Power supplies for industrial applications are similar in terms of their requirements. The following specifications should represent a typical application, for example an FPGA supply. Step by step the same design methodology can be used for similar designs in motor control, automation networks and machine vision.

The first step is to identify the possible voltages on the input of the power supply:

Vin nom operating: 24V
Vin operating: 18V.. 30.5V
Vin transient: 36V.. 42V
Tambient operating: -40°C..+85°C (without external heatsink or fan)

Electro Static Discharge (ESD), burst Electro Static Discharge (ESD), burst and surge, current injection requirements according to IEC 61132 PLC are not discussed here. The design of the power supply can be done independently of the protection circuitry needed for the system ruggedness against the above mentioned tests.

As the solution requires a multiple output solution, the WEBENCH Power Architect online tool can be used. After its launch the first screen (Configuration Step) will offer a mask to enter input and load specifications. Each load should be given the name actually used in the application ( Figure 1).

As it will be necessary to use as many modules as possible and check the option “Prefer Modules Solutions”. In industrial environments National Semiconductor power modules offer several advantages. These include: High reliability: The integration of the regulator and the inductor and even capacitors at critical nodes reduces the risk of individual failures. In addition, this offers the best thermal protection against over- temperature of each point of load regulator. The RoHS compliant TO263 package with leads guarantees high vibration ruggedness. This is highly advantageous when using the module directly attached to motors. In addition to that a standard soldering process can be applied during assembly and even hand soldering or rework during production; High Thermal Performance: The solid exposed pad ensures an optimum thermal coupling to the PCB. Very high ambient temperatures–up to 105°C–can be reached at maximum output current; System Integrity: National’s power modules guarantee electromagnetic emission performance respecting the international norm CISPR22 (Class B) and EN55022. The high integration and the use of a shielded inductor drastically simplifies the PCB layout and makes sure the buck regulator will not interfere with other sensitive analogue signal paths. The only layout requirement left is to place the input capacitor close to the input pin; and High Efficiency: The use of the synchronous buck topology enables efficiencies up to 93%. Please note that the efficiency is heavily dependent of the difference between Vin and Vout. For this reason a conversion in one stage from 24V directly to the low point of load voltage is not efficient and would cause thermal dissipation that is too high. So an interim bus voltage is recommended in many cases. Choosing the right combination of bus voltages and specific regulators optimised for those is very complex. Multiple output power supply design can take several hours while Webench Power Architect needs a few seconds to offer a variety of solutions.

Optimisation strategy
Back to the design example—Pressing the “Submit Project Requirements” button will trigger a complex calculation and optimisation process resulting in a display of several solutions presented by three different tools. In this first iteration the number of displayed solutions is limited in order to keep the result simple. The three displays are: A Table and Block Diagram: Each line represents a complete multiple output power supply design. The major system parameters are listed. One line is highlighted and its corresponding block diagram displayed on the right hand side. It is chosen based on a best compromise between efficiency and cost. Graphical Diagram (Figure 2): Each circle refers to one line of the table. The specific design can be identified by moving the mouse over the circle. The graph gives an easy overview of the footprint (y-axis) versus efficiency (x-axis) versus cost (circle size). TAn optimisation tuning dial (Figure 3) can be used to choose Lowest BOM Cost or Highest Efficiency. This will add additional design solutions in the graph and the table. The yellow circle is identifying the highlighted design in the table and the actual displayed block diagram. The other circle colours correlate to the background colour of the scale of the optimisation dial, indicating the optimisation strategy.

Power dissipation
At a glance the designer can see which effect on cost and efficiency the choice of a specific bus voltage or solution structure has. In the example we continue the next steps with the design highlighted and displayed by WEBENCH as best balanced design. The bus voltage is to be 5V. This is the most commonly used in terms of optimum efficiency and the best price to performance availability of parts specified to work from the 5V rail. The “View Project Details” button takes us to the next screen, displaying our final block diagram (Figure 4), schematic of the selected power module and pie charts visualising the contribution of power dissipation, price and footprint by each module. The colours in the charts identify the correlated module in the block diagram. At this point we can consider the design as finished. The chosen solution is only using power modules and thus offering all the advantages mentioned above. Power Architect offers another possibility for further optimisation of the chosen architecture. It allows selecting alternate parts within the chosen design. In our case we notice, from the project pie chart (Figure 5) that the module LMZ14203 in the first stage is contributing the most to the power losses. We click on the device and select “Alternate solutions” in the window below the pie charts. We sort by efficiency (clicking on the column) and select the LM3150 (Figure 6). This increases the total efficiency by 5% reaching a total of 79%. Another optimisation criterion may be the solution cost. The pie chart shows a quite significant contribution of the three modules with only 0.5A of load current. So we replace those within the “Alternate solutions” sorting by “BOM Cost $” with the LM3674. This still keeps our total system efficiency at high level (81%) and cuts the solution cost by more than half.

Summary
Multiple output power supplies can be designed within seconds by using the WEBENCH Power Architect with the complete flexibility of design optimisation on several individual criteria. National Semiconductor is in the process of releasing more voltage and power options for the power modules which will add more design flexibility in complex power supply solutions.
Figure 1 - Webench PA_Configure Stage.

Figure 2: Webench PA_Optimise Project Stage_Graph.

Figure 3: Webench PA_Optimise Project Stage_Dial.

Figure 4: Webench PA_View Project Stage.

Figure 5: Webench PA_View Project Stage.

Figure 6: Webench PA_View Project Stage.