efficient audio power amplification

Andersen Efficient Audio Power Amplification - Challenges

AES 27th International Conference, Copenhagen, Denmark, 2005 September 2–4 1

EFFICIENT AUDIO POWER AMPLIFICATION - CHALLENGES

MICHAEL A. E. ANDERSEN

Oersted-DTU, Technical University of Denmark, Lyngby, Denmark [email protected]

For more than a decade efficient audio power amplification has evolved and today switch-mode audio power amplification in various forms are the state-of-the-art. The technical steps that lead to this evolution are described and in addition many of the challenges still to be faced and where extensive research and development are needed is covered.

INTRODUCTION During the ‘80s and ‘90s as audio and TV sets became more and more popular there was a costumer demand for both more compact equipment and higher output powers from the audio power amplifiers. As the dominating audio power amplifier principle was the class-B/AB higher output power meant higher power losses and even larger heatsinks. Another trend that began at that time was the active loudspeakers, loudspeakers with built-in power amplifiers, in the extreme case one power amplifier per loudspeaker driver unit. As these active loudspeakers were typical very compact higher power amplifiers were needed to get a reasonable woofer output at lower frequencies. In the active loudspeakers the heatsink problem escalated, as the loudspeaker enclosure got smaller and the heat dissipation increased. Thus the demand for audio power amplifier principles with higher efficiencies and lower power losses increased.

1 AUDIO POWER AMPLIFIER PRINCIPLES Not that long after the invention of the semiconductor transistor the class B topology (Fig. 1) became the dominant power amplifier principle.

Vin Vout + -

E

E

Figure 1: Class B principle.

But as the output transistors were run in their active region half of the time with both voltage across them and current running through the transistors there were severe lower losses. There was no way these losses could be avoided and huge heatsinks were unavoidable.

However by adding some extra components (Fig. 2) it was possible to reduce the voltage across the output transistors when they were carrying current. And by adding more components (Fig. 3) one could go further. These were the class B2 and B3 principles.

Vin

Vout+ -

E

E

α

αE

E-

-

Figure 2: Class B2 principle.

Vin

Vout+ -

E

α

α E

E-

-

0 0.5 1

1

1β

E

βE

-E

Figure 3: Class B3 principle.

Extending this procedure lead to the class G principle (Fig. 4) where the power supply was now two voltages sources controlled by the input signal. These two controlled power sources could be made with linear regulators but then the only result would be that most of the power loss would be moved from the output

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transistors to the power supply, i.e. no net power loss reduction.

Vin Vout + -

Figure 4: Class G principle.

Power supplies with higher efficiencies were developed during the race for space. In space it is very difficult to get rid of heat and additional heat also implies additional mass, which is very costly to send into orbit. These power supplies [1] with higher efficiencies are the SMPS switch mode power supplies, often controlled by PWM pulse width modulation. Using SMPS’s as the two power supplies in (Fig. 4) class G meant that they would need to have a bandwidth at least that of the linear power amplifier, in itself a challenge. The switch mode principle is here to stay when converting electrical energy with high efficiency. During the late ‘70s Sony put out a new amplifier principle, the class D (Fig. 5) also called a PWM amplifier [2-5].

Ti me

5 0 u s 6 0 u s 7 0 u s 8 0 u s 9 0 u s 1 0 0 u s 1 1 0 u s 1 2 0 u s 1 3 0 u s 1 4 0 u s 1 5 0 u sV( GLI MI T1 : OUT) V( R1 : 2 )

- 1 . 0 V

0 V

1 . 0 V

SEL>>

V( V4 : +) V( V3 : +)- 1 . 0 V

0 V

1 . 0 V

Figure 5: Class D principle [11].

But the world wasn’t ready for this principle. Especially the power transistors weren’t good enough at that time. During the early ‘90s the power MOSFETs became better and better [6-10]. Their on-state resistance lowered and eventually the on-state voltage drop became lower than a diode forward voltage drop thus enabling the reverse conduction of the MOSFET (also called synchronous rectification by SMPS designers). The switching components for the class D principle had now matured [12-31]. The power losses and the efficiencies of the different audio power amplifier principles are shown in Fig. 6 and Fig. 7.

But in many companies it was a big obstacle that the audio power amplifiers designers that had done linear designs for maybe 20-30 years now were to do switch mode audio power amplifiers.

It is still in many companies different design teams doing the SMPS and the switching amplifier although they have very much in common.

0 0.2 0.4 0.6 0.8 10

0.1

0.2

0.3

0.4

0.5

Class BClass B2Class B3Class GClass D

x

Ploss/Pmax

Figure 6: Examples of relative power losses vs. relative

output level x for class B, B2, B3, G, and D.

x

Efficiency

0 0.2 0.4 0.6 0.8 10

0.5

1

Class DClass GClass B3Class B2Class B

Figure 7: Examples of efficiency vs. relative output level x for class B, B2, B3, G, and D.

Another switch mode topology that can be used for audio power amplifiers are the high frequency link inverter [11, 32-51] essentially an integration of a SMPS and a class D amplifier. An example is shown in Fig. 8.

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Figure 8: Example of a HL-link inverter [40].

2 POWER STAGE

2.1 Errors Although the MOSFETs have evolved a lot they are far from ideal. There are pulse timing errors (Fig. 9) and pulse amplitude errors (Fig. 10).

VPWM

VCC

-VCC

time

Figure 9: Pulse timing errors.

Figure 10: Pulse amplitude errors [12]. These errors and error sources will continue to exist and they can be reduced by feedback [11, 12] or by means of pre-compensation. In the future other control methods may be used e.g.:

Adaptive control techniques for power stage error reduction may be used.

Another very important factor is the diodes and intrinsic body diodes and their behaviour:

Reverse recovery characterisation and reduction are important.

It may have crucial influence on especially the overshoot and ringing in a layout [134]. Overshoot reduction, proper layout, and EMC reduction are very important issues in obtaining high quality.

2.2 Power switches For a decade now the MOSFET has been dominating the switching audio power amplifiers but:

IGBTs may be used for very high output powers. New power devices like:

The power JFET are claimed to be superior to the MOSFET.

It will be interesting to see how power JFET [52-53] performs in switching audio power amplifiers. When using the HF-link inverter type (Fig. 8) switching audio power amplifiers bi-directional switches are need for the output stage: Completely new power devices for audio power amplification is needed, the bi-directional switches.

These should preferably have no on-state diode forward voltage drop.

2.3 Output filter The output filter is used to attenuate the (Fig. 11) HF noise and HF components around multiple for the switching frequency fs but still allowing the audio frequencies within the bandwidth B to pass.

Figure 11: Power stage output frequency spectrum [12].

Figure 12: Power stage and output filter [11].

Usually the output filter I made as a second order filter (Fig. 12) with only moderate attenuation of the switching harmonics. Furthermore the output filter must not cause any distortion of the output signal. A higher order output filter would be favourable but ringing and/or feedback stability problems can occur.

2.4 Integration with loudspeaker When integrating the switching amplifier with an electro dynamic loudspeaker [11, 54-61] the output filter can be omitted. But special attention to the layout of the voice coil and magnetic structure of the electro dynamic loudspeaker has to be taken [11]. This is a trade-off of integration vs. versatility. Integrating other transducer types, like f.x. piezo transducers or NXT [62], may require other types of switching power stages [63-69].

3 DIGITAL INPUT Switching amplifiers with a digital input have to convert the PCM pulse code modulated input signal (from f.x. a CD) to a PWM signal. Doing a straight conversion

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would cause harmonic distortion, but techniques [70-115] have been developed to do this PCM-PWM conversion with only diminutive distortion.

Figure 13: PCM to PWM conversion.

4 ANALOGUE INPUT The waste majority of switching amplifiers with analogue input uses PWM. This is based on a fixed carrier frequency (Fig. 5) becoming the switching frequency of the output stage. In these solutions the PSRR power supply rejection ratio relies completely on the feedback and/or feed-forward circuitry. But in another kind of modulators the carrier is derived from the PWM-type output signal, which is integrated or low-pass filtered. These are the self-oscillation [116-119] type modulators, which can be based on hysteresis [120-128] on a controlled oscillation [129-133].

Modelling, analysis and optimization of self-oscillation type modulators can lead to improved designs.

4.1 Hysteresis type modulator

Figuer 14: Example of a hysteresis type modulator

[121].

4.2 Controlled oscillating type modulator Subsections and subsection titles should look like this.

Figure 15. Example of a controlled oscillation type

modulator [130].

4.3 Carrier distortion What especially became apparent during the research of the self-oscillating modulators was the carrier distortion. It seems that the controlled oscillation type modulators has a higher carrier distortion (Fig. 16) that is being attenuated be the loop gain. While the hysteresis type modulators has a more linear carrier (Fig. 16) thus not requiring that much loop gain for attenuation of the distortion.

Ti me

5 0 u s 5 5 u s 6 0 u s 6 5 u s 7 0 u s 7 5 u s 8 0 u s 85 u s 9 0 u s 95 u s 1 0 0 u sV( GAI N6 : I N) V( GLI MI T5 : OUT) V( GLI MI T7 : OUT)

- 8 0 0 mV

- 4 0 0 mV

0 V

4 0 0 mV

8 0 0 mV

Figuer 16: Input signal (green), hysteresis type

modulator (blue), and controlled oscillation type modulator (red) [11].

Designing for carrier distortion reduction will improve both fixed frequency PWM modulators and self-oscillating type modulators.

5 CONCLUSIONS Several major challenges that have been faced during the evolution of the switching audio power amplifiers have been addressed. In addition new areas of research and development have been pointed out. REFERENCES

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efficient audio power amplification

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