Bob Marchetti, Senior Product Manager at Vicor Corporation explores how fast on-line simulations are shaping advanced, modern power supply architectures
The task of the power system engineer grows steadily more complex. At the level of individual PCBs, semiconductor devices – microprocessors, FPGAs and the like – demand a range of discrete voltage levels, frequently at around 1V and at currents of tens of Amps.
The physics of distributing power at these levels mandates regulation as close to the point-of-load as possible: I2R losses are minute but inescapable inductances are always poised to degrade the performance of low-voltage, high-current paths that see rapid step-changes in power demand.
At the other end of the power-conversion chain, the first stage, as ever, is conversion from AC mains input to system DC. Between the two, the designer has a range of choices to make when defining the architecture of the power distribution scheme.
System-wide distribution, at rack level, is most commonly carried out at the well-established 48-VDC. A further level of distribution may be appropriate at back-plane and individual PCB level, at, perhaps, 12V or less.
Intermediate Bus Architecture or IBA, is the IBC, or Intermediate Bus Converter. This is a DC-DC conversion block that provides a fixed conversion ratio from input to output. In effect it is a ‘DC transformer’ and it permits significant reconfiguration of the supply architecture. For example, system-wide regulation can be relocated to the 48V level; lower rails derived with fixed-ratio IBCs are then regulated, by construction. (Figure 1).
The designer has new areas of freedom, and must evaluate different supply architectures for performance and efficiency. Adding to the task is the fact that, today efficiency must be evaluated not only at full rated power, but at reduced power and at standby levels. Depending on how the ultimate loads are structured and on the power they draw at full, partial and standby power levels, different configurations can – and almost certainly will – show variations in performance.
Each of the separate classes of functional module (DC-DC converters, IBCs, and regulators, for example) now perform at very high individual levels of efficiency and when concatenated into a complete PSU, easily outperform an equivalent supply of only a few years ago. But the additional design freedoms also mean that there is no one-size-fits-all, best solution.
In fact there are new degrees of freedom to consider: although efficiency is high, there are still some inescapable losses and, to a certain extent, the designer gets to choose where to dissipate the heat.
The wide range of possibilities
Evaluating the range of possibilities is a challenge in the design process. Each functional block or module comes with a typical application circuit configuration, a range of permitted values for its associated passive components, and formulae to determine its operating parameters.
Before building and testing a given configuration, a circuit simulation is a valuable step and company’s such as Vicor are addressing this need, for example with their IBC product offering.
The exact circuit configuration that lies within a module such as an Intermediate Bus Converter is proprietary, and for that reason, if no other, a complete circuit description of the IBC is not available for the engineer to incorporate into a circuit simulation. Even if it were revealed, it is likely that it would be of limited usefulness.
Achieving conversion efficiency in the close to 100 percent region is only achieved by circuit design, within the module, that manages every increment of charge as it moves between the active and passive components, on a nanosecond-timescale. Some general-purpose analogue circuit simulators may not handle the subtleties of the power switching waveforms with sufficient accuracy.
Against this background, the company set out to provide designers with a resource that provides the greatest amount of information about how an IBC would perform in a real circuit configuration, in the least amount of time. Users of the PowerBench online design centre can access the IBC Power Simulation tool too – in an interactive environment – accurately optimise the intermediate-bus portion of an IBA-architected power supply, before building any prototypes.
The DC-DC selector tool
Prior to carrying out a simulation, users of the tool are guided through a selection procedure to choose the optimum solution, or range of solutions, for the DC-DC conversion function itself. Users input the voltage and its allowed range, required output voltage – or conversion ratio if they already know they need a current multiplier – and power level (Figure 2).
The DC-DC selector tool returns a list of all devices that will meet the requirements, with a comprehensive spreadsheet of operating parameters; not only voltages and currents, but extending to matters such as power density, output capacitance required, PCB area, regulation, and cost – to mention only a few. All of the parameters can be exported to a spreadsheet, with an additional ‘compare’ function.
Having chosen one or more candidate products, the user then invokes the simulator. The tool presents the device in a nominal circuit configuration to match the initial conditions that the user specified
Simulation runs, which takes only seconds from any on-line browser, returns waveforms showing ripple levels on output and load voltages, against those for input and source voltages, with corresponding currents, for the power level requested.