Audio converter subsystem design in the 21st century – Part 1: Overview

Linear and switching PSUs
High-quality line-powered audio equipment has traditionally employed linear PSUs, but these have disadvantages of size/weight, heat and cost, and may need a manual line-voltage selector. But they do have the advantage (if properly designed) of not being a source of high-frequency interference into the analogue circuits. A switch-mode power supply (SMPS) eliminates these disadvantages, but must be carefully designed if it is not to become a major source of hostile switching noise. SMPSs have traditionally been feared by high-end audio designers.

A SMPS in any type of equipment must be designed to meet the appropriate EMC standards for conducted and radiated emissions. Nowadays this isn't very hard to do, with SMPS controller vendors providing application circuits designed to be compliant (usually only just, so as to save costs of filter components, etc.). But an unfortunate fact of life for audio equipment designers is that the same sorts of misbehaviours which might cause a SMPS to be non-compliant can play hell with audio performance, even at much lower levels. Thus SMPS design for audio applications can be elaborate.

Drawbacks of low-cost SMPSs
Since converter systems often need a large number of power rails, and may benefit from isolation between the digital parts and the analogue, a 'flyback' architecture is a popular choice because it conveniently offers these benefits - which may also be useful in DC-powered situations, as discussed later. A wide variety of other 'simple' SMPS architectures can be used instead – the problem for the uninitiated is generally how to choose between them.

In a flyback converter, DC (either directly input or rectified from the AC line voltage) is switched though the primary of the 'flyback transformer' by a transistor under the control of a device which regulates the duty cycle of a train of switching pulses in order to keep the various secondary outputs regulated. The secondaries are rectified and filtered to provide the power rails.

Figure 6 shows a flyback converter, and its switching waveform. Note that the HF oscillation at the point where the switch is turned off is caused by the stray capacitance Cd across the switch, and the leakage inductance of the primary Llk, and is a largely-unavoidable source of hostile RF. Its amplitude can be controlled with snubbers to protect the switch, but this can even make the interference worse. The LF oscillation prior to turning on the switch is a consequence of the stray capacitance Cd and the primary inductance Lp, and is interrupted at a randon voltage by the switch-on, causing potential interference from the large switching voltage and current.

So the main problem with the basic flyback topology (and most other basic topologies) is that the transistor switches hard and randomly, causing high levels of radiated and conducted interference to invade the analogue audio parts. We are now on a slippery slope: there isn't much we can do to reduce the source of the problem (we can keep the switching loops as small as possible to reduce radiation, we can optimise the ground topology and make critical tracks fat to reduce ground noise; we can tame the edge times with snubbers, but only at the cost of reduced efficiency and increased heat). So we end up having to take disproportionate steps in the vulnerable parts to make sure that the audio remains clean. These can include screening cans, split grounds, galvanic isolation, etc.

Another problem is the random frequency of the SMPS controller, which can produce interference at the beat frequency between itself and (for example) the audio sample rate, thus making it impossible to confine it to some inconspicuous part of the spectrum. A possible solution is to lock the switching frequency to some multiple of the sample rate in order to remove the beat frequency, but this can be problematic: the regulatory variation of duty-cycle can cause its own beat, and some types of SMPS controllers like to vary the switching frequency to effect regulation. You could even introduce a situation where a software bug or a wayward sample rate could collapse the power rails.

It's easy to see why we have been reluctant to use SMPSs in high-performance audio equipment, but in DC-powered situations we often have no choice, and SMPSs are small and cheap – so it would be good to find a way to tame them.

Resonant and quasi-resonant SMPSs
Ideally, to combine the benefits of a linear supply and a SMPS, we would like to find a way of passing a high-frequency sine wave through the transformer. An approximation to such a solution is a 'resonant' SMPS. In order for this to be achieved losslessly, it is usual to generate the sine wave by placing a resonant LC tank circuit in the primary – a neat trick is to use the primary inductance as the L part. Of course the stimulus for the waveform is still a hard-switching transistor under clever control, but the resonant circuit tunes the primary waveform to an approximation of a simple sinewave, with the switching happening at zero-voltage or zero-current moments, all of which leads to much less hostile switching noise.

On the other hand, resonant designs tend to be somewhat more costly and larger than flyback designs. A major drawback with many resonant architectures is that the LC tank circuit needs to be tailored to the switching frequency and DC input voltage in order to maintain resonant and zero-switching operation, which makes off-line universal input design problematic unless power-factor-correction (PFC) is incorporated.

A good compromise is a 'quasi-resonant' converter (QRC): the idea here is that since the problems in a simple flyback converter only happen at the moments of switching, it is only necessary to find a way of creating a resonant waveform at the switching points. This can be done quite simply by introducing primary resonance into an ordinary flyback topology, and making the controller clever about deciding when to switch. Figure 7 shows a zero-voltage-switching (or 'valley switching') QRC which is a low-cost way to cut SMPS emissions at source.

The tank circuit (in this case created by simply adding a large Cd across the switch) slows the switch-off rise time, and the controller arranges the switch-on instant to coincide with a 'valley' in the LF oscillation; this inherently makes the cycle period variable with an attendant 'spread spectrum' effect which can improve interference and certainly increases EMC margins, as shown in Figure 8.

SMPS topology selection is vital for audio performance
It is often difficult for the uninitiated to make the correct choice of SMPS architecture for audio, because most SMPS controller vendors organise their selection tables by power capability, on the assumption that the audio designer, like everyone else, will want to choose the cheapest solution for his power requirement. The recommended SMPS for low-power applications like this (say <20W) will usually be a very basic, hard-switching type because the power is low enough for its emissions to be retained below statutory EMC limits with minimal filtering, and for the losses resulting from its indiscriminate switching to be insufficient to set it on fire.

Resonant and quasi-resonant topologies tend to be recommended for higher power applications where the switching noise and losses HAVE to be controlled. But for audio, it's often a good idea to make a low-power implementation of a high-power topology in the interests of achieving minimum emissions – the extra cost can usually be saved in not having to armour plate the audio parts.

Other considerations
Cross-regulation is often a problem with multi-rail SMPS designs, since the regulatory mechanism of the controller can generally only operate on one rail. Varying load conditions on individual rails can cause the voltages of other rails to vary, an effect known as cross-regulation.

In performance-critical applications it may be necessary to provide linear post-regulation on analogue power rails from the SMPS. If so, care should be taken that the linear regulators are adequately cooled. It should also be noted that linear regulators can usually only regulate over a limited frequency range, and the switching products from the SMPS can easily exceed this, resulting in their passing straight through the regulator. It is therefore recommended to use ferrite beads ahead of linear post-regulators.

Line-powered linear and SMPS equipment usually draws current from the mains only during voltage peaks, which can cause the power line to be distorted with possible detriment to the performance of other sensitive audio equipment. It may be beneficial to design power-factor-correction (PFC) into your SMPS design in order to cause the least possible distortion to the power waveform (although you can be sure that all the other equipment around will probably be causing distortion anyway). Some PFC schemes allow tight control of the rectified voltage ahead of the SMPS, which can allow switching noise to be further reduced in resonant and quasi-resonant designs by ensuring zero-switching for any input voltage.

As well as applicable safety, EMC and disposal legislation, line-powered equipment is already, or will become, subject to various territorial legislation for standby power consumption (where applicable) and operating efficiency (for example under the EU Ecodesign Directive and the voluntary US EnergyStar program). Although territorial legislations vary, maximum standby power consumption of 0.5W and minimum operating efficiency of about 80% are typical for a 20W device.