When the designer is concerned about the distance between the power supply and the load, it is usually related to the IR drop (V = IR) or loss caused by noise pickup. The IR pressure drop problem can be solved by various strategies. These include the use of thicker wires, the use of four-wire Kelvin sensing on the load (although it will bring its own potential problems due to the feedback loop oscillation), or the use of a more distributed architecture powered by an intermediate bus converter. Multiple point-of-load (PoL) DC/DC converters for the load. The noise is attenuated by ferrite beads on the power cord and bypass capacitors placed close to the load.

However, the same physical law that characterizes voltage drop also states that power consumption P = I2R. Of course, this power is dissipated in the form of heat, which is not a problem in most designs. Compared with the overall system dissipation, the power loss in the cable and the resulting heat are negligible.

This situation is changing because data cables and even DC power cables are increasingly being routed together with other heat-dissipating cables, usually in locations with little or no convection cooling. In many commercial and industrial environments, large amounts of AC cables pass through risers and plenums, so there is almost no air flow.

In fact, the National Electrical Code (NEC) in the United States and similar regulations around the world define the maximum free air dissipation for these cables, and then add a derating factor for cables with little or no air flow. When these power cables are laid side by side with other cables, the situation becomes worse, so in addition to cutting off the airflow, there are adjacent heat sources.

Of course, placing cables carrying high currents in enclosed spaces is not new. There may be hundreds of amperes in the power cables of industrial cabinets used for motor control and data center racks. But these devices are designed to support the related thermal load, and their operating environment is bounded. No one will randomly "throw" another cable into a well-designed server rack that handles kilowatts.

In contrast, pushing one or two Ethernet cables into a plenum that is already full of AC cables seems harmless. However, the heat of these cables will degrade the insulation and electrical performance of the Ethernet cable.

The increased use of higher power Power over Ethernet (formerly known as PoE++, now officially known as IEEE 802.3bt) is exacerbating this situation because it allows more than 100 watts of power to be delivered to the load. Although most of it is dissipated on the load, some of it is dissipated along the Ethernet cable itself. For free air cables or relatively free spaces, this is not a problem; however, many of these PoE cables end up buried under carpets, or in tight risers, and narrow cables run side by side with AC cables (Figure 1) .

Figure 1 Power over Ethernet (PoE) provides flexibility for powering remote devices, but it also allows temporary wiring. (Image source: Intellinet Network Solutions)

These self-heating problems are modest compared to the problems associated with charging electric vehicles (EVs) at high-current charging stations.

Electric car chargers face severe thermal stress

In the case of electric car charging, although the distance between the charging station and the car is relatively short, we see hundreds of amperes and more current being pushed through the charging cable. In fact, the risk of the cable overheating is one of the many limitations that limit the charging rate (Figure 2). This is a problem that power system designers have solved, but there is more work to be done.

Figure 2 The key components of a typical DC electric vehicle charging system, taking the type 1 standard (J1772 AC + CCS) connector of the combined charging system as an example. (Image source: Purdue University)

A team from Purdue University analyzed, designed, and tested a method to increase the current-carrying capacity of ultra-fast electric vehicle charging cables from the current maximum of 500 A to more than 2400 A-an increase of nearly five times. They developed a This method is used to predict the heat transfer and pressure drop characteristics of laminar and turbulent flows through concentric circular annulus with uniformly heated inner walls and adiabatic outer walls. By capturing heat in both liquid and vapor forms, a liquid-to-vapor cooling system can remove at least ten times more heat than pure liquid cooling (Figure 3).

Figure 3 Schematic diagram of the annulus flow geometry and boundary conditions used by the researchers for the basic thermal model. (Image source: Purdue University)

Their solution involves pumping a highly subcooled dielectric liquid HFE-7100 through a concentric circular ring that simulates a section of an actual cable. The uniformly heated 6.35 mm diameter inner surface represents the electrical conductor, and the 23.62 mm diameter outer surface is used for heat insulation. In the external catheter. At these levels of power, heat, and fluid flow, testing and measuring "pipes" are complex, as are the various sensors and parameters of interest required for control and measurement (Figure 4).

Figure 4 A photo of the Purdue experimental facility, identifying its key components. (Image source: Purdue University)

You can read the full details in their two very long papers: "Refer to the Experimental Research on Supercooling Flow Boiling in the Ring by Thermal Management of Superfast Electric Vehicle Charging Cables" (20 pages) and "Comprehensive Theory of Supercooling" / Empirical prediction method "Refer to Circulation Boiling for Thermal Management of Ultra-Fast Electric Vehicle Charging Cables" (page 22). Both are behind the paywall, but an accessible copy is posted on the two-phase flow at Gwangju Institute of Science and Technology (South Korea) And Thermal Management Laboratory (TPFTML) web page.

Have you ever considered the self-heating of the power cord in your design? Is this an expected or unexpected surprise?

Intellinet network solutions, "What is Power over Ethernet?" Smart buildings, "Power over Ethernet (PoE): Power from the network" FS.com, "How to avoid overheating of PoE wiring?" Planet Technology USA, "How to keep PoE cables "Beam Cooling" International Association of Electrical Inspectors, "Wire Temperature Ratings and Terminals" Velo Project, "Conductor Ampacity and Terminal Ratings" Applied Thermal Engineering, "Cable Heat Reduction Model to Study Power Cabin Heat Dissipation and Airflow Distribution"

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns and product features.

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