I. Introduction As the performance requirements of avionics, military and space electronic systems increases, so do the demands on the power supplies for these systems. Successful system designs demand the power distribution architecture to be optimized for maximum efficiency with a minimum size and weight. Power system cost and development schedule must also be taken into consideration. One possible solution is to develop a custom power supply that meets all of the system load requirements. The downsides of this approach include long development time, high non-recurring engineering costs, and system flexibility limitations. Additionally, should the load specifications change during system development, then the power supply will need to be redesigned to accommodate changes, incurring even more time and cost
A better solution is to assemble the power distribution system using standard off the shelf DC-DC converter modules to meet the system load requirements. Power supply companies including VPT Incorporated offer an extensive line of isolated and non-isolated DC-DC converter modules that offer both high efficiency plus high power densities. These modules can be assembled with various front end modules, such as EMI filters or inrush current limiters, to provide a complete power distribution architecture that remains flexible for system design changes. If load specifications change in a system, then an engineer can simply change out a module or reprogram a module to match the new load requirements. With the high power density of the DC-DC converter modules, the complete power system can be much smaller and lighter than a custom discrete designed power system. II. Selecting the architecture When developing a power system using DC-DC converter modules, selecting the proper system architecture is critical to optimizing for efficiency, size and weight. Most systems attached to the 28V DC power bus require an isolation boundary, so the traditional approach for systems requiring multiple output voltages is to use an isolated DC-DC converter for each output voltage as shown in Figure 1.
Figure 1: Traditional power system using isolated DC-DC converters
The disadvantages of this approach include increased size and weight due to the unnecessary complexity of repeating the isolation boundary in each converter. One method of reducing the power system size and weight has traditionally been to use one isolated converter feeding several linear regulators to provide the lower output voltages. However, this approach results in poor system efficiency.
Figure 2: Improved approach using point of load converters.
A much better approach for systems with multiple low voltage load requirements is to use one isolated converter feeding several high efficiency, non-isolated point of load (POL) converters as shown in Figure 2. The comparison of the systems in Figure 1 and Figure 2 given in Table 1 shows that the POL approach significantly improves many parameters over the traditional, multiple isolated converter approach.
Table 1: Traditional and POL approach comparison
Additional benefits of the POL converter approach include a reduced bill of material component count, system flexibility, and lower system cost. The VPT POL converters output voltages are programmed with a single external resistor. This ensures that one part number can be used for all of the different load voltage requirements in the system, thereby simplifying the bill of materials. The POLs programmable output voltages also add greater system flexibility. For example, if a system voltage requirement changes during the design cycle, then the system change is limited to a single resistor value change. Finally, a simple POL converter cost is lower than the more complex isolated converters, resulting in a lower overall power system cost.