Device Application Note AN949

VISHAY SILICONIX
Power MOSFETs
Application Note AN-949
Current Rating of Power Semiconductors
TABLE OF CONTENTS
Page
What Is Current Rating?................................................................................................................................................................ 2
Current Ratings for Power Semiconductors ................................................................................................................................. 2
Continuous Current Rating ........................................................................................................................................................... 2
Switching “Duty Cycle” Ratings ................................................................................................................................................... 3
Junction Temperature under Pulsed Conditions .......................................................................................................................... 3
Peak Current Ratings .................................................................................................................................................................... 4
Appendix ....................................................................................................................................................................................... 5
Effect of Waveform Shape on RMS Value .................................................................................................................................... 5
The current rating of an electrical device, be that a circuit breaker or a motor or a transformer, is the current at which the
temperature within the electrical device reaches a value that may impair the reliability or functionality of the device itself. The
manufacturer knows the temperature limits of the materials used in the device, but he does not know the temperature of the
ambient in which the device will be used. So he makes an assumption on this temperature.
For technical questions, contact: [email protected]
www.vishay.com
1
APPLICATION NOTE
Document Number: 91419
Revision: 02-Jul-10
Application Note AN-949
Vishay Siliconix
Current Rating of Power Semiconductors
WHAT IS CURRENT RATING?
The current rating of an electrical device, be that a circuit
breaker or a motor or a transformer, is the current at which
the temperature within the electrical device reaches a value
that may impair the reliability or functionality of the device
itself. The manufacturer knows the temperature limits of the
materials used in the device, but he does not know the
temperature of the ambient in which the device will be used.
So he makes an assumption on this temperature. This has
two important consequences:
1. A current rating is meaningless without the rated
temperature
2. The temperature at which the rating applies may, or may
not be related to actual operating conditions. If it is, the
current rating can be used as an indication of the current
capability of that device in a real applications. If the
device is rated at a temperature that is not encountered
in a typical operating environment, e.g. 25 ºC, it cannot
be trusted to provide an indication of actual device
capability in an application. It can only be used to
compare the ratings of similar devices rated at the same
temperature.
The rating of electrical devices like motors and circuit
breakers are dictated by various agreements and
regulations. The ratings of many other devices, like
transformers, resistors and semiconductors are specified in
their data sheets. As a result, the user must do, at a
minimum, a verification that the device is capable of
operating:
a. at the maximum current
b. at the maximum ambient temperature
c. without exceeding its maximum temperature
APPLICATION NOTE
CURRENT RATINGS FOR POWER
SEMICONDUCTORS
Like any other electrical device, power semiconductors
must be operated within their maximum temperature. Since
the vast majority of power semiconductors operate at large
power densities, they need to be heatsunk. It is the task of
the designer to identify the heatsink, or other cooling
method, that fulfills the requirements a, b, and c of the
previous section. This task is normally referred to as
“thermal design.
”Power semiconductors have, however, some additional
limitations normally associated with their capability of
handling high voltages and high currents at the same time,
under static or dynamic conditions. These limitations are
peculiar to the specific type of semiconductor, e.g. SOA for
transistors, dV/dt, dI/dt and tq for thyristors, trr for diodes.
Information on these limitations are normally contained in
publications that are specific to the particular device.
Bipolar transistors have one additional limitation that is not
www.vishay.com
2
common to other power semiconductors: Gain. To operate
a bipolar transistor at its headlined “rated” continuous
current would require an inconveniently large amount of
drive current, and the saturation voltage and switching times
would be hard to live with in a practical design.
Other power semiconductors are not limited by gain. IGBT
for motor drive applications are, by design, limited in gain to
current levels much beyond normal operating conditions to
reduce the current under short-circuit conditions.
CONTINUOUS CURRENT RATINGS
The continuous rating of a power semiconductor is based is
heat removal when conducting a fixed amount of current.
This is determined by the fundamental equation for
temperature rise (see INT-936), with no switching losses
present. Rated ID, for a MOSFET is therefore:
ID =
T J (max.) - T C
---------------------------------------------R DS  on  x R th  JC 
where RDS(on) is the limiting value of the on-resistance at
rated TJ(max.), at the appropriate value of ID. Rth(JC) is the
maximum value of internal junction-to-case thermal
resistance, and TC is the case temperature.
Similarly, the continuous current rating of a diode, or a
thyristor, or an IGBT is calculated from the basic equation of
temperature rise. The power dissipation is calculated from
voltage drop and continuous current.
Except for water-cooled sinks, it is very difficult to keep the
case temperature of a power semiconductor at less than
90 °C. Thus, the usable continuous direct current of a power
device for most practical is whatever is applicable to a case
temperature of 90 °C to 110 °C. This allows a sufficient
differential between case and ambient temperature for the
heat dissipator to handle the heat transfer.
The “headlined” continuous current rating shown on the
data sheets of most power transistors is usually larger than
the above practically usable level of continuous drain
current. This is because the case temperature adopted by
the industry, to which the “headlined” continuous ID rating
applies, is 25 °C.
Figure 1 shows typical heatsinks for TO-247 and TO-220
packaged power MOSFETs that allow them to operate in a
40 °C ambient at a continuous direct drain current that is 60
% to 70 % of the rated continuous drain current at TC = 25
°C; the corresponding steady case temperature is about 100
°C. The continuous current rating of power transistors is,
however, of little direct use to the designer, other than as a
benchmark, for the following three reasons:
1. Power transistors are normally operated in switchmode,
with duty cycles considerably less than 100 %, and what
is really of interest is the current-carrying capability of the
device under the actual “switched” operating conditions.
For technical questions, contact: [email protected]
Document Number: 91419
Revision: 02-Jul-10
Application Note AN-949
Vishay Siliconix
Current Rating of Power Semiconductors
2. When operated in switchmode, power transistors have
switching losses, that have to be calculated and added to
the conduction losses, as indicated in INT-936.
3. The selection of the power device may be dictated by
surge requirements that make the continuous current
rating irrelevant.
And, if this were not enough, advances in the low-voltage
MOSFET technology have reduced conduction losses to the
point that the package has become the limiting factor in their
continuous current rating. This is explained in INTDT93-4.
SWITCHING “DUTY CYCLE” RATINGS
The basic thermal equation of INT-936 determines the basic
rating of a “thermal system” in a practical application. This
equation can be used to determine how much power can be
dissipated by a (known) thermal system or the junction
temperature for a given set of electrical operating conditions
(power dissipation). Since the power entered in that
equation is the “average” power, it remains valid as long as
the frequency of operation is high with respect to the
thermal inertia of the system. When the frequency is very
low (tens of Hz), the transient thermal response curve is
used, as described below in Section 5.
The power dissipation is normally divided in conduction and
switching. Conduction losses in a power MOSFET, being
resistive in nature, can be calculated as (IRMS)2 x R. The RMS
content of waveforms of different shape can be found in the
Appendix. Switching losses can be calculated from the
switching waveforms, from the gate charge or from
analytical methods. Conduction and switching losses for
IGBTs are more complex, as explained in INT-990.
JUNCTION TEMPERATURE UNDER PULSED
CONDITIONS
Document Number: 91419
Revision: 02-Jul-10
(a) Type 621-A heatsink give 4 A continuous rating for 5 CFM airflow
in 40 °C ambient.
For technical questions, contact: [email protected]
www.vishay.com
3
APPLICATION NOTE
Under surge conditions the junction temperature rises
exponentially, according to its thermal inertia. Rather than
using the thermal resistance, that is appropriate for steady
state operation, we use the Transient Thermal Impedance
(or, more correctly, Thermal Response Curve), as the one
shown in figure 2. For a surge of given duration (x axis), this
curve gives a thermal response factor (y axis). The peak
junction temperature due to the surge condition can be
calculated as indicated in the figure itself. The power
dissipation is normally calculated from the voltage and
current across the device during the surge.
This curve is also useful for determining the peak junction
temperature for power or pulses with a very low repetition
rate, when the method described in the previous section is
not applicable. The reason for this is illustrated by the
waveforms in figures 3 a and 3 b. Both sets of waveforms
are for the same power dissipation and duty cycle, but for
different operating frequencies. The cycle-by-cycle
fluctuations of junction temperature at 20 Hz (figure 3 a) are
clearly greater than at 200 Hz (figure 3 b). As frequency
increases, thermal inertia of the junction “irons out”
instantaneous temperature fluctuations, and the junction
responds more to average, rather than peak power
dissipation. At frequencies above a few kHz and duty cycles
above 20 % or so, cycle-by-cycle temperature fluctuations
become small, and peak junction temperature rise becomes
equal to the average power dissipation multiplied by the DC
junction-to-case thermal resistance, within 1 % or 2 %.
For pulses with low repetition rate the remaining curves in
figure 2 show effective thermal impedance at different duty
cycles. These curves are approximately related to the single
pulse curve, by the following relationship:
Effective normalized thermal impedance = D + (l - D) x
(transient thermal impedance for single pulse of duration t).
The thermal impedance, when multiplied by the power
dissipation during the conduction period t (i.e., the power
within the conduction pulse itself, not the power averaged
over the whole cycle), gives the value of the repetitive peak
junction-to-case temperature rise.
To determine the absolute value of the peak junction
temperature, it is, of course, necessary to know the case
temperature TC under steady-state operating conditions.
Because of thermal inertia, the heatsink responds only to
average power dissipation (except at extremely low
frequencies which generally will not be of practical interest).
TC is therefore given by: TC = TA + (Rth(CS) + Rth(SA)) PAV
where:
TA = ambient temperature
Rth(CS) = case-to-sink thermal resistance
Rth(SA) = sink-to-ambient thermal resistance
PAV = average power dissipation
also,
PAV = peak power x duty cycle for rectangular pulses of
power
Application Note AN-949
Vishay Siliconix
Current Rating of Power Semiconductors
Thermal Response (ZθJC)
10
1
0-0.5
0.2
0.1
0.1
0.05
PDM
0.02
SINGLE PULSE
(THERMAL RESPONSE)
0.01
t1
t2
NOTES:
1. DUTY FACTOR, D-t 1/t 2
10-2
(b) Type 641-A heatsink gives 3.5 A continuous rating with natural
convection cooling in 40 °C ambient.
2. PEAK T -P
10-5
J DM
0.1
10-3
10-2
t 1, Rectangular Pulse Duration (s)
10-4
x Z thJC + TC
1
10
Fig. 2 - Transient Thermal Impedance Curves for IRF530, SiHF530
Power MOSFET
100 W
POWER
t = 10 ms
ΔTJ = 64 °C
JUNCTION
TEMP.
RISE
(a) t = 10 ms D = 0.2
100 W
POWER
JUNCTION
TEMP.
RISE
t=1
ms
ΔTJ = 33.4 °C
AVERAGE
(b) t = 1 ms D = 0.2
(c) Type 689-75 e4 heatsink give 1 A continuous rating for IRF710,
SiHF710 with natural convection cooling in 40 °C ambient.
APPLICATION NOTE
Fig. 1 - Typical Heatsinks for Power MOSFETs
(Heatsink by Wakefield)
The transient thermal response curve assumes constant
case temperature. This is generally valid for pulses shorter
than 10 ms. For longer surges the case temperature starts
to rise and the results are of questionable accuracy. For
operation in free air, case temperature starts to rise within
few ms and this curve does not provide any useful
information. More sophisticated analytical methods that
take the entire thermal system into account are normally
used to calculate temperature rise under these conditions.
www.vishay.com
4
Fig. 3 - Waveforms of Power and Junction Temperature for
Repetitive Operation, showing that Peak Junction Temperature is
function of Operating Frequency.
PEAK CURRENT RATINGS
IGBTs and MOSFETs are able to carry peak current well in
excess of their continuous current rating, provided that the
rated junction temperature is not exceeded. There is,
however, an upper limit on the permissible current, defined
by the rated peak current. Most devices have a peak rating,
that is several times their continuous rating at TC = 25 °C.
Power transistors are fundamentally “linear” devices, as
opposed to “latching” devices. As current increases, the
point eventually is reached at which they go into “linear”
operation and start to act, in effect, as a current limiter. This
point depends upon the drive voltage applied to the gate,
the safe limit of which is determined by the thickness of the
oxide that insulates the gate from the body of the device.
Peak ratings of power devices are normally achievable with
an applied gate voltage that is equal to the maximum
permissible gate voltage of 20 V. They are repetitive ratings,
as long as the junction temperature is kept within the rated
For technical questions, contact: [email protected]y.com
Document Number: 91419
Revision: 02-Jul-10
Application Note AN-949
Vishay Siliconix
Current Rating of Power Semiconductors
TJ(max.). Peak junction temperature can be calculated from
the thermal impedance data for the device, as indicated
above.
It should be pointed out that the on-resistance of any
MOSFET does increase as current increases. As shown in
figure 4, the on-resistance of a 100 V rated power MOSFET
at its rated IDM with 20 V applied to the gate is typically 1.4
x the value at the rated ID; the corresponding multiplier for a
400 V rated power MOSFET is 2.9. This increase of
on-resistance must, of course, be taken into account when
making thermal calculations and designing for use of the IDM
rating.
3
400 V
power MOSFET
VGS = 10 V
100 V
VGS = 10 V
power MOSFET
2
2
2
I RMS (1) + I RMS (2) + ... + I RMS (n)
(2)
This is true to the extent that no two waveforms are different
from zero at the same time. In some applications such as
switching regulators, it is possible for the designer to control
the wave shape with topology or magnetic design. This can
be very beneficial in reducing the value for IRMS in the switch
for a given value of average current.
EFFECT OF WAVEFORM SHAPE ON RMS
VALUE
In a swith mode converter, the current waveforms through
the inductors, transformer windings rectifiers and switches
will appear as shown in figure A-1, ranging from a triangle to
a rectangle depending on the value of the averaging
inductor and load.
The RMS content of the current waveform changes
accordingly and this has a bearing on the MOSFET
conduction losses that are proportional to I2RMS.
2
PER UNIT RDS(on)
I RMS =
A measure of the squareness of the waveform can be
obtained from the ratio:
Ia
K = ---Ib
400 V
power MOSFET VGS = 20 V
1
100 V
power MOSFET
VGS = 20 V
It can be shown that:
Ia
K = ---- = f  L L C 
Ib
0
0
20
40
60
80
100
%ID M
Rated I D at T C = 25 °C
Fig. 4 - Typical Variation of On-Resistance with Drain Current
where:
L = inductance of the averaging choke.
APPENDIX
LC = 1 is the critical inductance for a particular input voltage
and load power.
Determining the RMS Value of ID Waveforms
As L is increased, K goes from 0 (triangle) to 1 (rectangle).
To accurately determine the conduction losses in a
MOSFET, the RMS value for ID must be known. The current
waveforms are rarely simple sinusoids or rectangles, and
this can pose some problems in determing the value for
IRMS. The following equations and procedure can be used to
determine IRMS for any waveform that can be broken up into
segments for which the RMS value can be calculated
individually.
From the above expression and
Ia + Ib
I avg = ----------------2
t 2
I RMS =
0 I  t dt----------------------
(1)
T
figure A-1 shows several simple waveforms and the
derivation for IRMS using equation (1).
If the actual waveform can be approximated satisfactorily by
combining the waveforms in figure A-1, then the RMS value
of the waveform can be calculated from:
Document Number: 91419
Revision: 02-Jul-10
2
2K
I a = ------------- I avg , I b = ------------- I avg
K+1
K+1
Substituting into the RMS expression for a trapezoidal
waveform, shown in figure A-1, we have:
2
I RMS = 2 D I avg
1 +K+K
---------------------------2
3 K + 1
For constant I(avg) and D, the normalized (IRMS = 1 for K = 1)
IRMS is as shown in figure A-3. This curve shows that, for
triangular current waveforms, the I2R losses are 32 % higher
than for rectangular waveforms. It is also apparent that for
Ia/Ib > 0.6, the improvement incurred by increasing L is only
2 %, so from a practical point of view, L need only to be
For technical questions, contact: [email protected]
www.vishay.com
5
APPLICATION NOTE
The RMS value of any waveform is defined as
we have:
Application Note AN-949
Vishay Siliconix
Current Rating of Power Semiconductors
about twice LC.
Ib
Increasing the value of Ia/Ib increases the switch turn-on
losses but decreases the turn-off losses. Since the turn-off
losses tend to be larger than the turn-on losses, increasing
Ia/Ib reduces the total switching loss also.
SMALL L
For the case of discontinuous inductor current (L < LC),
Ia/Ib = 0 and is no longer relevant, since the waveforms are
now triangles.
Ib
Ia
I1
I1
IRMS =
2
FULL WAVE
SINUSOIDAL
T=T
T
LARGE L
I1
D
2
IRMS = I
1
PULSED
SINUSOIDAL
T
T
D=
T
Fig. A-2
I1
IRMS = I D + sin
1
2
PHASE
CONTROLLED
SINUSOIDAL
T
(1-D) cos
2Π
Π
(1-D)
1.16
1.14
t
D=1- 1
T
1.12
T
IRMS = I
1
I1
1/2
IRMS NORMALIZED
t1
T
D
T
D=
T
1.10
1.08
1.06
1.04
T
1.02
T
1/2
Ib
2
IRMS =
Ia
D ( Ib + Ia Ib + Ia
T
D=
T
2)/3
1
0
T
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
K = I a / Ib
RECTANGULAR
Fig. A-3
T
I1
IRMS = I
1
D=
APPLICATION NOTE
T
D
3
T
T
TRIANGLE
Fig. A-1
www.vishay.com
6
For technical questions, contact: [email protected]
Document Number: 91419
Revision: 02-Jul-10