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Class-D amplifier

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Block diagram of a basic switching or PWM (class D) amplifier

A class-D amplifier or switching amplifier is an electronic amplifier where all power devices (usually MOSFETs) are operated as binary switches. They are either fully on or fully off. Ideally, zero time is spent transitioning between those two states.

Output stages such as those used in pulse generators are examples of class D amplifiers. However, the term mostly applies to power amplifiers intended to reproduce signals with a bandwidth well below the switching frequency.

Basic operation

Class D amplifiers work by generating a square wave of which the low-frequency portion of the spectrum is essentially the wanted output signal, and of which the high-frequency portion serves no purpose other than to make the wave-form binary so it can be amplified by switching the power devices.

A passive low-pass filter removes the unwanted high-frequency components, i.e., smoothes the pulses out and recovers the desired low-frequency signal. To maintain high efficiency, the filter is made with purely reactive components (inductors and capacitors), which store the excess energy until it is needed instead of converting some of it into heat. The switching frequency is typically chosen to be ten or more times the highest frequency of interest in the input signal. This eases the requirements placed on the output filter. In cost sensitive applications the output filter is sometimes omitted. The circuit then relies on the inductance of the loudspeaker to keep the HF component from heating up the voice coil. It will also need to implement a form of three-level (class BD) modulation which reduces HF output, particularly when no signal is present.

The structure of a class D power stage is essentially identical to that of a synchronously rectified buck converter, a type of non-isolated switched-mode power supply. Whereas buck converters usually function as voltage regulators, delivering a constant DC voltage into a variable load and can only source current (one-quadrant operation), a class D amplifier delivers a constantly changing voltage into a fixed load, where current and voltage can independently change sign (four-quadrant operation). A switching amplifier must not be confused with any amplifier that uses an SMPS. A switching amplifier may use any type of power supply but the amplification process itself operates by switching.

Theoretical power efficiency of class D amplifiers is 100%. That is to say, all of the power supplied to it is delivered to the load, none is turned to heat. This is because an ideal switch in its on state will conduct all current but has no voltage across it, hence no heat is dissipated. And when it is off, it will have the full supply voltage standing across it, but no current flows through it. Again, no heat is dissipated. Real-life power MOSFETs are not ideal switches, but practical efficiencies well over 90% are common. By contrast, linear AB-class amplifiers are always operated with both current flowing through and voltage standing across the power devices. An ideal class B amplifier has a theoretical maximum efficiency of 78%.

Terminology

The term "class D" is sometimes misunderstood as meaning a "digital" amplifier. While some class D amps may indeed be controlled by digital circuits, the power stage deals with voltage and current as a function of time. The smallest amount of noise, timing uncertainty, voltage ripple or any other non-ideality immediately results in an irreversible change of the output signal. The same errors will only lead to incorrect results when they become so large that a signal representing a digit is distorted beyond recognition. Up to that point, non-idealities have no impact on the transmitted signal. The difference between digital and analogue signals is that digital signals are subsequently interpreted as numbers whereas in analogue signals the exact waveform matters.[1]

Nevertheless, the term "digital amplifier" has gained currency to denote class D amplifiers with significant amounts of digital processing in them.

Signal modulation

The binary waveform is derived using pulse-width modulation (PWM), pulse density modulation (sometimes referred to as pulse frequency modulation), sliding mode control (more commonly called "self-oscillating modulation" in the trade.[2]) or discrete-time forms of modulation such as delta-sigma modulation.[3]

The most basic way of creating the PWM signal is to use a high speed comparator ("C" in the block-diagram above) that compares a high frequency triangular wave with the audio input. This generates a series of pulses of which the duty cycle is directly proportional with the instantaneous value of the audio signal. The comparator then drives a MOS gate driver which in turn drives a pair of high-power switches (usually MOSFETs). This produces an amplified replica of the comparator's PWM signal. The output filter removes the high-frequency switching components of the PWM signal and recovers the audio information that the speaker can use.

DSP-based amplifiers which generate a PWM signal directly from a digital audio signal (e. g. SPDIF) either use a counter to time the pulse length e. g.[4] or implement a digital equivalent of a triangle-based modulator. In either case, the time resolution afforded by practical clock frequencies is only a few hundredths of a switching period, which is not enough to ensure low noise. In effect, the pulse length gets quantized, resulting in quantization distortion. In both cases, negative feedback is applied inside the digital domain, forming a noise shaper which has lower noise in the audible frequency range.

Design challenges

Distortion and switching speed

Two significant design challenges for MOSFET driver circuits in class D amplifiers are keeping dead times and linear mode operation as short as possible. "Dead time" is the period during a switching transition when both output MOSFETs are driven into Cut-Off Mode and both are "off". Dead times need to be as short as possible to maintain an accurate low-distortion output signal, but dead times that are too short cause the MOSFET that is switching on to start conducting before the MOSFET that is switching off has stopped conducting. The MOSFETs effectively short the output power supply through themselves, a condition known as "shoot-through". Meanwhile, the MOSFET drivers also need to drive the MOSFETs between switching states as fast as possible to minimize the amount of time a MOSFET is in Linear Mode, the state between Cut-Off Mode and Saturation Mode where the MOSFET is neither fully on nor fully off and conducts current with a significant resistance, creating significant heat. Driver failures that allow shoot-through and/or too much linear mode operation result in excessive losses and sometimes catastrophic failure of the MOSFETs.[5]

Electromagnetic interference

The switching power stage generates both high dV/dt and dI/dt, which give rise to radiated emission whenever any part of the circuit is large enough to act as an antenna. In practice, this means the connecting wires and cables will be the most efficient radiators so most effort should go into preventing high-frequency signals reaching those:

  • Avoid capacitive coupling from switching signals into the wiring.
  • Avoid inductive coupling from various current loops in the power stage into the wiring.
  • Use one unbroken ground plane and group all connectors together, in order to have a common RF reference for decoupling capacitors
  • Include the equivalent series inductance of filter capacitors and the parasitic capacitance of filter inductors in the circuit model before selecting components.
  • Wherever ringing is encountered, locate the inductive and capacitive parts of the resonant circuit that causes it, and use parallel RC or series RL snubbers to reduce the Q of the resonance.
  • Do not make the MOSFETs switch any faster than needed to fulfil efficiency or distortion requirements. Distortion is more easily reduced using negative feedback than by speeding up switching.

Power supply design

Class D amplifiers place an additional requirement on their power supply, namely that it be able to sink energy returning from the load. Reactive (capacitive or inductive) loads store energy during part of a cycle and release some of this energy back later. Linear amplifiers will dissipate this energy away, class D amplifiers return it to the power supply which should somehow be able to store it. In addition, half-bridge class D amps transfer energy from one supply rail (e.g. the positive rail) to the other (e.g. the negative) depending on the sign of the output current. This happens regardless of whether the load is resistive or not. The supply should either have enough capacitive storage on both rails, or be able to transfer this energy back.[6]

Error control

The actual output of the amplifier is not just dependent on the content of the modulated PWM signal. The power supply voltage directly amplitude-modulates the output voltage, dead time errors make the output impedance non-linear and the output filter has a strongly load-dependent frequency response. An effective way to combat errors, regardless of their source, is negative feedback. A feedback loop including the output stage can be made using a simple integrator. To include the output filter, a PID controller is used, sometimes with additional integrating terms. The need to feed the actual output signal back into the modulator makes the direct generation of PWM from a SPDIF source unattractive.[1] Mitigating the same issues in an amplifier without feedback requires addressing each separately at the source. Power supply modulation can be partially canceled by measuring the supply voltage to adjust signal gain before calculating the PWM [7] and distortion can be reduced by switching faster. The output impedance cannot be controlled other than through feedback.

Advantages

Despite the complexity involved, a properly designed class D amplifier offers the following benefits:

  • Reduction in size and weight of the amplifier,
  • Reduced power waste as heat dissipation and hence smaller (or no) heat sinks,
  • Reduction in cost due to smaller heat sink and compact circuitry,
  • Very high power conversion efficiency, usually better than 90% above one quarter of the amplifier's maximum power, and around 50% at low power levels.
Boss Audio mono amp. The output stage is top left, the output chokes are the two yellow toroids underneath.

Uses

  • Home Theatre systems. In particular the economical "home theatre in a box" systems are almost universally equipped with class D amplifiers. On account of modest performance requirements and straightforward design, direct conversion from digital audio to PWM without feedback is most common.
  • Mobile phones. The internal loudspeaker is driven by up to 1 W. Class D is used to preserve battery lifetime.
  • Powered speakers
  • High-end audio is generally conservative with regards to adopting new technologies but class D amplifiers have made an appearance[8]
  • Active subwoofers
  • Sound Reinforcement and Live Sound. The Crest Audio CD3000, for example, is a class-D power amplifier that is rated at 1500 W per channel, yet it weighs only 21 kg (46 lb).[9] Similarly, the Powersoft K10 is a class-D power amplifier that is rated at 6000 W per 2-ohm channel, yet it weighs only 12 kg (26.5 lb).[10]
  • Bass amplifiers Again, an area where portability is important. Example: Yamaha BBT500H bass amplifier which is rated at 500 W, and yet it weighs less than 5 kg (11 lb).[11] The Promethean P500H by Ibanez is also capable of delivering 500 W into a 4 Ohm load, and weighs only 2.9 kg.

See also

References

  1. ^ a b Putzeys et al. All Amplifiers etc., Presented at the AES 120th convention
  2. ^ The generic analysis of sliding mode control is quite math heavy. The specific case of 2-state self-oscillating class D amplifiers is much more intuitive and can be found in Globally Modulated Self-Oscillating Amplifier with Improved Linearity, 37th AES Conference
  3. ^ The Analog Devices AD1990 Class-D audio power amplifier is an example.
  4. ^ Sandler et al., Ultra-Low Distortion Digital Power Amplification, Presented at the 91st AES convention
  5. ^ Analytical and numerical analysis of dead-time distortion in power inverters
  6. ^ irf.com - IRAUDAMP7S, 25W-500W Scalable Output Power Class D Audio Power Amplifier Reference Design, Using the IRS2092S Protected Digital Audio Driver, page 26, 2008-08-29
  7. ^ Boudreaux, Randy, Real-Time Power Supply Feedback Reduces Power Conversion Requirements For Digital Class D Amplifiers
  8. ^ Group review of "high end" class D offerings and round-table discussion with amplifier designers
  9. ^ Crest Audio CD3000 Specifications
  10. ^ Burning man Festival K10
  11. ^ Yamaha BBT 500H Specifications
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