Designing for complex power systems

In addition to the efficiency, density, and thermal improvements, a new generation of power components targeting the three key parameters of efficiency, power density and thermal performance also feature larger voltage step-down ratios and integrated magnetic structures. A further benefit is that they facilitate the emergence of new power distribution designs, including factorized power architecture (FPA), as well as supporting new applications such as High-Voltage DC (HVDC) power distribution, which enable further improvements in efficiency and the use of alternative energy. 

This article shows how these devices, which are fully optimised for efficiency, power density, transient response and EMI, help engineers to configure high-performance power systems predictably and cost-effectively using a proven design methodology. By taking this approach, rather than developing power chains using discrete components, the designer benefits from the fact that all the key parameters are already optimised and ready for the designer to use for a best-fit solution for any power design project. In addition, the configuration of these blocks is completely re-useable for future designs, leading to considerable savings in time and effort.

There are three steps in this power component design methodology: identification, architecture and implementation. Identification can be summarised as the designer’s overview of a project’s power requirement, defining elements such as the number of rails, voltages and current needs, as well as timing considerations. These requirements can be drawn up in a list, at the same time as initial consideration is given to the type of products that are available to meet these requirements.

There are many sources of information to aid the selection process for products to meet these requirements. For example, Vicor provides a solution selector tool that searches the database of components available and recommends solutions that meet the customer’s input and output needs. Using such an intelligent tool reduces the time taken to produce a shortlist of possible components to almost zero, and makes it easy to select the best one for a particular design based on the criteria that are most important to the application.

The first of these criteria is power delivery. Here power components are required to take a high-voltage DC or AC source and bring it to a Safety Extra-Low Voltage (SELV). In many high-performance applications engineers are working with high voltages and high currents to deliver power to their system, resulting in high levels of heat dissipation. Hence choosing components that are thermally adept is critical. These components may be mounting on a chassis or on the motherboard, and the appropriate cooling for each will need to be considered.

The next consideration is the delivery of power from the 48V rail to the point of load. Here, engineers need to be careful in the selection of the proper rail for their application, since having too many stages in the conversion process will reduce overall efficiency. In recent years power designs have started to move from a 12 V rail to a 48 V rail to deliver higher efficiency systems. The challenge here is selecting the best components that deliver the proper performance at the highest efficiency: a process that is greatly speeded by tools like Vicor’s Whiteboard, which helps engineers evaluate the performance using different bus voltages for their design.

Finally there is the selection of the point-of-load components. Based on the bus voltage selected, the engineer needs to pick the components required to reach the point-of-load requirements, which can be as low as 1V at high current. Depending upon the application requirements, engineers can select isolated or non-isolated PoL devices.

Designers can also make use of components designed for factorized power architectures, where the regulation and voltage transformation/isolation functions are separated. Choosing the latter enables the designer to have high power density, which translates to the ability to convert a lot of power in a small space.

Architecture
The first step in developing the system architecture is to create a block diagram of the power system, starting at the output and then working backwards towards the input. It is generally better to start at the lowest power level and work up from there, so that the power component class can be reviewed and changed wherever necessary as the power level grows.

It is important to select the right component class for the appropriate power level. For example, at low power levels, ‘System in Package’ products (SiPs) such as the Vicor ZVS Buck Regulators are the best solution. At higher power levels a better approach might be to use Vicor’s ChiP (Converter housed in Package) products. Depending on the complexity of the number of rails required to drive the loads, a mixture of SiPs and ChiPs may be an appropriate choice. This approach will help to provide the optimum combination of maximum power density and cost effectiveness, as well as maintaining the high-efficiency operation of every device in the system.

It is clear that the first three rails (MR#1, 2 and 3) are the rails that require the highest power class devices, while the last five (MR#7 to AR#2) are the lowest in power class. The remainder (MR#4 to MR#6) fall in between. Designers will need to use their discretion and judgment in terms of device selection here. Working back from the outputs, it is possible to start to build up a picture in the system block diagram of the classes and power levels of the power blocks that are needed. Continuing to work back, it is possible to identify the classes of components necessary to deliver the power levels in each rail. In this example we have chosen to use PRM Regulators and VTM Current Multipliers, together with ZVS Buck PoL regulators. At this stage it is important to bear in mind that the power levels required to ensure that the loads are balanced and the power capacity of each individual device are utilised.

Implementation

Once the blocks have been finalised, the designer needs to match part numbers to these blocks, as well as noting any special circuits for implementing functions and simulating individual chains of power conversion components. Additional circuitry that needs to be developed might include filters, holdup circuitry and power sequencing. The engineer also needs to think about thermal, termination and packaging considerations at this stage of the design.

There are some special requirements for the power supply: a delay on MR #3 until auxiliary rails are up; and tight regulation on MR #3, which will require the use of a remote sense loop. It would also make sense to consider configuring the PRM regulators for the exact load current limit and other parameters to match exactly the rail and load requirements. For engineers who need to tweak the design using the PRM, Vicor provides a PowerBench simulator that assists further understanding of the performance of the system.

In the past, engineers have made component selection and have analysed the power system efficiency of each stage (and of the total system performance) by calculations that reference the device datasheets. Although completely satisfactory, this method can become more than a little tedious. In order to streamline the design process and save time, Vicor has recently introduced the PowerBench Whiteboard: an online tool for the design and analysis of power systems with the appropriate set of power conversion components. With The Whiteboard it is no longer necessary to wade through the operating and efficiency parameters contained in the datasheets. Instead, an engineer simply draws the blocks in the online tool and all calculations are done within milliseconds. By keeping the familiar metaphor of sketching a system on a whiteboard, while also adding automatic parameter lookup and calculation, the Whiteboard further reduces the time to complete a design when using the power component design methodology.

In addition, Vicor’s Solution Selector is tightly coupled to the Whiteboard tool, which means that a design recommended by the solution selector can automatically import the design into the Whiteboard so that engineers do not need to draw the system themselves. At this point, they can also tweak the design to further meet their needs and quickly understand the efficiency of the design.