PWM Audio Amplifiers

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15 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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PWM Audio Amplifiers

Zhiming Deng Chinwuba Ezekwe Dimitrios Katsis

Outline


PWM Basics



Digital Modulator Signal Flow



Pulse Edge Delay Error Correction



Volume Control

PWM Basics


Efficient modulation
method when only two
states (ON & OFF) are
available


Linear modulation:
Average pulse ON time
proportional to signal


Extract modulating signal
with low pass filter


Modulation depth (M<1):
signal peak/triangle peak

www.powerdesigners.com

Natural PWM (NPWM)


What about PCM input?

www.powerdesigners.com


Straightforward analog
implementation

Uniformly Sampled PWM
(UPWM)

Jorge Varona,
http://www.eecg.toronto.edu/~kphang/ece1371/pwrdac.pdf


Sample modulating signal at the “edges” of the sawtooth


Use this to estimate the pulse width


Problem: Introduces an error!

Weighted PWM (WPWM)


Estimates pulse width using the information from two
consecutive samples:


NPWM approximation in
the digital domain


Yields improved THD
compared to UPWM

Morten Kjaer Johansen et al, “A review and comparison of digital PWM methods for digital PMA
systems”, Proceedings of the 107
th

AES Convention, 1999.

Digital Modulator Signal Flow


Oversampling gives better dynamic range


WPWM is used to simulate NPWM in order to decrease THD


Noise shaping also helps lower the bit
-
rate while keeping a high
resolution

Steen Munk et al, “State of the Art Digital Pulse Modulated Amplifier System”, AES 23
rd

International Conference, 2003.

Sigma
-
Delta Noise shaper


Use noise shaping
to preserve in
-
band
SNR

Amplification Errors in Analog
Power Stage


Non
-
stable supply

causes amplitude errors.


Dead time in switches

causes delay of rising edge of output
PWM signal.


Variation of load

changes the frequency characteristics of the
demodulation filter.


Finite On resistance

of semiconductor switch and output filter
resistance lead to output resistance.


Non
-
linearity of the demodulation filter

leads to errors in the
demodulated output signal.

Error Correction


Analog signal referenced control system


NOT POSSIBLE in a digital PMA because of the absence of
analog reference signal.



Pulse Edge Delay Error Correction (PEDEC)


PEDEC is a pulse referenced control system that eliminates
all types of error by re
-
timing the edges of the PWM inputs.


Pulse Edge Delay Error Correction

Karstem Nielsen, “Digital Pulse Modulation Amplifier (PMA) topologies based on PEDEC Control”,
Proceedings of the 106
th

AES Convention, 1999.


Pulse referenced control system


Eliminates error by retiming pulse edges

Edge Delay (ED) Unit

Steen Munk et al, “State of the Art Digital Pulse Modulated Amplifier System”, AES 23
rd

International Conference, 2003.

I
C
p
PEDEC
V
V
t
t
K
0
2

I
e
V
v

for


Makes absolute correction


No quantization error




Limited correction range→
saturates if correction range is
exceeded


Increasing phase lag of feedback
signal w.r.t. reference signal
leads to saturation for very short
or long pulses

Voltage Feedback before
Demodulation Filter


Feedback from
switching
power stage
output Vp.


Feedback path
compensator
A(s) is a
simple
attenuation.


No reference
signal shaping

Karstem Nielsen, “Digital Pulse Modulation Amplifier (PMA) topologies based on PEDEC Control”,
Proceedings of the 106
th

AES Convention, 1999.

Voltage Feedback before
Demodulation Filter

Karstem Nielsen, “Digital Pulse Modulation Amplifier (PMA) topologies based on PEDEC Control”,
Proceedings of the 106
th

AES Convention, 1999.


1st order reference
shaping with matched
1st order output
feedback shaping


Voltage Feedback after
Demodulation Filter


Using global
feedback from
demodulator
output Vo.


2
nd

order reference
shaping


This is the ONLY
topology that
eliminates the
errors in the
demodulation
filter

Karstem Nielsen, “Digital Pulse Modulation Amplifier (PMA) topologies based on PEDEC Control”,
Proceedings of the 106
th

AES Convention, 1999.

Volume Control


A digital volume control is simple to implement but will
decrease the dynamic range as the signal is attenuated.



An analog volume control can retain the dynamic range,
but it is not applicable to a digital PMA.



GOAL: a volume control system that will not decrease
dynamic range.




Intelligent Volume Control (IVC)

Intelligent Volume Control (IVC)


Use multiple supply
voltages in power
stage for coarse
control


Use digital
attenuation through
modulation depth
modification for
fine adjustment

Steen Munk et al, “State of the Art Digital Pulse Modulated Amplifier System”, AES 23
rd

International Conference, 2003.

IVC (two
-
level example)

Steen Munk et al, “State of the Art Digital Pulse Modulated Amplifier System”, AES 23
rd

International Conference, 2003.

IVC Advantages


Improved dynamic range



Improved edge related noise



Improved efficiency



Improved electromagnetic interference (EMI)
characteristics

Summary


Improve THD with WPWM



Reduce noise by noise shaping



Correct various errors through PEDEC



Improve SNR of volume control by using
multiple supply voltages