Power MOSFET
Maximum Ratings
Power MOSFET in Detail
3.
Maximum Ratings
3.1
Definition
The maximum current, voltage, and allowable power dissipation are specified as maximum ratings
for power MOSFETs.
When designing a circuit, it is very important to understand the maximum ratings to ensure the
most effective operation and reliability of the power MOSFETs for the required period of time.
The maximum ratings are the values which must not be exceeded to ensure the power MOSFET’s
life and reliability. Note that the maximum ratings mean the absolute maximum ratings.
The absolute maximum ratings are the values which must never be exceeded during operation even
for a moment.
If the maximum ratings are exceeded, some characteristics may be deteriorated in an unrecoverable
manner. Therefore, pay much attention to fluctuations of supply voltage, characteristics of electrical
components, ambient temperature, and input voltage, as well as maximum rating violation during
circuit adjustment so that no maximum rating value is exceeded.
Parameters that must be specified for the maximum ratings include current for power MOSFET’s
drain, voltage between each pair of pins, power dissipation, channel temperature, and storage
temperature. These parameters cannot be considered individually since they are closely related to
each other, and they are subject to change due to the circuit’s environmental conditions.
3.2
Voltage Ratings
3.2.1
Maximum Drain-Source Voltage Rating
There are four methods of specifying the drain-source breakdown voltage of a power MOSFET in
accordance with gate-source bias conditions.
Figure 3.1 shows the various drain-source breakdown voltage modes.
(1) VDSS: the drain-source voltage with zero gate-source bias (the last “S” means short).
(2) VDSX: the drain-source voltage with reverse bias (for example, when a voltage of VGS = −3 V is
applied with an N-channel MOSFET).
(3) VDSR: the drain-source voltage with resistance shunted between the gate and source (the “R”
means shunt resistance).
1
Power MOSFET in Detail
When a Power MOSFET, except a trench type MOSFET, is the enhanced type, the relationship
VDSS ∼
− VDSX is established in the above three breakdown voltage modes. In addition, there is
almost no difference in breakdown voltage from the VDSX mode, even when the drain-source
breakdown voltage (VDSR mode) is measured with a large resistance inserted between the gate
and source, and the relationship VDSS ∼
− VDSX ∼
− VDSR is established.
For a trench type MOSFET, the relationship VDSS >
= VDSX is established because breakdown
voltage at VDSX mode fall depending on applied voltage to gate-source.
Consequently, there is no method for regulating a drain-source breakdown voltage larger than
the VDSS value, which is therefore used as the maximum rating. Thus, caution must be exercised
to avoid exceeding the VDSS value, even instantaneously.
(4) VDSO: drain-source voltage with the gate open
The input impedance of a power MOSFET is extremely high; therefore, bias is applied between
the gate and source by static electricity inductance, etc., and the device is destructed because of
malfunction. Avoid use in this mode.
(a) VDSS mode
(b) VDSX mode
(c)
VDSR mode
(d) VDSO mode
The device can
sometimes fail
D
G
D
D
G
G
S
S
R
−VGS
D
G
S
S
Figure 3.1 Drain-Source Breakdown Voltage
3.2.2
Maximum Gate-Source Voltage Rating
VGSS: the gate-source voltage when the drain and source are shorted, which is determined by the
degree of withstand of the gate oxide film. The maximum rating for a MOSFET should be
based on the side of the maximum sustainable practical voltage or, in consideration of
reliability.
3.3
Current Ratings
For power MOSFETs, the DC current that flows in the forward direction is referred to as ID and the
pulse current that flows is referred to as IDP. The values of the reverse-direction (diode direction) DC
current (IDR) and pulse current (IDRP) are defined as the same as the corresponding currents flowing
in the forward direction under the ideal heat radiation condition.
The forward-direction DC and pulse currents are subject to the effects of the power loss caused by
drain-source ON-resistance. The reverse-direction currents are subject to the effects of power
dissipation caused by the diode’s forward voltage. Hence, current ratings are determined by the heat
radiation conditions; the channel temperature maximum rating value (Tch) must not exceed 150°C.
2
Power MOSFET in Detail
3.4
Temperature Ratings
Maximum channel temperature Tch is defined
according to the constituent material and
reliability requirements. It must be considered
not only in terms of device operation, but also in
0.6
conjunction with such factors as allowable
0.4
Degradation of power MOSFETs generally
accelerates as the channel temperature
increases. The following relationship is known
to exist between the mean service life Lm
(hours) and channel temperature Tch (K)
ℓog Lm ∼
− A+
Defect rate (%/1000 h)
0.3
degradation and minimum service life.
0.2
0.15
0.1
0.08
0.06
Silicon
PNP
0.04
0.03
B
........................................ (1)
Tj
0.02
Silicon NPN
0.015
0.01
0.1
Where A and B are constants inherent to
0.2
0.3
0.4 0.5 0.6
0.8 1.0
Junction temperature Ta normalized by
reference temperature To
power MOSFETs
For a power MOSFET required to have a
long-term guaranteed service life, the upper
limit of the allowable channel temperature is
Ta =
Tj − To
Tjmax − To
defined according to the power MOSFET defect
Figure 3.2 Relationship between Lifespan
rate and reliability. Generally, the channel
and Junction Temperature of
temperature is below 150°C.
Storage temperature Tstg is the temperature
Bipolar Transistor
(source: MIL-HDBK-217A)
range within which non-operating power
MOSFETs can be safely stored. This rating is
also defined by the power MOSFET’s constituent material and reliability. Figure 3.2 shows a typical
relationship between the lifespan and junction temperature of a bipolar transistor.
3.5
Power Ratings
The power dissipation in a power MOSFET is converted into thermal energy which in turn causes
the power MOSFET internal temperature to rise.
Internal power dissipation of a power MOSFET operating at a certain point is represented by the
equation PD = (ID･VDS).
The primary parameters limiting maximum power dissipation PDmax in a power MOSFET are
maximum channel temperature Tchmax, described above, and reference temperature To (ambient
temperature Ta or case temperature Tc). These parameters are known to be correlated by thermal
resistance θ (or Rth).
T
− To
.......................................................................(2)
PDmax = chmax
θ
3
Power MOSFET in Detail
Thermal resistance represents the ratio at which the channel temperature increases per unit
amount of power dissipation. That is, it is a physical quantity that indicates a difficulty in radiating
heat. To allow for large power dissipation, therefore, it is necessary to choose a power MOSFET with a
large PDmax. Knowing how to design for heat radiation in power MOSFETs is especially important.
The rated value of PDmax is normally defined with respect to Ta = 25°C, or with respect to Tc = 25°C
when the use of a heat sink is anticipated. In either case, Equation (2) can be used to determine
thermal resistance between a power MOSFET channel and the external air or between the channel
and the case.
3.6
Safe Operating Area
3.6.1
Forward Bias Safe Operating Area (SOA)
Because current concentration does not readily occur in power MOSFETs due to their structure, in
principle, unlike in bipolar transistors, secondary breakdown does not occur in the high-voltage area.
Thus, for power MOSFETs the safe operating area (SOA) can be expressed by a constant-power line
which is limited by thermal resistance with pulse width as a parameter (Figure 3.3). The device can
be safely operated over a very wide range within the breakdown voltage between the drain and source
without narrowing the high-voltage area in the SOA.
Along with the higher precision of cells developed, some power MOSFETs exhibit a phenomenon
similar to secondary breakdown.
The safe operating areas of these power MOSFETS are defined individually. Figure 3.4 shows the
safe operating areas which are different from the typical safe opeating areas limited by the
constant-power lines as shown in Figure 3.3.
Safe Operating Area
Safe Operating Area
100
50
30
100
ID max (pulsed)*
ID max (pulsed)*
100 μs*
ID max (continuous)
1 ms*
10
10
Drain current ID (A)
Drain current ID (A)
1 ms*
10 ms*
5
DC operation
(Tc = 25°C)
3
1
0.5
*: Single pulse Tc = 25°C
0.3
Curves must be derated
linearly with increase in
temperature.
0.1
0.1
0.3
1
10 ms*
1
0.1
Curves must be derated
linearly with increase in
temperature.
VDSS max
3
Drain-source voltage
10
30
*: Single pulse Ta = 25°C
0.01
0.01
100
VDS (V)
0.1
VDSS max
1
Drain-source voltage
Figure 3.3 Power MOSFET Safe Operating Area
Figure 3.4
(2SK2782)
100
VDS (V)
Power MOSFET Safe Operating Area
(TPC8009-H)
4
10
Power MOSFET in Detail
Reverse Bias Safe Operating Area
When a switching device is used in the
(a) Reverse safe operating area
field of power switching for applications
such as switching power supplies, the
VCEX (sus) mode
L (coil) = 200 μH
IB1 = 2 A
IB2 = variable
< 1%
Duty =
load becomes inductive. In this case,
both the forward and reverse safe
operation ranges become a problem.
6
Normally, when bipolar transistors
5
(A)
are used in switching power supplies,
forced reverse bias is applied between
IB2 = −1 A
Collector current IC
the base and emitter to reduce switching
loss, base reverse current IB2 is applied,
and tstg and tf are shortened. However,
if IB2 is increased, the reverse bias safe
−2 A
4
−3 A
The reverse safe
operating area of
power MOSFETs
3 does not become
narrower with −VGS
as is the case with
bipolar transistors.
2
operating area becomes narrow, as
shown in Figure 3.5, and the load curve
operation range is restricted during turn
Power MOSFET reverse
safe operating area (for
the 2SK2610)
Measurement conditions:
L = 200 μH
VGS = 15 V
1
OFF.
With power MOSFETs, on the other
0
0
200
hand, tf and toff can be shortened by
400
600
VCEX (sus)
800
(V)
applying reverse bias between the gate
and source. However, since power
Figure 3.5 Reverse Safe Operating Area
MOSFETs are majority carrier devices
and there is essentially no carrier storage effect, the reverse bias SOA does not become narrow even if
gate reverse voltage −VGS increases. (However, breakdown voltage for trench type power MOSFET
falls depending on an applied VG; hence, reverse bias SOA becomes narrow.)
L
D.U.T.
ID
VDD
VDS
P.G.
ZDi Di
Drain current ID (A)
3.6.2
VDSS = VZ
Drain-source voltage
Figure 3.6 Measurement Circuit for Reverse Safe Operating Area
5
VDS
Power MOSFET in Detail
3.7
Derating
When designing power MOSFET circuits, you determine the appropriate heat radiation condition
from the absolute maximum ratings (maximum ratings) listed in the technical datasheets to ensure
that the parameters-voltage, current and power (channel temperature)-are each within the maximum
rating. However, it is a common practice to derate these maximum values in consideration of
reliability requirements prior to using them in a circuit design.
To balance maximum ratings against reliability and economy, the following derating methods are
generally recommended:
●
Voltage: The worst-case voltage (including surge) must be no greater than 80% of the maximum
rated voltage.
●
Current: The worst-case current (including surge) must be no greater than 80% of the maximum
rated current.
●
Power: The worst-case power (including surge) must be no greater than 50% of the derated
maximum power dissipation at the maximum ambient temperature of the equipment in
which the device is used.
●
Temperature: The maximum operating channel temperature Tch (including surge) must be 70%
to no greater of the 80% of the Tchmax.
The power dissipation of power MOSFETs used in switching circuits must be such that the peak
values (including surges) of voltage, current, power and channel temperature do not exceed the
absolute maximum ratings (maximum rating). However, when using these power MOSFETs under
derated conditions, with respect to reliability, power dissipation can be considered in terms of average
values. Safe operating areas before and after derating are expressed as the formulas shown in Figure
3.7.
6
Power MOSFET in Detail
100
Drain current ID (A)
−1
y = cx
10
−1
y = cdTx
dT
−b
y = ax
−b
y = adTx
1
bd
T
0.1
0.1
1
10
Drain-source voltage
100
VDS (V)
Figure 3.7 Temperature Derating in Safe Operating Range
The derating ratio dT of the constant-power line is expressed by using the following definition
equation:
− Ta
T
.....................................................................................(3)
PD = chmax
rth (ch − a )
as follows:
dT =
PD(Tch °C )
T
− Tch
...............................................................(4)
= chmax
PD(25°C )
Tchmax − 25
When Tch = 25°C, the constant-power line is:
y = cx −1 .....................................................................................................(5)
The constant-power line after derating is:
y = cd T x −1 ................................................................................................(6)
When the line limited by the phenomenon similar to secondary breakdown at Tch = 25°C is:
y = ax − b ....................................................................................................(7)
The derating ratio and the equation after derating are expressed as follows:
d PS/B = b d T .............................................................................................(8)
y = ad T x − b ................................................................................................(9)
7
Power MOSFET in Detail
<Example of derating (TPC8108)>
Using TPC8108 as an example,
−100
derate the safe operating area
ID max (pulse)
from Ta (= Tch) = 25°C to Ta =
1 ms
Drain current ID (A)
125°C.
Read the coordinates of the
points marked with a circle (o) in
the safe operating area (in the
individual technical data) at Ta =
−10
10 ms
−1
25°C. Substitute the read
coordinates for the values in
VDSS max
−0.1
−0.1
equations (3) and (5). The
−1
−10
Drain-source voltage
constant-power lines are
calculated as follows:
−100
VDS (V)
Figure 3.8 TPC8108 Safe Operating Area (Ta = 25°C)
y = 470x −1
(Ta = 25°C, t = 1 ms)............ (8)
y = 76.2x −1
(Ta = 25°C, t = 10 ms).......... (9)
Lines limited by the phenomenon similar to secondary breakdown are calculated as follows:
y = 1500x −1.73
(Ta = 25°C, t = 1 ms) .................. (10)
y = 282x
(Ta = 25°C, t = 10 ms) ................ (11)
−1.73
When derating from Ta = 25°C to Ta = 125°C, the derating ratio dT of the constant-power line is
dT = 0.2.
Constant-power lines at Ta = 125°C are calculated as follows:
y = 407x −1 = 81.4 x −1
(Ta = 125°C, t = 1 ms) ................ (8)
y = 76.2x
(Ta = 125°C, t = 10 ms) .............. (9)
−1
= 15.2x
−1
Lines limited by the phenomenon similar to secondary breakdown are calculated as follows:
y = 1500d x −1.73 = 300x −1.73
(Ta = 125°C, t = 1 ms) ................ (10)
y = 282d x
(Ta = 125°C, t = 10 ms) .............. (11)
−1.73
= 56.4 x
−1.73
Figure 3.9 shows the safe
operating area plotted using
−100
equations from (8) to (10).
Drain current ID (A)
ID max (pulse)
−10
1 ms
10 ms
−1
−0.1
−0.1
VDSS max
−1
Drain-source voltage
−10
−100
VDS (V)
Figure 3.9 TPC8108 Safe Operating Area (Ta = 125°C)
8
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