Don’t Make the Cable the Weakest Link in Delivering Power Supply Efficiency

Amateur cyclists can spend considerable sums on the very latest, ultra-light-weight, high-performance bikes in order to shave seconds off their personal best times. And, while investment in the newest tech is no doubt going to enhance performance, it may be that in some cases a rider could do even better if they simply lost those few extra kilos that they have been carrying since the holiday season!

But what does this have to do with delivering energy-efficient power designs?

Well, the situation with the cyclist and the bike is not so different to that of the power engineer who puts effort and resource into creating the most efficient AC/DC or DC/DC power converter, only to find that the in-the-field efficiency is reduced by ‘real-life’ factors. One of those factors is losses from the cable that connects the supply to the circuit it is powering.

Factors to Consider When Choosing Cables

Consider the example of a modern 60W or 100W power adapter in which a highly efficient AC/DC power supply is used to quickly charge a mobile phone or other portable device via USB.

If the power supply is built around a topology such as Eggtronic’s QuarEgg and SmartEgg, designers can expect conversion efficiency of up to 96% at full load and 92% at light load.

However, those efficiency benefits can quickly be eroded by compromising on the USB cable. The voltage drop resulting from the wrong choice of cable may mean that the device does not work as originally intended - charging more slowly than expected or, in extreme cases, not working at all.

Of course no cable technology can be completely loss-free and even on a good cable it is not unusual to see a power dissipation of around 2W within a 100W charging scheme. The key to mimimizing this dissipation is to keep the I2R loss as low as possible, which is why cable resistance is fundamental.

Because a conductor’s resistance is determined by the equation R = ρ*L/A (where ρ is material resistivity, L is conductor length and A is cross-sectional area) then resistance will depend on cable length, diameter and the type of material used. Though this does not automatically mean (as people sometimes assume) that the more expensive the material the lower the resistivity. Gold, for instance, with a resistivity of 2.45 x 10-8 ohm-meter, has a higher resistivity than both silver (1.64 x 10-8 ) and copper (1.72 x 10-8 ).

However, these are only minimum requirements and, in reality, the best solutions should deliver better performance than the basic specification. At Eggtronic, for example, when it comes to high current capability we avoid cables longer than 2m to reduce the power lost. through the cable length.

Let’s consider a simple example and suppose that you have an Eggtronic 45W adapter capable of 95% efficiency. When compared to a standard efficiency of 91% you have saved 1.8W power. However, if you then have a poor quality cable exceeding the USB limit - e.g. 750 mV on VBUS and 500 mV on GND resulting in an extra drop of 500mV (compared to the maximum limit) - then at 3A output full load power loss (I*V) will be 1.5W. As a result the improved results in adapter efficiency enabled by the advanced converter technology are almost all eliminated thanks to poor cable performance.

* The maximum allowable voltage drop is 500mV for VBUS and 250mV for GND.

The table above shows the results of some research that Eggtronic has conducted on various cables in its own engineering labs. As this illustrates, saving money on cables is a false economy when it comes to delivering high-efficiency adapters.