Comparison of total losses of 1.2 kV SiC JFET and BJT
in DCDC-DC converter including gate driver
J.-K. Lim1,2, G. Tolstoy2, D. Peftitsis2
J. Rabkowski2, M. Bakowski1, H.-P. Nee2
2
1 Acreo AB, Electrum 236, SE-164 40, Kista, Sweden
EES/EME, KTH, Royal Institute of Technology, Stockholm, Sweden
e-mail: Jang-Kwon.Lim@acreo.se
Abstract
The 1.2 kV SiC JFET and BJT devices have been investigated and compared with respect to total losses including the gate driver losses in a DC-DC converter configuration. The buried grid,
Normally-on JFET devices with threshold voltage of -50 V and -10V are compared to BJT devices with ideal semiconductor and passivating insulator interface and an interface with surface recombination velocity of
4
4.5·10 cm/s yielding agreement to the reported experimental current gain values. The conduction losses of both types of devices are independent of the switching frequency while the switching losses are
proportional to the switching frequency. The driver losses are proportional to the switching frequency in the JFET case but to a large extent independent of the switching frequency in the BJT case. The passivation
of the emitter junction modeled here by surface recombination velocity has a significant impact on conduction losses and gate driver losses in the investigated BJT devices.
Switching performance
How to improve the efficiency of Power module
40
Bipolar Junction Transistor
Buried grid Junction Field Effect Transistor
1Ω :15Ω
1Ω :10Ω
5Ω :15Ω
1Ω :15Ω
Peak gate current
30
Voltage-controlled type
Current-controlled type
Negative gate voltage (N-on type)
Considerable base current
500
VDS (V)
Bipolar device ( electrons & holes )
Jgate (A/cm2)
Unipolar device ( electrons )
600
no
cap
a
(no 22nF)
20
400
5Ω
300
200
10
5,0e-8
1,0e-7
1,5e-7
2,0e-7
0
0,0
2,5e-7
5,0e-8
1,0e-7
1,5e-7
Strong
Ig
:15Ω
1Ω :1
5Ω
1Ω :1
0Ω
100
0
0,0
JFET TurnTurn-on time (tf)
cito
r
tf
2,0e-7
2,5e-7
Weak
R1, Cap
R2
Strong
Weak
R1, Cap
R2
time (sec)
time (sec)
20
0.8Ω :1Ω
0.8Ω :0.5Ω
1.2Ω :1Ω
0.8Ω :1Ω
-40
-. Gate drive circuit
-. The nature of the load
-60
0,0
1,0e-7
1,5e-7
2,0e-7
0
0,0
2,5e-7
:0.5
Ω
0.8 Ω
5,0e-8
10
-. Packaging and Cooling system
Peak base current
0.2Ω : 0.2Ω
0.2Ω : 0.5Ω
0.5Ω : 0.2Ω
0.2Ω : 0.2Ω
1) Power devices used as switches and 2) gate drive circuit determine
switching losses (efficiency of power conversion)
4
2
2e-8
4e-8
6e-8
8e-8
300
0
0,0
1e-7
5,0e-8
1,5e-7
-2
-4
0.5
-6
-8
Peak base current
0
W cell/2 = 1.7 µm
2e-8
4e-8
6e-8
8e-8
0
0,0
1e-7
tdrift = 6.3 µm
tbase = 0.5 µm
Values for
ton = toff = 200ns
tcollector = 10 µm
W cell/2 = 5.0 µm
Nbase = 2.6×1017 cm-3
Sp = 4.5×104 cm/s
1) 4H4H-SiC JFETs ( -50 Vth , -10 Vth ) and 2) BJT s ( ideal , state of the art )
are compared
Summary of driving conditions
Device types
Nch = 1.9×1016 cm-3 (Vth -10 V)
Strong
Weak
R1
R2, Cap
Strong
Weak
R1,R2
Cap
BJT TurnTurn-off time (tr)
:
4Ω Ω
0.2 0.24
:
0Ω
-Ib
Strong
R2
Strong
Weak
R1, R2
Cap
tr
5,0e-8
1,0e-7
1,5e-7
2,0e-7
Weak
R1, Cap
2,5e-7
time (sec)
time (sec)
Nch = 1.2×1017 cm-3 (Vth -50 V)
2,5e-7
Ω
0.5
400
200
Lch / W ch = 1.6 / 1 µm
tf
2,0e-7
0
no .24 Ω
ca :0
pa .2
cit 4 Ω
or
600
(no 22nF)
0
VCE (V)
Jbase (A/cm2)
1,0e-7
Ib
time (sec)
0.24Ω : 0.24Ω
0.24Ω : 0.50Ω
0.50Ω : 0.24Ω
0.24Ω : 0.24Ω
2
Device design factors
Cap
BJT TurnTurn-on time (tf)
0.5Ω
:0
0.2Ω .2 Ω
:0.5 Ω
0
no .2 Ω :0
ca
.2
Ω
pa
cit
or
100
0
R1, R2
400
200
0
Weak
600
time (sec)
Carrier lifetime 0.1µs
2,5e-7
500
6
4
Simulated JFET and BJT structures
2,0e-7
R2
Strong
tr
(no 22nF)
VCE (V)
Jbase (A/cm2)
Energy efficiency
1,5e-7
Weak
R1, Cap
time (sec)
8
Capacitor 22 nF
1,0e-7
Strong
-Ig
200
time (sec)
-. Passive components
JFET TurnTurn-off time (tr)
(no 22nF)
Peak gate current
5,0e-8
400
ca
p
0.8 acito
Ω
r
:1 Ω
12
Ω
:1 Ω
2
-20
no
-. Fast switching speeed
600
0
VDS (V)
Parameters
-. Low on-state resistance
Jgate (A/cm )
Driver
Configuration
R1 : R2 : Cap
-. High blocking voltage capability
JFET
(Vth=-10 V)
BJT
(ideal)
BJT
(state of the art)
1
15
0.2
1
0.20
0.20
0.13
0.13
1.37
6.30
1.16
6.51
1.20
5.74
1.80
5.00
JFET
(Vth=-50 V)
Turn--on
Turn
R1, [ ]
R2, [ ]
Qgate/base, [µC]
Eon, [mJ]
Turn--off
Turn
R1, [ ]
R2, [ ]
Qgate/base, [µC]
Eoff, [mJ]
0.8
1
0.5
1
0.24
0.24
0.5
0.5
-3.50
11.50
-1.73
16.10
-1.28
14.50
-0.63
17.8
VGS, VBE [V]
-60 to 2
-15 to 2
0 to 5
Cap [nF]
22
22
22
0 to 5 (on)
0 to 10 (off)
Conditions
Conduction losses
Switching losses
22
Driver losses
JFET
Comparison of Ron & Current gain
120
Ron (m
cm2)
N-on JFET
(Vth = -50 V)
1.00 (VGS=0 V)
0.91 (VGS=2 V)
N-on JFET
(Vth = -10 V)
2.50 (VGS=0 V)
1.67 (VGS=2 V)
BJT
(ideal)
BJT
(state of the art)
3.00
100
Current emitter gain
Device types
7.89
80
no interface states (RT)
no interface states (150oC)
interface states (RT)
o
interface states (150 C)
BJT
110
Conclusions
98
5 µm, W half of cell width
Power losses = Conduction + Switching + Gate driver losses
4 types of devices at 200A/cm2
4 types of devices at 600V/200A & 10kHz
60
40
45
33
20
0
10-1010-9 10-8 10-7 10-6 10-5 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 105
2
log collector current density (A/cm )
For N-on BG Junction FETs,
-.Very low Ron ( -50Vth, -10Vth) due to small Wcell
high Nch
-.Ron depends on Vth and is lower for high Vth
For Bipolar Junction Transistor,
-. The surface recombination strongly influneces
Ron and
N-on JFET with high negative Vth shows good static performance (Ron)
Surface recombination is demonstrated as a limiting factor (Ron & ) in BJT
The switching losses of both JFET and BJT &
driver losses of the JFET are propotional to the switching frequency
The conduction losses of both JFET and BJT &
most of the BJT driver losses are independant of the switching frequency
Acknowledgements: This work was funded jointly by VINNOVA and the Swedish Energy Agency (STEM)