September 2017 59
Resistor: Pd = I2 x R = V2 / R = V x I
Diode, SCR, or TRIAC: Pd = Vf x Iavg where Vf is the
component’s forward voltage drop across its internal
junction(s), and Iavg is the average forward current through
the junction. This is why power rectifiers get so hot; for
every amp of average current through them, they are
dissipating at least 0.7 watts.
Transistors: Pd = VCE x IC = VGS x ID. The transistors act
like variable resistors as far as heat generation goes. This is
a particular concern where the transistor is used to regulate
current, such as in a voltage regulator’s pass transistor. This
forms the basis of our experiment coming up later.
Inductor, Capacitor, Transformer: Pd = I2 x RLOSS.
Capacitors have loss? You bet! It’s called the equivalent
series resistance (or ESR) and its created by loss in the
dielectric or electrolyte and from resistive losses in the
metal electrodes. ESR is quite important when choosing
a capacitor for use in a switchmode supply or converter
where the AC currents can be quite high.
In these equations, if the current is AC, use RMS
values. For either DC or AC currents that are intermittent,
multiply Pd by the duty factor of the current. For example,
if a resistor only carries current in pulses that are on three-quarters of the time, calculate peak Pd and multiply by .75.
When you are using an IC, sum up all of the power
dissipation from each significant source of heat. These
are usually the IC’s outputs. Include the power dissipated
inside the IC by multiplying the power supply voltage times
the current drawn by the IC.
2 — Determine Maximum Power or Temperature
“Bulk” components — such as resistors and capacitors
— generate heat throughout or along their bodies.
A maximum continuous power dissipation, Pdmax,
is specified, such as for a half watt resistor. An
ambient temperature is also specified for Pdmax,
because that determines the temperature at one
end of the component’s thermal resistance.
Semiconductors, on the other hand,
generate heat in the very small volume around
the junctions or channels that conduct current.
This small source of heat is generally referred
to as “the junction,” whether it’s an actual P-N
junction or an FET channel or something else. For
semiconductors, thermal resistance is specified
between the heat-generating junction and the
surrounding or ambient conditions, as qja. This
is referred to as the junction-to-ambient thermal
If the semiconductor is intended to be used
with a heatsink, qjc is specified as the thermal
resistance from the junction to the case of the
transistor, which might be a metal tab or just
the external plastic surface. The total thermal resistance,
junction-to-ambient, qja, then becomes the sum of qjc
and whatever thermal resistance exists from the case to
ambient conditions, qca.
The internal structure of semiconductors must be kept
below some maximum temperature, Tjmax, or it will be
destroyed. For devices made from silicon, this is usually
150°C. For a given amount of power dissipation, the
junction temperature, Tj, will be:
Tj = Tambient + P x qja
If you calculate Tj and find it to be less than Tjmax, you can
You have to be careful when assuming an ambient
temperature. In an enclosure, the actual ambient
temperature may be quite a bit higher than room
temperature. You also need to include a safety factor when
deciding on what an acceptable junction temperature will
be; 25 percent or 35°C is reasonable.
If you find Tj to be on the high side, you must reduce
either P or qja. Reducing P depends on the application
of the component. Reducing qja means helping the
component pass heat more effectively through its outer
surface since qjc, the thermal resistance from the junction
to the outer surface, is fixed.
3 — Selecting a Method to Get Rid of the Heat
There are two common methods of removing excess
heat from a component. The first is to cool it by moving
enough air across the component to keep Tambient as low
as possible, i.e., blow on it. This will work for relatively
low power dissipations of up to a watt or so with small
PRACTICAL TECHNOLOGY FROM THE HAM WORLD
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n FIGURE 2. Several common heatsinks for power transistors,
voltage regulators, and small amplifier modules. These have
thermal resistances from 15 to 55 °C/W. Some clip directly onto
a transistor case (the ones in the middle), while others are held on
with screws, such as the mounting hardware shown at lower right.