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Power-Supply Issues And Remedies
Class D technology introduces some new issues of its own. It's often forgotten that Class D amplifiers are more sensitive to the quality of the power supply. Unlike their linear counterparts, whose bandwidths are limited to the audio range, Class D output stages switch at frequencies of several hundred kilohertz.
To accurately produce these square pulses,
the supply must be able to increase or decrease its current output very rapidly without ringing or a drop in the output voltage. This requires reservoir capacitors that can hold enough charge to prevent a current surge from causing the supply voltage to drop. Because any parasitic resistance or inductance will impede a rapid delivery of the stored charge, these capacitors should feature a low effective series resistance (ESR).
Adding a small, low-ESR capacitor in parallel with a larger, conventional electrolytic capacitor
isn't sufficient. Because all of the output power is delivered in short bursts, all capacitance must be low-ESR. Parasitic resistances and inductances in pc-board copper tracks are equally detrimental and should be minimized by placing the reservoir capacitors as close to the output stage as possible.
Arranging for the transistors in the different output stages to switch on sequentially rather than simultaneously can alleviate demands on the supply's transient behavior. To this end, advanced PWM modulators feature a built-in "PWM phase-shift" function that introduces a fixed delay between the PWM signals for each output channel.
At a fraction of a PWM cycle, this delay is far too short to make an audible difference to the output, but it spreads switching transients around the PWM cycle (
Fig. 2). This technique diminishes instantaneous load-current changes by a factor that increases with the number of channels.
Many Class D amplifiers also provide little or no power-supply rejection (PSR), effectively using the supply as a voltage reference. To prevent mains or audio-band ripple from modulating the signal, the supply must be regulated. A switching power is the best answer in this case.
With their fast built-in load regulation often operating up to fairly high frequencies, these supplies eliminate the need for linear voltage regulators and help shrink reservoir capacitors. Essentially, the capacitor only needs to store enough charge to keep the supply voltage stable under load until the regulator kicks in. Moreover, switching supplies are more power-efficient than linear supplies, further reducing cooling requirements.
Class D And EMI
Another common problem with Class D amplifiers is electromagnetic interference (EMI) caused by the rapid switching of currents in the output stage. The simplest way to reduce EMI is to keep pc-board tracks or cables leading to the output stage as short as possible. Supplies and output stages should be located on the same pc board if at all feasible.
Speaker cables pose more of a challenge. In systems with built-in speakers, such as boom boxes, short speaker cables are an effective measure against EMI-and it won't add to the materials cost. Wherever external speakers are used, though, the cables' length is outside the designer's control, and EMI filtering becomes indispensable. Still, achieving sufficient EMI suppression without affecting sound quality can be difficult because the EMI spectrum is relatively close to the audio range. It consists primarily of the PWM switching frequency and its harmonics.
Consequently, designers face a difficult choice. Should they choose a filter with a low cutoff frequency that will suppress EMI but also attenuate the treble? Or, should they use a filter with a high cutoff frequency that will preserve a flat audio response at the cost of increased EMI (
Fig. 3)? Higher-order filters can better separate the different frequency ranges, but often the cost of the high-quality inductors is prohibitive.
Adding a digital speaker equalizer to the PWM modulator chip offers a way out of this dilemma. When programmed as a treble boost, such an equalizer counteracts the effect of a basic low-order, low-cutoff EMI filter on the audio signal, keeping the frequency response flat across the audio range. Different equalizer settings also can be used for matching an existing filter design to speakers with different impedances, maximizing design reuse.
