POWER SUPPLY DESIGN Five steps to quick and efficient power supply design

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Power supply designs are usually complex, particularly as they typically require multiple output voltage rails. However, development can be made faster and easier, even for less experienced designers, by currently available design tools. This article reviews how these tools can be deployed through five key power supply design steps.

Achieving the optimum design for a power source is essential, yet it is also complex. There is no single typical application, while most systems need multiple DC power rails. Although total power supply design automation is not yet possible, novices and expert power supply designers alike can benefit from the availability of a comprehensive range of semi-automated design tools. Designers can deploy these tools through five critical steps of the power supply design process, as reviewed below.

Step one: Creating the power supply architecture

This step starts with developing a simple block diagram of the target power supply, showing the input voltage, plus the required output voltage rails and their values. In general, classic step-down switching buck converters make good sense for converting from (say) 24 V to 5 V – but should 3.3 V or other lower-voltage rails be generated directly from the 24 V input, or should they be derived from the 5 V converter output?

The decision is important, because it impacts the design’s efficiency – always an important consideration for power supplies today. However, it is not obvious; using an intermediate converter adds that inverter’s inefficiency to the supply chain, yet the alternative – a voltage converter that can handle wider input to output voltages directly – is usually more expensive, with reduced efficiency.

One available solution comprises Analog Devices’ LTpowerPlanner, which is part of their LTpowerCAD development environment. This tool can solve such issues, while making evaluation of different architectures quick and easy. It can create a power supply architecture that allows for the available energy, the input voltage, the maximum input current, and the voltages and currents to be generated. Other considerations include size, financial budget, thermal dissipation, EMC requirements (including both conducted and radiated behaviors), expected load transients, changes in the supply voltage, and safety. The tool also allows designers to rapidly accommodate late changes to the system’s design; a reprogrammed FPGA needing more power, for example.

Step two: Selecting integrated circuits for each DC-DC converter

Today’s power supply designs use integrated circuits rather than multiple discrete components. However, the market offers many different switching regulator ICs and linear regulators, all of which are optimized for one specific property. Accordingly, changing a device after the initial design-in can be challenging, unless a tool like LTpowerCAD is used to handle the recalculations needed.

LTpowerCAD also allows parametric searches into its database of integrated circuits, and offers selections based on input criteria like input voltage, output voltage, and required load current. Additional features, like an ‘Enable’ pin or galvanic isolation can also be specified.

Step three: Circuit design around the individual DC-DC converters

Once the converter ICs have ben chosen, the power supply circuit can be built up and optimized by suitable choices of external passive components. The analysis and calculation required can be greatly simplified by using the LTpowerCAD design tool. The tool can recommend external components based on the specifications entered, which optimize conversion efficiency. It also calculates the control loop transfer function, easing implementation of the best control bandwidth and stability.

The stability calculations performed in LTpowerCAD are a highlight of its architecture. The calculations, performed in the frequency domain, are much faster than time domain simulations. Accordingly, parameters can be changed on a trial basis and an updated Bode plot is provided in a few seconds.

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Running a simulation in the time domain would take many minutes or even hours. Additionally, a stable circuit response can be ensured in all operating modes together with very good output voltage dc precision.

After this, the tool reliably simulates the behaviors of a real circuit as calculations are based on detailed models of external components, not just ideal values. For instance, the equivalent series resistance (ESR) of a capacitor and the core losses of a coil are taken into consideration.

Some designs may call for additional converter input and output filters. Filter components must be selected to ensure an acceptable voltage output ripple, while input filters must keep conducted emissions below specified EMC limits. Additionally, interaction between the filter and the switching regulator must not ever cause instability.

Step four: Simulating the entire circuit in the time domain

Once the circuit design is complete, the entire power supply can be simulated in the time domain, using a tool like Analog Devices’ LTSpice - based on the SPICE program originated by the University of California at Berkeley. Individual signals are checked against time. The interaction of different circuits can also be tested on a printed circuit board. Additionally, it is possible to integrate parasitic effects into the simulation. This makes the simulation results very accurate, but simulation times are longer.

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Step five: Testing the hardware

While automation tools are valuable for power supply design, hardware testing remains essential. The converters use currents with a very high switching rate. Because of the circuit’s parasitic effects — particularly from the printed circuit board (PCB) layout—these switched currents cause voltage offset, which generates radiation.
However, there is support available for creating an optimal PCB layout. The switching regulator ICs’ data sheets usually provide information for a reference design. For most applications, this suggested layout can be used.

Testing the hardware to its intended temperature limits is also important. Although temperature effects arising from temperature-related changes to component values can be simulated, the simulation results are only as good as specified parameters. Accordingly, it is often desirable to evaluate the hardware through physical testing.

The hardware must pass electromagnetic interference and compatibility (EMI and EMC) tests during the later stages of system design. While these tests must be passed with real hardware, simulation and calculation tools can be extremely useful in gathering insights. Since EMC tests are costly and time-intensive, utilizing software such as LTspice or LTpowerCAD in the early design stages can help achieve more accurate results prior to testing, thus speeding up the overall power supply design process and reducing costs.

Conclusion

The tools now available to aid power supply design offer high performance with simple to use interfaces. They allow designers with any level of expertise to develop complex power supplies reliably and quickly.

For a more in-depth discussion of these tools and how to use them, please refer to Analog Devices’ White Paper: ‘Improving Power Supply Design Using Semi-Automation — Five Steps to Quick and Efficient Design’ (see button above).

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