Back to Basics: Heat Pipes

Heat Pipes

Most circuit designers would have used heatsinks in their circuits to dissipate heat generated by Regulators, MOSFETs, Drivers, etc. Heatsinks are used to dissipate heat to the surroundings whereas heat pipes are designed to transfer heat from one region to the other where cooling can take place. It is used in spaces where there is not enough space to have a large area of heatsink right next to the heat source. You would probably have seen them as closed copper-coloured tubes on processor heatsinks or inside laptops.

At their most basic level, heat pipes are simply sealed tubes filled with a liquid that is designed to absorb and transport heat. When one end of the heat pipe is exposed to a heat source, the liquid inside the tube absorbs the heat and evaporates into a gas. This gas then travels down the length of the tube to a cooler area, where it condenses back into a liquid and releases the heat it has absorbed(Via a heat sink or fan or any cooling mechanism). The liquid then flows back to the hot end of the tube by a wick. What makes them extremely good is that they have a very large(at least 10x) thermal conductivity than that of a pure metal block like Copper or Aluminium because the liquid inside can change to vapor and carry heat faster. They rely on evaporation on one side and condensation on the other side. They can also operate over long distances, allowing heat to be transferred away from critical components to a remote location.

Power Transfer Heat Pipe

Heatpipes do come with a drawback, since it relies on fluid moving from a colder section to a hotter section in the liquid phase via wicking, their performance of heat transfer drops significantly if it’s made to work against gravity. If you see the charts you will find that how heat transfer properties fall as you change the orientation angles. So, when using heatpipes, it’s important to consider the orientation of the product to maximize heat transfer

For more Recommended Reading: Wakefield Thermal Design guide, ACT Cooling & Celsiainc website

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Back to Basics: Skin Depth

Skin effect is a phenomenon that occurs in conductors carrying alternating current (AC), in which the current density is highest near the surface of the conductor and decreases towards the centre. It’s measured as the distance from the outer surface of a conductor to the point where the current falls to 37%(1/e) of the max current value. It depends on the frequency of the signal travelling in the cable. Higher the frequency, lower the depth from the surface it travels. For a DC signal(f=0Hz), it travels through the entire cross-section of the signal.

So Why does it occur? When an alternating current flows through a conductor, it generates a changing magnetic field around the conductor. This magnetic field, in turn, induces an opposing electric field within the conductor, which creates eddy currents that flow in a circular path around the conductor. These eddy currents produce a secondary magnetic field that opposes the original magnetic field. This field is strongest in the centre and hence it pushes the current towards the outer surface of the conductor.

This means that to carry a high-frequency signal, it’s relatively wasteful to have a very thick conductor as most of the current is on the outside surface. Hence you would see the usage of multi-stranded wires than solid core wires to carry these types of signals. Induction cooktops you use to cook food in your kitchen also rely on high-frequency switching coils. Due to its high freq, the skin depth kicks in and heating only happens in a thin region of the bottom surface of your pan. Skin depth depends inversely on the conductivity of the material and magnetic permeability. Better a conductor, lower the skin depth. Similarly, more magnetic a material, lower the skin depth. For PCB designs in very high frequencies, Skin depth usually would need to be factored in the field solver as impedance will get affected.

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