IXAN0017 - IXYS Corporation

New 1600V BIMOSFET™ Transistors Open
Up New Applications
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by
Ralph E. Locher
IXYS Corporation
Santa Clara, CA
Introduction
There are many applications today using high
voltage MOSFETs and IGBTs, which would benefit from a higher voltage part. Examples are
sweep circuits, radar pulse modulators, capacitor discharge circuits, solid state relays, auxiliary power supplies on traction equipment and
other high voltage switch mode power supplies.
MOSFETs are connected in series-parallel strings
to overcome their voltage and high RDS(on) limitations. High voltage IGBTs are too slow for
some applications. A new family of high voltage BIMOSFETTM transistors is fulfilling these
needs.
The conventional construction for both
MOSFETs and IGBTs is commonly referred to
as DMOS (double-diffused-metal-oxide-silicon),
which consists of a layer of epitaxial silicon
grown on top of a thick, low resistivity silicon
substrate, as shown in Fig. 1b. However, at voltages in excess of 1200V, the thickness of the Nsilicon layer required to support these blocking
voltages makes it more attractive and less costly
to use a non-epitaxial construction as illustrated
in Fig. 1a. This type of construction is also known
as "homogeneous base" or "non-punch through"
(NPT).
Referring to Figure 1a, 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 'BIMOSFETTM transistor.' The turn-off behavior of the BIMOSFETTM
transistor is controlled by the amount of collector shorting. In order for the diode to be usable
and not cause commutating dV/dt problems, the
lifetime of the minority carriers must be reduced
by irradiation. The end result is a device, which
can be optimized for either high frequency or low
frequency switching by tailoring its collector
short pattern along with suitable amounts of ir-
(a)
(b)
Figure 1. Comparison of the BIMOSFETTM IXBH40N160 cross-section (a)
to an IGBT with epitaxial construction (b).
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Table 1: Comparative Electrical Performance
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1600V
1600V
Vge(th)
5-9V
5-9V
7V
2.5V
Qg(on)
121nC
108nC
Ic(on) @ Vge = 15V
110A
100A
td(on)
50ns
50ns
tri
195ns
168ns
E(on)
0.78mJ
0.66mJ
trv
195ns
335ns
tfi
240ns
1980ns
E(off)
3.0mJ
29mJ
DC Parameters (Tj = 25C)
Vce(sat) @ Ic = 25A
Switching (Tj = 125C)
Turn-on (Note 1)
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Notes:
1. Turn-on test conditions: VC = 960V; IC = 30A; RG = 2.7Ω; Resistive load
2. Turn-off test conditions: VC = 1440V; IC = 25A; RG = 22Ω; Inductive load
radiation.
Since there are many applications in which the
electrical characteristics of the intrinsic diode are
not optimum for the application, e.g. high onvoltage or reverse recovery current or too high
power dissipation, a modified fabrication process
has been developed to block the intrinsic diode
without impacting the effectiveness of the collector shorts. The first member of the family with
the diode blocked is the 1600V rated
IXLH45N160 BIMOSFETTM transistor intended
for high current applications with low repetition
rates. This part has a much lower saturation voltage (3.5V at Ic = 30A) because it is not irradiated. Its switching speed is controlled by the
amount of collector shorting; more shorting re-
sults in higher saturation voltages due to loss of
conducting area but faster switching performance.
DC Electrical Performance
It is foreseen that the BIMOSFETTM transistor
family will span the range of high voltage applications, from a simple high voltage switch to
increasing the upper frequency performance of
high voltage IGBTs. Table 1 offers a comparison of their electrical performances.
In examining this table and some of the figures below, we can note the following:
1. The typical threshold voltage of the
BIMOSFETTM transistor family is higher than
normal IGBTs but its Qg(on) is comparable. This
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Figure 3a: VCE(sat) of the IXLH45N160 showing the
effect of increasing TJ.
Figure 2a: Gate charge
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Figure 2b: IXLH45N160 output current vs. gateemitter voltage
is due to its relatively low Miller gate capacitance resulting in low Miller gate charge as can
be seen in Figure 2a. In one sense, a high threshold voltage can be considered as an advantage in
electrically noisy environments. The low VCE(sat)
version also has a 2V higher VGE(on). Figure 2b
plots its transconductance at room and elevated
temperatures and shows that the output current
vs. VGE is relatively independent of temperature.
2. The VCE(sat) of the IXLH45N160 is almost
one-third of the IXBH40N160, 2.5V and 7.0V
respectively at IC = 25A. The saturation voltage
of both parts has a strong, positive temperature
coefficient as evidenced in Fig. 3a, depicting the
VCE(sat) curves for the IXLH45N160. Fig. 3b plots
the diode voltage drop of the IXBH40N160 and
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Figure 3b. Forward voltage drop of the
IXBH40N160 intrinsic diode.
shows that it too has a positive tempco. Consequently it is easier to operate BIMOSFETTM transistors in parallel than either DMOS IGBTs or
MOSFETs.
3. In order to survive short circuit testing
(SCSOA) at higher voltages, low
transconductance, yielding low short circuit current ICE(on) is required. So with ICE(on) values in the
order of 100A, the BIMOSFETTM transistors can
be used in applications where survivability to this
type of fault is a must.
4. However in many pulse applications, the capability to conduct high peak currents is more
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Figure 4. Saturation voltage curve for the
IXBH45N160. Note increased output current
capability and lower VCE(sat) voltage with 20V
gate drive.
important than SCSOA. One way to overcome low transconductance is to increase
gate voltage. Figure 4 shows that IC(on) almost doubles from 100A to over 200A
as VGE is increased from 15V to 20V,
while gate charge only increases by 20nC.
This is easily done using either discrete
MOSFETs or bipolar transistors in the
gating circuit or by using commercially
available IC drivers, such as the Telcom
TC4431 or TC4432 MOSFET drivers.
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Figure 5. Turn-off current and voltage waveforms
of the IXBH40N160.
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Switching Performance Comparison
Both BIMOSFETTM transistors switch
exceptionally fast for 1600V rated parts.
The resistive turn-on time of the IXLH45N160
with a 2.7Ω gate resistor is typically 168ns, which
edges out the IXBH40N160 (tri = 195ns) because
the latter is irradiated. But Figure 5, illustrating
the IXBH40N160 turning off a 20A inductive
load into a 1000V clamp at the elevated temperature of 125OC, shows where it shines. There is
relatively little tail current so that the E (off) is
2.4mJ, which is 50% less than a comparable
IGBT. Figure 6 plots its turn-off energy as a function of the series gate resistor RG. This resistor
primarily determines the rate-of-rise of collec-
Figure 6. Turn-off energy versus gate resistor RG
for the IXBH40N160.
Figure 7. Series connection
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1. Static voltage sharing resistors RS due to unequal leakage currents of the two switches;
2. Dynamic voltage sharing capacitors CS to
compensate for differences in turn-on and
turn-off times;
3. Resitor RC may also be required to dampen
voltage ringing or to limit capacitor in-rush
current at turn-on;
4. Zener diodes Z1 to protect the IGBTs
against overvoltage transients;
5. Duplicate gating circuit components RG, Z2
and RE.
Eight of these components can be eliminated
when using only one high voltage switch! In addition, the pulse transformer is easier to wind
since now there is only one secondary winding.
When one switch does not have the current
handling capability, semiconductor switches are
used in parallel. While both MOSFETs and
IGBTs are used in parallel, both require matching to achieve satisfactory operation. The
Figure 8a: Bi-directional AC switch.
Figure 8b: AC current control using diode bridge.
tor voltage, which increases as RG decreases and
correspondingly E(off) decreases. The low VCE(sat)
IXLH45N160 has a much longer tail current,
which is only marginally affected by RG. Consequently its operating frequency range is less than
5kHz.
Applications
Some of the many applications have already
been mentioned but let us a review a few to see
the advantages of the availability of high voltage switches.
One fast growing type of application is capacitor discharge circuits, such as found in laser
power supplies, defibrillators, spot welders and
similar circuits. The use of high voltage is an
advantage because energy stored in a capacitor
is proportional to voltage squared and fast current rise times are easier to achieve. Figure 7
shows a typical circuit using two IGBTs in a series string. Note the necessity of the following
duplicate components:
Figure 9a: Dynamic break configuration.
Figure 9b: Boost configuration.
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BIMOSFETTM transistor family facilitates paralleling due to its positive voltage temperature
coefficient of both its saturation voltage and forward voltage drop of the intrinsic diode as shown
in Figure 3b..
A traditional usage of thyristors is in AC solid
state switches. Two possible circuits are shown
in Figures 8a and 8b. Figure 8a shows the connection diagram for two IXLH45N160
BIMOSFETTM transistors and two high voltage
diodes while Figure 8b circuit uses one
BIMOSFET TM transistor inside a full-wave
bridge. Both circuits can be used on AC mains
up to 600V(RMS) and both also provide the additional functions of precise current control and
overcurrent protection. The circuit in Figure 8a
can carry more current because the current is
shared by the two BIMOSFETTM transistors and
will be more efficient because current only flows
through one diode. The circuit in Figure 8b will
cost less because there is only one BIMOSFETTM
transistor.
Finally Figures 9a and 9b show the usage of
the BIMOSFETTM transistors in two rapidly
growing applications, namely AC motor control
featuring dynamic braking and boost inverters.
Again due to the high voltage and fast switching
capability of the IXBH40N160, one can now
design these circuits to operate up to 600V(RMS)
or produce output DC voltages up to 1200V.
Just as it is anticipated that the applications
for BIMOSFETTM transistors will proliferate,
IXYS will continue to grow the BIMOSFETTM
transistor family by the additions of both higher
and lower current devices with a range of switching speeds to meet the requirements of the power
conversion market.
(Acknowledgement: The author wishes to recognize and thank his co-workers, Messrs. M.
Arnold, T. Jankovic, A. Lindemann and O.
Zschieschang, for their contributions and suggestions to make this article possible.)
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