L i n e a r Mo d... S a f e O p e r... P o w e r - M O SF E...

L i n e a r   Mo d... S a f e   O p e r... P o w e r - M O SF E...
L in e a r Mo d e O p e r at io n a n d
S af e O p e r a t in g D i a g r a m o f
P o we r - MO SF E T s
J. Schoiswohl
A pp l ica t io n N o t e
V0.92 June 2010
I FN A I M M S M D P M D
Published by
Infineon Technologies AG
81726 Munich, Germany
© 2011 Infineon Technologies AG
All Rights Reserved.
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AP99007
Description of Application Note
Table of Contents
Table of Contents
1
Introduction ………………………………………………………………………………………..p. 4
1.1
Applications driving MOSFETs in linear mode operation ……………………………….. p. 4 - 6
2
The safe operating area digram ………………………………………………………………..p. 6
2.1
Safe operating area diagram limit lines ………………………………………………………p. 7 - 12
3
Choosing the right MOSFET for linear mode operation……………………………………p. 12
4
Summary …………………………………………………………………………………………..p. 12
1
Introduction
The development of modern Power-MOSFETs
MOSFETs has focused on ultra
ultra-fast
fast switching and ultra-low
ultra
ohmic devices
leading to continuously decreasing area specific on
on-resistances. Consequently, compared to Power-MOSFETs
Power
from
10 years ago today’s
oday’s state of the art Power
Power-MOSFETs utilize a much smaller silicon
on die area for any
a given RDS(on).
Thus the power handling capabilities of devices for a specific RDS(on) generally have decreased. Especially in linear
operation mode when MOSFETs handling
ing high power dissipation due to the simultaneous occurrence of voltage at
the pins and current in the MOSFET this is of importance
importance. The Safe Operating Area diagram (SOA
(SO diagram) defines
the allowed maximum current-voltage
voltage range. It must be considered during the design-in
design
phase of the
Power-MOSFETs especially in cases with
ith linear mode operation.
Many applications exist with the Power--MOSFET being operated primarily in linear
inear mode. Linear mode operation
refers to the current saturation region in the output characteristics
characteristics. The drain current (IDS) is nearly independent of
the drain to source voltage (VDS) for a given gate to source (VGS) voltage. It depends then directly on the VGSvoltage of the MOSFET. In Fig. 1 the area of linear mode operation is indicated as red shaded area.
Contrary to the region of linear mode operation is the so called ohmic region (indicated as blue shaded area in
Fig. 1). The relationship between VDS and IDS follows Ohm’s law.
As shown in Fig. 1 MOSFET operating at low VGS voltage (e.g. 2.9V) and high VDS (e.g. VDS=4V) voltage is able to
carry considerable currents (e.g. ~200A)
A). Here, the
he MOSFET operates in linear mode. The power dissipation is very
high (e.g. 800W) which bares the potential for MOSFET destruction as being discussed later. The
T SOA diagram is a
tool to check for a given parameter set of VDS, IDS and stress time whether or not the Power
ower-MOSFET operates
within specification. This is important as operation outside the specification can deteriorate the device and
decreases its reliability. In severe cases of specification violation a catastrophic failure can occur.
occur To verify that the
MOSFET stays within the SOA it is necessary to un
understand the application and the time dependent voltage and
current functions (VDS=f(t), IDS=f(t)).
Fig. 1:: Output characteristics of a Power
Power-MOSFET (OptiMOS™ 25V, BSC010NE2LS [1]).
). The blue shaded area
indicates the operation in the ohmic region
region, the red
d shaded region represents linear mode operation.
1.1
Applications driving MOSFETs in linear mode operation
Many applications exist where the MOSFET enters linear mode operation. It will be shown that even fast switching
MOSFETs operate briefly and repetitivel
repetitively in linear mode. Consequently, when the switching speed of a MOSFET is
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4
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being slowed down by adding an external gate resistor Rg the MOSFET will operate in linear mode for a longer time
and could violate by SOA limitations.
Traditional linear mode operation – battery charger, fan controller
A fan controller arrangement where the MOSFET is used as current source is shown in Fig. 2. The fan speed (or in
case of a battery charger the charging current) is controlled by the current flow through the fan. The current is
exclusively controlled by the MOSFET. This means that the MOSFET must be operated in linear mode. Changing
the VGS voltage will change the current flowing through the motor and thus the fan speed. On the right hand side of
Fig. 2 the output characteristics of the MOSFET is shown. The point where the MOSFET is being operated is shown
as red dot. According to the diagram the voltage across the MOSFET will be the input voltage minus the voltage
drop across the fan and might be as high as VDS=7.5V, for example. The current flowing through the MOSFET could
be ~15A depending on fan speed. Therefore the power dissipation in the MOSFET can be as big as 112.5W. The
choice of a thermally good performing package (low RthJC and RthJA values) and an adequate thermal management
(cooling with airflow and/or heatsink) are obviously necessary to ensure safe operation of the MOSFET. When the
MOSFET is used as a fan speed controller the linear mode operation could last forever and the current (fan speed)
is only controlled via the VGS voltage. As the MOSFET is operated exclusively in linear mode, the RDS(on) of the
MOSFET is completely irrelevant when calculating the power dissipation. The power dissipation in the MOSFET
depends only on the voltage drop across the MOSFET and the current flow: Pdiss=VDS ⋅ IDS.
fan
gate
Ids (A)
Vdd
100
90
80
70
60
50
40
30
20
10
0
Vgs=4.0 V
operation point
Vgs=3.6 V
Vgs=3.3 V
Vgs=3.1 V
Vgs=3.0 V
Vgs=2.9 V
0
5
10
15
20
25
30
Vds (V)
gnd
Fig. 2: MOSFET used as constant current source. The current flow through the fan is controlled exclusively by the
VGS voltage at the MOSFET gate.The red point in the output characteristics diagram on the right hand side indicates
a potential operation point of the MOSFET in the linear region.
Partial linear mode operation – e-fuse, load-switch
In e-fuse applications or load-switches the MOSFET can be used to maintain slow turn on avoiding high inrush
currents. For e-fuse applications an e-fuse controller senses the current and controls the current flow by varying the
MOSFET VGS voltage. In doing so the MOSFET briefly operates in linear mode. The VGS voltage is being increased
only slowly until the full current flows and finally the MOSFET is being operated in the ohmic region. The diagram on
the right part of Fig. 3 illustrates the three operation states of the MOSFET in the output characterstics diagram.
At first the MOSFET is off and the entire voltage drops across the MOSFET. Then the VGS voltage is being
increased continiously and drain current starts to flow. At this point the MOSFET operates in linear mode which is
nd
shown as 2 operational state in the diagram. Here the SOA becomes relevant. Finally the MOSFET is fully
enhanced (turned on) and it is being operated in the ohmic region. The same three operational states occur when
the MOSFET is used as a load switch. Critical for the MOSFET is the time in linear mode which depends on the
load-switch controller (or the e-fuse controller) timing. Typical timings are in the range of µ-seconds and may even
Application Note
5
3/24/2011
reach milli-seconds. It is vital to understand this timing in detail so that time dependent voltage and current stress on
the MOSFET can be derived to check for potential violation of the SOA.
12V bus
gate
Ids (A)
Ids(A)
e-fuse
controller
current
sense
gnd
CPU
100
90
80
70
60
50
40
30
20
10
0
Vgs=4.0 V
on
Vgs=3.6 V
linear mode
Vgs=3.3 V
Vgs=3.1 V
Vgs=3.0 V
Vgs=2.9 V
off
0
5
10
15
20
25
30
Vds (V)
Vds(V)
gnd
Fig. 3. MOSFET used in e-fuse application (left). In e-fuse application (during turn-on) the MOSFET passes the
linear mode operation region for a significant period of time. The SOA must be checked to verify the MOSFET can
withstand the thermal stress.
Short linear mode operation – buck converter, general switching
When the MOSFET is used as high-side switch in a buck converter for example it also enters linear mode operation
for a very short period of time. Linear mode operation starts exactly when the MOSFET’s VGS voltage is at the
threshold voltage (VGS(th)) and ends with the drain-source voltage reaching zero. This corresponds to the end of the
Miller plateau. In other words: during the switching event the MOSFET is in linear mode operation. Modern power
MOSFETS, however, demonstrate dramatically short switching times. State of the art 25V-250V MOSFETs are
capable of switching thourgh the linear region within a few nano-seconds (<10ns), depending on the specific
product. The period of linear mode operation is usually as short as a few ns for MOSFETs utilized as high-side
switches in high-performance buck converters. The example also demonstrates that any reduction of switching
speeds by introducing external gate resistance (Rg) or utilizing a slow driving concept (e.g. MOSFET gate is
charged from a constant current source with low current rating) the SOA diagram could become relevant. Therefore,
whenever soft-turn on or reduction of ringing by slowing down the switching speed is implemented the SOA diagram
should be considered.
2.0
The Safe Operation Area diagram
In section 1 the SOA diagram has been introduced as crucial diagram for designs with MOSFETs in linear mode
operation. Now the SOA will be explained in detail. The origin of the limit-lines in the SOA diagram will be
discussed. Furthermore, it will be shown that one must pay attention to the conditions the SOA diagram is given in
the datasheet versus the application conditions.
Application Note
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3/24/2011
2.1
SOA limit-lines
In Fig. 4 the SOA diagram of an Infineon power MOSFET (BSC010NE2LS) is shown. The five limit-lines defining
the SOA diagram are the RDS(on) limit-line
line (blue line), the current limit-line (red line), the maximum power limit-line
limit
(dark green line), the thermal instability limit
limit-line
line (light green line) and the breakdown voltage limit-line
limit
(yellow line).
Within these limit-lines
lines the green shaded area gives the area where the MOSFET can be safely operated. In this
example the SOA diagram is calculated for a constant case temperature of Tc=25°C and a single pulse with
duration 100µs. The datasheet provides additional limit
limit-lines
lines for various pulse lengths up to continuous operation
(DC operation). To calculate limit-lines for other case temperatures and/or continuous pulsed operation the origins
of the limit-lines must be understood. The
he following paragraphs give insight to the derivation of the SOA diagram
limit-lines.
Fig. 4 Safe operating area diagram of a 25V Power MOSFET (BSC010NE2LS). The SOA is shown for 100µs pulse
time and the limiting lines are indicated.
RDS(on)-limit-line (blue line)
As seen in Fig. 4 the RDS(on) limit-line
line gives a linear dependency between VDS and IDS. The slope of the line is simply
the maximum RDS(on) of the MOSFET at Tj=150°C and VGS=10V. Therefore the RDS(on)-limit-line
line is given as
@ , °
eq.1
This means
• For lower VGS voltages the RDS(on)-limit-line will be lower since the RDS(on) value increases with lower VGS
voltages. The dependency of RDS(on) vs VGS voltage can be found in the MOSFET datasheet.
• In case Tj is known to be lower than 150°C the RDS(on)-limit-line
line will move up again since the RDS(on) value
decreases with lowering Tj.
Application Note
7
3/24/2011
Therefore the RDS(on) limit-line can easily be recalculated using equation 1 and datasheet parameters for RDS(on) and
the relevant Tj.
Package-limit-line (red line)
When following the RDS(on) limit-line towards higher currents and voltages the current-limit-line is found. This
limit-line is typically defined by the maximum current the package can handle. Therefore the amount of this current
will be strongly package dependent. Packages with bond wires (DPAK) will show different maximum current
handling capabilities compared to packages with clip bonding technology (SuperSO8). Also the die size will impact
the current handling capability of the package since it determines the bonding scheme (number of bond wires, bond
wire diameter, clip dimension). The package limit-line will not change in case application conditions such as ambient
temperature vary.
Maximum-power-limit-line
After the package limit-line the maximum power limit-line follows. It is calculated from the maximum power the
system is allowed to generate to reach a stable junction temperature Tj of 150°C in thermal equilibrium and
Tc=25°C. It is obvious that the cooling concept of the system and therefore thermal variables like case temperature
(Tc) and thermal impedance (ZthJC) will strongly impact this limit-line. The limit-line can be calculated by assuming
thermal equilibrium:
!"
#"$"%
!"
The equation for Tj in a simple thermal model:
&' &( + *!+, ∙ !"
the IDS as function of VDS is given by
∆/01
23456 !789:; ∙
eq. 2
This equation illustrates some important dependencies:
•
The IDS depends on the maximum allowable temperature rise for Tj. The maximum allowable temperature
rise ∆Tmax will depend on Tc and the maximum Tj the device is permitted to sustain.
•
The IDS value is influenced by the value of the thermal impedance ZthJC. For short pulses the value of ZthJC
depends on the pulse length and its duty cycle. ZthJC can be taken from the corresponding diagram in the
datasheet. The SOA diagram visualizes that an increase of pulse length shifts the maximum thermal limitline downwards. It reflects the higher thermal impedance at longer pulse times and/or higher duty cycle.
•
Due to the increased power dissipation with increasing VDS the IDS also depends with indirect proportionality
on the applied VDS voltage.
Application Note
8
3/24/2011
The SOA diagram shown in Fig. 4 has been calculated for the BSC010NE2LS MOSFET:
•
tpulse=100µs ZthJC=0.102 K/W from datasheet
•
Tc=25°C and Tjmax=150°C ∆Tmax=125°C
<
.<∙
eq.3
For an application where Tc is greater than 25°C and/or the maximum allowed Tj is less than 150°C the maximum
power limit-line will naturally be lower and can be calculated with the help of equation 3. The SOA diagram in the
datasheet is given at dutycycle D=0 for various pulse lengths. Exposing the MOSFET to repetitive pulses results in
D≠0. In that case the thermal impedance diagram ZthJC=f(tp, D) has to be used to get the correct ZthJC value.
Then, the maximum power limit-line can be calculated by equation 2 according to the needs of the engineer.
Thermal (in-)stability limit-line
Following the maximum-power limit-line reveals a point at which the slope of the limit-line changes. This point
indicates the on-set of the thermal-instability limit-line. To understand the origin of this limitation it is necessary to
consider the criterium for thermal instability. A MOSFET (or generally a system) is considered to be thermally unstable in case the power generation (Pgenerated) rises faster than the power dissipation (Pdissipated) over temperature.
That means
>#"$"% !" > !"
>
>&
>&
In such a condition the temperature of the system is not stable and the system is not in thermal equilibrium unlike
for the maximum-power limit-line. With the help of
#"$"%
!"
@ ∙ and
!"
&' − & BC
*!+,
the inequality can be rearranged to
@ ∙
>
1
>
>&
*!+, EFG" For further elaboration here it will be assumed that VDS is being constant over temperature.
The equation above defines the range where the MOSFET may encounter thermal instability. The term
Temperature Coefficient. Since VDS>0 and
23456 !FG"
HI
H
is called
>0 thermal instability can occur only if the temperature
coefficient of the drain to source current is positive. The question under what conditions such a positive temperature
coefficient can be found is answered by the transfer-funtion graph in the MOSFET datasheet showing IDS over VGS
for various temperatures. An example for such a graph is given in Fig. 5. Comparing the currents at VGS=2V for
Tj=25°C and Tj=150°C indicate that in case of VGS=2V the current will rise over temperature. This means the
temperature coefficient for VGS=2V is positive. At VGS=3.5V the current will decrease over temperature and the
corresponding temperature coefficient is negative. In between is a crossover for the transconductance curves of
Application Note
9
3/24/2011
Tj=25°C and Tj=150°C. This crossover
over point is being referred to as Zero Temperature Coefficient (ZTC) point.
Clearly, thermal instability can occur only for VGS below the VGS of the ZTC point.
Fig. 5 Transfer characteristic of a Power--MOSFET for Tj=25°C (blue line) and Tj=150°C (red line).
The change of temperature coefficient from positive to negative over VGS is caused by two
wo competing effects. On
one hand the resistance of a MOSFET increases over temperature due to lower electron mobility in the crystal
lattice. On the other hand the threshold voltage of the MOSFET decreases over temperature since more electrons
have been excited into the conduction band of the MOSFET. At low temperatures the effect of decreasing threshold
voltage over increasing temperature dominates and currents increase over temperature. At higher temperatures the
increase of RDS(on) over temperature dominates and currents decrease over temperature.
As shown above thermal instability will occur for VGS being below the ZTC point. Thus, MOSFETs with a ZTC at
high currents and high VGS voltages will be more prone to thermal instability. The ZTC point is in direct relationship
to the MOSFET transconductance. With increasing transconductance the ZTC point will move towards higher VGS.
Modern power MOSFETs exhibit ever increasing transconductances and therefore also ZTC-points
ZTC
at higher VGS .
When comparing comparing the transconductance cu
curves
ves in datasheets for 25V and 150V MOSFETs (eg 30V
OptiMOS™ vs 150V OptiMOS™ 3) one
ne will realize that for the 150V technology the ZTC point is located at lower
currents and VGS voltages. This is because for higher voltage class MOSFETs (e.g. 150V) the increase of RDS(on)
over temperature will dominate the transconductance behavior over temperature as they have thicker EPI layers,
lower cell pitch and generally a lower doping level compared to lower voltage class MOSFETs (25V).
(
Having discussed the origins of thermal instability the destruction mechanism in case of thermal instability can be
explained as follows: Generally, it is assumed that the temperature distribution over the MOSFET
MOSFE is uniform so that
Tj is the same over the entire chip. In reality this is not the case
case. Small
mall inhomoginities of, for example, solder
thickness between die and package exist. They will lead to minimal higher local temperatures. This local
temperature differences will develop in
n dependence on the operation point of the MOSFET with respect to the ZTC
point:
•
For operation above ZTC the temperature coefficient is negative. Hotter spots will draw less current and the
spot cools down. The
he chip stabilizes itself and initial temperatur
temperature variations become irrelevant.
For operation below ZTC the temperature coefficient is positive. In this case the local hot spots will draw
more current as they heat up. This will lead to increased local power dissipation and further heating.
heating
Ultimately this
is results in thermal runaway and local destruction of the chip.
Finally, the question of how to adjust the thermal instability line for application conditions different
differ
from the
datasheet must be answered. The on-set
set of the thermal instability line is giv
given by
•
Application Note
10
3/24/2011
@ ∙ *!+, EFG" ∙
>
1
>&
This means that the temperature coefficient must be known. The latter cannot be easily calculated.
calculated The most
straight forward approach to de-rate
rate the thermal-instabiliy
instabiliy limit line is to displace the line vertically according to the
following procedure:
First, the RDS(on) limit-line, package limit
limit-line and maximum power consumption limit-line can be calculated and
plotted in a SOA diagram for any given Tc, maximum Tj and pulse lengths as explained above.
above The formula for the
thermal-instability limit line in the form:
UVW
X ∙ log
@ ) Y
can be derived by utilizing two points (point A, point B) on the limit line as indicated in Fig.6. In Fig. 6: point A is at:
IDS=105A and VDS=8V; point B: IDS=12A and VDS=25V. The two equations
log 105 X ∙ log
8 ) Y
log 12 X ∙ log
25 ) Y
give the parameters α and β of the thermal
thermal-instability-limit-line (here: α A1.904, β 3.740..
Now the formula for the de-rated
rated limit line can be found by simply determining a new β as the slope α does not
change upon de-rating.. This is done by using point C on the derated power limit-line
line at the VDS of the connection
point of power limit-line and thermal
mal instability limit
limit-line as given in the datasheet. In Fig. 6: point C: IDS=65A and
VDS=8V.
Fig. 6 Method for derating
erating of the thermal instability limit-line.
Therefore
βLMNOPML log
65 ) 0.585 ∙ log
8 3.532
This finally provides the de-rated thermal
thermal-instability limit line for Fig. 6:
log
I α ∙ log
log
Vds ) βLMNOPML A1.904 ) log
@fg ) 3.532
Breakdown voltage limit-line
On the right hand-side
side the limit of the SOA is given by the MOSFETs maximum applicable
icable VDS voltage. This is
generally the breakdown voltage Vbr(DSS) of the device and will depend on the Tj. The relationship between Vbr(DSS)
Application Note
11
3/24/2011
and Tj is given in a separate graph in the MOSFET datasheet. It can be seen that the BVDSS voltage might be
significantly reduced at low temperatures. Therefore care must be taken in case the application is located in an
environment of low ambient temperatures. For example, this could be the case for Telecom Base station
equipment.
Violating the maximum applicable VDS voltage will lead to MOSFET junction breakdown and without any current or
power limiting scheme the MOSFET will face thermal destruction.
So far the various limit-lines in the SOA diagram have been discussed. The datasheet typically provides the SOA
diagram for Tc=25°C, Tjmax=150°C and various single pulse lengths. Now recalculation of the SOA diagram is
possible to adapt to application conditions different from datasheet conditions.
3.0
Choosing the right MOSFET for linear mode operation
As discussed in section 1 linear mode operation will operate the MOSFET at high VDS and IDS simultaneously.
Consequently, power dissipation will be high. The SOA diagram is a tool to evaluate the suitability of the MOSFET
operation in linear mode. In section 2 the various limit-lines of the SOA have been discussed and formulas have
been presented that allow for recalculation of the SOA diagram for various application conditions. These equations
also enable the deduction of some general rules for selecting the best suited MOSFET for linear mode operation.
At first it is important to understand whether the MOSFET will be operated continuously in linear mode (e.g. current
source) or the MOSFET passes the linear mode operation region for a period of time before it is fully turned on.
•
Continuous operation in linear mode
In case of continuous operation in linear mode the MOSFET’s RDS(on) and dynamic parameters are
irrelevant to judge power dissipation but the SOA is the key design criterium to the power engineer.
Thermal design is most important and therefore MOSFETS with low ZthJC are most suitable. To remove the
generated heat an appropriate package and cooling concept must be applied. Thermal instability can be
avoided by utilizing MOSFETs with low ZTC point. This means that MOSFETs of previous technology
generations and/or higher voltage classes will be more suitable for this kind of application.
•
Limited operation time in linear mode
The MOSFET will pass the linear mode operation region for a given period of time. The applied VDS voltage,
the IDS current and the pulse length must be known to understand if the MOSFET can handle the generated
power. It has to be verified that the MOSFET is being operated within the SOA. If the application conditions
differ from the SOA datasheet conditions (Tc temperature, VGS voltage, duty cycle …) the SOA diagram
needs to be recalculated according to the formulas given in section 2. Generally, a MOSFET with low ZthJC
and very low RDS(on) will be a suitable product in cases where linear mode operation ruggedness is needed.
4.0
Summary
The application note has shown that the safe operation area (SOA) of MOSFETs is an important design criterion.
Especially, when the MOSFET is in linear mode operation the SOA diagram has to be considered. This can be
either in applications where the MOSFET is being operated continuously in linear mode (e.g. MOSFET as current
source) or in applications where the MOSFET passes through linear mode operation for a certain time (e.g. e-fuse).
The SOA diagram in datasheets is only valid for the given conditions. For different application conditions a derating
to the SOA has to be performed as discussed here.
References
[1] BSC010NE2LS OptiMOS™ PowerMOSFET data sheet; www.infineon.com
Application Note
12
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