Electronic Circuit Directory

This paper describes the analysis and derivation of an optimum heat

sink design for maximizing the thermoelectric cooling performance

of a laboratory liquid chiller. The methods employed consisted of

certain key changes in the design of the heat sink in order to improve

its thermal performance. Parametric studies were performed in order

to determine the optimized cooling system design per dollar."

"The objective of this project was to analyze the thermal performance

of an initial simple heat sink design and improve cooling

performance while reducing the cost and overall size of the cooling

system. Several changes were examined in an effort to improve the

thermal performance and/or to reduce overall cost. The result

obtained has provided some guidelines for the selection/design of the

most effective and economical heat sink configuration. These results

were somewhat surprising since they are contrary to what one might

instinctively expect without the benefit of the detailed analysis

presented in this paper.

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Modern portable electronics have seen component heat loads

increasing, while the space available for heat dissipation has

decreased, both factors working against the thermal designer.

This requires that the thermal management system be optimized

to attain the highest performance in the given space. While

adding fins to the heat sink increases surface area, it also

increases the pressure drop. This reduces the volumetric airflow,

which also reduces the heat transfer coefficient. There exists a

point at which the number of fins in a given area can be optimized

to obtain the highest performance for a given fan. The primary

goal of this paper is to find the optimization points for several

different fan-heat sink designs. The secondary goal is to find a

theoretical methodology that will accurately predict the

optimization point and the expected performance.

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The basic structure of a Solid State Relay includes an internal power semiconductor mounted to an electrical insulator which in turn is mounted to the Solid State Relay’s base plate. To form an assembly, the SSR with an accompanying thermal interface material placed on its base plate is then torque mounted to the Heat Sink.

The thermal model representing the above configuration includes the following elements:

A.

The selected SSR with specified thermal impedance (RΘ ssr), forward voltage drop (Vf), and maximum allowed internal operating temperature (Tj).

B.

The thermal interface material placed between the SSR and the Heat Sink and its specified thermal impedance (RΘ tp).

C.

The calculated minimum Heat Sink thermal impedance rating (RΘ hs) required for proper SSR operation.

D.

The operating environment’s max ambient air temperature in °C (TA ).

In certain instances, once the heat sink requirements for a SSR in a particular application have been determined and installed, it may be desirable to verify that the system does indeed provide adequate cooling to ensure reliable SSR operation.

The following is a relatively simple method to check this suitability, and essentially uses some of the calculations from SELECTING A SUITABLE HEAT SINK (above) in a reverse manner. This technique may also be used on existing systems in the field that might have been more or less “empirically” designed, to gain information on their performance and potential reliabilty. This method involves determining the temperature of the internal power devices

Heat Sinks are required to insure the proper operation and long term reliability of Solid State Relays because they provide a means to dissipate the power that is normally developed by the SSR into the surrounding ambient air and maintain a safe operating temperature. Selecting the correct Heat Sink for any given SSR application involves coordinating form factor, size, mounting and thermal impedance rating. This paper discusses “Why Heat Sinks are Required for Reliable Solid State Relay Operation”, how the minimum required Heat Sink thermal impedance rating is calculated based upon application operating conditions, and includes an example calculation.

Due to the forward voltage drop of the output SCRs, solid state relays generate an internal power loss. The amount of power generated is afunction of the load current. The manufacturer provides power loss curves, as shown in Fig 1. At normal load currents the power loss can be estimated at 1 Watt for every 1 Arms of load current. In order to maintain an acceptable power switch junction temperature, some form of

heatsink must dissipate the heat generated by the power loss. For most printed circuit board types, the relay current rating is established by measuring the thermal impedance, from the dissipating elements to air, using the relay package as the heat sink. Some printed circuit board types are available with an integral heatsink; their ratings reflect the additional effects of the integral heatsink.

Many parameters contribute to a design's thermal circuit, including the

device's maximum power consumption for the design, the maximum

environment temperature, package characteristics, and airflow at the

device.

Maximum Power Consumption (P)

Use the power calculator values from design simulations in the Altera

Quartus® II software (or the device's power calculator at

http://www.altera.com) to estimate the maximum power consumption

of the device. Once a prototype design is available, measure the actual

power consumption and use this value for thermal calculations.

Maximum Temperature (TJ & TA)

The maximum ambient and junction temperatures are found in the data

sheet for the device under Device Absolute Maximum Rating and the

operating junction temperature is found under Device Recommended

Operating Conditions. The temperature must be kept within the

maximum conditions or damage could occur. The junction temperature

should be kept within the recommended operating conditions to ensure

the device achieves the performance reported by the Quartus II software.

The rate at which heat is conducted

through a material is proportional

to the area normal to the heat flow

and to the temperature gradient

along the heat flow path. For a one

dimensional, steady state heat flow

the rate is expressed by Fourier’s

equation:

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All semiconductor devices have some electrical resistance, just

like resistors and coils, etc. This means that when power

diodes, power transistors and power MOSFETs are switching

or otherwise controlling reasonable currents, they dissipate

power — as heat energy. If the device is not to be damaged by

this, the heat must be removed from inside the device

(usually the collector-base junction for a bipolar transistor, or

the drain-source channel in a MOSFET) at a fast enough rate

to prevent excessive temperature rise. The most common way

to do this is by using a heatsink.

To understand how heatsinks work, think of heat energy itself

as behaving very much like an electrical current, and

temperature rise as the thermal equivalent of voltage drop. We

also have to introduce a property of materials and objects

known as thermal resistance, which behaves in a very similar

way to electrical resistance: the more heat energy ‘flowing’

through it, the higher the temperature rise across it. As you

might imagine metals like copper and aluminium have very low

thermal resistance, while air tends to have a relatively high

resistance. So do many plastics and ceramic materials.

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The objective of thermal management programs in electronic packaging is the efficient removal of heat from the semiconductor junction to the ambient environment. This process can be separated into three major phases:

1) heat transfer within the semiconductor component package;

2) heat transfer from the package to a heat dissipater (the initial heat sink);

3) heat transfer from the heat dissipater to the ambient environment (the ultimate heat sink)

Charles Nogales, VP of Engineering at Emulex, talks about the effects of heat on electronic circuits and devices, such as HBAs, and how heatsinks play a role in keeping networking and server product

Thermal management is an important design consideration with complex

devices running at high speeds and power levels as these devices can

generate significant heat. Proper thermal management can increase

product performance and life expectancy. The thermal management

requirements for a programmable device depend on its application.

AlteraR packages are designed to minimize thermal resistance

characteristics and maximize heat dissipation. However, in some cases,

complex designs require heat dissipation greater than packages provide.

This application note discusses ways to dissipate heat, how to calculate

the heat dissipation of a device, and how to determine if a device requires

a heat sink in an application.