unwanted elements from affecting the common electrical grid. In fact, switch-mode power supplies are better suited to complying with this standard.

A power supply equipped with PFC (Power Factor Correction) reduces the harmonics fed back into the grid by means of an active filter that corrects the waveform of the input current, making it sinusoidal and in phase with the voltage.

Another advantage of a switch-mode power supply is its higher efficiency (that is, better use of the input current and voltage at the output stage). Greater efficiency reduces the amount of energy converted into heat, so we can find power supplies that run cool, which also extends the life of the electronic components.

Of course, switch-mode power supplies also have their problems, such as their difficulty in withstanding transient voltage spikes, whereas a linear supply can withstand them up to a magnitude of 10 (that is, it can withstand spikes of up to 10 times the nominal value).



The current situation



If switch-mode power supplies are already a reality in personal computers and other small appliances, what about audiovisual equipment?

The popularization of digital amplification has given the term "switch-mode power supply" greater prominence. In fact, the practical synergy between the two technologies is very favorable, since both solutions bring advantages in weight and efficiency. But in most cases, switch-mode power supplies are found in low-consumption equipment, which includes players, tuners and televisions. Why are they not included in high-power amplifiers?

This question is answered by Keiichi Onodera, designer of Pioneer Corporation's latest high-end amplifiers. "Switch-mode power supplies have not yet reached their full potential, and are not very suitable for high-end technological solutions," says Onodera. "In the Pioneer VSA-AX10Ai A/V receiver we use a traditional power supply, as it seemed to us the best option for our purposes."

The people at Linn do not think so, as their catalog includes several products equipped with switch-mode power supplies. Examples include their Linn Classik Movie DI complete A/V system, capable of delivering up to 75 W RMS (4 ohms) per channel, and their multichannel power amplifiers, thus getting ahead of the European directives with cutting-edge, technologically advanced solutions.