The eGaN FET-Silicon Power Shoot-Out Vol. 4

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DESIGNfeature
JOHAN STRYDOM, PH.D., V.P., Applications, and
ANDREW FERENCZ, Consulting Engineer, Efficient Power Conversion Corporation
The eGaN® FET-Silicon Power
Shoot-Out Vol. 4: Brick Converters
I
solated brick converters are widely used in telecommunication systems to provide
operating power to network equipment. These bricks come in a variety of standard
sizes and input and output voltage ranges. Most can operate over large input voltage ranges of 2:1 or even as high as 4:1. Their modularity, power density, reliability
and versatility has simplified and to some extent commoditized the isolated power
supply market.
As these brick converters are of a strictly defined size, designers are forever
coming up with innovative ideas to increase their output power (and power density).
Although these ideas are numerous and varied, they are all related to system efficiency. Consider an eighth brick converter as an example -- although there are numerous
input and output voltage configurations, topologies and output range tolerances (regulated, semi-regulated, unregulated), they all have very similar maximum power loss
numbers at full power (i.e. between 12-14 W). This is a physical limit based on the
fixed volume of the converter and the method of heat extraction. Thus, for an eighth
brick converter that is 90% efficient (η = 0.9) at full load, the maximum output power
(assuming 14 W loss) is shown in Fig. 1 and represented by the equation:
A prototype eGaN FET based
fully regulated eighth brick
converter was compared to
a similar MOSFET-based converter. The eGaN FET showed
improved efficiency and 15%
more output power at a 33%
higher switching frequency.
14 W @ 90%
140 W
126 W
Fig. 1. Maximum output power for an eighthbrick converter at 90% efficiency.
174 W
160 W
(28% more)
14 W @ 92%
Fig. 2. A 2% efficiency improvement provides
28% more power.
If the efficiency can be improved by just 2%, the output power is increased to 160
W - 28% more output power (Fig. 2)!
As shown in a previous Shoot-Out article [1], it is possible to reduce the power
loss in the magnetic components (up to a point) by increasing the operating frequency.
However, this is not normally done because the increase in frequency-dependent
semiconductor losses outweighs the potential improvement. On the contrary, the
operating frequency is typically reduced to the point where the magnetic structure
size is maximized within the overall brick size constraints.
COMPARING CONVERTERS
100
98
Efficiency (%)
96
94
92
90
88
86
Quarter Brick
Eighth Brick
84
82
80
0
5
10
15
20
25
30
Output Current (A)
Fig. 3. Comparison of eighth brick and quarter brick efficiencies [2,3]
18
Power Electronics Technology | July 2011
35
In previous Shoot-Out articles, eGaN FETs and silicon
MOSFETs were compared using devices in identical circuits
and using power MOSFETs with similar RDS(ON) values as the
eGaN FETs. This simple “apples-to-apples” comparison makes
evaluating results straightforward and conclusions can readily
be drawn from the resultant data.
When it comes to isolated brick converters, however, this
simple yet effective approach breaks down. Even when limiting our comparison to regulated 12 V output, eighth-brick
converters only, there are still a significant number of variations between commercial designs. Over time, advances in
devices, materials, construction and other innovations have
allowed greater output power. The resultant structure, layout
www.powerelectronics.com
L
12 V/15 A
Cin
Gate Drive
36 to 75 V
Gate Drive
Fig. 4. 180 W, eighth-brick fully
regulated, phase-shifted fullbridge (PSFB) topology, with fullbridge synchronous rectification
(FBSR) using eGaN FETs.
Cout
G
EIGHTH BRICK SHOOT-OUT
The eGaN FET-based prototype eighth brick converter can be compared against a similar 48 V to
12 V fully regulated production converter shown in
Fig. 6. Efficiency and power loss results are shown
in Figs. 7 and 8, respectively. Despite the eGaN
FET prototype operating at 33% higher frequency,
it is able to produce 15% more output power for
the same power loss. Also of note is the full-bridge
synchronous rectifier, using 100 V eGaN devices,
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Gate Drive
Gate Drive
and topology changes
with power level and
even between manufacturers for the same
EPC2001 x 4
EPC2001 x 4
power level as what is
considered optimum is
Controller
interpreted differently.
Determining the best
Isolation
solution is an iterative
process. Even so, the
Feedback
isolation
efficiency achieved in a
specific brick converter
-- as good as it may be -- can easily be improved simply
could be changed to a center-tap with two devices in parallel
by allowing the converter to increase in size. This is clearly
(similar to the MOSFET based design). Instead of the outshown in Fig. 3 by comparing eighth-brick and quarter brick
put inductor current flowing through two devices in series,
it would then flow through two devices in parallel. This
efficiency for the same generation products.
would reduce the secondary-side device conduction losses
An eGaN FET based converter was developed that is not
by 75% (1.3 W, or roughly 10% of total power losses) at 14
necessarily an optimal solution. The design goal was to delibA output current.
erately push the operating frequency much higher than curThe same eGaN FET-based prototype was operated
rent commercial systems to show that eGaN devices could
at 500 kHz with the efficiency results shown in Fig. 9. It
enable someone skilled in power supply design to develop
shows that even at twice the switching frequency of the
state-of-the-art next-generation products.
similar MOSFET-based converter it still has equal or betFor the 48 V to 12 V eGaN FET based eighth brick
ter performance at 36 VIN and even at 60 VIN, the full
converter, a phase-shifted full-bridge (PSFB) converter with
a full-bridge synchronous rectifier (FBSR) topology was choload efficiency is still within half a percentage point of the
sen as shown in Fig. 4. A more complete schematic is shown
production eighth brick converter. Although 500 kHz is
in Fig. 5. The aim was to show that, due to their
TABLE 1. COMPARISON OF NEXT GENERATION
relatively small device size, a significant number of
EIGHTH
BRICK SILICON MOSFET DEVICES AND
eGaN FETs can be used within the restrictive eighth
EQUIVALENT eGaN FETS.
brick size limitations. The choice of transformer
SILICON MOSFETs
eGaN FETs
RELATIVE eGaN FET
turns ratio (6:3) meant that, at 75 Vin, the secondary
IMPROVEMENT
side winding voltage would be 38 V (too close for 40
Primary Side
HAT2174
¼ x EPC2001
V devices) and therefore 100 V devices were used
Device Voltage Rating
100V
100V
Same
on both the primary the secondary sides. The actual
22
22
Same
RDS(ON) (mΩ)
prototype is shown in Fig. 6 and compared side-by22#
8.9
60% less
QOSS (nC @ 50VDS)
side to a similar [4] silicon-based converter.
8.4
0.56
93% less
Q (nC @ 50V )
DS
Switching FoM (RDS(ON)×
QGD) (pC.Ω)
Secondary Side
184
12
93% less
HAT2266
EPC2001
Device Voltage Rating
60V
100V
40V more
Total RDS(ON) (mΩ)
9.5
5.6
40% less
QOSS (nC @ 50VDS)
18#
35
17nC more
QG (nC)
25
8
17nC less
SRFoM [RDS(ON) ×
(QG+QOSS)] (pC.Ω)
237
59
75% less
# Calculated from datasheet graphs
July 2011 | Power Electronics Technology
19
gallium nitridEfets
VIN
C1 0.1 µF
L1 0.68 µH
C3, C13, C14
2.2 µF
GND
C17
0.22 µF
5Vpri
C2
0.1 µF
OUT1
OUT2
C12
22 pF
Q1
EPC2001
GND
Bias supply
RT
VIN
VCC
SS
COMP
VFB OUT1
OUT2
CS
GND
5Vpri
VDD LOH
HB
LOL
HGH VSS
HGL
LI
HS
HI
U5
LM5030
R10
7.5k
R1
Zero
U1
LM5113
C9
4.7 µF
Q2
EPC2001
T1
ER18
C5
4.7 µF
6 turns
R3
Zero
Q4
EPC2001
Q3
EPC2001
Deadtime adjust
R11
2.0 k
C6 0.1 µF
R6
Zero
U3
LM5113
5Vsec
Q5
EPC2001
VDD LOH
HB
LOL
HGH VSS
HGL
LI
HS
HI
T1
3 turns
Q6
EPC2001
GND
Deadtime adjust
Isolation
and feedback
R8
Zero
C9
4.7 µF
ER18
Q7
EPC2001
C10
4.7 µF
Q8
EPC2001
Isolation
and logic
Deadtime adjust
eighth brick converter has an output power increase of
67%, to 240 W, with a peak efficiency two percentage
points higher than the converter used in our comparison with eGaN FETs. This impressive performance was
achieved through multiple improvements (Fig. 10 shows a
Next GeNeratioN eiGhth Brick
visual comparison between these two converters.) Some key
A recently released [3] next-generation MOSFET-based
changes are:
1. Switching frequency wasreduced by
30%, to 180 kHz. Core cross-sectionTransformer
al area for both the transformer and
Primary
output inductor were increased to
Output
Secondary Side
Side
Bias
Inductor
SR
HB
Supply
accommodate the lower frequency.
2. SSecondary side center-tap synMOSFET-based brick (Top View)
MOSFET-based brick (Bottom View)
chronous rectifier MOSFET device
Input
voltage was reduced to 60 V from
LDO
Capacitor
Transformer
100 V. These new MOSFET devices
Secondary Side
PSFB
Output
have about half the COSS x RDS(ON)
FBSR
Controller
Inductor
product than the previous generation
devices and 25% lower RDS(ON).
3. SPrimary side topology was changed
eGaN FET based brick (Bottom View)
eGaN FET based brick (Top View)
to full bridge (FB) from half-bridge
(HB).
Fig. 6. comparison between the 48V to 12V eGaN Fet-based eighth brick converter prototype (lower image) and
4.SPrimary side MOSFET devices were
comparable silicon-based converter (upper image) [4] showing top and bottom views (scale in inches).
20
Primary Side FB
Output Capacitors
not an optimum operating frequency for this converter, it
emphasizes the reduction in switching losses gained by using
eGaN FETs.
Power Electronics Technology | July 2011
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R2 Zero
HI
HS
LI
HOL
HOH VSS
LOL
HB
VDD LOH
U2
LM5113
GND
Deadtime
adjust
R4
Zero
OUT2
OUT1
L2
1.2 µH
ER14.5
C7 0.1 µF
12 V
5Vsec
R7 Zero
5Vsec
Fig. 7. (right, top)
Efficiency comparison between eighth
brick converters
showing an eGaN
FET-based prototype
vs. a commercial silicon MOSFET-based
solution.
92
250 kHz
MOSFET
90
88
HI
HS
LI
HOL
HOH VSS
LOL
HB
VDD LOH
GND
Bias
supply
C8,
C15, C16
22 µF
0V
Fig. 8. (right) Power
loss
comparison
between eight brick
converters showing
an eGaN FET-based
prototype vs. a
commercial silicon
MOSFET-based solution.
86
www.powerelectronics.com
48 V MOSFET
60 V MOSFET
60 V eGaN FET
84
0
14
10
2
4
6
8
Output Current (A)
10
12
14
10
12
14
36 V MOSFET
36 V eGaN FET
48 V eGaN FET
48 V MOSFET
60 V eGaN FET
60 V MOSFET
8
6
250 kHz
MOSFET
4
333 kHz
eGaN FET
2
0
0
2
4
6
8
Output Current (A)
94
500 kHz
eGaN FET
250 kHz
MOSFET
92
Efficiency (%)
doubled in number (for FB). Also, a smaller die size was
chosen to have ½ the COSS and about 1/3 the QGD of the
current eighth brick converter devices, but double the
RDS(ON).
5. STo accommodate these 60 V devices, it is calculated
that the transformer turns ratio was changed from 4:3:3
(HB:CT) to 9:3:3 (FB:CT). This requires a 100%+ duty
cycle at 36 VIN to maintain regulation and, at 75 VIN, the
secondary winding voltage is 50 V.
6. SThe use of a digital controller reduced the required
board area for control, but also enabled nearly a 100%
duty cycle.
Considering 2, 5 and 6 above, there were significant
advantages from going to a lower RDS(ON) secondary side
devices , whereas 1 and 4 improved efficiency by reducing
primary side switching losses.
To see what eGaN FETs can offer to further improve
this benchmark performance, consider the comparison in
Table 1. To make direct comparison possible, the equivalent
eGaN FETs have been scaled to match the RDS(ON) of the
MOSFETs.
Using eGaN FETs for the primary side devices, a 60%
lower QOSS losses and a staggering 93% reduction in switch-
36 V MOSFET
36 V eGaN FET
48 V eGaN FET
12
U4
LM5113
R9
Zero
333 kHz
eGaN FET
94
Power Loss (W)
5Vpri
96
Efficiency (%)
Fig..
5.
(left)
Simplified schematic
of eGaN FET-based
eighth brick operating at 333kHz with
36 to 75V input and a
[email protected] output.
C2 0.1 µF
90
88
36 V MOSFET
36 V eGaN FET
48 V eGaN FET
86
48 V MOSFET
60 V MOSFET
60 V eGaN FET
84
0
2
4
6
8
10
12
Output Current (A)
Fig. 9. Efficiency comparison between eighth brick converters showing an eGaN
FET-based prototype operating at twice the switching frequency of the commercial silicon MOSFET-based solution.
ing figure of merit (FOM) (RDS(ON) x QGD) can be achieved.
The actual switching loss improvement is dependent on
gate drive strength and layout. To put this in perspective,
the changes in QOSS and QGD between the next-generation
eighth brick and the previous version is devices are 45%
July 2011 | Power Electronics Technology
21
GALLIUM NITRIDEfets
and 42%, respectively. The eGaN FET
Output
Full
Inductor
reduces these numbers further by 60%
Bridge
Transformer
and 90%, without having to increase
Half
Transformer Bridge
RDS(ON). Although the improvement
Output
Inductor
is QOSS doesn’t offer much of an efficiency improvement, it is estimated
that the reduction in switching time Fig. 10. Comparison of present [4] (left) and next generation (right) silicon MOSFET-based eighth brick converter [3,5] .
can reduce the primary side switchover Ethernet,” Power Electronics Technology, March 2010, http://powerelecing losses by as much as 2.3 W at full load. The equivalent
tronics.com/power_semiconductors/egan-fet-viable-efficient-201103/
[2] Ericsson BMR453 series 48 V to 12 V quarter brick converter, Ericsson
eGaN FET has 40% lower RDS(ON) while offering 100 V
website, http://www.ericsson.com/ourportfolio/products/bmr453-series-quarcapability.
ter-brick
[3] Ericsson BMR454 series 48 V to 12 V eighth brick converter, Ericsson webThe sidebar, “ EPC Moves Ahead,” discusses the compasite, http://www.ericsson.com/ourportfolio//products/bmr454-series-eighthny’s work on medium and high-voltage devices and ongoing
brick-intermediate-bus-converter
[4] Ericsson PKB4000-C series 48 V to 12 V eighth brick converter, Ericsson
efforts to improve the on-resistance of its eGaN FETs.
REFERENCES
[1] Johan Strydom, “eGaNTM FET-Silicon PowerShoot-Out Part 3: Power
website, http://www.ericsson.com/ourportfolio//products/pkb-c-series-eighthbrick
[5] Picture of Ericsson BMR454 eighth brick converter taken from press release
http://www1.ericsson.com/solutions/news/powermodules/2009/index.shtml
■ EPC MOVES AHEAD
FROM ITS START IN 2007,Efficient
Power Conversion, EPC, has come a
long way toward boosting the image and
applications for its eGaNTM FETs. These
devices are produced at Episil in Taiwan,
a commercial wafer foundry also used to
produce CMOS, BCDMOS, and bipolar
ICs side-by-side with EPC’s eGaN wafers.
EPC’s wafers use standard well-known
CMOS processes, which provides the
advantage of producing eGaN FETs at
costs competitive with current MOSFETs.
This “cost competitiveness” separates
eGaN from other alternative materials.
Today, EPC’s eGaN FETs cover the
range from 40V to 200V. It will be
launching 600V products in 2011.
Depending on customer interest, it may
launch 900V and 1200V products in
2012.
The eGaN FETs have extremely low
on-resistances. Today, EPC’s eGaN FETs
have an RDS(ON) as low as 4 mΩ for 40V
devices; by scaling the size of the device,
much lower values can be achieved. EPC
already has benchmarked RDS(ON) with
the EPC2010 200V, 25 mΩ FET. This
device has an area less than 6 mm2.
Because of the wide band gap of
the gallium nitride crystal, active eGaN
devices can be made to operate beyond
300°C. Special devices for high temperature operation are now in development
22
at EPC. The present devices are rated
at 125°C or 150°C, because they mount
directly to a PCB with a typical temperature limitation of about 100°C.
Monolithic ICs consisting of GaN
transistors integrated with GaN FETs
experience no degradation in FET performance. In contrast, monolithic ICs
consisting of integrated silicon transistors and MOSFETs experience degraded
MOSFET performance. This has forced
manufacturers to use multi-chip modules
to achieve optimum performance from
circuits consisting of silicon MOSFETs
and transistors.
EPC does not produce eGaN subsystems, but is designing certain eGaN FET
subsystem reference designs that can be
used by its customers to get products to
market sooner and with less engineering
effort.
MIL SPECS
EPC and Microsemi have partnered
together to market eGaN FETs to high
reliability, defense, and space applications. EPC’s eGaN FETs have demonstrated extraordinary capability to operate
in high-radiation environments such as
those experienced by commercial, military, and research satellites. With EPC
as a partner, Microsemi is expecting to
release a full range of mil spec products
Power Electronics Technology | July 2011
in the coming year.
The eGaN FETs are also capable of
meeting the AEC-Q100 automotive specs
but the testing and infrastructure are not
yet in place. The company expects the
first appearance of eGaN FETs in automotive applications in three to five years,
starting with entertainment, navigation,
comfort, and convenience functions and
later spreading to safety and drive train
applications.
EPC is planning to integrate various
protection and driver function monolithically with its eGaN power FETs. Expect
to see those in 2012.
The company now has two application-engineering groups in California and
an EPC device characterization group
also in California. Its reliability testing
and reliability R&D is done in outside
labs in both the US and Taiwan, and it
periodically uses various industry experts
as consultants and contractors for product characterization, and applications
development.
EPC now has a pool of highly talented
and experienced engineers and continues to hire top technical people in the
field of power management. In particular
it is looking for device engineers, material scientists, and applications engineers
with advanced degrees.
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