EB205 Motorola GaAs Rectifiers Offer High Efficiency in a 1 ... - ECA

SEMICONDUCTOR ENGINEERING BULLETIN
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by EB205/D
By Scott Deuty
Motorola Inc.
Efficient power conversion circuitry requires rectifiers that
exhibit low forward voltage drop, low reverse recovery current,
and fast recovery time. Silicon has been the material of choice
for fast, efficient rectification in switched power applications.
However, technology is nearing the theoretical limit for
optimizing reverse recovery in silicon devices. A new material
is required to increase switching speed.
To increase speed, materials with faster carrier mobility are
needed. Gallium Arsenide (GaAs) has a carrier mobility which
is five times that of silicon [1]. Since Schottky technology for
silicon devices is difficult to produce at voltages above 200 V,
development has focused on GaAs devices with ratings of
180 V and higher. The advantages realized by using GaAs
rectifiers include fast switching and reduced reverse recovery
related parameters. An additional benefit is the variation of
parameters with temperature is much less than Silicon based
rectifiers.
Motorola’s 180 V and 250 V GaAs rectifiers are being used
in power converters that produce 24, 36 and even 48 Vdc
outputs. Converters producing 48 Vdc are especially popular
in telecommunications and mainframe computer applications.
To show the advantages of the GaAs parts, a 48 Volt DC–DC
converter was constructed and the performance of the GaAs
rectifiers was compared to similar silicon–based parts at a
switching frequency of 1 MHz. An output power level of
450 watts was chosen to profile the MGR1018 near its 8 amp
current rating. The converter was tested with an input voltage
level of 400 volts to simulate the commonly used rail produced
by Power Factor Correction boost converters.
For efficient power conversion at the chosen switching
frequency, power and voltage levels, the chosen topology for
the 48 Vdc converter testbed is the half bridge, zero voltage
switched, converter shown in Figure 1. This configuration
would allow for MOSFETs with voltage ratings to handle Vin or
400 Vdc (as opposed to 2 Vin) and allow the MGR1018 diodes
to operate near rated levels at a switching frequency of 1 MHz.
In addition, a testbed could quickly be developed using the
existing Motorola MC34076 high performance, resonant
mode, control IC and available demonstration board [2].
RT
CT
VCC
Vin
ROSC
R1
R2
OSC CHARGE
1
COSC OSC
RC
2
IOSO
3
RVFO
E/A 6
OUT
Cint
E/A INV
7
E/A NON INV
5.1 V VREF
8
5
MC34067
16
11
SOFT
START
ONE
SHOT
15
VCC
OUTPUT A (5)
14
PWR
GND
13
12
OUTPUT B (1)
10
1N5819 x 2
FAULT
INPUT
Cb
(9)
(10)
(7)
Rg1
18 V
ZENERS
Rg2
(6)
18 V
ZENERS
Rb
MTP8N50E
N CHANNEL
Ra
GATE
GATE
SOURCE
DRAIN
SOURCE
1N5819 x 4
DRAIN
Q2
Q1
MTP8N50E
N CHANNEL
Rc
Ns
T3
Cin3
Cin2
T3
Np
T3
Cin1
Np
2
1
MGR1018
D2
T1
3
Ns
4
5
Ns
6
D1
MGR1018
L
Figure 1. Half Bridge, Zero Voltage Switched Converter
© MOTOROLA
Motorola, Inc. 1995
1

GaAs PERFORMANCE RESULTS
Performance results for output power levels of 73 watts to
460 watts are presented in Figures 2 through 5 and Tables 2
and 3.
In Table 1, shows how 10 amp, MGR1018 GaAs rectifiers
offer an advantage over silicon parts with a 20 amp current
rating. (Note: only one side of the silicon device was used
for equal comparison.) The efficiency gains of the GaAs
rectifiers decrease with output power somewhat due to its
larger forward voltage drop. However, the silicon devices
did not perform above 143 watts. This was attributed to the
large amount of ringing due the large reverse recovery peak
exhibited by the silicon devices. The peak current
generated a high voltage spike that in turn forced the
MBR20200 into a zener mode and destroyed the part.
Figure 2 shows the efficiency achieved from the
converter over a power output of 96 to 460 watts. The
switching frequency of the inductor was 1.2 MHz throughout
this power range. Through use of zero voltage switching by
the MC34067 IC and the fast recovery times of the rectifiers,
the converter was able to achieve efficiencies in excess of
91% with a maximum of 95.4%! Note that the increase in
GaAs efficiency in Figure 2 over Table 1 is due to the use of
both leads of the MGR1018 part in Figure 2. In Table 1, one
outer lead is connected on the single die MGR10180 (each
outer leg is attached to the single die) and only one outer
lead is connected on the dual die MBR20200. This allows
for comparison of a silicon MBR20200, die rated at 10 amps
(one half of device) to a 10 amp rated MGR1018. After
running the silicon comparison, both outer leads of the test
socket were connected for maximum efficiency realized in
Figure 2.
Table 1. Performance Benefits Realized
When Using GaAs versus Silicon Rectifiers
(400 Volts Input, 1.1 to 1.29 MHz Converter Frequency)
Output
Power
Center Tap
Si
MBR20200
Efficiency
Single
GaAs
MGR1018
Efficiency
GaAs
Advantage
73 Watts 81.6% 84.2% 2.6%
95 Watts 87.2% 91.2% 3.9%
119 Watts 91.4% 91.9% 0.5%
143 Watts 92.7% 92.7% 0.0%
96
MGR1018 Efficiency and Regulation at 375 Vdc Input Level
95.5
95.4%
95
94.5
94.5%
EFFICIENCY (%)
94
93.5
93
92.5
92
91.5
91
96 119 143 166 189 237 264 287 309 343 380 427 449 455 462
OUTPUT POWER (WATTS)
Figure 2. GaAs Offers High Efficiency at
Switching Frequencies Above 1 MHz
(Input Voltage Ranging from 375 Vdc to 405 Vdc)
2 MOTOROLA

Component
Table 2. Parts List
Reference
Designator Part Value 1 Value 2
Transformer T1 Core
Main Power Path (Bolded on Schematic)
Magnetics Inc. (1)
K 43515–EC
Transformer T1 Primary Turns: Np = 46
Transformer T1 Secondary Turns: Ns = 13
Inductor L Core Magnetics Inc. 55121–A2
Inductor L Windings Turns: NL = 12
Wire:
1–180 strand, #44 AWG
Litz (2)
Wire:
1–1000 strand, #48 AWG
Litz (2)
Wire:
1–175 strand, #38 AWG
Litz (2)
Transistors Q1 & Q2 MTP8N50E 8 Amp 500 V
Rectifiers D1 & D2 MGR1018 10 Amp 180 V
Output Capacitors (3)
Input Voltage Divider
Capacitors
Cout
T491X685M050AS
(Qty 6)
6–6.8 µF 50 V
Cin1 & Cin2 polypropylene 4–0.1 µF 400 Vdc, 250 Vac
Input Capacitor Cin3 ceramic 0.01 µF 400 V
Gate Drive
Transformer T3 Core Magnetics Inc. K EP7
Transformer T3 All Windings Turns: 8 #38 AWG Magnet
Resistors Rg1 & Rg2 film resistor 5.2 Ω 1/8 watt
Zeners 1N4747 18 V
Clamp Diodes 1N5819 40 V
Current Sense
Filter Resistor Ra film 800 Ω
Sense Resistor Rb film 670 Ω
Impedance Matching Rc film 100 Ω
Filter Cap Cb ceramic 47.5 pF
Transformer
T3
Coilcraft
H7919–A
Np = 1
Ns = 200
Rectifiers 1N5819 40 V
Control IC U1 MC34067
Divider Resistors R1 & R2 film resistor
Control IC/Support Components
Motorola Inc.
P. O. Box 20912
Phoenix, AZ 85036
R1 = 10 kΩ
R2 = 1.2 kΩ
Integrator Cap Cint ceramic 4700 pF
Timing Cap CT ceramic 220 pF
Timing Res RT film 2.3 kΩ
Oscillator Cap COSC ceramic 220 pF
Oscillator Cap ROSC film 31.77 kΩ
Soft Start Cap Soft Start ceramic 47 pF
Control Res RVFO film 7.47 kΩ
(1) Magnetics, Inc., P. O. Box 391, Butler, PA, 16003–0391; (412) 282–8282
(2) Kerrigan Lewis Wire Products, 4421 W. Rice Street., Chicago, IL 60651–3487 (312) 772–7208
(3) Kemit Electronics Corporation, P. O. Box 5928, Greenville, SC 29606, (803) 963–6300
High Performance, Zero
Voltage Switch, Resonant
Mode Controller
MOTOROLA
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I
In order to illustrate the speed of the GaAs part, observe the
waveform of Figure 3 (Figure 4 is an expanded portion of
Figure 3). Note the fast recovery, reduced ringing, and low
peak reverse recovery current GaAs technology offers. The
actual values of the fast recovery GaAs rectifiers are shown in
Table 3.
At maximum output power, the converter obtained an
efficiency of close to 93% at a switching frequency of 1.2 MHz
while the diode recovered in 52 nsec at a peak recovery
current of 1.34 amps.
Table 3. Performance Offered By GaAs
at 1.29 MHz, 460 Watts
Parameter
Value
Frequency (MHz) = 1.29
Current Slew Rate (Amps/nsec) = –0.0711029
Current Slew Rate (Amps/µsec) = –71.102941
trr (nsec) = 52
Ipk (max) = 10.24
IRMpk (max) = –1.34
GaAs rectifiers not only increase converter efficiency, they
also allow operation at switching frequencies in excess of
1 MHz. Figure 5 shows the smooth waveforms of the
converter’s primary side components. The zero voltage
switching results in a smooth drain to source waveform while
the primary current shows how the rectifier’s fast recovery
results in low peak stress on the switching transistors which
enhances the reliability of the converter and reduces
generated EMI.
p (AMPS)
2
1
0
Primary Current and Drain to Source Voltage
IP
VDS
500
400
300
200
VDS (VOLTS)
12
MGR10180 Current
– 1
100
CURRENT
10
8
6
4
– 2
0
1
0
2 3 4
TIME (SECONDS)
Figure 5. GaAs Diodes Offer Clean
Primary Side Waveforms
CURRENT
2
0
– 2
0
10
8
6
4
2
0
1000
2000
TIME (ns)
3000 4000
Figure 3. GaAs Rectifiers Produce Very
Clean Waveforms; Even at 1.2 MHz!!
MGR10180 Expanded Current
– 2
1800
1850
1900
TIME (ns)
1950 2000
Figure 4. Performance Advantage Offered by
GaAs Rectifiers is Shown in This Expanded View
of the Reverse Recovery Current Waveform
SUMMARY
New GaAs technology in rectifiers allows efficient power
processing at high frequencies. The 180 V platform offered by
Motorola can increase power density in 48 Vdc
telecommunications and mainframe computer applications.
Densities as high as 90 Watt/cubic inch have been achieved
using GaAs rectifiers. [1] These devices allow designers to
switch converters at 1 MHz without generating large amounts
of EMI.
ACKNOWLEDGMENTS
The author wishes to thank Mike Horgan of Magnetics Inc.
for his contributions to this design. Mike was responsible for
designing and providing materials for the power transformer,
inductor and gate drive transformer. Special thanks goes out
to Allen Richter of Kerrigan Lewis who provided all of the Litz
wire for the transformer and Nancy Reynolds of Kemet
Electronics for the tantalum chip capacitors used on the
output. Finally, the efforts of Jeff Morud and Chris Gass of
Motorola were greatly appreciated as they proved vital in
supporting the MC34067 IC performance.
4 MOTOROLA

REFERENCES
[1] S. Delaney, A. Salih, C. Lee, “GaAs Diodes Improve
Efficiency of 500 kHz DC–DC Converter,” pp 10,11,
Power Conversion Intelligent Motion, August 95.
[2] Chris Gass, et. al. “A New High–Performance Control IC
for Zero Voltage Switching Resonant Mode Controller,
HFPC, May 1992 proceedings.
[3] “MC34067 Data Sheet,” Motorola Linear/Interface ICs
Data Book DL128/D Rev 4, Vol. I 1993, p 3–278.
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