1 power supply Design Seminar Accelerating Power-Supply Compliance to specification Reproduced from 2012 Texas Instruments power supply Design Seminar SEM2000, Topic 6. TI Literature Number: SLUP308. 2012, 2013 Texas Instruments Incorporated power Seminar topics and online power - training modules are available at: Accelerating Power-Supply Compliance to specification John Rice Abstract Efficient and reliable power conversion is the foundation for everything electronic today. From automobiles to mobile phones and everything in-between, power electronics drives our world. Developing an efficient and reliable power converter necessitates a systemic understanding of the complexities of power electronics and the system-level design considerations for which the power converter is being designed.
2 Equipped with this knowledge, mathematical models and/or simulations can help achieve a comprehensive understanding of the circuit under nominal and worst-case conditions. This topic offers a practical approach for advancing a Power-Supply design through SPICE-based Power-Supply analysis and advanced measurement techniques. I. Converter specification Specifications Design Objectives Analysis and Analysis Method A. Specifying Circuit Behavior Gain/Phase Margin, Input/Output SPICE. Impedance Regardless of complexity, writing a power - supply specification is the essential first step to Conducted Susceptibility SPICE. advancing a robust design. From three-terminal Input Conducted Emissions Math linear regulators to complex switching regulators, Input Filter Damping, Attenuation SPICE.
3 The specification sheet should detail the design UVLO Threshold/Latch Off, No Math requirements throughout the product lifetime a Oscillations design affected by layout, packaging, component Output Ripple Math/. selection, interconnects, environment and more. SPICE. Table 1 lists the typical design objectives in a Switching Frequency Tolerance/Sync SPICE. Power-Supply worst-case circuit analysis (WCCA), Efficiency Math/. a rigorous approach to verifying functional SPICE. reliability through comprehensive mathematical- Primary Overcurrent Trip Thresholds Math and simulation-based analysis. A WCCA Short-Circuit Analysis/Turn- SPICE. Off (Survival). encompasses not only the obvious considerations Topic 6. but more subtle behavior influenced by circuit and Output Overvoltage SPICE.
4 Component impedances. Isolation Resistance and Capacitance Math Although a power converter typically regulates Bus Off State Leakage Current Math output voltage, power or current, it is circuit Load Step (Natural and Forced SPICE. impedance that you are controlling. Transient Responses). response, electromagnetic compatibility (EMC) Output Response Bus Transient SPICE. and input/output stability are a few foundational On/Off Command Overshoot/Delay/Rise SPICE. Power-Supply considerations strongly influenced In-Rush/Out-Rush SPICE. by circuit impedance, both controlled and parasitic. Gate Drive Analysis SPICE. Thus, my topic focuses heavily on understanding Magnetics Characteristics Math and quantifying circuit and component impedances power Sequencing SPICE. and their effect on Power-Supply operation.
5 Regulation Math Table 1 Design specification checklist. Texas Instruments 6-2 SLUP308. B. Circuit Analysis Options When analyzing complex or subtle circuit To perform a successful Power-Supply analy- behavior, behavioral modeling coupled with sis, you should be familiar with the subtleties of targeted mathematical analysis offers the most power converter design control theory, EMC, comprehensive approach with which to execute a component selection and component stress toler- Power-Supply analysis. It should be emphasized ance, to name a few. Figure 1 illustrates the speed that simulation is not a method for designing a of an analysis relative to the model abstraction power supply but rather a means of analyzing it. level. And although experienced designers may not want or need simulation, simulation tools today along with the computers they run on have evolved in ways that make it an efficient means of verifying a Power-Supply design .
6 Designers unfamiliar with simulation and the mathematics behind power conversion may rely on highly intelligent design synthesis and analysis tools. Design tools like power 4-5-6 from Ridley Engineering, PowerESIM and Texas Instruments'. WEBENCH circuit designer help novice and experienced designers quickly develop power - supply solutions and analyze behavior without the burden of building a custom analysis. These tools offer an effective means for Figure 1 Analysis run time vs. abstraction. starting a design, as they integrate many power - supply design considerations including stability, If a device or circuit model is built purely in thermals, magnetic component selection, circuit mathematics, the abstraction level from physical board size and efficiency. However, even the best behavior will be high and the analysis run time Power-Supply synthesis tools cannot anticipate all will be fast (not inclusive of the time to build the of the considerations and complexities of your analysis), particularly when applied to switch- design.
7 As such, every design built in synthesis mode power conversion. In contrast, transistor- must be understood and validated on the bench level simulations used in the design of integrated before it is committed to production. circuits reveal subtle circuit behavior over To summarize, a thoroughly specified, temperature and process, but the simulations can analyzed and tested power converter is the take hours and in some cases even days to principle method of advancing a robust power complete. converter. Topic 6. To address long simulation run time at the circuit level, behavioral modeling has evolved to C. To Simulate or Not to Simulate . where transistor-level functionality is replaced As Figure 1 illustrates, there are many with controlled voltage and current sources that simulation tools to choose from.
8 However, your define device behavior as logical and/or simulator choice may be mandated by the mathematical functions. These behavioral company you work for. Standardization within a constructs although abstractions themselves . company facilitates engineering collaboration and can greatly simplify the development of complex re-use, but with so many power -centric software device models and accelerate circuit development tools available, this mandate can work against the and simulation run time while also achieving Power-Supply designer. For example, SIMPLIS, excellent fidelity. IsSpice, PSIM and TINA are simulation tools that Texas Instruments 6-3 SLUP308. may not be endorsed by an organization, but may to consider this effect can result in an abrupt be far more useful in a Power-Supply analysis than reduction in control-loop phase margin and other top-tier simulation software tools.
9 Instability. Although the location of the RHPZ can SPICE is arguably the most widespread be expressed mathematically , a small-signal simulation program for circuit analysis. With SPICE simulation like the one presented in dozens of variants to choose from several that reference  facilitates a far greater understanding are free of charge it is also an economical choice. of the RHPZ and its impact on stability. Another reason to choose SPICE over other Other effects often missed in a Power-Supply simulators is that the vast majority of analysis are capacitor ESR variance over semiconductor and component suppliers make temperature and DC bias . These effects can SPICE models of their devices freely available. represent the difference between a stable converter Beware, however just because a model is and one that is unpredictable under variations in available does not mean it is suitable for your temperature, DC bias and life.
10 Developing a model application. It is up to you as the designer to that accurately represents a capacitor's behavior is understand and validate a model's behavior and essential for accurate modeling of a power augment it where necessary. converter . Ultimately, it is the integration of In general, SPICE models are defined to Power-Supply experiences into an intelligent represent typical behavior, so they are generally simulation that makes it a powerful approach in not appropriate for worst-case or extreme value- advancing a robust design. Over time, your type simulations. In most cases the only way to understanding of SPICE and your confidence in achieve a representative WCCA in SPICE is to simulation will improve; this will facilitate design develop your own parameterized component creativity and design insight that ultimately will models and circuit simulations.