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Power device selection and a comparative study of transistor
technologies for a Zero Voltage Switching Buck Converter
Pål Andreassen
Abstract— The new ultra thin wafer technology makes it possible to
make Non Punch Through (NPT) IGBTs in the 600V range, with a
lightly doped collector. This new IGBT technology is tested and
compared with other competitive transistor technologies. The case
study of the Zero Voltage Switching (ZVS) Buck Converter is
performed because this topology until now has favored the use of
MOSFETs.
Tore M. Undeland
When using an external capacitor, the dv/dt is reduced and the current is
almost zero when the voltage starts to rise. The tail-bump of the PT IGBT is
larger in the beginning and decreases faster compared to the High Speed
NPT IGBT. The tail-bump of the SKB15N60HS is low, and decreases very
slowly.
Introduction-The ZVS QSW Buck Converter
The specific ZVS topology examined is known as the Quasi Square Wave
(QSW) Converter. Two of the advantages of this topology are Zero Voltage
Switching for both transistors and bidirectional power flow. The
disadvantages are high transistor peak current and high input/output current
ripple. High turn-off current of the main switch tend to increase the turn off
losses, especially when minority-carrier devices such as IGBTs are used.
Therefore, this topology has not been regarded desirable with IGBTs, at
least with older generations of IGBTs.
+
-
VQ1
a) IRG4BC30KD
D1
ILr
Lr
Q1
Co
Vin
Q2
D2
Ro
b) SKB15N60HS
Fig 4. Turn-off-transient soft switching
The turn-off losses are measured for different values of the external
capacitor. It shows that the turn-off losses of the SKB15N60HS and the
SPB20N60S5 is low even without the use of an external capacitor, and is
not reduced much when using an external capacitor.
Cr
Fig 1. QSW converter topology and switching waveforms
Capacitive turn-off losses at Lr=30uH, Ioff=16A
0,2
Measurement and calculation of transistor turn-off losses
0,18
0,16
0,14
Eoff[mJ]
The transistors subjected to comparison are two Punch-Through (PT)
IGBTs, IRG4BC30KD and FGB30N6S2D, two Non Punch Through (NPT)
IGBTs, SGB15N60 and SKB15N60HS, and the CoolMOS SPB20N60S5.
The SKB15N60HS is the one optimized for high speed switching.
FGB30N6S2D
0,12
IRG4BC30KD
0,1
SGB15N60
SPB20N60S5
0,08
SKB15N60HS
0,06
0,04
0,02
0
0
10
20
30
Cr[nF]
Fig 5. Capacitive turn-off losses at Ioff=16A as a function of external Cr.
Calculation of Transistor losses at operating point
a) IRG4BC30KD
b) SKB15N60HS
Fig 2. Turn-off transient hard switching
It is interesting to see from the turn-off transient of the high speed IGBT
that, due to stray capacitance, Cce, and low output inductance the turn-off is
capacitive.
For comparative study the losses in the transistor Q1, referred to Fig. 1, is
calculated at the operating point,Vin= 300V, Vo=150V,Iout= 8A, fs,max=
160 kHz.The ideal option with regard to the conduction losses would be to
keep the Cr as low as possible. With regard to this we compare the
transistor losses with the internal stray capacitance as resonant capacitor,
Cr.
0,3
Turn off losses
0,25
0,45
0,2
0,4
0,35
0,15
0,3
0,1
Eoff[mJ]
IRG4BC30KD
0,25
FGB30N6S2D
SGB15N60
0,05
SPB20N60S5
0,2
Ec[mJ]
SKB15N60HS
0,15
Eoff[mJ
]
IR
G
4B
C3
0K
D
(P
T)
FG
P3
0N
6S
2D
(P
SP
T)
B2
0N
60
S5
(F
ET
SG
)
B1
5N
60
HS
(N
PT
)
SG
B1
5N
60
(N
PT
)
0
0,1
0,05
0
0
5
10
15
20
25
30
35
Fig 6. Q1 transistor losses at the operating point
Ioff[A]
Fig 3. Hard switching turn off losses as a function of turn-off current
Conclusion
Since the turn-off current of the ZVS QSW buck converter will be at least 2
times the current of a hard switched converter, the rate of increase in the
turn-off losses with an increase of turn-off current is of interest. The rate of
increase in turn-off losses in the SKB15N60HS is less than for the others,
hence promising for use in the ZVS QSW converter.
For the specific operating point the CoolMOS has the lowest loss. However,
the new high speed NPT technology looks promising for use in the ZVS
QSW converter due to the low increase in turn-off losses with increasing
turn-off current.
NORPIE 2004
06.14.04