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Putting power forward

Steve Pimpis, AMP Group -October 06, 2016


Introduction


Designers of advanced computing systems are no longer able to consider the power supply as a “black box” that can be plugged in at the end of the project. Giving due consideration to power design at an early stage is essential given the growing complexity of server boards, demands for greater power and efficiency, and the need to plan for multiple product generations. On the other hand, engineers also need flexible power solutions in order to respond to system design changes and adopt a platform approach to the power design, which can help streamline future development.  The ability to easily configure, control and monitor power delivery functions is a valuable characteristic enabled by digitally configurable power modules.

 

Pressure on power design


High-performance computer boards such as data-center servers present increasingly complex routing and component-placement challenges as designers seek to maximize data-processing and storage capabilities in the minimum possible area to comply with the standard rack dimensions. With a mix of advanced processors, ASICs and FPGAs that feature large numbers of I/Os and multiple power domains, the PCB can incorporate 20 layers or more for timing-critical high-speed signal traces and power distribution.


Up to 40 or 50 power rails can be needed, which call for a large number of point-of-load (POL) converters that are powered by intermediate bus converters (IBCs) fed by an AC/DC front-end power supply (Figure 1).  It is often the case that multiple power rails are used to provide power to a single IC and the order in which the rails power up and down is important so as to not destroy the IC.  The required sequencing between the power rails involves routing signals to communicate the status of the different power supplies.




Figure 1 The proliferation of supply rails at the board level has resulted in an intermediate bus architecture (IBA) that requires multiple POL converters on the system board.

 

The design of the power-delivery infrastructure is becoming increasingly exacting. Multiple power planes are implemented to minimize parasitic inductance and resistance, and there is also demand for large numbers of decoupling capacitors. These must be placed close to the load to provide tight voltage tolerance and ensure stability in the event of sudden load changes. Without adequate decoupling, such load changes can cause voltage rail transients that may then result in undesirable events such as spurious resets. In addition, the power connections must coexist with the signal traces and not interfere with their precision routing designed to ensure accurate control of path lengths for guaranteed timing.


As the constraints on power delivery become increasingly strict, designers need to consider the selection and placement of power modules and associated components at an early stage.  Implementing power delivery routing early in the design process enables the PCB traces for power delivery to conform to signal integrity guidelines similar to those used for high-speed signals.  Squeezing in the power-distribution circuitry late in the project seldom affords the opportunity for clean power delivery and often results in reduced performance of the product.


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