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

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