Easing The Conversion To Digital Power
The benefits of digital power are well documented, with countless technical articles and analyst reports filling an array of engineering publications. The problems it solves are recognised by both large and small OEMs from the medical, industrial, automotive and almost every other market sector. But, there are barriers to the acceptance of digital power. By Bruce Rose, Technical Marketing Manager for CUI Inc.
We live in an increasingly digital world, but there are some functions that resist moving out of the analog domain, and in some cases analog may still be the best alternative. But for the majority of functions a digital solution does exist and offers many benefits, even if it has not yet achieved wide-scale adoption. Digital power is definitely in this latter class.
Power supply design has traditionally been the province of analog experts, with recent technologies and circuit trends placing increased demands on their skills. One of the most challenging functions is point-of-load regulation used to deliver supply current to advanced microprocessors and FPGAs. These parts — with their internal clock frequencies of hundreds MHz to several GHz — and their power dissipation requirements mean the supply must react properly in the face of step-changes in load of tens of Amps, on a nanosecond timescale while maintaining, for example, 1V output to a fine tolerance. This demands a very wide bandwidth in the regulator’s control loop; the function can equally well be thought of as a wide-band amplifier that is set to a fixed-DC-level output.
To achieve this, an analog designer may apply a linear control loop topology, contriving high gain at DC, and applying standard control-loop techniques to place poles and zeros in the loop’s response so that it responds to step-changes in load rapidly, but without ‘ringing’; remaining stable under all load conditions. They might analyse their circuit from first principles to determine its gain/phase response profile; or they might measure it with a network analyser; or in less demanding cases they might reach a solution empirically.
The Digital Approach
Conversely, the digital approach uses analog-to-digital converters, which digitise the output voltage, and a microcontroller core that runs software algorithms to implement adaptable digital control loop schemes. The digital control loop then generates signals to drive pulse-width-modulated (PWM) outputs to control the power switches, typically MOSFETs.
With the digital control loop comes the ability to adapt the control algorithms to optimally react to the immediate input voltage and output voltage and current conditions. All of the ‘secondary’ PSU (power supply unit) functions such as sequencing, ramping, margining, fault response and reporting of voltage, current and status, and external control, become exercises in software — taking delivery of stable voltage to be the primary role.
The power management bus (PMBus) is therefore the default medium to communicate with the digital power component and this gives engineers significant flexibility. Indeed, this level of flexibility can be shown by looking at the specification sheet of a digital control IC made by Zilker Labs (Intersil), for example. The list of PMBus output commands (Vout mode/command/trim and so on) comprises 15 entries: just for time-related commands (delay, rise time, fall time) there are five commands; overall there are well over 100 commands in the set.
CUI's GUI enables the simple implementation of a digital power supply
Removing Perceived Complexity
For both individuals and companies, the barrier to adoption of digital power has been perceived as relatively high. This is particularly true when the context is not that of a large IT infrastructure rack, or a telecoms routing centre, but rather in smaller projects, or when only a proportion of the task demands advanced capabilities. Both engineers and their management have often concluded that while the benefits of digital power would be ‘nice to have’, the cost and the risk of adopting a new technology is too high.
Much of the literature and design support material that has been published about digital power has assumed a large-scale conversion to the digital philosophy. What has been lacking is an approach that would allow exploitation — selectively at first — of the benefits and performance of digital power without going through a steep learning curve and without putting a project schedule at risk by moving a complete power supply system design over to an unknown (to that design team) technology.
To bridge this gap in skills and confidence, a ‘toe in the water’ approach to digital power adoption is essential in order to allow power engineers, system architects, product marketers and program managers to assess if the move to digital power suits a project.
One approach, which has been implemented in the NSM2P series of Novum Advanced Power modules, is a No-Bus approach. This lets OEMs benefit from efficiency gains, auto-compensation advantages inherent in digital power systems without incorporating a digital bus into their system.
But prioritising ease of use doesn't mean sacrificing performance. The NSM2P delivers industry-leading performance in a package that is easier to implement than an analog point of load module. Users do not have to write any code, but instead use an intuitive GUI to configure the required input and output conditions, plus functions such as voltage sequencing, voltage margining, and voltage tracking. Once the module has been configured, any further connection to the bus is no longer required: the module exceeds the performance of an analog regulator while occupying a smaller footprint.
For OEMs seeking to implement a fully digital power system, CUI has developed the NDM2P and NDM2Z series, which opens up the entire suite of digital features to the user. In doing so, organisations can benefit from the telemetry and power management advantages of digital power while leveraging the ‘plug and play’ aspect of a power module design.
No matter the iteration however, perhaps the greatest benefit — and the one that addresses one of the most difficult issues in the analog/digital skills gap — is automatic compensation; therefore this function is present in all of the Novum Advanced Power modules. Auto compensation enables the module to dynamically set optimum stability in real time as conditions change. It eliminates the need for manual loop compensation, one of the most labor intensive, time-consuming aspects of analog power design, and the need to build-in margins for component ageing, manufacturing variations, temperature and other factors. The realised benefits for the design engineer include improved performance, faster time to market and lower overall cost.
Thanks to recently released module developments such as auto-compensation and the No-Bus platform, digital power continues to address the perceptions that have limited the technology’s wide-scale adoption. Now, a greater number of OEMs will see the opportunity to migrate to digital and benefit from improved system efficiency and auto-compensation algorithms. This industry shift towards simple-to-use power modules can only be a good thing.