[Shi Shuo Design] These tools can improve your power supply design in just 5 steps~

Creating a suitable power supply architecture is a decisive step in power supply design. This step is made more complex by increasing the number of required voltage rails. At this point it is decided whether and how many intermediate circuit voltages need to be created. Figure 1 shows a typical block diagram of a power supply. On the left side, the 24 V supply voltage for industrial applications is shown. This voltage must now be converted to 5 V, 3.3 V, 1.8 V, 1.2 V, and 0.9 V and supplied with the corresponding currents. What is the best way to generate the individual voltages? To convert from 24 V to 5 V, the classic step-down switching converter is the best choice. But how to generate the other voltages? Does it make sense 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 the conversion efficiency, it is important to keep the efficiency as high as possible when choosing the architecture.

Figure 1. Creating a power architecture.

If the intermediate voltage (such as the 5 V in the example of Figure 1) is used to generate other voltages, the energy used for the 3.3 V must have passed through two conversion stages. Each conversion stage can only achieve a limited efficiency. For example, assuming that each conversion stage has a conversion efficiency of 90%, the efficiency of the 3.3 V energy that has passed through two conversion stages is only 81% (0.9 × 0.9 = 0.81). Can the system tolerate such low efficiency? This depends on the current required for this 3.3 V rail. If only a few mA of current is required, the low efficiency may not be a problem at all. However, for higher currents, this lower efficiency may have a greater impact on the overall system efficiency and is therefore a significant disadvantage.

However, from the above considerations it does not follow that it is always better to convert directly from a higher supply voltage to a lower output voltage in a single step. Voltage converters that can handle higher input voltages are usually more expensive and also become less efficient when the difference between the input and output voltages is large.

In power supply design, you can use an architecture tool such as LTpowerPlanner® to find the best architecture. This tool is available free of charge from Analog Devices and is part of the LTpowerCAD® development environment that can be installed on your computer. The LTpowerPlanner tool allows you to quickly and easily evaluate different power supply architectures.

Determining the final specification is extremely important in power supply design. All other development steps depend on this specification. Often, the accuracy requirements of the power supply are unknown until the rest of the electronic system is designed. This often adds another layer of time constraints to the power supply design development. Specifications often change late in the development phase. For example, if it is discovered during the final programming design that the FPGA requires additional power, the voltage of the DSP must be reduced to save energy, or the originally planned 1 MHz switching frequency must be avoided because it couples into the signal path. Such changes can have very serious impacts on the architecture, especially on the power supply circuit design.

Specifications are usually adopted at an early stage. This specification should be designed to be as flexible as possible so that changes can be made relatively easily. Choosing multifunctional integrated circuits is helpful in this regard, and using development tools is particularly helpful. This allows the power supply to be recalculated within a short time. In this way, specification changes can be done more easily and, most importantly, more quickly.

Specifications include available energy, input voltage, maximum input current, and the 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 provides all the necessary functions to create a power system architecture. It is very simple to use, so concept development can be done quickly.

First define the input energy source, then add the individual loads or consumers. Then add the individual DC-DC converter modules. This can be a switching regulator or a low dropout (LDO) linear regulator. All components can be assigned their own names. The expected conversion efficiency is stored and used to calculate the overall efficiency.

There are two major advantages to using LTpowerPlanner. First, a simple architecture calculation can determine the configuration of the individual conversion stages that will benefit the overall efficiency. Figure 2 shows two different architectures for the same voltage rail. The bottom architecture has slightly higher overall efficiency than the top architecture. This is not obvious without doing detailed calculations. When using LTpowerPlanner, the difference is immediately apparent.

The second advantage of LTpowerPlanner is the clear documentation it provides. The graphical user interface provides a clear architectural sketch, and this visual tool is very useful when discussing and documenting development work with colleagues. The documentation can be stored as a paper copy or a digital file.

Figure 2. Two competing architectures, both featuring efficiency calculations.

When designing power supplies today, integrated circuits are used instead of 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 certain characteristic. Interestingly, all integrated circuits are different and are only interchangeable in rare cases. Therefore, the selection of an integrated circuit is a very important step. Once an integrated circuit is selected, the characteristics of the circuit are fixed for the rest of the design process. If it is later discovered that a different IC is more suitable, it is necessary to start over and integrate the new IC. This development work can be very time-consuming, but design tools can alleviate some of the workload.

Using tools is essential to effectively selecting integrated circuits. One such tool is the parametric search on analog.com. Searching for components in LTpowerCAD is even more efficient. Figure 3 shows the search window.

Figure 3. Searching for a suitable switching regulator IC using LTpowerCAD.

Figure 4. LTpowerCAD power supply calculation tool.

To use this search tool, simply enter a few specifications. For example, you can enter the input voltage, output voltage, and desired load current. Based on these specifications, LTpowerCAD generates a list of suggested solutions. Entering additional criteria can further narrow the search. For example, in the "Optional Features" category, you can select from features such as enable pins or electrical isolation to find the right DC-DC converter.

Step 3 is circuit design. External passive components need to be selected for the chosen switching regulator IC. The circuit is optimized in this step. This usually requires a careful study of the data sheet and making all the required calculations. Using the comprehensive design tool LTpowerCAD greatly simplifies this step of the power supply design and allows for further optimization of the results.

LTpowerCAD was developed by Analog Devices to simplify circuit design. It is not a simulation tool, but a calculation tool. It can provide suggestions on optimized external components in a very short time based on the entered specifications. The conversion efficiency can be optimized. The transfer function of the control loop can also be calculated. This helps to effectively control the bandwidth and stability easily.

After opening a switching regulator IC in LTpowerCAD, the main screen will show a typical circuit with all the required external components. Figure 4 shows the main screen for the LTC3310S as an example. This step-down switching regulator has an output current of up to 10 A and a switching frequency of up to 5 MHz.

Yellow fields on the screen display calculated or specified values. Blue fields allow the user to configure settings.

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 losses of the coil are taken into account. To select external components, click on the blue external components shown in Figure 4. A new window will open with a long list of components that may be suitable. 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 recommended component list and select the Show all option to choose from more than 4660 capacitors.

This list is constantly being expanded and updated. Although LTpowerCAD is an offline tool and does not require an internet connection, regular updates of the software (using the update function) will ensure that the database of integrated switching regulator ICs and external components is always up to date.

Figure 5. List box of different output capacitors for the LTC3310S.

After selecting the optimal external components, the switching regulator conversion efficiency can be checked using the Loss Estimate & Break Down button.

An accurate diagram of the efficiency and losses is 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 the 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 satisfactory, you can change the switching frequency of the switching regulator (see the left side of Figure 6), or change the selected external coil. The efficiency will then be recalculated until a satisfactory result is obtained.

After selecting the external components and calculating the efficiency, the control loop is optimized. The loop settings must ensure that the circuit is reliable and stable, without oscillation or even instability while providing high bandwidth, that is, it can respond to input voltage changes and especially to load transients. In LTpowerCAD, stability factors can be taken into account through the Loop Comp. & Load Transient tab. In addition to the Bode plot and the output voltage response curve after the 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 is used and the user can adjust any parameter without in-depth knowledge of control engineering. Figure 7 shows the LTpowerCAD screen when setting up the control loop.

The stability calculations performed in LTpowerCAD are a highlight of this architecture. The calculations are performed in the frequency domain and are very fast, much faster than time domain simulations. Therefore, parameters can be changed on an experimental basis and updated Bode plots are provided in seconds. Time domain simulations usually take many minutes or even hours.

Depending on the specifications, additional filters may be required at the input or output of the switching regulator. Inexperienced power supply developers in particular face a huge challenge. They need to answer questions such as: How must the filter components be selected to ensure a certain voltage ripple at the output? Is an input filter required and, if so, how must it be designed to keep conducted emissions below certain EMC limits? In this regard, interactions between the filter and the switching regulator leading to instabilities must not be allowed under any circumstances.

Figure 8 shows the Input EMI Filter Design, a sub-tool in LTpowerCAD. This tool is accessed from the first page of optimizing external passive components. Launching this filter design tool will display the filter design using passive ICs and an EMC plot. The plot plots the conducted emissions with and without the input filter and are within the appropriate limits of various EMC specifications such as CISPR 25, CISPR 22, or MIL-STD-461G.

The filter characteristics in the frequency domain and the filter impedance can also be displayed graphically alongside 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 taken into account in LTpowerCAD and in-depth knowledge of them is not required. The filter design is automatically provided using the Use Suggested Values ​​button.

Of course, LTpowerCAD also supports the use of filters at the output of the switching regulator. This filter is usually used in applications where only very low output voltage ripple is allowed at the output voltage. To add a filter in the output voltage path, click the LC filter icon on the Loop Comp. & Load Transient page. After clicking this icon, a filter is displayed in a new window as shown in Figure 9. The parameters of this filter can be easily selected here. The feedback loop can be connected either before or after this additional filter. Here, despite the good DC accuracy of the output voltage, the stable response of the circuit is guaranteed in all operating modes.

Figure 8. Filter design tool in LTpowerCAD for minimizing conducted interference at the input of a switching regulator.

Figure 9. At the output of the switching controller

The LC filter is selected to reduce the voltage ripple.

After the circuit design is completed using LTpowerCAD, the next simulation is extremely important. Usually the simulation is performed in the time domain. Each signal is checked against time. The interaction of different circuits can also be tested on the printed circuit board. Parasitic effects can also be integrated into the simulation. In this way, the simulation results become very accurate, but the simulation time is longer.

Generally speaking, simulation is a good way to gather additional information before implementing real hardware. It is important to understand the potential and limitations of circuit simulation. It may not be possible to find the optimal circuit through 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 is not always clear to the simulation user whether the circuit has reached the optimal state. Calculation tools such as LTpowerCAD are better suited for this purpose.

LTspice® from Analog Devices is a powerful circuit simulation program. It is widely used by hardware developers worldwide because of its ease of use, extended user support network, optimization options, and high-quality, reliable simulation results. In addition, 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 the University of California, Berkeley. SPICE is an acronym for Simulation Program for Integrated Circuits. Many commercial versions of the program are available. Although originally based on Berkeley's SPICE, LTspice has made considerable improvements in the convergence of circuits and the speed of simulation. Other features of LTspice include a schematic editor and a waveform viewer. Both tools are intuitive to operate, 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 and contains a large database of simulation models for nearly all of Analog Devices' power ICs, as well as external passive components. As mentioned earlier, once LTspice is installed, it can be used offline. However, regular updates ensure that the latest models for switching regulators and external components are loaded.

To start an initial simulation, select an LTspice circuit (e.g., LT8650S evaluation board) in the Power Products folder on analog.com. These are typically circuits that fit the available evaluation boards. In the specific product folder on analog.com, double-click the relevant LTspice link and LTspice will launch the complete circuit locally on your PC. This includes all the external components and presets required to run the simulation. Then, click the Run Program icon shown in Figure 10 to launch 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 the hardware. You can also use LTspice to change and optimize the circuit. In addition, you can simulate the interaction of the switching regulator with other circuit parts on the printed circuit board. This is particularly useful for discovering interdependencies. For example, you can simulate multiple switching regulators at once. This will increase the simulation time, but you can also check certain interactions.

In conclusion, LTspice is an extremely powerful and reliable tool used by IC developers today. Many ICs from Analog Devices have been developed with the help of this tool.

Figure 10. LTC3310S simulation circuit generated using LTspice.

Figure 11. LTC3310S circuit simulation results using LTspice.

While automated tools are useful in power supply design, the next step is to perform a basic hardware evaluation. Switching regulators switch currents at very high rates. The voltage biases caused by these switching currents can generate radiation due to parasitics in the circuit (especially the printed circuit board layout). Such effects can be simulated using LTspice. However, to do this, precise information about the parasitic characteristics is required. This information is not available in most cases. You must make many assumptions that reduce the value of the simulation results. Therefore, a thorough hardware evaluation must be completed.

Printed circuit board layout is often called a component. It is important, for example, it is not possible to operate a switching regulator for testing by jumper wires like a breadboard. This is mainly because the parasitic inductance in the path of the switching current will cause voltage bias, making it impossible to operate in this way. Some circuits may also be damaged by excessive voltage.

LTspice supports the creation of an optimal PCB layout. Switching regulator IC data sheets usually provide information about a reference PCB layout. For most applications, this recommended layout can be used.

During the power supply design process, the conversion efficiency can be used to determine whether the switching regulator IC is operating within the allowed temperature range. However, it is important to test the hardware under the expected temperature limits. The ratings of the switching regulator IC and even the external components will change within the allowed temperature range. These temperature effects can be easily taken into account during simulation with LTspice. However, such simulations are only as good as the given parameters. If these parameters have actual values, LTspice can perform Monte Carlo analysis to get the desired results. In many cases, it is still more practical to evaluate the hardware through physical testing.

In the later stages of system design, the hardware must pass electromagnetic interference and compatibility (EMI and EMC) tests. Although these tests must be performed using real hardware, simulation and computational tools are very useful for gathering insights. Different scenarios can be evaluated before hardware testing. Of course, there are parasitic factors 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 modify the hardware if the initial EMC tests do not pass. Since EMC testing is costly and time-consuming, using software such as LTspice or LTpowerCAD in the early design stages can help 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, easy-to-use interfaces. Therefore, these tools are helpful to designers at any level of expertise. Both experienced developers and inexperienced novices can use these programs for daily power supply development.

It’s amazing how far simulation capabilities have come. Using the right tools can help you build advanced, reliable power supplies faster.