Advance Power Electronics Design

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A 48 VDC to 1 VDC converter using conventional silicon-based power MOSFETS is usually realized in two stages; a preliminary stage that converts the input voltage to a 12 V intermediate voltage, and a point of load (POL) converter that performs the second stage conversion from the intermedi- ate voltage to the required 1 V output. This dual- stage technique is required since a single stage 48 V to 1 V converter will need to operate at a small duty cycle of about 2%. At high frequencies (250 kHz and above) the pulse widths required to operate the converter fall below 100 nsec which is prohibitively short for Silicon based MOSFETs. EPC1001(100 V, 5.6 mΩ) and EPC1007 (100 V, 24 mW) GaN-on-silicon transistors have been shown to operate at pulse widths well below 100 ns. Turn-on and turn-off times of about 4 ns are achieved using an industry standard pin driver.

EPC1001 and EPC1007 Parameter Overview

The EPC1001 and EPC1007 enhancement mode Gallium Nitride power transistors are the first of a new generation of devices that go beyond the limitations of Silicon technology. Whereas similar voltages and RDS(on) can be found in sili- con power MOSFETs, the typical gate charge required for switching EPC1001 and EPC1007 devices, 11 nC and 2.7 nC respectively, com- bined with this voltage and RDS(on) is well beyond silicon’s reach. Table 1 compares the capability of these two devices with the benchmark de- vices on the market today. As can be seen, the product of RDS(on) and Gate Charge (RDS(on) x QG product) is six times better than the best perfor- mance achieved in silicon.

In addition to the superior RDS(on) x QG product, EPC’s GaN transistors have an integrated re- verse diode with a VF of about 2 V and no reverse recovery charge.

EPC1001 and EPC1007 transistors are available as bumped “flip-chip” devices. Because of the in- novative Gallium-Nitride-on-Silicon technology used by EPC, the substrate is isolated from the active device area by at least 300 V (This parame- ter is being characterized and may be revised at a future date). This affords an additional oppor- tunity for power density improvements com- pared with packaged Silicon transistors as the device can be directly connected to a heatsink without an intermediate insulating layer.

Single-Stage 48 V – 1 V DC-DC Conversion Simplifies Power Distribution While Significantly Boosting Conversion Efficiency

Edgar Abdoulin and Alex Lidow, Ph.D.

Efficient Power Conversion Corporation’s (EPC) hyper fast Gallium Nitride (GaN) power tran- sistors offer performance enhancements well beyond the realm of silicon-based MOSFETs. Standard power converter topologies can greatly benefit from the added performance and leap to areas not attainable with current MOSFET designs; improving converter efficiency, while maintaining the simplicity of converter designs.

Table 1 – Comparison Between EPC1001/EPC1007 GaN Transistors and Silicon Benchmark Devices

In Brief A first-generation buck converter that delivers high efficiency while converting from 48 V to 1 VDC has been designed and characterized. The use of enhancement mode, Gallium Nitride power transistors from EPC has made this practical for the first time.

• New generation of power transistors outperforms silicon in high frequency switching applications by a wide margin

• Promises to open many new doors to applications previously dominated by power MOSFETs.

• EPC is planning a rapid set of introductions through 2009 and 2010 to cover a broad spectrum of power applications:

– Isolated and non-isolated DC-DC conversion

– Synchronous rectification – Class-D Audio – Motion control – Cell phones and base stations

Manufacturer Part Number Voltage RDS(on) max QG max (RDS(on) x QG)

IR IRLB4030

100

4.5 130 585

IR IRLSL4030 4.5 130 585

Fairchild FDP054N10 5.5 203 1117

IPP050N10LG 6.4 163 1043

Fairchild FDMS86101 8.0 55 440

IPD068N10N3G 12.3 68 836

IR IRLR3110ZPbF 14.0 48 672

BSC159N10LSFG 21.5 35 753

Fairchild FDMC86102 24.0 18 432

BSC205N10LS 28.0 41 1148

Vishay SUD06N 10-225L 225.0 3 608

EPC EPC1001 100

7 11 77

EPC EPC1007 30 2.7 81

EFFICIENT POWER CONVERSION

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EPC1001 and EPC1007 Gate Characteristics

The equivalent circuit of the gate characteristics of the GaN power transistor is depicted in Fig 1. The gate consists of a small resistor (RG ~ 0.5 Ω), and a Capacitor, QG, with a breakdown above 6 VDC. Full enhancement of the device channel is achieved by 5 VGS and it is important to maintain a gate drive level that will not exceed the 6 VDC absolute maximum. The EPC1001 required QG to turn on is only about 10 nC and EPC1007 requires about 2.7 nC.

Gate Driver Considerations.

Due to the hyper-fast switching characteristics of the GaN power transistors, high dV/dt’s are present when the device switches from one state to another. These high dV/dt’s can cause high currents to flow in the miller capacitor (CGD). In a half bridge topology a small driver with a relatively high RDS(on) could cause an undesired turn-on of the lower device when the actual requirement is to keep the device off. This phenomenon will increase the risk of shoot-thru currents and result in excessive loss- es. Therefore, the selection of a proper driver is not only driven by the current/switching time requirement, but also by the need to provide a low impedance path for stray currents gener- ated by the high dV/dt’s.

As an example, with a CGD of ~100 pF and a dV/ dt of 12 V/ns, the current injected in the driver equates to about 1.2 A. Since the minimum threshold of the EPC1001 is 0.7 V, the RDS(on) needed to avoid turning the lower device on is:

Driver

High Current in Driver

High dV/dt on Drain

5 V

CGD

GaN

Fig 2 – High dV/dt can cause high currents to flow in the gate driver.

Gate

Source

Fig 1 – EPC1001 and EPC1007 Gate Structure

RDS(on) (Driver) < 0.7 V/1.2 A = 0.58 Ω

CH1: Lower Gate | CH2: Lower Device VDS | dV/dt ~ 12 V/nsec

Fig 3 – Half Bridge Topology with high gate drive RDS(on) – High dV/dt causes “Bump” in Lower gate drive.

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EPC1001/EPC1007 Evaluation Circuit

The EPC1001/EPC1007 evaluation circuit is a non-isolated “buck” converter capable of oper- ating with up to 72 VDC input, while generating 0.5 to 3.5 VDC output at up to 15 A. A function generator is required to set the PWM pulses for various output voltages and current combina- tions. Dead time adjustments are accomplished by two on-board potentiometers to optimize performance vs. various dead time settings. The EPC transistors are covered by a detachable heat sink. Due to the internal body diode of the EPC1001, an external re-circulating diode is not necessary.

Fig 4a – Schematic of EPC1001/EPC1007 Evaluation Circuit

Fig 4b– Schematic of EPC1001/EPC1007 – Dead time generator

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Table 4 – 48 VDC to 1 VDC conversion

f(kHz) VIN (V) IIN (mA) VOUT (V) IOUT (A)

200

252.5 1.001

10.006

82.64

300 258.8 1.005 80.95

400 264.7 1.007 79.30

500 270.3 1.006 77.58

Table 5 – 24 VDC to 1 VDC conversion

f(kHz) VIN (V) IIN (mA) VOUT (V) IOUT(A)

200

23.97

484.7 1.007

10.006

86.73

300 488.1 1.007 86.12

400 492.0 1.006 85.35

500 497.4 1.007 84.51

EPC1001/ EPC1007 Evaluation Circuit Performance

Typical performance data for the EPC1001/ EPC1007 evaluation circuit has been measured over a range of input voltages, output cur- rents and operating frequencies. Tables 2 and 3, as well as Fig 5, show efficiencies at 24 VDC and 48 VDC to 1 VDC conversion from 2 A to 12 A. All measurements were performed at 250 kHz switching frequency.

100

70 0 2 4 6 8 10 12 14

y ( %

)

IOUT (ADC)

Conver y vs Output Current (f=250 KHz)

24 VDC to 1 VDC

48 VDC to 1 VDC

Fig 5 – Converter efficiency vs. input voltage and output current

Table 2 – 48 VDC to 1 VDC conversion

f(kHz) VIN (V) IIN (mA) VOUT (V) IOUT(A)

250

48.06 54.8 1.004 2.0053 76.44

48.05 126.2 1.004 5.0066 82.89

48.05 203.8 1.005 8.0079 82.18

48.04 258.5 1.004 10.006 80.90

48.03 318 1.005 12.008 79.01

Table 3 – 24 VDC to 1 VDC conversion

f(kHz) VIN (V) IIN (mA) VOUT (V) IOUT(A)

250

24.02 92.58 1.007 2.0053 90.81

24 231.6 1.006 5.0066 90.61

23.97 380.6 1.003 8.0079 88.04

23.97 483.6 1.002 10.006 86.49

23.96 598.3 1.005 12.008 84.18

100

70 100 200 300 400 500 600

y (%

)

Frequency (kHz)

y vs Switching Frequency (VOUT =1 VDC, IOUT=10 ADC)

24 VDC to 1 VDC

48 VDC to 1 VDC

Fig 6 – Converter efficiency vs. switching frequency.

Tables 4 and 5 and Fig 6 depict operation of the converter at 24 and 48 VDC inputs over a range of operating frequencies.

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Fig. 7 shows the loss distribution in the con- verter for both 24 V and 48 V inputs operating at 250 kHz. Top and bottom device losses in- clude conduction as well as switching losses. Other losses include PCB resistive losses, out- put capacitor ESR, and parasitic losses due to skin effects and layer to layer capacitances.

Top Device Losses Bottom Device Losses Inductor Losses Other

Loss Distribution

Top Device Losses Bottom Device Losses Inductor Losses Other

Loss Distribution

Fig 7 – Distribution of losses in the EPC1001/EPC1007 Evaluation Board (48 or 24 VDC(IN), 1 V/10 AOUT )

Typical Operating Waveforms

Typical waveforms obtained during actual operation are shown in Fig’s 8 thru 11. Measurements were performed at 250 kHz operating frequency.

Fig 8 – Overall converter operation (48 VIN, 1 V and 10 AOUT ) – 250 kHz

Fig 9 – Low side device gate drive and drain voltage (48 VIN, 1 V and 10 AOUT ) Fig 10 – Top device turn on/off (48 VIN, 1 V and 10 AOUT )

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Fig 11 – Top device turn on and turn off times (48 VIN, 1 V and 10 AOUT)

Conclusions

A first-generation buck converter that delivers high efficiency while converting from 48 VDC to 1 VDC has been designed and characterized. The use of enhancement mode, Gallium Nitride power transis- tors from EPC has made this practical for the first time.

This new generation of power transistors outperforms silicon in high frequency switching applica- tions by a wide margin and promises to open many new doors to applications previously dominated by power MOSFETs.

EPC is planning a rapid set of introductions through 2009 and 2010 to cover a broad spectrum of power applications including isolated and non-isolated DC-DC conversion, synchronous rectification, Class-D Audio, motion control, cell phones, and base stations.

http://www.epc-co.com