Design of Class-D Audio Amplifiers

Chun-Hsien Su中央大學電機系 蘇純賢

chunsu@ee.ncu.edu.tw

May 17, 2006

Chun-Hsien Su 2

OUTLINE

1. Introduction of Class-D Audio Amplifier

2. Large-Power Class-D Applications

4. Sigma-Delta Class-D Audio Amplifiers

5. Conclusions

3. Integrated Class-D Audio Power Stages

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Introduction of Class-D Audio Amplifier

η = 30% at temp rise test condition.

– Traditional Class-AB Amplifier

Class AB amplifier uses linear regulating transistors to modulate output voltage.

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Introduction of Class-D Audio Amplifier– Class-D Audio Amplifier

Class D amplifier uses MOSFETs that are either ON or OFF.

PWM technique is used to express analog audio signals with ON or OFF states in output devices.

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Basic PWM Operation

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Audio Output Spectral Density

LC-filtered

Carrier and modulated signalsFundamental

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Topology Comparison: Class AB vs. Class D

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Efficiency: Class AB vs. Class D[From TI’s report]

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Measured Efficiency [From TI’s data]

– TI-TPA2000D4

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Temperature vs. Output Power– Class-D Advantage: Less Heat

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Battery Life

A head to head test ofTI’s class-AB and class-D~1W mono solutions:Class-AB - TPA731Class-D - TPA2001D

Both set-up on the PnPplatform powered by3x1.2V NiMH batteriesdriving a typical 8-Ωspeaker used in wirelessapplications.

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Battery Life

A head to head test ofTI’s class-AB and class-D~1W mono solutions:Class-AB - TPA731Class-D - TPA2001D

Both set-up on the PnPplatform powered by3x1.2V NiMH batteriesdriving a typical 8-Ωspeaker used in wirelessapplications.

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Analogy to Buck DC-DC Converter

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Loss in Power Device

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Full Bridge versus Half Bridge

– Full Bridge

– Full Bridge

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Full Bridge versus Half Bridge (Cont.)

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Major Cause of Imperfection

Gate Driver

PWM

Feedback

Audio source

Dead timeDelay time

ArchitectureAnalog/digitalw/SDM

Modulation errorQuantization error

Single-bridge/Full bridgeFilterless modulation scheme

Switch nonidealities

Finite RonBody diode Recovery(EMI)

Nonlinear LC

Poor PSRRNoise coupling

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THD and Dead Time

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Shoot Through and Dead Time

- Shoot through charge increases rapidly as dead time gets shorter.

- Need to consider manufacturing tolerances and temperature characteristics.

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Power Supply Pumping

- Significant at low frequency output- Significant at low load impedance- Significant at small bus capacitors- Largest at duty = 25%, and 75%

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EMI consideration: Qrr in Body Diode

1. Low side drains inductor current2. During dead time body diode of low side conducts and keep inductor current flow3. At the moment high side is turned ON after dead time, the body diode is still conducting to wipe away minority carrier charge stored in the duration of forward conduction. This current generates large high frequency current waveform and causes EMI noises.

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Gate Driver: Why is it Needed? Gate of MOSFET is a capacitor to be charged and discharged. Typical effective capacitance is 2nF.

High side needs to have a gate voltage referenced to it’s Source.

Gate voltage must be 10-15V higher than the drain voltage.

Need to control HS and LS independently to have dead time.

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Functional Block Diagram Inside Gate Driver

With the addition of few components, they provide very fast switching speeds and low power dissipation.

High side needs to have a gate voltage referenced to it’s Source.

Need to control HS and LS independently to have dead time.

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Bootstrap High Side Power Supply

When Vs is pulled down to ground through the low side FET, the bootstrap capacitor (CBOOT) charges through the bootstrap diode (Dbs) from the Vcc supply, thus providing a supply to Vbs.

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Power Dissipation in Gate DriverWhenever a capacitor is charged or discharged through a resistor, half of energy that goes into the capacitance is dissipated in the resistor. Thus, the losses in the gate drive resistance, internal and external to the MGD, for one completecycle is the following:

GSWG QfVP

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Power Dissipation in Gate Driver (Cont’d)

The use of gate resistors reduces the amount of gate drive power that is dissipated inside the MGD by the ratio of the respective resistances.

These losses are not temperature dependent.

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MOSFET Power Switch

A MOSFET is a voltage-controlled power switch.A voltage must be applied between Gate andSource terminals to produce a flow of current inthe Drain.

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Key Parameters of Power MOSFETs

(1). Voltage Rating, BVDSS : Drain-source breakdown voltage. Temperature dependent.

(2). Gate Charge, Qg : Directly related to MOSFET speed Temperature dependent.

(3). D-to-S On-Resistance, RDS(ON) : Directly related to MOSFET conduction losses. Temperature dependent.

(4). Body Diode Reverse Recovery Characteristics, Qrr, trr , Irr , and S factor. Influence THD, EMI, and Efficiency. Temperature dependent.

(5). Package : Power dissipation capability, current capability, internal inductance, internal resistance, electrical isolation, and mounting process.

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Voltage Rating, BVDSS

This is the drain-source breakdown voltage (with VGS = 0). BVDSS should be greater than or equal to the rated voltage of the device, at the specified leakage current, normally measured at Id=250uA.

This parameter is temperature-dependent and frequently ∆BVDSS/∆Tj (V/°C) is specified on datasheets.

BVDSS MOSFET voltages are available from tens to thousand volts.

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Gate Charge, Qg

parameter is directly related to the MOSFET speed and is temperatureindependent. Lower Qg results in faster switching speeds and consequently lower switching losses.

The total gate charge has two main components: the gate source charge, Qgs and, the gate-drain charge, Qgd (often called the Miller charge).

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D-to-S On-Resistance, RDS(ON)

This is the drain-source resistance, typically specified on data sheet at 25°C with VGS = 10V. RDS(ON) parameter is temperature-dependent, and is directly related to the MOSFET conduction losses. lower RDS(ON) results in lower conduction losses.

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Body Diode Reverse Recovery Characteristics, Qrr, trr , Irr , and S Factor

Power recovery characteristics are temperature-dependent and lower trr, Irr and Qrr improves THD, EMI and Efficiency η.

Power MOSFETs inherently have an integral reverse body-drain diode. This body diode exhibits reverse recovery characteristics. Reverse Recovery Charge Qrr, Reverse Recovery Time trr, Reverse Recovery Current Irr and Softness factor (S = tb/ta), are typically specified on data sheets at 25°C and di/dt = 100A/us.

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Package

MOSFETs devices are available in several packages as SO-8,TO-220, D-Pak, I-Pak, TO-262, DirectFET™, etc.

The selection of a MOSFET package for a specific application depends on the package characteristics such as dimensions, power dissipation capability, current capability, internal inductance, internal resistance, electrical isolation and mounting process.

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Choosing the MOSFET Voltage Rating

– Topology (Full Bridge or Half Bridge)

MOSFET voltage rating for a Class D amplifier is determined by:

– Desired POUT and load impedance (i.e. 250W on 4Ω)

– Modulation Factor M (80-90%)

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Trends in Class-D Audio Amplifiers

– Make it smaller! higher efficiency smaller package Half Bridge

– Make it sound better! THD improvement fully digitally processed modulator

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– Patent Blooming since Y2KUS Patent Class 330/207A – Class-D Amplifier

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Trends in Class-D (1)

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– Systems by ICs

Trends in Class-D (2)

Bipolar

CMOS

DMOS (double-diffused MOS)

• It is a (lateral) double-diffused MOS transistor.• The device is asymmetrical.• A lightly doped Nwell(extended drain) region supports high drain voltages.• The thin gate oxide allows a high device but restricts• The Pbody is shorted to source of device in metal 1.

DMOS features:

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– Full versus Half, Performance versus Cost

Trends in Class-D (3)

Full Bridge

Half Bridge

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– Performance Improved by Analog FB Loop

Trends in Class-D (4)

This structure improves output waveform & PSRR.

This patent can be avoided by using different architecture.

US Patent : 6300825, issued 2001/10/9“PWM with feedback loop Integrator”

Similar: Yokoyama 4504793 1985/3/12

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– Filterless Modulation for Portable Applications

Trends in Class-D (5)

Information in pulse width Information in phase

Limits the development of small-power (~1W) class-D amplifier.

Can be solved by using different modulation schemes.

Gate Driver

US Patent : 6262632, issued 2001/7/17“Concept and method to enable filterless, efficient operation of Class-D amplifiers”

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– Performance Improvement by Sigma-Delta?

Trends in Class-D (6)

PWM

Delta

Sigma-Delta

The power efficiency and the signal-to-noise ratio (SNR) for a amplifier based on the above scheme are compared with the values typical of linear and PWM amplifiers of the same rated power. The power efficiency of the solution is an intermediate value between the linear and PWM amplifiers, whereas SNR mainly depends on the modulator type adopted. A SNR around 60–70 dB was estimated by adopting a fourth-order modulator. However, this value can be significantly improved by increasing the switching frequency or the modulator order.[Dallago, Tran. CAS-I, Aug, 1997]

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Trend in Class D Amplifiers

(1). Reduction of power circuit parasitic capacitance by technological improvement of both semiconductor and magnetic devices;

In any case, to extend the power range of high-frequency modulation, future studies should be oriented in the following direction:

(3). development of extended modulation techniques (for example of multilevel type or nonconstant switching period type or, generally, of hybrid type) allowing a lower switching frequency under the same baseband performance.

(2). Adoption of soft-switching schemes able to perform a larger exploitation of converter parasitic parameters under a load range that is as wide as possible;

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1-bitPDM

PWMClass-D CT SDM

Comparator Run-free Clocked

Output PWM 1-b PDM

Min. pulse Width

1/fsfs<1MHz

1/fsfs>1MHz

Drive H-bridge

OK May burn out

For second-order Mfs=40kHzx128=5.12MHz

Gate Driver

High-orderSingle-bit

M

Audio source Bit flipping(bit grouping)

Open-loop, Full-bridge, low-OSR M with bit grouping

(digital) (digital)(d & a)

THD

PWM versus PDM

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Class-D Amplifier

Gate Driver

M

Audio source

Dual noise-shaper architecture

+-

Digital

- Simple, almost all digitized

m-bitAD

WaveformRepresent

Grouping

Interpolator Noise Shaper

1/m-bitDAC

CTfilter

Class AB/Class DAmplifier

SpeakerPCM

Interpolator Noise Shaper

Class DAmplifier

SpeakerPCM

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Conclusion

– All-digital Sigma-delta class-D Future work.

– Highly efficient Class D amplifiers now provide similar performance to conventional Class AB amplifiers - If key components are carefully selected and the layout takes into account the subtle, yet significant impact due to parasitic components.– Constant innovation in semiconductor technologies helps the growing Class D amplifiers usage due to improvements in higher efficiency, increased power density and better audio performance. Trends in class-D: half-bridge filterless scheme, analog-feedback loop to improve performance.

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References[1]. M. Berkhout, “An integrated 200-W Class-D audio amplifer,” IEEE J. Solid-State Circuits, vol. 38, pp. 1198–1206, July 2003.

[2]. H. Ballan, M. Declercq, and J. U. Duncombe, “12 V - Class-D amplifier in 5 V CMOS technology,” in Proc. IEEE CICC, 1995, pp. 559–562.

[3]. E. Dallago, “Advances in high-frequency power conversion by deltasigma modulation,” IEEE Trans. Circuits Syst. I, vol. 44, pp. 712–721, Aug. 1997.

[4]. Marco Berkhout, “An integrated 200 W class-D audio amplifier,” IEEE J. Solid-State Circuits, vol. 38, no. 7, pp. 1198–1206, Jul. 2003.

[5]. Jun Honda and Jorge Cerezo, IRA Technical Report.