IXAN0014 - IXYS Corporation

Technical Application
IXAN0014
Comparative Performance of
BIMOSFETs in Fly-Back
Converter Circuits
One of the typical applications for a flyback converter is the auxiliary power supply for the IGBT gate driver in an inverter. The
essential requirement for a switch of a flyback converter in a drives inverter is a high breakdown voltage combined with fast
switching speed. To minimize the predominant switching losses, the switch-on and -off energies have to be low. The main
advantage of the BIMOSFET lies first in its lower turn- on losses and secondarily in its lower conduction losses. A comparison
of the total energy losses between a MOSFET and a BIMOSFET results in 35 % less total losses for the BIMOSFET.
Flyback Operation
Flyback Application
The Flyback Converter is one of the most simple converter
types. The minimum configuration consists of only a switch, a
transformer, a diode and two capacitors as shown in fig. 1. The
One of the typical applications for a flyback converter is the
auxiliary power supply for the IGBT gate driver in an inverter.
This application has all the requirements, which can be fulfilled
ideally by a flyback converter.
IS
D1
DC - Bus
C2
Input Rectifier
U1
500-800 V
U2
C1
C1
DC 1
AC
Brake Chopper
400 V
3~ C2
50 Hz
UDS
C3
T1
DC 2
.
.
.
DC Start up Circuit
DC n
Inverter
U
DC
V
& Driver
BIMOSFET
UDS
AC
AC
400 V
3~
0-50 Hz
W
Inrush Current Limitation
U1
IGBT Driver
0
DC 2
DC 3
T
DC 1
t
DC 4
Micro Controller
DC 5
IS
t
0
T
t
Figure 1: flyback
energy in this converter type is stored in the air gap of the
ferrite core. Primary current ramps up during the on state of
the switch storing magnetic energy, which is then transferred
to the output by the diodes when the switch turns off. The power
range for this converter type is limited to approximately 300 W.
The advantages of this circuit are the very wide input-output
voltage ratio and the feasibility of adding more secondary
windings to create multiple output voltages. Furthermore, it is
advantageous to have galvanic insulation between primary and
secondary side. The disadvantages are the high breakdown
voltage required for the switch and the RFI emission generated
by the air gap in the transformer. The flyback converter can not
work without load or closed regulation loop as otherwise the
output voltage will exceed allowable limits.
Figure 2: inverter
The shaded area in fig. 2 shows a converter with the start-up
circuit as part of the drives inverter. The auxiliary power supply
can be built very cost effectively with relatively few elements.
Since the input voltage for the converter is the DC-power bus,
there is a wide voltage variation. During the precharge of the
bus capacitors, the power supply has to work properly with very
low DC-bus voltages, as well as under braking operation of the
motor, when the bus voltage reaches high values up to 750 V.
The output voltage can easily be regulated by varying the
transistor duty cycle.
All the insulated DC- outputs can be generated by adding more
separated secondary windings. For example the 5 V supply for
the micro controller, ± 15 V for current sensors, a common +15V
supply for the driver of the three lower IGBTs and three separate +15V supplies for the upper IGBT drivers.
IXAN0014
Requirements for the Switch
The essential requirement for a switch of a flyback converter in
a drives inverter is a high breakdown voltage. In a flyback
converter the maximum voltage applied to the switch is
approximately two x the input voltage. Therefore, the minimum
breakdown voltage must be higher than 2 x Vin. For standard
inverters for motor control used off mains of 400 V, the DC-bus
voltage can reach up to 750 V in the motor braking mode of
operation. Here a breakdown voltage of at least 1600 V is
needed.
Flyback converters normally run with switching frequencies
between 50 to 100 kHz. To minimize the predominant switching
losses the switch-on and -off energies have to be low. To achieve
this, a high switching speed by the switch is obvious. A common
trick to avoid switch-on losses in a fly back topology is not to
turn on the transistor until the current in the output diode has
reached zero (discontinuous mode). There must be a deadtime
until the next cycle starts. The advantage here is less transistor
and diode commutation losses, which allows higher switching
frequency in order to reduce the size of the transformer.
BIMOSFETTM Chip Technology
Standard high voltage IGBTs are too slow for flyback
applications. The new family of high voltage BIMOSFET
transistors is fulfilling these needs.
The conventional construction for both MOSFETs and IGBTs is
commonly referred to as DMOS (double-diffused-metal-oxidesilicon), which consists of a layer of epitaxial silicon grown on
top of a thick, low resistivity silicon substrate, as shown in Fig.
3a.
Referring to Fig. 3b, the typical pnpn structure for the IGBT has
been maintained, but note that an N+ collector- short pattern
has been introduced in order to reduce the current gain of the
PNP transistor and consequently its turn-off switching behavior.
However, now there is a “free” intrinsic diode from emitter to
collector, not unlike that found in a MOSFET, which led us to
coin the acronym BIMOSFET transistor. The turn-off behavior
of the BIMOSFET transistor is controlled by the amount of
collector shorting. In order for the diode to optimize reverse
conduction and not cause commutating dV/dt problems, the
lifetime of the minority carriers must be reduced by irradiation.
There are two types of BIMOSFETs: The standard type has been
designed for IGBT like control with VGE=15V/0V, while the “G”
type can be operated with the same gate voltages as a MOSFET,
as is explained in the following section. Besides, static and
dynamic behaviour of both types are the same.
Driver requirements
a) Standards BIMOSFET
Our tests have shown there is a significant influence on the losses
by gate resistor and gate voltage. As a rule, we have found that
a series gate resistor of less than 30 W has a tendency to oscillate
while switching above 50 W increases mainly the turn-on losses.
Therefore, the IXBH 9N160 BIMOSFET operates best at 15 V’s
gate drive and by using a gate resistor between 30 and 50 W.
To achieve full conduction, a gate drive of 15 V is necessary,
because the threshold voltage of 6 V is relatively high compared
to MOSFETs.
Mosfet 1500V, 2A:
T_vj=125°C
10
I_D [A]
9
8
7
6
5
4
3
2
Epitaxial IGBT crossection
1
00
Figure 3a: Cross section
However, at voltages in excess of 1200 V, the thickness of the
N-silicon layer required to suppport these blocking voltages
makes it more attractive and less costly to use a non-epitaxial
construction as illustrated in Fig. 3b. This type of construction
is also known as “homogeneous base” or “Non Punch Through”
(NPT).
5
10
15
20
V_GS=6 V
V_GS=15 V
V_GS=4 V
V_GS=5 V
25
30
V_DS [V]
Figure 4a: output characteristics
IXBH9N160;
T_vj=125°C
10
I_C [A]
9
8
7
6
5
4
3
2
1
00
Homogeneous Bimosfet crossection
Figure 3b: Cross section
5
10
V_GE=7V
V_GE=9 V
15
20
V_GE=11 V
V_GE=15 V
25
30
V_CE [V]
Figure 4b: output characteristics
2
IXAN0014
b) BIMOSFETTM “G”- Type
The threshold voltage of “G” type BIMOSFETsTM is typically 4V
and thus lower than of the standard type. Due to this fact it is
possible to drive the part with an on state gate voltage of 10 V.
This way a BIMOSFETTM may for example replace a 1000V
MOSFET in a flyback converter. Because of the blocking voltage of 1400/1600V the snubber capacity may be reduced or even
omitted. Nevertheless a drive voltage of 15 V is also applicable
and will reduce switch on.
Type designations of “G”-Types end with “G”. First parts are IXBF
9N140G and IXBF 9N160G.
Static behavior
When comparing the output curves we can see the linear
characteristic for the MOSFET (fig.4a) and the bipolar behavior
for the BIMOSFET (fig.4b).
As can be seen from figure 4a, the MOSFET can conduct 2 A
with only 6 V gate drive. Comparing this to the BIMOSFET output
characteristic in fig. 4b, one sees that there is no current flow with
7 V gate drive. Here lies the major difference for the BIMOSFET.
We need at least 11 V to switch on properly at currents below 5 A.
For higher peak currents, we need 15 V gate voltage for proper
conduction. There is a significant difference in the on-state losses.
At 2 A and 15 V gate drive, the MOSFET has a voltage drop of 18
V and the BIMOSFET has only 4 volt drop. This leads to 4.5 times
less conduction losses. We also can see the much higher current
capability of the BIMOSFET, which can easily conduct more than
10 A compared to the MOSFET, which is limited to 3 A.
Switching behavior
We have done several comparative measurements to quantify
the performance of a standard high voltage MOSFET and the
BIMOSFET. The figures 5a and 5b show a full switching cycle
and allow the calculation of the total losses. The parameters drain
current, drain voltage and gate voltage have been measured. The
power dissipation and the total energy have been calculated from
these data.
The test equipment was a double pulse tester, in which the
freewheeling diode is still conducting when the MOSFET is
switched on. Consequently, the turn-on waveform is impacted by
the diode’s recovery behavior. However, the performance is
comparable because the diode‘s influence on the MOSFET or
BIMOSFET is the same.
The conditions are as follows:
Turn-off current amplitude = 4 A
Voltage = 800 V
Gate drive = 15 V, 40 W
Junction temperature = 125°C
Mosfet 1500V, 2A: R_G=40 Ohm, T_vj=125°C
5
The time from t0 to t1 is the end of the conduction phase. At the
end of this phase we can see a rise in the energy curve (solid
line below), which is caused by the higher on state losses of
the MOSFET.
0
0
t0
t1
t2
I_D [1 A/div]
V_DS [200 V/div]
V_GS [5 V/div]
Energy [0,2 mJ/div]
t3
t4
t5
t [250 ns/div]
The next step (t1 to t2) is the turn-off. The dotted line (Ptot) below
shows no significant difference in turn-off losses although they
might be slightly less for the BIMOSFET.
After turn- off (t2 to t3), there is no visible tail current for the
BIMOSFET. The slight increase in energy that we can see during
the off-state might be a measurement error, since we have the
same for the MOSFET, which definitely has no tail current.
The next phase is turn-on from t3 to t4. We easily can see that the
major losses occur during turn-on. The upper solid line shows a
high peak current, which is mainly caused by commutation of the
diode. In comparison, the turn-on time for the MOSFET is longer
than for the BIMOSFET. The peak power for the MOSFET is
approximately 4 kW for 250 ns. The peak power for the BIMOSFET
is 5 kW but only for a duration of 130 ns. Therefore, the total
switch on energy for the MOSFET is 0.5 mJ and only 0.4 mJ for
the BIMOSFET. This is 20% less for the BIMOSFET.
IXBH9N160; R_G=40 Ohm, T_vj=125°C
5
The last 500 ns, from t4 to t5, are the beginning of the conduction
phase. The energy curve for the MOSFET shows a rise caused
by the high on resistance. The BIMOSFET curve is almost flat,
which is a sign for a low saturation voltage (compare fig.4b).
0
0
t0
t1
t2
I_C [1 A/div]
V_CE [200 V/div]
V_GE [5 V/div]
Energy [0,2 mJ/div]
t3 t4
t5
t [250 ns/div]
Figure 5: switching curves
3
IXAN0014
Product range
Type
BVCES
min.
V
VCEsat @ 25°C
max
V
I C25
tr
tf
A
ns
IXBH 9N140
IXBH 9N160
1400
1600
7.0
9
IXBH 9N140G
IXBH 9N160G
1400
1600
7.0
IXBF 9N140G
IXBF 9N160G
1400
1600
IXBH 15N140
IXBH 15N160
Housing
ns
Gate
drive
V
60
40
15
TO247
9
200
70
10
TO247
7.0
7
200
70
10
i4-PacTM
1400
1600
7.0
15
60
40
15
TO247
IXBH 20N140
IXBH 20N160
1400
1600
6.5
20
60
40
15
TO247
IXBH 40N140
IXBH 40N160
1400
1600
7.1
28
60
40
15
TO247
IXBF 40N140
IXBF 40N160
1400
1600
7.1
28
60
40
15
i4-PacTM
In summary
The main advantage of the BIMOSFET lies first in its lower turn- on losses and secondarily in its lower conduction
losses. The total energy loss per pulse is shown at time t5 where we see that the MOSFET value is 0.95 mJ and for the
BIMOSFET, only 0.62 mJ. This results in 35 % less total losses for the BIMOSFET.
IXYS Semiconductor GmbH
Edisonstr. 15, D-68623 Lampertheim
Telefon: +49-6206-503-0, Fax: +49-6206-503627
e-mail: [email protected]
Publication DE 0103 E Printed in Germany (06.01 • 5 • Ha)
IXYS Corporation
3540 Bassett Street, Santa Clara CA 95054
Phone: (408) 982-0700, Fax: 408-496-0670
e-mail: [email protected]
http://www.ixys.net
IXAN0014