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December 2013
Bridging
Mansour Izadinia
Senior Vice President
of Micrel
the Gap:
How Micrel fuses its
resources to achieve
power innovation.
Curiosity Rover
Power Source
Low Power
Converters
Power Developer
CONTENTS
4
TECH COLUMN
How to GaN: eGaN FETs for High Frequency Switching
All the forces in the world are
not as powerful as an idea
whose time has come.
10
TECH ARTICLE
Using Nuclear Energy to Power the Mars Curiosity Rover
—Victor Hugo, 1800
16
COVER INTERVIEW
Mansour Izadinia - Senior Vice President of Micrel
Power Developer contains new
ideas that come every month.
22
TECH COLUMN
Perfection in Power: Mini Power Supplies Lower Consumption
—Power Developer Editors, 2013
P
OWER
D
EVELOPER
Read Power Developer, the
monthly newsletter for Engineers:
http://www.embeddeddeveloper.com/news_letter/
3
Power Developer
TECH COLUMN
How To GaN:
eGaN FETs for High
Frequency Switching
Alex Lidow
CEO of Eficient Power Conversion (EPC)
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4
5
5
Power Developer
TECH COLUMN
The previous columns in this series discussed the advantages of
eGaN FETs in high frequency resonant and soft-switching designs
were presented. In this installment a return to hard-switching
converters is made, but with a push to higher frequencies – beyond
the practical limits of silicon technology. One such particular
application is envelope tracking. The aim of this article is to show
what power and efficiency levels are readily realizable using
current eGaN FETs and the LM5113 eGaN FET half-bridge driver
and present some new devices to extend their high frequency
switching range capabilities.
Concept of Envelope Tracking
while realizing the required bandwidth likely will
require some series or parallel linear element
[4]. One practical solution to implementing
an ET supply is through multi-phase buck
converters (with or without linear element to
improve bandwidth). The realizable bandwidth
for these buck converters are typically limited
to ~1/5th the effective switching frequency,
but can be increased through non-linear
control techniques [5].
The idea behind envelope tracking (ET) or
sometimes called envelope elimination and
restoration (EER) for linear power ampliiers
(PA) are not new, but only in recent years
has new technology emerged to make ET
viable. The technology development is driven
by the need for continuous improvements
in cell phone battery life and base station
energy eficiency as data rates keep rising
exponentially [1]. The key to envelope
tracking’s ability to improve eficiency lies
in the ampliier’s peak to average power
(PAPR) requirements [2]. As shown in the
conceptual drawing (Figure 1) it is possible
to achieve peak PA eficiencies as high as
65% with a ixed supply, but given PAPRs as
high as 10:1, the average eficiency is likely to
be lower than 25%. Through modulation of the
PA supply voltage, this can be improved to
over 50% - essentially doubling the eficiency
and reducing PA losses by two thirds. This
will not only reduce power consumption,
but also lower the cost of operation, cooling
requirements, size etc. [3].
Figure 1:
Conceptual PA eficiency vs. output power for ixed
supply and ET operation
Experimental High Frequency Buck
Converters
The high PAPR that make ET possible also
means that average output voltage is
typically between 30% - 50% of the buck
converter supply voltage with excursions
below and above this average in the multi
megahertz range. For device performance
demonstration purposes a single phase steady
state buck converter running at a similar duty
cycle can be used to determine the eficiency
and thermal requirements of a multi-phase
ET buck converter. Based on the active area
of the EPC2000 series eGaN FETs, the most
representative ET application would be Digital
Video Broadcasting (DVB) transmitters, such as
that implemented by ET specialist Nujira. The
experimental setup speciications are listed in
Table 1. To show the impact of active area
reduction, two demonstrators using different
sized devices were constructed.
High Frequency Switchers
The best method for implementing envelope
tracking is still under debate. However, to
achieve the eficiency requirements, some
form of high frequency switcher is required,
Table 1:
Speciications for experimental high frequency buck
converter for envelope tracking
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Power Developer
TECH ARTICLES
The demonstrator switching frequencies
were pushed as high as practically possible,
given their respective die sizes
.
The respective
eficiency results and loss breakdown are
shown in Figures 2 and 3 respectively.
Experimental Result Implications
Figure 3 clearly shows how the devices losses
are concentrated in the hard-switch control
FET and that switching losses are limiting further
frequency increases. The smaller devices
(higher on-resistance) were capable of a
proportional four times increase in switching
frequency, although at a signiicant reduction
in output power of roughly 40%, thus requiring
2.5 times more phases to achieve the same
total output power. Furthermore, a four times
increase in switching frequency resulted in
almost doubling the overall converter losses
when the increase in number of phases
required are taken into account.
Table 2:
A new family of reduced active area high frequency eGaN FETs
Figure 4:
Eficiency and loss results for an EPC8005
based demonstrators operating at 42 V
IN
, 20 V
OUT
.
Although these results are suitable for high
power applications, a large market for
envelope tracking in cellular base stations
require lower power and higher bandwidth.
Thus smaller, higher on-resistance devices are
required to address these higher switching
frequency applications, such as the
new
family of high frequency GaN power transistors
listed in Table 2. Through their reduced size
and structural and device improvements, they
offer switching transition speeds in the sub
nano-second range, making them capable
of hard switching applications beyond 10
MHz. As an example, a 10 MHz, 42 V to 20 V
buck converter’s eficiency results using one
of these devices are shown in Figure 4.
MHz operation. eGaN FETs continue to enable
exciting new applications not possible with
the aging Si MOSFET.
eGaN is a registered trademark of Eficient
Power Conversion Corporation.
Figure 2:
Eficiency and loss results for EPC2001 and EPC2007 based buck
demonstrators operating both at 45 V
IN
, 22 V
OUT
.
References
[1] Cisco white paper, “Cisco Visual Networking
Index: Global Mobile Data Trafic Forecast
Update, 2012–2017”, February 2013.
[2] OpenET alliance, Introduction to envelope
tracking, http://www.open-et.org/Intro-to-ET-
pa-712.php
Summary
[3] J. Hendy, Transmitter power eficiency, http://
broadcastengineering.com/infrastructure/
transmitter-power-eficiency-1109/
Envelope tracking is an emerging application
that can signiicantly improve the performance
of PA’s ubiquitous in modern and future
technologies. To enable envelope tracking,
higher speed devices are required. In this
column high frequency hard-switching eGaN
FET based buck converters suitable for high
power envelope tracking were presented.
The results showed over 97% and over 94%
eficiency at 1 MHz and 4MHz operation
respectively. To further increase switching
frequency, reductions in die active area and
output power per phase are required. A new
family of eGaN FETs was presented, with initial
results showing peak eficiency of 89% at 10
[4] O. Garćıa, et al., “Overview of fast DC–
DC converters for envelope ampliier in RF
transmitters,” IEEE transactions on Power
Electronics, vol. 28, no. 10, Oct. 2013, pp.
4712-4722
[5] M. Norris, D. Maksimovic, “10 MHz Large
Signal Bandwidth, 95% Eficient Power Supply
for 3G-4G Cell Phone Base Stations,” Applied
Power Electronics Conference (APEC) 2012,
Feb. 2012, Orlando, Florida. ■
Figure 3:
Loss breakdown for EPC2001 and EPC2007 based buck demonstrators
operating at full load, 45 V
IN
, 22 V
OUT
. (I
OUT
= 15 A and 6 A respectively)
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