Fast, Online Simulations Shape Advanced 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 1 V 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 and 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, 12 V or less.

To assist in shaping this architecture, the designer can call on a cast-of-characters of power supply modules that is re-defining how this task is carried out. The fundamental functions of the power supply remain transformation (from AC to DC and from one voltage level to another), regulation, and isolation. Rather than working with pre-defined combinations of these basic functions, today’s market offerings allow the engineer to locate voltage level conversion, isolation and regulation precisely where they are most effective, anywhere along the conversion chain. Some of the terminology is well-established: the POL (point-of-load) regulator, and the NIPOL (non-isolated) equivalent, for example, appear at the end of the chain and are fed in turn by various configurations of DC-DC conversion.

Intermediate Bus Architecture

Figure 1: Intermediate Bus Architecture

A recent addition serving this, the 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 re-located to the 48 V level; lower rails derived with fixed-ratio IBCs are then regulated, by construction. (Figure 1)

The designer therefore 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. 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 Vicor has recently addressed this need for its 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.

DC-DC Selector

Figure 2: DC-DC Selector

Against this background, Vicor 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 center can access the IBC Power Simulation tool to – in an interactive environment – accurately optimize the intermediate-bus portion of an IBA-architected power supply, before building any prototypes. Prior to carrying out a simulation, a user of the freely-accessible online tool is guided through a selection procedure to choose the optimum solution, or range of solutions, for the DC-DC conversion function itself. Users enter input 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.

IBC Circuit Configuration

Figure 3: IBC Circuit Configuration

 

IBC Thermal Simulation

Figure 4: IBC Thermal Simulation

 

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 (Figure 3); all of the passives appear with initial, default values, but they can all be altered. Similarly, a default cooling airflow is assumed, but that, too, can be adjusted at will – as can the ambient temperature. Simulation runs, which take only seconds from any on-line browser, return waveforms showing ripple levels on output and load voltages, against those for input and source voltages, with corresponding currents, for the power level requested. Simulation runs for detail performance span thermal (returning steady-state device temperature, power loss and efficiency); steady-state operation (Figure 4);  response to applying input voltage, or an input voltage step; response to a load step (i.e. load transient response) (Figure 5); and start-up/shutdown behaviour when the input voltage is present but the unit is switched by its enable pin As an example of the detail available, the enable-pin start-up simulation – over a default 200 µsec – presents charts of output voltage as it rises in response to the enable-pin signal; load and output current, with complete transient behaviour, over the same time interval; together with input voltage and current transient waveforms (Figure 6). All of the charts are interactive, with cursors that read off instantaneous values at any time.

Load Step 32 to 64A

Figure 5: IBC Load Step 32 to 64A

 

Enable Turn-on

Figure 6: IBC Enable Turn-on

With intimate knowledge of the design and operation of the IBC modules, Vicor has been able to characterize the detailed behaviour of each one, over its complete operating range, and with all allowed passive component values. With this data set underpinning the simulation tool, there is no need for it to perform a full circuit-simulation run each time a user invokes it; it is both accurate and fast, even with many users on-line, requesting simulations at the same time. From the designer’s perspective, this methodology of on-line simulation delivers completely accurate data, charting every parameter that would be instrumented if the development was by way of hardware prototyping – but in a timescale that allows multiple design configurations to be conceived, evaluated and compared in the course of a single day.

Vicor’s PowerBench Continued Improvements

The PowerBench online design center is constantly being updated with new products and capabilities. You can design your own DC-DC converters and power supplies using our proprietary simulator or using hundreds of predefined designs. Visit PowerBench now. 

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