Selecting heat sinks for dc/dc converters

BY CHRIS SOULE
Aavid Engineering, Inc.
Laconia, NH

When selecting heat sinks for dc/dc converters, design engineers need to know
both thermal management and what types of heat sinks are available to do the
job. Dc/dc converters offer the designer significant advantages in distributing
necessary voltages to various system points. As manufacturing processes and
components improve, dc/dc converter efficiencies are increasing from 50% to 60%
up to 75% to 85%. However, the demand for higher output power is offsetting
these efficiency increases. Thus, as output power increases, the amount of heat
to be dissipated increases proportionally.

Many dc/dc converters can generate over 100 W in a package smaller than 6
in.³ With watt-densities now reaching 50 W/in.³ and higher,
cooling the converters becomes increasingly difficult.
Although most converter manufacturers can build extrusions into their
converters to help remove heat, such measures are limited in both natural and
forced convection. Forced airflow can help reduce the baseplate temperature at
the extended surfaces to further remove heat. Applications requiring either
higher than average power outputs or increased MTBFs require more effective
heat removal techniques–namely, the use of heat sinks.

Ultimately the limitations of cooling any electronic product are based on the
temperature of the semiconductor die or, in the case of dc/dc converters, the
maximum temperature of the baseplate (usually specified by the manufacturer).
With base temperature limitations of 85° to 100°C, the amount of
output power is quickly calculated based solely on the thermal management
scheme and the efficiency of the converter as it changes input power to output
power at various voltages.

Calculating thermal management
The nominal temperature rise (over ambient room temperature) of any cooling
method is limited to the product of the thermal resistance of the heat sink and
the input power times the efficiency of the converter minus 1. This is
represented as:

Temperature rise =
(Thermal resistance)
x (Input power) (Efficiency – 1)

By rearranging this formula algebraically to express the relationship in terms
of output power, the designer can use it to estimate the thermal resistance
required to produce a given output power:

Thermal resistance =

Temperature rise above ambient
—————————————-
(Power output/Efficiency) – Power output

As an example, assume the designer needs an output power of 100 W from a
converter that is 80% efficient. With a maximum allowable baseplate temperature
of 85 degreesC and an ambient air temperature of 35°C, the allowable
rise is 40°C. Using the above formula, the thermal resistance is found
to be 1.6°C/W.

The types of heat sinks
Various types of heat sinks are available to remove the excess heat that is
generated by dc/dc converters. Here are descriptions of some of the most common
heat sinks.

Extruded aluminum heat sinks
Conventional extruded aluminum heat sinks (see Fig. 1) can be made in various
converter heights and fin spacings.
The number of fins on a converter-size heat sink limits the amount of surface
area per cubic inch. Designing the optimum extruded heat sink also depends on
whether the part will be used in natural or forced convection. The number of
fins and the spacing between them is limited by the aluminum extrusion process.
Fins can run in either direction on the part. However, fins spaced closer
together than 0.2 in. tend to block natural convection airflow and cannot be
optimized for use in forced convection. This limitation in heat removal
constrains the power output to maintain a given baseplate temperature.
In forced convection, the amount of surface area is proportional to the amount
of heat that can be removed. The number of fins, hence the amount of heat
removed, are limited by extrusion technology and the application where the part
will be used.
In the standard dc/dc module size of 2.4 by 4.6 in., thermal resistances for
conventional extruded heat sinks can be as low as 1.20°C/W with
forced-air cooling at 500-ft/min air speed.

Pin-fin heat sinks
Pin-fin heat sinks (omnidirectional) are commonly extruded of aluminum. Cross
cutting of an extrusion can improve performance in some forced-convection
applications by 10% to 15%. This is due to the increase in turbulence of the
air stream as it makes its way through the heat sink. Fins can run in either
direction on the part. The cost of the cross-cutting adds to the piece part
price as well as to the static pressure drop required to move air through the
pin-fin field.
True omnidirectionality can be achieved by manufacturing round or elliptical
pins using a die-casting process. An alternative method to creating square
pin-fin heat sinks comes in the form of vacuum-assisted die casting. This
process is valuable when the design of the heat sink has been established and
the quantities of the part to be made are high. The die-casting process
requires manufacturing a relatively expensive die cavity to form the parts.
However, the cost of the parts is very low and will, in most cases, be produced
as net-shaped parts requiring little or no finished machine work. A
vacuum-assisted process is required to ensure that the parts are produced with
as few voids, porosity, and inclusions as possible.
The thermal resistance of pin-fin heat sinks varies widely depending on the pin
size, shape, and quantity, but will reduce thermal resistance to approximately
1.0°C/W for the typical 2.4 x 4.6-in. module. This resistance is in
forced air at 500 ft/min.

Bonded-fin heat sinks
Additional heat-removal surface area can increase the amount of cooling. This
is essential for use in forced-air cooling either where lower junction
temperatures and higher reliability are required or where dictated by higher
output power levels. Bonded-fin heat sinks offer one such solution.
These heat sinks (see Fig. 2) have an extruded aluminum base with highly
conductive fins epoxy bonded into grooves. The fins can be spaced very closely
together and can be made in any practical height. This method of increasing
surface area is a way of circumventing the fin extrusion limitation set by the
extrusion process. Bonded-fin parts offer two to three times the surface area
of extrusions and can provide greater cooling in smaller volumes. The parts can
be made from either aluminum or copper.

In the standard dc/dc module size of 2.4 x 4.6 in., thermal resistances for the
bonded-fin style can be as low as 0.60°C/W with forced-air cooling at
500-ft/in. air speed. Bonded-fin sinks tend to be heavier than extruded sinks
because of the thickness of the extruded base section. The base section must be
thick enough to form a strong structural joint with the fins. The base section
must therefore be at least six times the fin thickness. Bonded-fin parts are
also 25% to 35% more expensive than equally sized extrusions.

Folded-fin heat sinks
Folded-fin assemblies are heat sink assemblies made from a relatively thin
base section and a set of fins folded into corrugated sections (see Fig. 3).
The base section can be very thin (0.06 in.) to reduce mass or thicker to act
as a heat spreader. The folded fins act as a heat-transfer area, allowing the
forced-air stream to remove heat from the baseplate cooling the converter.
Folded-fin heat sinks offer the maximum potential in surface area and reduced
weight. Thinner base sections can help where limited head room is available and
shoc and vibration requirements must be met. The thinner base sections must be
used with caution to reduce localized heating effects. Forced-air cooling is
mandatory because of the close-fin configuration.
Thermal resistances as low as 0.40°C/W can be achieved with folded-fin
assemblies in forced-air cooling at 500 ft/min of air velocity. The drop in
static pressure increases significantly with folded-fin parts and must be taken
into account during design.

CAPTIONS:

Fig. 1. Conventional aluminum extruded heat sinks can be made in various
converter heights and fin spacings. The number of fins determines the amount of
surface area per cubic inch.

Fig. 2. Bonded-fin heat sinks have an extruded aluminum base with highly
conductive fins epoxy bonded into grooves. The fins can be spaced very closely
together and can be made in any practical height.

Fig. 3. Folded-fin heat sinks are made from a relatively thin base section and
with fins folded into corrugated sections.