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MOTOROLA

SEMICONDUCTOR ENGINEERING BULLETIN

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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].

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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)

EFFICIENCY (%)

MGR1018 Efficiency and Regulation at 375 Vdc Input Level

96

95.4%

95.5

95

94.5%

94.5

94

93.5

93

92.5

92

91.5

91

OUTPUT POWER (WATTS)

Figure 2. GaAs Offers High Efficiency at Switching Frequencies Above 1 MHz

(Input Voltage Ranging from 375 Vdc to 405 Vdc)

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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

MGR10180 Current

TIME (ns)

Figure 3. GaAs Rectifiers Produce Very

Clean Waveforms; Even at 1.2 MHz!!

MGR10180 Expanded Current

TIME (ns)

Figure 4. Performance Advantage Offered by GaAs Rectifiers is Shown in This Expanded View of the Reverse Recovery Current Waveform

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.

Primary Current and Drain to Source Voltage

TIME (SECONDS)

Figure 5. GaAs Diodes Offer Clean

Primary Side Waveforms

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.