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Keep It Cool

Written by George Novacek

Electronics Cooling Essentials

The reliability of your electronic design is typically inversely proportional to its operating temperature. So, how can you keep electronics cool? George covers some common cooling techniques.

The trend towards smaller, more powerful electronics continues. This quest for miniaturization, accompanied by higher performance expectations, brings with it a nagging problem—the heat, which is the cause of reliability loss and affects life expectancy. It’s known that the reliability of electronic equipment is, inversely, often exponentially proportional to its operating temperature. Small and nonmetallic packaging is inefficient in absorbing and dissipating that internally generated heat. Hand-held instruments can get uncomfortably hot. And, as I said, the reliability plummets.

So how do we keep electronics cool? In this article, I’ll cover the essentials.

THE HEATSINK

The classic solution is the well-known heatsink. Heatsinks enhance heat dissipation from a hot surface, such as an integrated circuit (IC) package to a cooler, usually the ambient air. A heatsink should maintain the IC’s temperature below the maximum allowed by the IC manufacturer by increasing the area dissipating the heat to the coolant, most often air

There are several terms we need to establish first. Q is the power or rate of the heat to be dissipated in watts (W). TJ is the maximum allowable junction temperature of the device in degrees Celsius. It generally ranges from about 115°C to 180°C (239°F to 356°F) as defined by the manufacturer’s specification. For reliable operation the operating TJ should be less than the allowed maximum.

TC is the maximum case temperature. TS is the sink temperature. TA is the coolant temperature. (All temperatures are in degrees Celsius.) This is analogous to a voltage divider, while the thermal resistance between surfaces is analogous to ohmic resistance (see Figure 1).

Figure 1 Calculation of requirements of a heatsink
Figure 1
Calculation of requirements of a heatsink
THERMAL RESISTANCE

The RJC is thermal resistance between the semiconductor junction and the case.

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It is specified by the device manufacturer and usually given as a constant. RCS is the thermal resistance between the case and the heatsink.

It depends on many factors, such as the quality of the surfaces, whether a thermal compound is used, and so forth. RSA is the thermal resistance between the heatsink and the coolant, such as ambient air. Similarly:

The total junction to coolant thermal resistance is:

RSA depends on the heatsink material, construction, and so forth. The heatsink manufacturer provides it. All the thermal resistance values are expressed in °C/W.

To select a heatsink, you first calculate:

Manufacturers of heatsinks give this value in their catalogs. Similarly, manufacturers of thermal compounds and interface materials publish RCS in their catalogs.

HEATSINK EFFICIENCY

One method for improving heatsink efficiency is forced coolant convection. Commonly, the air, but other coolants too, are blown across heatsink fins to increase removal of the heat. The air is usually propelled by a fan or a blower. The fundamental difference between a fan and a blower is that a fan moves the air parallel to its axis, while the blower relies on centrifugal forces and moves air perpendicular to its axis. A fan, reminiscent of a propeller, moves a high volume of air at a lower pressure. Blowers, such as “squirrel cage” types used in furnaces, move lower volume but at higher pressure.

Fans look deceptively simple, but in reality, they act as aircraft propellers, having to have a proper airfoil cross section. To design an efficient fan, one must be familiar with aerodynamics. High volume movement is achieved, for instance, by parallel and/or serial combination of fans.[1,2] Fans for moving large amounts of air, such as those in aircraft to cool avionic racks, resemble small turbofan engines. They are in fact true turbines driven by high speed brushless DC motors at 10,000 to 20,000 RPM.

When specifying a fan, we need to solve the equation:

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Q is the amount of heat transferred in watts. CP is specific heat of air in J/(kg × K). m is mass flow rate of air in kg/s. ∆T is the desired air temperature differential (compared to outside air) in K. (K is the temperature in Kelvins, where 0 K = –273.15°C = –459.666°F.)

Mass flow rate is calculated as:

G is the volumetric flow rate in m3/s. Ρ is air density in kg/m3. This is a rough estimate of the required airflow at sea level to dissipate the given amount of heat. The amount of heat removed depends on the mass flow rate, not the volumetric flow rate. Accordingly, the calculations need to be performed with the altitude and air temperature in mind. (Additional computations can be found in “All You Need to Know About Fans,” which is listed in the Resources section at the end of this article.)

Several basic rules should be followed when designing a cabinet to house electronics. Blow the air through a filter into the cabinet (i.e., pressurize it) to keep dust out. Locate components with the highest heat dissipation close to the air exit. The enclosure air inlet and exit vents should be at least as large as the fan’s venturi opening. There should be enough free volume in the enclosure for the air to not exceed 7 m/s velocity. Hot spots need to be avoided. An additional small fan may be needed for the spot cooling. Components with critical temperature sensitivity need to be the nearest to the air inlet.

PELTIER MODULES

Sometimes more active cooling has to be employed inside the cabinet. One of the methods is thermoelectric cooling (TEC) with Peltier modules. Peltier module is like a solid-state heat pump. It is essentially a thermocouple. When two thermoelectrically dissimilar metals are connected in a circuit with two junctions at different temperatures, a voltage will develop between the two ends. Conversely, should we induce DC current into such circuit, one junction will become warmer, the other cooler. Seebeck coefficient tells us the magnitude of voltage developed per unit temperature of different junction materials.

Peltier modules differ from thermocouples in that the metal junctions are replaced with P and N type semiconductors (see Figure 2). The amount of heat transferred by the module is directly proportional to the DC current flowing through it. The current also heats up the module, so more heat has to be dissipated by the hot side than is absorbed by the cool side. Consequently, there is a point beyond which the Peltier module becomes inefficient.

Figure 2 Principle of a TEC cell
Figure 2
Principle of a TEC cell

The eTEC series HV14 modules, for example, can remove 1.4 W of heat at 25°C (77°F) ambient temperature through 1 mm2 (1.55 × 10–3 in2) area of the module.[1] The modules can also work in reverse, generating power when a temperature differential exists between the two sides.

There are advantages to Peltier modules. Because they are semiconductor devices, there are no moving parts and can perform well in harsh environments. By reversing their current flow, they can be turned from cooling to heating, thus electronic components fitted with a TEC could be operated well outside their specified temperature range by cooling or heating them as necessary. TECs disadvantage is their need for additional power, which may be substantial, while dissipation of the resulting heat to the environment may become impossible. The design engineer, therefore, must weigh many factors before making a decision to use TECs.

HEAT PIPES

Heat pipe technology is a two-phase cooling system. It has been widely used in laptop computers. A tubular heat pipe filled with water as the cooling fluid operating at 150°C (302°F) has thermal conductivity several hundred times that of a copper rod of the same dimensions, but weighs less. Similar to the TEC, the heat pipe transfers the heat from a component, such as a microcontroller to an external heat sink radiator.

A heat pipe comprises a vacuum-tight pipe, a wick, and a working fluid (see Figure 3). The evacuated pipe is filled with a small amount of the working fluid, just enough to saturate the wick, and sealed. This results in an internal equilibrium of vapor and liquid.

Figure 3  Principle of a heat pipe
Figure 3
Principle of a heat pipe

Heat from the cooled component warms up the evaporator end of the pipe. This upsets the equilibrium by generating more vapor, which is at slightly higher pressure. The higher pressure vapor travels to the condenser which is at a lower temperature, where it condenses, giving up its latent heat. The condensed fluid then travels back to the evaporator through the capillary action of the wick. Wicks made of sintered powder work even against the gravity.

The heat pipe operation is passive. It does not require any additional source of energy other than the heat dissipated by the cooled device. It is simple to design and manufacture. Its MTBF is estimated to be in excess of 100,000 hours. There are many coolant fluids, among the common ones would be water and ethanol (alcohol). Fluorocarbon FC-77, for example, is suitable for high-performance operation from about –95°C to 200°F (–139°C to 392°F).

The thing to remember is that the heat pipe is not a heat pump like the TEC. Its condenser must be at a lower temperature than the evaporator.

STAY COOL

In this article, I covered the most common cooling techniques. Keep your electronics cool! 

REFERENCE
[1] Laird Technologies, “Thermoelectric Modules,” THR-BRO-THERMOELECTRIC-MOD 0113, 2013.

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RESOURCES
Electronics Cooling, “All You Need to Know About Fans,” 1996, www.electronics-cooling.com/1996/05/all-you-need-to-know-about-fans/.
———, Blowers/Fans/Filters, www.electronics-cooling.com/category/applications/blowers-fans-filters/
Transterm, “Overview of Heat Pipe Basics,” www.transterm.ro/Overview.htm.

PUBLISHED IN CIRCUIT CELLAR MAGAZINE • MAY 2016 #310  – Get a PDF of the issue

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George Novacek was a retired president of an aerospace company. He was a professional engineer with degrees in Automation and Cybernetics. George’s dissertation project was a design of a portable ECG (electrocardiograph) with wireless interface. George has contributed articles to Circuit Cellar since 1999, penning over 120 articles over the years. George passed away in January 2019. But we are grateful to be able to share with you several articles he left with us to be published.

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Keep It Cool

by George Novacek time to read: 6 min