### Computing IC Temperature Rise

```COMPUTING IC TEMPERATURE RISE
IC temperature TJ is determined by ambient temperature TA, heat
dissipated PD, and total thermal resistance Rθ . This total thermal
resistance is comprised of three individual component resistances: chip RC, lead frame RL, and heat sink RS.
WHY IC TEMPERATURES RISE
Heat is the enemy of integrated circuits—particularly power
devices. Here’s how to use thermal ratings to determine safe IC
operation.
Excessive heat shortens the life of an IC and reduces its operating
capability. Until recently, ICs were capable of operating only in lowpower applications requiring perhaps a few milliwatts of power. But
now, ICs handle several amperes and drive devices such as relays,
solenoids, stepping motors, and incandescent lamps. These high
power levels may increase IC temperatures substantially and are capable of destroying devices unless appropriate precautions are taken.
THERMAL CHARACTERISTICS
The thermal characteristics of any IC are determined by four
parameters. Maximum allowable IC chip junction temperature TJ and
thermal resistance Rθ are specified by the IC manufacturer. Ambient
temperature TA and the power dissipation PD are determined by the
Reprinted by permission from the June 9, 1977 issue of MACHINE DESIGN,
Application Note
29501.4
APPLICATIONS
INFORMATION
COMPUTING IC
TEMPERATURE RISE
user. Equation 1 expresses the relation of
these parameters.
TJ = T A + PDRθ
(1)
Junction temperature TJ usually is limited
to +150°C for silicon ICs. Devices may
operate momentarily at slightly higher temperatures, but device life expectancy decreases exponentially for extended hightemperature operation. Usually, the lower
the junction operating temperature, the
greater the anticipated life of the IC.
limited either to 70°C or 85°C for plastic dual
in-line packages (DIPs) or 125°C for hermetic
devices. Again, the objective is to operate at
as low a junction temperature as practical.
Thermal resistance Rθ is the basic
thermal characteristic for ICs. It is usually
expressed in terms of °C/W and represents
the rise in junction temperature with a unit of
power applied in still air. The reciprocal of
thermal resistance is thermal conductance,
or derating factor, Gθ expressed as W/°C.
Thermal resistance of an IC consists of
several distinct components, the sum of
which is the specified thermal resistance.
For a typical IC, these components of thermal
resistance are 0.5°C/W per unit thickness of
the silicon chip, 0.1 to 3°C/W per unit length
of the lead frame, and up to 2,000°C/W per
unit thickness of still air surrounding the IC.
DIPs are used more than any other type of
packaging for ICs and copper-alloy lead
frames provide a superior thermal rating over
the standard iron-nickel-cobalt alloy (Kovar)
lead frames. However, power ICs are also
available in other packages such as PLCCs,
SOICs, and power-tabs.
Total IC power to be dissipated depends on input current, output
current, voltage drop, and duty cycle. Thus, for many industrial digitalcontrol ICs, logic-gate power Pl (typically less than 0.1 W) and output
power P0 must be determined to find the total power to be dissipated.
Total power dissipation for these logic devices is the sum of Pl and P 0.
Pl = n(VCCICC)
(2)
P0 = n(V CE (SAT)IC)
(3)
where VCC = logic-gate supply voltage, ICC = logic-gate supply ON
current, VCE(SAT) = output saturation voltage, IC = output load current,
and n = number of logic gates. Manufacturers usually list typical and
maximum values for these voltages and currents. For thermal considerations it is best to use the maximum values so that worst-case power
dissipation is determined.
If the duty cycle of the device is longer than 0.5 s, the peak power
dissipation is the sum of the logic-gate power Pl and output power P0
for the logic ON state alone. If the ON time is less than 0.5 s, however, average power dissipation must be calculated from instantaneous ON and OFF power PON and POFF from
PD = DPON + (1-D) POFF
(4)
CORRECTIVE ACTIONS
If the junction temperature or the required power dissipation of the
IC is calculated to be greater than the maximum values specified by
the manufacturer, device reliability and operating characteristics
possibly will be reduced. Possible solutions are:
1.
Modify or partition the circuit design so the IC is not required to
dissipate as much power.
2.
Reduce the thermal resistance of the IC by using a heat sink
or forced-air cooling.
3.
Reduce the ambient temperature by moving heat-producing
components such as transformers and resistors away from the
IC.
4.
Specify a different IC with improved thermal or electrical
characteristics (if available).
The power PD that an IC can safely
dissipate usually depends on the size of the
IC chip and the type of packaging. Most
common copper-frame DIPs can dissipate
about 1.5 W, although some special-purpose
types have ratings as high as 5 W.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
COMPUTING IC
TEMPERATURE RISE
SETTING UP THE CIRCUIT
MEASURING IC TEMPERATURE
Sometimes IC junction temperature cannot be calculated
be made when there is insufficient data with which to calculate,
when the effects of external variables such as forced-air cooling
or enclosure size must be determined, or as a check on the
manufacturer’s specifications regarding package thermal
resistance.
The most popular technique of measuring IC temperature
uses the characteristic of a diode to reduce its forward voltage
with temperature. Many IC chips have some sort of accessible
diode—parasitic, input protection, base-emitter junction, or output
clamp. With this technique, a “sense’’ diode is calibrated so that
forward voltage is a direct indicator of diode junction temperature.
Then, current is applied to some other component on the chip to
simulate operating conditions and to produce a temperature rise.
Because the thermal resistance of the silicon chip is low, the
temperature of the sense diode is assumed to be the same as the
rest of the monolithic chip.
CALIBRATING
THE SENSE DIODE
The sense diode should be calibrated over at least the
expected junction operating temperature. Apply an accurately
measured, low current of about 1 mA through the sense diode
and measure the forward voltage in 25°C increments after
stabilization at each temperature. This calibration provides
enough data for at least six points to construct a diode-forwardvoltage versus junction-temperature graph at the specified
forward current. A typical 25°C forward voltage is between 600
and 750 mV and decreases 1.6 to 2.0 mV/°C.
For power levels above 2 W, it may be necessary to use
more than a single transistor if only the device saturation voltage
and sink current are used. If higher power is desired, keep the
output out of saturation.
Measuring the sense-diode forward voltage may require a
considerable waiting period (10 to 15 minutes) for thermal equilibrium. In any event, at the instant of measurement, the heating
power may have to be disconnected because erroneous readings
may result from IR drop in circuit common leads. Various circuit
connections (such as four-point Kelvin) may be arranged to
reduce or eliminate this source of error.
The IC junction temperature can be determined by comparing
the voltage measurement with the internal power source against
the voltage measurement with the temperature chamber.
COMPUTING IC
TEMPERATURE RISE
FINDING SAFE OPERATING LIMITS
Here’s how to calculate the safe operating limits for an
IC. The first two examples are simple calculations involving maximum allowable power and are straightforward.
The third and fourth examples are more complex and
involve logic power, output power, and duty cycle.
Problem: Determine the maximum allowable power
dissipation that can be handled safely by a 16-lead Kovar
DIP with an Rθ of 125°C/W in an ambient temperature of
70°C.
Solution: From Equation 1, the maximum allowable
power dissipation PD for this lC is
PD =
150°C - 70°C
TJ = 70°C + (1.225 W) x (16.67 mW/°C)
= 143.5°C
Problem: Determine the acceptable duty cycle for a
power driver with a thermal resistance of 100°C/W in an
ambient of 85°C and which is controlling load currents of
250 mA on each of four outputs.
Solution: From Equation 1, the allowable average
power dissipation PD for this IC is
PD =
125°C/W
= 0.64 W
150°C - 85°C
100°C/W
= 0.65 W
Problem: Determine the maximum allowable power
dissipation that can be handled by a 14-lead copper DIP
with a derating factor Gθ of 16.67 mW/°C in an ambient of
70°C.
Solution: Because the derating factor Gθ is the
reciprocal of thermal resistance Rθ the maximum allowable power dissipation PD, from Equation 1 is
PD = (150°C - 70°C) x (16.67 mW/°C)
= 1.33 W
Problem: Calculate the maximum junction temperature for a quad power driver wlth a thermal resistance of
60°C/W in an ambient of 70°C and which is controlling a
250 mA load on each of the four outputs.
Solution: To determine the maximum (worst case)
junction temperature for this IC, the maximum total power
dissipation must be determined from the data listed on the
IC data sheet. The specifications are usually listed as
typical and minimum or maximum values. It is important
to use maximum voltage and current limits to ensure an
adequate design. Common maximum values for an
industrial power driver are VCC = 5.25 V, ICC = 25 mA, and
VCE(SAT) = 0.7 V, and IC = 250 mA. From Equations 2 and
3, worst case logic and output power dissipation are
Pl = 4 (5.25 V x 25 mA)
= 525 mW
Thus, the total worst case power dissipation PD is
525 mW plus 700 mW, or 1.225 W. From Equation 1,
maximum junction temperature TJ is
This means that there is 0.65 W limit on average power,
but, not instantaneous power. If the duty cycle is low
enough, and the ON time is not more than about 0.5 s, the
average power dissipation can be considerably lower than
the peak power. The ON, or peak power, is determined
from the data sheet maximum values of VCC, ICC, and
VCE(SAT) at the specified load current of 250 mA. From
Equations 2 and 3, logic-gate power Pl and output power
P0 for the ON state are
Pl = 4 (5.5 V x 26.5 mA)
= 583 mW
P0 = 4 (0.7 V x 250 mA)
= 700 mW
Instantaneous ON power PON is the sum of Pl and P 0 for
the ON state, or 1.283 W. The OFF power is primarily the
power dissipated by the logic in the OFF state, and is
found by using the ICC maximum rated current listed on
the specification sheet. The power dissipated in the
output stage can be calculated from the leakage current IC
and supply voltage VCE. From Equations 2 and 3, Iogicgate power Pl and output power P0 for the OFF state are
Pl = 4 (5.5 V x 7.5 mA)
= 165 mW
P0 = 4 (100 V x 0.1 mA)
= 40 mW
P0 = 4 (0.7 V x 250 mA)
= 700 mW
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Worcester, Massachusetts 01615-0036 (508) 853-5000
COMPUTING IC
TEMPERATURE RISE
Instantaneous OFF power POFF is the sum of Pl and P 0 for the off state,
or 205 mW. From equation 4, acceptable duty cycle D is
PD - P OFF
D=
P ON - POFF
=
0.65 W - 0.205 W
1.283 W - 0.205 W
= 41%
WHAT THE CURVES SHOW
The junction temperature of an IC depends on several factors, including the thermal resistance of the IC and the
operating duty cycle. Graphs showing the relationship of these factors are often useful in specifying an IC.
Typical thermal-resistance ratings for ICs in still air range from
60°C/W to 140°C/W. The slope of each curve on this graph is equal
to the derating factor Gθ, which is the reciprocal of thermal resistance Rθ. For an ambient temperature of 50°C, a typical 14-lead
flatpack with an Rq of 140°C/W can dissipate about 0.7 W. A typical
DIP, however, with 14 copper-alloy leads can dissipate almost
1.7 W at 50°C.
The highest allowable package power dissipation shown here is
2.5 W. Other special-purpose DIP packages are available with
power dissipation ratings as high as 3.3 W at 0°C (R θ = 45°C/W). If
not for package limitations, IC chip dissipation might be greater than
9 W at an ambient temperature of up to 70°C.
Although the curve for plastic DlPs goes all the way to +150°C,
they ordinarily are not used in ambients above +85°C because of