Søren Poulsen Ph.D. Student Ørsted ·DTU Technical University of Denmark [email protected]

NORPIE 2004 1

Søren Poulsen, [email protected]

Ørsted·DTU, AutomationTechnical University of Denmark

Integrating switch mode audio power amplifiers and electro dynamic loudspeakers for a

higher power efficiency

Søren Poulsen

Ph.D. Student

Ørsted·DTU

Technical University of Denmark

[email protected]

Michael A. E. Andersen

Professor

Ørsted·DTU

Technical University of Denmark

[email protected]

NORPIE 2004 2



Integrating switch mode audio amplifier and loudspeaker

• Using the voice coil inductance as output filter

NORPIE 2004 3



Loudspeaker impedance• Maximum phase shift ~ 65°

– Real part caused by eddy current losses in the magnetic system

• Resonances at high frequencies– Voice coil inductance

– Voice coil capacitance

NORPIE 2004 4



PWM modulation schemes

2-level modulation 3-level modulation

NORPIE 2004 5



PWM HF output spectra

2-level modulation 3-level modulation

M=0.01

M=1

NORPIE 2004 6



Pushing up ressonance frequency

• Layer to layer

capacityd x

x

b

h

d

2

2

1

l

xUCxdW l

2

2

0

2

3

22

2

1

p

CUdx

l

x

p

UCl

W ll

l

WUCll 2

2

1

23

14

p

pCpC l

ll

NORPIE 2004 7



Pushing up voice coil resonance frequency

• Minimizing L·C– L N2

– C 1/number of winding layers

• Fewer turns

• More winding layers

NORPIE 2004 8



Voice coil fill factor

2

2

14

tråd

Cu

vikling

Cu

d

d

A

Af

234

2

2

2

tråd

Cu

vikling

Cu

d

d

A

Af

isoCu

Cu

vikling

Cu

td

d

A

Af

3

NORPIE 2004 9



Efficiency improvement with low impedance voice coils

• Efficiency constant for same fill-factor

• Higher fill factor benefits efficiency

2

2

2

DC

DC

DC DC

Bl N

R N

Pi

R

P Bl NBl i Bl konst

R R N

NORPIE 2004 10



Power stage losses, 2-level modulation

• Output stage losses• Using same DIE area

and voice coil fill factor for different supply voltages

• Ripple current included

Single ended and full bridge output stage

5V, 10V, 20V, 40V supply

NORPIE 2004 11



Improving the magnetic system

• Modifying magnetic system with other materials near the voice coil

• Ferrite– Exellent HF properties– Bmax0,5T

• Powder iron– Exellent HF properties– Bmax 1-1,4T

NORPIE 2004 12


Ørsted·DTU, AutomationTechnical University of DenmarkPrototype magnetic system,

FEM simulation-26 iron powder material used

NORPIE 2004 13



Standard magnetic system

Blue: Above gap

Green: In gap

Red: Below gap

Short voice coil

NORPIE 2004 14



Prototype magnetic system

Blue: Above gap

Green: In gap

Red: Below gap

Short voice coil

NORPIE 2004 15



Standard and prototype magnetic system

Blue: Prototype system

Green: Std. system

Full length voice coil

NORPIE 2004 16



Magnetic system power loss vs. fs, 2-level modulation

Blue: M=0 Green: M=0.25 Red: M=0.5 Magenta: M=0.75 Orange: M=1

Prototype magnetic systemStandard magnetic system

NORPIE 2004 17



Magnetic system power loss vs. fs, 3-level modulation

Green: M=0.25 Red: M=0.5 Magenta: M=0.75 Orange: M=1

Prototype magnetic systemStandard magnetic system

NORPIE 2004 18



Audio quality

• Listening tests carried out– ABC blind test

• No prove of sound degradation

NORPIE 2004 19



Conclusion• Using the voice coil inductance as output

filter for the amplifier possible

• Eddy current losses in the magnetic system can be reduced significantly

• Higher voice coil fill factor possible

• Dedication favours low impedance interface

• Significant improvement of power efficiency achievable

Documents

Søren Poulsen Ph.D. Student Ørsted ·DTU Technical University of Denmark [email protected]