By Frederik Dostal, Senior Field Applications Engineer, Power Management, Analog Devices
With no typical application, designing the right power supply is both important and complex. While power supply design has not yet been fully automated, a range of semi-automated tools already exist. This article details how to use semi-automated design tools through five key steps in the power supply design process. These tools are valuable for both novice and expert power supply design engineers.
Power Design Step 1: Create a Power Architecture
Creating a suitable power supply architecture is a decisive step in power supply design. This step is further complicated by increasing the number of voltage rails required. At this point it is decided whether and how much an intermediate Circuit voltage needs to be created. Figure 1 shows a typical block diagram of a power supply. The left shows the 24 V supply voltage for industrial applications. This voltage must now be converted to 5 V, 3.3 V, 1.8 V, 1.2 V, and 0.9 V and provide the corresponding current. What is the best way to generate a single voltage? To convert from 24 V to 5 V, it is best to choose a classic buck switching converter. But how to generate other voltages? Is it reasonable to generate 3.3 V from the already created 5 V, or should we convert directly from 24 V to 3.3 V? Answering these questions requires further analysis. Since an important characteristic of a power supply is conversion efficiency, it is important to keep the efficiency as high as possible when choosing an architecture.
Figure 1. Creating a Power Architecture
If an intermediate voltage (5 V in the example in Figure 1) is used to generate other voltages, the power for 3.3 V must have passed through both conversion stages. Each conversion stage can only achieve limited efficiency. For example, assuming a 90% conversion efficiency for each conversion stage, the 3.3 V power that has passed through both conversion stages is only 81% efficient (0.9 × 0.9 = 0.81). Can the system afford such inefficiencies? It depends on the current required for that 3.3 V rail. If you only need a few mA of current, the inefficiency may not be an issue at all. However, for higher currents, this lower efficiency can have a larger impact on the overall system efficiency and is therefore a big disadvantage.
However, from the above considerations, it cannot be concluded that it is always better to switch directly from a higher supply voltage to a lower output voltage in one step. Voltage converters that can handle higher input voltages are generally more expensive and less efficient when the voltage difference between the input and output voltages is large.
In power supply design, an architecture tool such as LTpowerPlanner® can be used to find the best architecture. This tool is available free of charge from Analog Devices as part of the LTpowerCAD® development environment and can be installed on your computer. Using the LTpowerPlanner tool, different power architectures can be quickly and easily evaluated.
Determining final specifications is extremely important in power supply design. All other development steps depend on this specification. Often, the accuracy requirements of the power supply are not known until the rest of the Electronic system is designed. This often adds another layer of time constraints to power supply design development. Specifications also often change later in the development phase. For example, if the FPGA is found to require additional power in the final programming design, the voltage on the DSP must be reduced to save energy, or the intended 1 MHz switching frequency must be avoided because it couples into the signal path. This change can have a very serious impact on the architecture, especially on the power circuit design.
Specifications are usually adopted at an early stage. This specification should be designed to be as flexible as possible so that changes are relatively easy. In this regard, the selection of multifunction integrated circuits is helpful, and the use of development tools is especially helpful. This allows the power to be recalculated for a short period of time. In this way, specification changes can be made easier and, most importantly, faster.
Specifications include available energy, input voltage, maximum input current, and voltage and current to be generated. Other considerations include size, financial budget, heat dissipation, EMC requirements (including conducted and radiated behavior), expected load transients, supply voltage variations, and safety.
LTpowerPlanner as an optimization aid
LTpowerPlanner provides all the necessary functions needed to create a power system architecture. It is very simple to operate, so concept development can be done quickly.
Define the input energy before adding individual loads or consumers. Then add a single DC-DC converter module. Can be a switching regulator or a low dropout (LDO) linear regulator. All components can specify their own names. The storage expected conversion efficiency is used to calculate the overall efficiency.
There are two major advantages to using LTpowerPlanner. First, through simple architectural calculations, it is possible to determine the configuration of the individual conversion stages that is beneficial to the overall efficiency. Figure 2 shows two different architectures for the same voltage rail. The overall efficiency of the bottom architecture is slightly higher than that of the top architecture. This is not obvious without detailed calculations. When using LTpowerPlanner, this difference is immediately apparent.
A second advantage of LTpowerPlanner is its well-organized documentation. The graphical user interface provides clear architectural sketches, a visual tool that can be useful when discussing and documenting development work with colleagues. Documents can be stored as paper copies or as digital files.
Figure 2. Two competing architectures with efficient computing capabilities
Power Design Step 2: Select ICs for Each DC-DC Converter
Power supplies are designed today using integrated circuits rather than discrete circuits with many individual components. There are many different switching regulator ICs and linear regulators on the market. They are all optimized for a specific characteristic. Interestingly, all integrated circuits are different and only interchangeable in rare cases. Therefore, choosing an integrated circuit is a very important step. Once an integrated circuit is selected, the characteristics of that circuit are fixed during the subsequent design process. If other ICs are later found to be more suitable, the integration of the new ICs needs to be restarted. This development effort can be time-consuming, but some of the work can be relieved using a design tool.
The use of tools is critical to the efficient selection of integrated circuits. This tool is available for a parametric search on analog.com. Searching for components in LTpowerCAD is even more efficient. Figure 3 shows the search window.
To use this search tool, just enter some specifications. For example, the input voltage, output voltage and desired load current can be entered. Based on these specifications, LTpowerCAD generates a list of suggested solutions. Enter additional criteria to further narrow your search. For example, in the “Optional Features” category, you can choose from features such as enable pins or electrical isolation to find the right DC-DC converter.
Figure 3. Searching for a suitable switching regulator IC using LTpowerCAD
Figure 4. LTpowerCAD Power Calculation Tool
Power Design Step 3: Circuit Design of a Single DC-DC Converter
The third step is circuit design. External passive components need to be selected for the selected switching regulator IC. The circuit is optimized in this step. This usually requires careful study of the data sheet and doing all the required calculations. Using the comprehensive design tool LTpowerCAD greatly simplifies this step of power supply design and further optimizes the results.
LTpowerCAD as a powerful calculation tool
LTpowerCAD was developed by Analog Devices to simplify circuit design. It is not a simulation tool, but a computational tool. It can provide recommendations for optimized external components in a very short period of time, based on the input specifications. Conversion efficiency can be optimized. The transfer function of the control loop can also be calculated. This makes it easy to efficiently control bandwidth and stability.
After opening the switching regulator IC in LTpowerCAD, the main screen will Display a typical circuit with all required external components. Figure 4 shows the main screen using the LTC3310S as an example. This step-down switching regulator has output currents up to 10 A and switching frequencies up to 5 MHz.
A yellow field on the screen displays the calculated or specified value. The user can configure the settings using the blue fields.
select external components
LTpowerCAD is based on detailed external component models, not just ideal value calculations, so it can reliably simulate the behavior of real circuits. Ltpower includes a large database of integrated circuit models from multiple manufacturers. For example, the equivalent series resistance (ESR) of the capacitor and the core loss of the coil are taken into account. To select an external component, click on the blue external component shown in Figure 4. A new window will open showing a long list of possible components. For example, Figure 5 shows a list of recommended output capacitors. This example shows 88 different capacitors from different manufacturers. You can also exit the list of recommended components and select the Show all option to choose from over 4660 capacitors.
This list is constantly expanding and updating. Although LTpowerCAD is an offline tool and does not require an internet connection, regular software updates (using the update function) will ensure that the integrated switching regulator IC and external component databases are always up to date.
Figure 5. LTC3310S List Box for Different Output Capacitors
Check conversion efficiency
Once the optimal external components have been selected, the conversion efficiency of the switching regulator can be checked using the Loss Estimate & Break Down button.
Precise graphs of efficiency and losses are then displayed. In addition, the junction temperature reached in the IC can be calculated based on the thermal resistance of the case. Figure 6 shows the calculation page for conversion efficiency and thermal behavior.
Once you are satisfied with the circuit response, you can proceed to the next set of calculations. If the efficiency is not ideal, the switching frequency of the switching regulator can be changed (see left side of Figure 6), or the choice of external coil can be changed. Efficiency is then recalculated until a satisfactory result is obtained.
Optimize control bandwidth and check stability
After selecting the external components and calculating the efficiency, the control loop is optimized. The loop setup must ensure that the circuit is reliable and stable, not oscillating or even unstable while providing high bandwidth, that is, being able to respond to input voltage changes, especially to load transients. In LTpowerCAD, stability can be considered through the Loop Comp. & Load Transient tab. In addition to the Bode plot and output voltage response curve after a load transient, there are many setting options.
Figure 6. Efficiency calculation and thermal response of the circuit
Figure 7. Setting up the control loop in LTpowerCAD
The Use Suggested Compensation button is the most important. In this case, optimized compensation can be used, and the user can adjust any parameter without a deep knowledge of control engineering. Figure 7 shows the LTpowerCAD screen when setting up the control loop.
Performing stability calculations in LTpowerCAD is a highlight of this architecture. Calculations are performed in the frequency domain and are fast, much faster than time domain simulations. As a result, parameters can be changed on a trial basis and updated Bode plots are provided in seconds. Time domain simulations typically take many minutes or even hours.
Check EMC response and add filter
Depending on the specification, additional filters may be required at the input or output of the switching regulator. Especially inexperienced power supply developers will face enormous challenges. They need to address the following questions: How must filter components be selected to ensure a certain amount of voltage ripple at the output? Is an input filter required, and if so, how must this filter be designed so that conducted emissions are below a certain EMC limit? In this regard, under no circumstances should the interaction between the filter and the switching regulator be allowed to cause instability.
Figure 8 shows the Input EMI Filter Design, a subtool in LTpowerCAD. This tool can be accessed from the first page of Optimizing External Passive Components. Launching this filter design tool will reveal the filter design using passive IC and EMC diagrams. The graph plots conducted disturbances with or without input filters, and all within the appropriate limits of various EMC specifications such as CISPR 25, CISPR 22, or MIL-STD-461G.
Figure 8. Filter Design Tool in LTpowerCAD to Minimize Conducted Disturbance at the Input of a Switching Regulator
The filter characteristics and filter impedance in the frequency domain can also be displayed graphically next to the plot of the input conducted EMC response. This is important to ensure that the total harmonic distortion of the filter is not too high and that the filter impedance matches the switching regulator impedance. Impedance matching problems can cause instability between the filter and the voltage converter.
These specific factors are considered in LTpowerCAD and do not require an in-depth knowledge of them. Use the Use Suggested Values button to automatically provide filter designs.
Of course, LTpowerCAD also supports the use of filters at the output of switching regulators. This filter is typically used in applications where the output voltage only allows very low output voltage ripple. To add a filter to the output voltage path, click the LC filter icon on the Loop Comp. & Load Transient page. After clicking this icon, a filter will be displayed through a new window, as shown in Figure 9. The parameters for this filter can be easily selected here. The feedback loop can be connected either before or after this additional filter. Here, the stable response of the circuit is guaranteed in all operating modes, despite the good DC accuracy of the output voltage.
Figure 9. Choosing an LC filter at the output of the switching controller to reduce voltage ripple
Power Supply Design Step 4: Simulate the Circuit in the Time Domain
After completing the circuit design with LTpowerCAD, the next simulation is extremely important. Simulations are usually performed in the time domain. Check individual signals against time. The interaction of different circuits can also be tested on a printed circuit board. Parasitics can also be integrated into the simulation. In this way, the simulation results become very accurate, but the simulation time is longer.
In general, simulation is suitable for gathering additional information before implementing real hardware. It is important to understand potentials and limits for circuit simulation. The optimal circuit may not be found by simulation alone. During simulation, parameters can be modified and the simulation restarted. However, if the user is not an expert in circuit design, it is difficult to determine the correct parameters and then optimize. Therefore, it may not always be clear to the simulation user whether the circuit has reached its optimum state. Calculation tools such as LTpowerCAD are better suited for this purpose.
Emulate Power Supply Using LTspice
LTspice® from Analog Devices is a powerful circuit simulation program. Its ease of use, extensive user support network, optimization options, and high-quality and reliable simulation results are widely used by hardware developers worldwide. Additionally, LTspice is free and can be easily installed on a personal computer.
LTspice is based on the SPICE program, which was born in the Department of Electrical Engineering and Computer Science at UC Berkeley. SPICE is an acronym for Integrated Circuit Simulation Program. Many commercial versions of this program are available. Although originally based on SPICE at Berkeley, LTspice offers considerable improvements in circuit convergence and simulation speed. Other features of LTspice include a schematic editor and a waveform viewer. The operation of both tools is intuitive, even for beginners. These features also provide a lot of flexibility for experienced users.
LTspice is designed to be simple and easy to use. The program is available for download at analog.com, where a large database contains simulation models of virtually all of Analog Devices’ power ICs, as well as external passive components. As mentioned before, LTspice can be used offline once installed. However, regular updates ensure that the latest models of switching regulators and external components are loaded.
To start the initial simulation, select an LTspice circuit (for example, the LT8650S evaluation board) in the Power Products folder on analog.com. These are usually circuits suitable for available evaluation boards. In a specific product folder on analog.com, double-click the relevant LTspice link and LTspice will launch the full circuit locally on your PC. This includes all external components and presets required to run the simulation. Then, click the run program icon shown in Figure 10 to start the simulation.
After simulation, all voltages and currents of the circuit can be accessed using the waveform viewer. Figure 11 shows a typical schematic of the output voltage and input voltage as the circuit ramps up.
SPICE simulation is primarily useful for understanding power circuits in detail so that there are no surprises when building hardware. Circuits can also be changed and optimized using LTspice. In addition, the interaction of the switching regulator with other circuit components on the printed circuit board can be simulated. This is especially useful for discovering interdependencies. For example, multiple switching regulators can be simulated simultaneously. This increases simulation time, but also checks for some interactions.
Finally, LTspice is an extremely powerful and reliable tool used by IC developers today. Many of ADI’s ICs are developed with this tool.
Figure 10. LTC3310S simulation circuit generated using LTspice
Figure 11. LTC3310S circuit simulation results using LTspice
Power Design Step 5: Hardware Testing
While automated tools are useful in power supply design, the next step is to perform basic hardware evaluation. Switching regulators switch current at very high rates. The voltage bias caused by these switching currents can radiate due to parasitics of the circuit (especially the printed circuit board layout). Such effects can be simulated using LTspice. However, to do this requires precise information about the parasitic characteristics. This information is not available in most cases. You have to make many assumptions that degrade the value of your simulation results. Therefore, a full hardware evaluation must be completed.
Printed Circuit Board Layout – Important Components
A printed circuit board layout is often referred to as a component. It’s important, for example, that it doesn’t work like a breadboard with jumpers to operate a switching regulator for testing. Mainly because the parasitic inductance in the path of the switching current causes voltage bias, making this operation impossible. Some circuits can also be damaged by excessive voltage.
LTspice supports the creation of optimal printed circuit board layouts. Switching regulator IC data sheets often provide information on reference printed circuit board layouts. For most applications, use this suggested layout.
Evaluate hardware over specified temperature range
During power supply design, conversion efficiency can be used to determine whether a switching regulator IC is operating within the allowable temperature range. However, it is important to test the hardware within the expected temperature limits. The ratings of switching regulator ICs and even external components can vary over the allowable temperature range. These temperature effects can easily be considered during simulation with LTspice. However, such a simulation is as good as the given parameters. If these parameters have actual values, LTspice can perform a Monte Carlo analysis to get the desired results. In many cases, it is still more practical to evaluate hardware through physical testing.
EMI and EMC Considerations
In the later stages of system design, hardware must pass electromagnetic interference and compatibility (EMI and EMC) testing. While these tests must be performed using real hardware, simulation and computational tools are useful for gathering insights. Different scenarios can be evaluated before hardware testing. Of course, there are some parasitics involved that are not usually modeled in simulation, but general performance trends related to these test parameters can be obtained. In addition, the data obtained from such simulations can provide the necessary insights to quickly make modifications to the hardware if the initial EMC test fails. Due to the high cost and length of EMC testing, using software such as LTspice or LTpowerCAD in the early design stages can help to obtain more accurate results before testing, thereby speeding up the entire power supply design process and reducing costs.
Tools for power supply design have become sophisticated and powerful enough to meet the needs of complex systems. LTpowerCAD and LTspice are high-performance tools with simple and easy-to-use interfaces. Therefore, these tools will be of great help to designers of any professional level. Both experienced developers and inexperienced newbies can use these programs for day-to-day power development.
The extent to which simulation capabilities have evolved is astounding. Using the right tools can help you build advanced and reliable power supplies faster.