Abstract for PCIM Europe 2010
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J. Everts / KULeuven
A Hard Switching VIENNA Boost Converter for
Characterization of AlGaN/GaN/AlGaN Power DHFETs
J. Everts1, P. Jacqmaer1, R. Gelagaev1, J. Das2, M. Germain2, J. Van den Keybus3 and
J. Driesen1
1
2
KULeuven, Dept. ESAT/Electa
IMEC
3
TRIPHASE
Kasteelpark Arenberg 10
Kapeldreef 75
Romeinse Straat 18
3001 Leuven, Belgium
3001 Leuven, Belgium
3001 Leuven, Belgium
1.4 Wide Bandgab Devices (SiC etc.)
Preferred Presentation Form: Oral Presentation
Abstract
A high frequency, hard switching boost converter (VIENNA topology) was constructed for
characterizing new power AlGaN/GaN/AlGaN Double Heterojunction Field Effect Transistors.
This converter enables us to accurately measure the, in power circuit design, most important
device parameters (dynamic on-resistance Rdyn, gate charge Qg, Miller charge Qgd, switching
times, FOM, …). A circuit for improving measurement accuracy/resolution was constructed.
For meeting the drive requirements of the tested AlGaN/GaN/AlGaN DHEFT devices, two
possible high speed gate drivers were developed and tested. Measurements of dynamic onresistance together with gate charge measurements were performed on AlGaN/GaN/AlGaN
DHEFT prototypes, showing promising results.
Synopsis
Recent technological improvements made GaN-based heterojunction field effect
transistors (HFET’s) a good replacement candidate for the currently used silicon devices in
power electronic converters [1 - 2]. One of the biggest concerns when using GaN-based
HFETs as a power switch is the possible increase of the “dynamic” on-resistance (Rdyn) when
applying high voltage swings to the drain [3, 4]. A second important parameter of interest is
the gate-drain charge (Qgd), often used in discussions concerning switching speed and driver
design [3]. When multiplying Qgd with Rdyn, a figure of merit (FOM) is obtained, making
comparison of different devices possible. This paper presents a hard switching VIENNA
converter, enabling fast and accurate measurement of the two key quantities (Rdyn and Qgd ).
Also switching times, currents and temperatures can be measured. Fig. 1 shows the main
power circuit diagram of the VIENNA converter (3.5 kW, 600V out, 0.1 - 2 MHz). The GaNbased transistors that we used in our test setup are AlGaN/GaN/AlGaN DHFETs (Double
Abstract for PCIM Europe 2010
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J. Everts / KULeuven
Heterojunction Field Effect Transistors) [5]. The heterostructure is grown by MOCVD on
Si<111> substrates. The devices fabricated for this study have a gate length of 1.5 µm and
total gate width of 57.6 mm.
Drive circuit: The DHFET prototypes are “normally on” devices [1]. As a result, being able
to supply negative gate voltages is a first requirement of the gate drive circuit. A second
requirement is that the driver must be able to reach very high frequencies (up to 2 MHz).
Fig. 2 shows the principle scheme of the gate drive circuit, constructed to meet these
requirements. A fully controllable drive-voltage range (VB) is obtained: VB = [0 - Voff / Vdd - Voff].
An alternative way to drive the AlGaN/GaN/AlGaN DHFET, using its internal gate-source
diode (Dint) will be presented in the full paper.
Dynamic on-resistance measurements: Determining the dynamic on-resistance of a
transistor by measurement is based on Ohms law, saying that: Rdyn=(Vds/Id)on, where Vds and
Id are respectively the measured drain-to-source voltage and drain-current during on-state of
the transistor [3]. When putting the voltage probes directly on the drain and source terminals
of the transistor, a measurement resolution problem occurs. When the off-state voltage
(Voff,max) is 400 Volts for example, the maximum resolution is 400/256=1,56 V (8-bit analogto-digital converter (ADC)). Consequently resolution can be improved by keeping Voff,max low.
Fig. 3 shows the circuit (based on a Wilson current mirror [6]) constructed for doing this by
clamping the measured drain-source voltage to a diode voltage drop when the transistor is in
its “off” state (measurement points are A and B). Fig. 4 shows an example of a dynamic onresistance measurement on an AlGaN/GaN/AlGaN DHFET where figure (a) shows the
difference between the measured Vds with (“projected Vds”) and without (“real Vds”) using the
resolution improvement circuit of figure 3. The off-state drain-source voltage was clamped to
2.4 V (Vd_clamp) by a Zener diode (a), giving a very good measurement resolution of 2.4 V/256
= 0.0094 V. The dynamic on-resistance (c) was calculated out of the projected Vds,on and Id,on
(=Ishunt,on) from (b), resulting in a very low Rdyn = 0.2 ohm.
Gate-drain charge measurements: Switching time of a power electronic transistor in a
converter is determined by charging and discharging times of the gate-drain capacitance (Cgd
= Miller Capacitance), requiring a gate-drain charge (Qgd) [3]. Switching loss is proportional to
Qgd, being determined by integrating the gate drive current during the turn-on switching. The
gate drive current was measured from the voltage drop across the external gate resistance
Rg (Fig. 2), using a differential probe.
Fig. 5 shows an example of a gate charge measurement on an AlGaN/GaN/AlGaN DHFET.
The gate-drain charge Qgd was measured to be 0.65 nC. The threshold voltage Vth was -2.09
V and the total gate charge Qg was 2.46 nC. By multiplying Rdyn with Qgd, a figure of merit is
obtained, providing the possibility to compare different devices. The figure of merit from the
presented measurements is: 0.2 [ohm] x 0.65 [nC]= 0.13 [ohm x nC], which is very low [3].
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J. Everts / KULeuven
D3
(SiC)
L
drv_g1
3.5
GaN
HFET 1
Cout 1
GaN
HFET 2
Cout 2
RL 1
95,2 µF
Vin
24 µH
Cin
D2 drv_g2
RL 2
95,2 µF
D4
(SiC)
Fig.1. Main power circuit diagram of the hard
switching VIENNA converter.
Projected Vds [V], Ishunt [A]
D1
3
“Off”-state
Ishunt
Vds
“On”-state
2.5
2
1.5
1
0.5
0
-0.5
2
2.2
2.4
2.6
2.8
3
3.2
Time [s]
3.4
3.6
3.8
4
x 10
-6
(b)
0.4
0.35
R1
60 ohm
R2
60 ohm
0.25
0.2
0.15
0.1
WILSON current mirror
T1
BC557C
T2
BC557C
0.05
0
10mA
10mA
Vcc = 15V
0.3
Rdyn [ohm]
Fig.2. Principle scheme of the gate drive circuit
that was used in the VIENNA converter.
T3
BC557C
2
2.2
2.4
2.6
2.8
3
3.2
Time [s]
3.4
3.6
ground
Fig.4. Dynamic on-resistance measurement
(Vds,off=90 V, f=300 kHz, D=50% and Iin,avg=1.5 A).
Drain to source voltage with (“Projected”) and
without (“Real”) resolution improvement circuit is
shown in (a). Figure (b) presents the drain
current and the projected drain to source voltage
during on-state, resulting in a Rdyn of
approximately 0.2 ohm (figure c).
B
Clamping diodes
Source DHEMT
Off-state
D2
BY527
Drain DHEMT
On-state
Fig.3. Circuit for improving accuracy when
measuring high off-state voltages.
0
Vth
-1 = -2.09 V
Miller plateau
-2
100
-3
80
Vgs [V]
Real
Projected
90
70
-4
-5
Qgd = 0.65 nC
Vds [V]
60
-6
50
-7
40
Resolution improvement
30
-8
Vd_clamp
20
10
0
-10
-6
(c)
A
D1
BY527
4
x 10
C
R3
1260 ohm
3.8
0
0.5
1
1.5
2
2.5
3
Time [s]
(a)
3.5
4
4.5
x 10
5
-6
-9
Qg = 2.46 nC
0
5
10
Qg [C]
15
20
x 10
Fig.5. Gate charge (Qg) and gate-drain charge
(Qgd) measurement (Rg=220 ohm, Vgs= [-9.28 /
0.32V], Vds,off=30 V, f=300 kHz, D=50% and
Iin,avg=1.5 A).
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Abstract for PCIM Europe 2010
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J. Everts / KULeuven
Conclusion
The characterization of AlGaN/GaN/AlGaN DHFETs, using a hard switching VIENNA
boost converter was demonstrated. Gate drive requirements for this type of devices were
met by two dedicated gate drive circuits, offering a maximum flexibility for testing. Methods
for fast and accurate measurement of dynamic on-resistance (Rdyn) and gate-drain charge
(Qgd) were proposed and demonstrated. A circuit for improvement of measurement resolution
showed its value. It was also shown that GaNAlGaN/GaN/AlGaN DHFET have promising
figures of merit (FOM), making them a good replacement candidate for conventional device
types (MOSFET and COOLMOS) in power electronic converters.
References
[1] L. F. Eastman, and U. K. Mishra, “The toughest transistor yet,” IEEE Spectrum, vol. 39,
Issue 5, pp. 28-33, May 2002.
[2] W. Yifeng, M. Jacob-Mitos, M. L. Moore, and S. Heikman, “A 97,8% efficiënt GaN HEMT
boost convertor with 300- W output power at 1 MHz,” IEEE Electron Device Letters, vol. 29,
Issue 8, pp. 824-826, Aug. 2008.
[3] W. Saito, T. Nitta, Y. Kakiuchi, Y. Saito, K. Tsuda, I. Omura, and M. Yamaguchi,
“Suppression of Dynamic On-Resistance Increase and Gate Charge Measurements in HighVoltage GaN-HEMTs With Optimized Field-Plate Structure,” IEEE Trans. on Electron
Devices, vol. 54, NO. 8, Aug. 2007.
[4] G. Meneghesso, G. Verzellesi, R. Pierobon, Fabiana Rampazzo, A. Chini, U. K. Mishra,
C. Canali, and E. Zanoni, “Surface-Related Drain Current Dispersion Effects in AlGaN–GaN
HEMTs,” IEEE Trans. on Electron Devices, vol. 51, Issue 10, pp. 1554-1561, Oct. 2004.
[5] D. Visalli, M. Van Hove, J. Derluyn, S. Degroote, M. Leys, K. Cheng, M. Germain, and G.
Borghs,, “AlGaN/GaN/AlGaN Double Heterostructures on Silicon Substrates for High
Breakdown Voltage Field-Effect Transistors with low On-Resistance”, Jpn. J. Appl. Phys.,
vol. 48, 04C101, Apr. 2009.
[6] P. Horowitz, and W. Hill, “The Art of Electronics,” Cambridge University Press, Second
Edition, 1994.
Biography
Jordi Everts received the M.Eng. degree in electromechanical engineering from University
College KHLim, Diepenbeek, Belgium, in 2007. From then on he worked as a projectengineer at the KHLim on MicroGrid applications. In 2008 he also obtained a post-academic
degree in energy management in buildings from the University of Gent, Belgium. In January
2009, he started a PhD at the department of electrical engineering (ESAT) from the
University of Leuven, Belgium. He has been engaged in the research and the development
of power electronic converters for wide bandgap transistors.