Thermal Resistance in Electronics

Thermal resistance is the ratio between temperature difference and power dissipated. In electronics, Thermal resistance is a great interest for engineer. It is due to every electronic equipment produce heat and need to be cooled. If they cannot be cooled properly, it will be harmed because of overheat problem.

Thermal resistance can be analogized by electrical resistance. Current represent heat flow, voltages represent temperature differences, and Resistor represent thermal resistance. It can be simplified: R_th=∆T/Q is analogized by R=V/I.

There are several kind of thermal resistance, such as conductive thermal resistance, convective thermal resistance, and spreading thermal resistance. Thermal resistance is basically needed to calculate heat transfer from one point/surface to another point or surface.

In electronic, Thermal resistance is the parameter that informs how effective heat dissipated can be transferred to the ambient. Moreover, it can be used to determine heat source temperature, usually chip junction temperature. In other words, heat source temperature can be known if thermal resistance is already known.

Let’s consider a simple electronic device with heat sink. Heat transfer is started from junction to casing and finished at ambient. This construction results several thermal resistances, those are junction to casing thermal resistance, casing to ambient thermal resistance through heat sink. Total thermal resistance of this system is R_total= (T_j-T_amb)/Q= R_JC+R_CA

Power dissipated from electronic device is assumed that all electricity is converted to heat. Therefore heat dissipation is : Q=VxI, where Q represent heat dissipation, V represent Voltage, and I represent Current. For LED heat dissipation, sometimes optical power from LED is considered, so heat dissipation becomes, Q= P_electrical-P_optical = (V*I)-P_optical.

Types of fan

There are several air moving device, those are: fan, blower, and compressor. All of their function is similar, that is to create air flow. Some expert classified them based on their pressure. Fan is the lowest, less than 1000mmAq. Second is blower, 1000-10000mmAq. The highest is compressor, more than 10000mmAq.

Fan is used at many kind of the equipments, especially cooling system equipments. For example, air conditioner, cooling system in electronics, or even just fan for summer. All of them are used to enhance heat transfer from the object to the ambient.

Based on the air flow direction, fan can be divided into two types, centrifugal and axial fan. In centrifugal fans, air comes from the axes direction of rotation and exhaust at periphery. In axial fan, air comes from behind and exhaust to the front.

There are three kinds of centrifugal fan. First is sirocco fan, this fan has forward curved blade, low RPM, Low noise, Low static pressure, and well suited for residential HVAC application. Second is air foil fan, this fan has backward curved blade, high RPM, Low noise, and usually used for clean environment. Third is turbo fan, this fan has backward curved blade, high RPM low noise, high static pressure, and high efficiency.

There are also three kinds of axial fan. First is propeller fan, this fan has propeller only, large flow rate, low static pressure, low efficiency. Second is tube axial fan, it is similar with propeller fan but improved by casing, higher total pressure capability than propeller fan. Third is vane axial fan, this fan same with tube axial fan but have vanes to straighten, this fan has high efficiency.

Thermoelectric cooler

Thermoelectric cooler is a device that converts from electricity to temperature difference. This device consists of p-type and n-type materials. When voltage is applied, current flows and result the difference temperature between its surfaces . Hot and cold surfaces can be reversed by reversing the direction of current.

Thermoelectric

Thermoelectric cooler is one of the methods to produce low temperature. Comparing with conventional method (vapor compression), thermoelectric cooler has advantage such as: thermoelectric can be a cooler or heater by change the direction of electrical current, no vibration, quite, reliable, and no refrigerant (environmental friendly)

However, disadvantage of thermoelectric cooler performance is very low. Coefficient of performance of Thermoelectric cooler is only about 0.3 (vapor compression COP : 3-4).

Due to its advantages, thermoelectric cooler can solve the problem that conventional method cannot be applied to it, such as electronics cooling.

The application of thermoelectric has been introduced in many aspects. We can find thermoelectric cooler system at small refrigerator or even thermoelectric air conditioner. In electronics, thermoelectric cooler also is one of the solutions to maintain chip temperature below allowable maximum temperature. Thermoelectric can be attached to the chip and cool the chip directly.

Performance of thermoelectric will decrease if heat dissipation from hot side is not released properly.  Heat sink can be attached to the hot side of thermoelectric cooler to support heat transfer from hot side to the ambient. Moreover, thermoelectric cooler can produce very low temperature by making cascade thermoelectric cooler. It means hot side thermoelectric cooler is attached by another thermoelectric cooler.