DESIGN OF POWER
ELECTRONIC CIRCUITS
Heatsink
THERMAL RESISTANCE

THERMAL RESISTANCE

THERMAL RESISTANCE

THERMAL RESISTANCE

TRANSIENT THERMAL IMPEDANCE
In some situations, the user of power devices must be
concerned with the transient thermal response of the device
being used. For example, during transient overloads or at
power-up or power-down of a system containing power devices,
the instantaneous dissipation in the devices may greatly exceed
the average power rating of the device.
TRANSIENT THERMAL IMPEDANCE

TRANSIENT THERMAL IMPEDANCE

TRANSIENT THERMAL IMPEDANCE

TRANSIENT THERMAL IMPEDANCE
In real devices where the heat must flow through several
dif ferent layers (the left figure), the equivalent circuit is more
complicated as shown (the right figure).
TRANSIENT THERMAL IMPEDANCE

TRANSIENT THERMAL IMPEDANCE

TRANSIENT THERMAL IMPEDANCE

TRANSIENT THERMAL IMPEDANCE
If P(t) is not a rectangular pulse, the approach just outlined can
still be used if an equivalent rectangular pulse can be fitted to
the actual P(t). Consider the half-sine power pulse shown in
figure below.
TRANSIENT THERMAL IMPEDANCE

HEAT SINKS
Keeping the junction temperature of a power device within
reasonable bounds is the joint responsibility of the device
manufacturer and the designer. The manufacturer minimizes
the thermal resistance R θjc between the interior of the device
where the power is dissipated and the outside of the case
enclosing the device. The designer must provide a heat
conduction path between the case of the device and the
ambient so that thermal resistance R θca between the case and
the ambient (where the heat generated by device operation will
ultimately be dissipated) is minimized in a cost-effective
manner.
HEAT SINKS
The designer’s responsibility is made easier by the wide
availability of extruded aluminum heat sinks of various shapes
that are used for cooling of the power semiconductor devices. If
the heat sinks are cooled by natural convection, the distance
between each fin, such as is shown in the figure, should be at
least 10-15 mm. A coating of black oxide results in a reduction
of the thermal resistance by 25%, but the cost may be higher by
almost the same factor.
HEAT SINKS

HEAT SINKS
The choice of the proper heat sink depends on the allowable
junction temperature the device can tolerate. For a worst-case
design, the maximum junction temperature T j,max , the maximum
ambient temperature Ta,max , the maximum operating voltage,
and maximum on-state current are specified. The maximum onstate losses in the power device can be calculated if the
maximum duty ratio, maximum on-state current, and maximum
on-state resistance (obtainable from the data sheets
corresponding to T J.max and the maximum current) are known.
The switching losses can be obtained by integrating the
instantaneous power loss with respect to time and averaging it
over the switching time period. Therefore, P loss , which is the
sum of the on-state losses and the average switching losses can
be estimated.
HEAT SINKS

HEAT SINKS
A proper heat sink can then be selected based on the
information provided by the heat sink manufacturer's data
sheets such as shown in the figure.
Heat sink no.
1
2
3
4
5
6
7
8
9
10
11
12
Rθsa (oC/W)
3.2
2.3
2.2
0
2.1
1.7
1.3
1.3
1.25
1.2
0.8
0.65
Vol. (cm3)
76
99
181
0
198
298
435
675
608
634
695
1311
HEAT SINKS
When using any of these heat sinks, it is imperative that the
manufacturer's instructions be followed closely. Improper
mounting of the power device on the heat sink could result in
R θca being much larger than anticipated and thus intolerably
high values of junction temperature of the device during normal
operation. For example, a small amount of thermal grease
should be used to increase the contact area between the device
and the heat sink. Application of the proper torque to the
mounting bolts and nuts will also help ensure good contact
between the device and the heat sink.
HEAT SINKS

HEAT SINKS
Heat sink number 7 in the 22nd slide has a thermal resistance
of 1 .3 °C/W, which is acceptable for this application. In fact
using this heat sink will lower the junction temperature to
122.6°C, which is slightly cooler than assumed. The power
dissipation in the transistor will then be somewhat smaller as a
result and thus will lower the real junction temperature
somewhat as well, perhaps to less than 120°C. If the converter
that uses this heat sink is to be mass produced, it might make
economic sense to look for a heat sink with R θsa = 1 .39 °C/W
since it will be lighter and smaller than number 7.