Power-Developer-AMETEK-Spread-.pdf
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February 2014
AMETEK
Shawn Smith
Vice President & Business
Unit Manager of AMETEK
Programmable Power
Programmable Power
DIFFERENTIATED
POWER SUPPLIES
Powering Audio
Amplifiers
Power Developer
CONTENTS
4
TECH COLUMN
eGaN FETs for High Performance Class-D Audio Ampliiers
All the forces in the world are
not as powerful as an idea
whose time has come.
10
TECH ARTICLE
Power Factor & Power Factor Correction
—Victor Hugo, 1800
18
COVER INTERVIEW
Shawn Smith - Vice President of
AMETEK Programmable Power
Power Developer contains new
ideas that come every month.
24
FEATURED ARTICLE
Compensation Methods in Voltage Regulators
—Power Developer Editors, 2013
32
TECH COLUMN
Tackling Problems with Innovative Concepts:
Extra Flexibility thanks to two-stage driver solutions
P
OWER
D
EVELOPER
Read Power Developer, the
Perfection in Power
monthly newsletter for Engineers:
http://www.embeddeddeveloper.com/news_letter/
3
Power Developer
TECH COLUMN
Alex Lidow
CEO of Eficient Power Conversion (EPC)
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5
5
Power Developer
TECH COLUMN
level digital signal presented to the power
stage of the ampliier. This distortion is due to
excessive propagation delays and dead-times
based on (a) gate-to-source charge required
to charge gate to the threshold voltage (Q
GTH
),
(b) slow slew rates based on turn on charge
(Q
GS2
+ Q
GD
), common source inductance
(CSI), (c) resistive drops based on R
DS(on)
, (d)
reverse recovery delays and ringing based on
diode reverse recovery charge (Q
RR
), and (e)
ringing of the open loop power stage.
The quality of sound reproduced by an audio amplifier, measured
by critical performance parameters such as THD (Total Harmonic
Distortion), damping factor (DF), and T-IMD (Inter-modulation
Distortion), is influenced by the characteristics of the switching
transistors used. Class-D audio amplifiers typically use power
MOSFETs, however, lower conduction losses, faster switching
speed, and zero reverse recovery losses provided by enhancement-
mode GaN (eGaN) FETs enable a significant increase in the sonic
quality, and higher efficiency that can eliminate heatsinks. The
result is a system with better sound quality in a smaller form
factor that can be built at a lower cost.
Figure 1:
Comparison of MOSFET and eGaN FET
switching waveforms
Power stages using eGaN FETs come much
closer to synthesizing the ideal power digital
signal compared with power stages using
MOSFETs because they are far superior
to silicon MOSFETs in every characteristic
that contributes to the distortion of the
class-D power stage. The same factors
that contribute to deviation from the ideal
waveform are responsible for power losses in
the ampliier, so the eGaN FET-based class-D
audio ampliier will naturally be a much more
eficient ampliier.
TOTAL HARMONIC DISTORTION (THD)
Musical sound usually contains multiple
frequencies, all of which must be ampliied
equally to truly reproduce sound without
inducing distortion and impacting the sound
quality. Factors such as cross over distortion,
slew rate, and current gain of transistors can
make ampliiers non-linear. The signal delays
with negative feedback systems make the
ampliier incapable of correcting distortion
when subjected to fast transient signals
commonly found in audio applications [1,
3]. These items cause harmonic distortion and
intermodulation distortion in the reproduced
signals. As we will show, the improved
performance of
eGaN FETs
compared with
MOSFETs signiicantly improve the overall sonic
performance of Class-D ampliiers.
Total Harmonic Distortion is a primary
measurement of the quality of an audio
ampliier. THD is measured at a constant audio
frequency (typically 1 kHz) over a range of
power levels. With power MOSFETs in a class-D
ampliier, a fairly long period of dead-time is
required to prevent “shoot-through” between
the high-side and low-side FETs, as well as to
account for the body diode reverse recovery
time. These reverse recovery oscillations add
additional noise and distortion. Since eGaN
FETs switch many times faster than power
MOSFETs and do not have any reverse
recovery charge, the dead-time – typically 25
ns for silicon power MOSFETs – can be reduced
by 80%, to 5 ns or less.
To read the previous installment, click
the image below:
Figure 2:
R
DS(on)
and Q
GS1
comparison : eGaN FETs vs.
Si FETs (100 V)
THD is also inluenced by the switching and
delay times of the FET. eGaN FETs have much
lower gate charge, Q
GS
, compared to a
silicon MOSFET of similar R
DS(on)
. This means
that the turn-on and turn-off delay, as well
as the rise and fall times for eGaN FETs, will
be much faster and contribute less to signal
distortion. Figure 1 shows a typical switching
waveform of a power MOSFET and eGaN FET
illustrating the faster switching speed and
absence of diode recovery.
OPEN LOOP DISTORTION & EFFICIENCY
PWM generators do an excellent job of
synthesizing the low level analog waveform
in digital format. Of course a higher PWM
frequency will generate a higher quality
digital representation of the analog signal.
The primary source of open loop distortion in a
class-D audio power ampliier is the deviation
of the ampliied digital signal from the low-
Figure 3:
R
DS(on)
and (Q
GS2
+ Q
GD
) comparison : eGaN
FETs vs. Si FETs (100 V)
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Power Developer
TECH ARTICLES
TRANSIENT INTERMODULATION
DISTORTION (T-IMD)
In addition, the losses in the eGaN FETs
were so small that heatsinks could be
completely eliminated, while delivering
150 W into an 8-ohm speaker (or 250 W
into a 4-ohm speaker). Figure 5 shows the
efficiency of the EPC2016 [5] eGaN FET-
based class-D power converter.
A second type of distortion is Transient
Intermodulation Distortion (T-IMD), which
is the result of introducing frequencies not
present in the original audio [1]. With power
MOSFETs, higher open-loop impedance
from the higher R
DS(on)
, Q
GD
, and Q
GS
of the
switching device, as well as the longer
dead-time, requires excessive feedback
to improve audio performance. Excessive
feedback limits bandwidth, and introduces
intermodulation distortion [3]. The low on-
resistance and low capacitances as well as
reduced dead time of eGaN FETs enable
simpler and lighter feedback providing
substantial reduction in T-IMD as well as
higher eficiency in Class-D systems.
SUMMARY
Lower conduction losses, faster switching speed
and zero reverse recovery losses provided by
high switching speed enhancement-mode
GaN (eGaN) FETs enables a step forward in
designing class-D audio ampliiers with superior
sonic performance and lower cost.
REFERENCES
[1] Non-linear distortion and intermodulation
distortion, white paper, Klippel, GmbH
http://
www.klippel.de/measurements/nonlinear-
distortion/intermodulation-distortion.html
A SIMPLE FIGURE OF MERIT TO COMPARE
TECHNOLOGIES
Given their impact on performance and
eficiency described above, perhaps the best
way to gauge the relative performance of
the an eGaN FET and a MOSFET in a Class-D
audio system is by calculating two igures of
merit (FOM) based on, (a) the product of the
on-resistance, R
DS(on)
, and the gate-to-drain
turn -on charge, (Q
GS2
+ Q
GD
), and (b) R
DS(on)
and the gate-to-source charge (Q
GS1
) [2].
These comparisons are illustrated in Figures 2
and 3, where it can be seen that eGaN FETs
are signiicantly better than state-of-the-art
MOSFETs in both FOMs.
Figure 4: THD+N of 0.003% at 8 Ω, 1 kHz, with the eGaN FET class-D ampliier
[2] J. Strydom: “Comparing Figure of Merit,”
http://powerelectronics.com/discrete-power-
semis/egantm-silicon-power-shoot-out-part-
1-comparing-igure-merit-fom
[3] L. Butler, “Intermodulation performance and
measurement of intermodulation components,”
VKSBR, Australia.
http://users.tpg.com.au/users/
ldbutler/Intermodulation.htm
[4] Eficient Power Conversion, “
EPC9106 –
Demo Circuit Print
,” data sheet
[5] Eficient Power Conversion, “
EPC2016
– Enhancement-mode Power Transistor
”,
data sheet
CLASS-D AMPLIFIER EXAMPLE
To demonstrate the advantages of eGaN
FETs in class-D applications, a Bridge-
tied Load (BTL) output class-D ampliier
demonstration system was built and tested
[4]. Each channel was designed to deliver
150 W into 8 ohms, or 250 W into 4 ohms load,
with less than 0.1% THD, when powered from
a ±27 V nominal power supply.
eGaN® FET is a registered trademark of
Eficient Power Conversion Corporation.
■
» CLICK HERE
Faster switching speeds, shorter dead-times,
and the absence of the diode recovery
enable very low THD+N as shown in igure 4,
while minimizing T-IMD and the EMI emissions
from the ampliier.
Figure 5:
eGaN FET Class-D ampliier eficiency - 4 Ω and 8 Ω ohms speakers
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