Broschüre
Auswahlhilfe für Flex-Leistungsmodule
In the world of gambling, the worst thing is to chase losses, risking your shirt and worse. In power delivery network design, you are gambling with your job if you don’t. Every watt dissipated unnecessarily is a dollar and environmental cost, multiplying up with its increased cooling overhead.
Current produces dissipation in resistance, so, as with your utility AC supply, distribution at high voltage/low current is the way to go. For example, the recent trend toward increasing bus voltage in server blades from 12 V to 48 V reduces current by a quarter, and power lost in connection resistance by a useful 16x, due to that handy ’square law’ — that’s if the PCB designers don’t steal your gains with thinner tracks.
GPU current draw can peak at over 1000 A in AI applications
High currents right at the end load are inevitable though, with latest GPUs, FPGAs and CPUs in some cases gulping down over 1000 A, but at a fraction of a volt. So, although we can sneak tracks around at 48 V with maybe 15 A rating for this power level, we still need Point of Load (PoL) converters to generate the low voltage.
A traditional difficulty is that conversion tends to be less efficient for non-isolated DC/DCs such as PoLs with high input to output voltage ratios, as shown in the figure below. As a consequence, a second intermediate bus might be used at say 5 V or 12 V, but we do then have more converters with potentially more losses. However, an examination of the conversion efficiency curves of DC/DCs might show that there is a ‘sweet spot’ where multiple stages with lower conversion ratios can be as efficient as a single stage. More converters are a higher cost, but this is offset by savings in cooling systems if overall efficiency is higher.
Figure 1: A DC/DCs efficiency can vary strongly with voltage conversion ratio
We shouldn’t get too hung up on % efficiency values though — it’s power dissipated that matters. For example, although the efficiency in the graph above varies widely, these figures are for a converter delivering 40 A, or 24 W at 0.6 V and 132 W at 3.3 V. Actual losses are then largely the same in each case, between 4.5 and 5.5 W.
