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Posted on 01 January 2019

Revolutionary Design Experience Beyond Spice Simulations

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Advanced external component optimization

Great steps ahead have been made over the past years in process technology to enhance performance of power converters; novel topologies and innovative IC control logics have renovated electronic architectures leading to smaller, faster and more sophisticated systems. Furthermore, fast performing portable battery powered devices and high energy price with severe CO² emission concordats emphasized the importance of more efficient sub-systems, pushing further to novel power supply architectures and optimized system designs.

By Principal Michele Sclocchi, Application Engineer, National Semiconductor

 

To accomplish these sophisticated designs, engineers must heavily rely on software design tools that reduces cost and design iterations. This article aims to highlight the advantages of electrical circuit calculators and simulation tools that help to design switching power supply systems.

System knowledge

Designing a switching power supply is not as simple as it seems, since it embraces all the aspects of engineering: EMI noise, closed loop analysis, power losses, thermal analysis, layout issues, magnetic design. Power supply design is not longer the responsibility of experts, but can become a task for any system designer. Fortunately, a wide selection of integrated switching regulators and design tools are available to simplify and accelerate the design process of a power supply.

The first important step to design an optimized power supply system is to well understand the system in which the power supply has to be designed in. As for anything else in life, there is not one magic “recipe” that fits well for all the applications. The engineer has to well understand trade-offs to be taken between, efficiency, size and cost.

Typical power supply trade offs, size cost, efficiency

For example, portable equipments such as mobile phones, PDAs and MP3 players need high efficiency designs in order to extend the battery life as much as possible while providing a small size solution. Then the designer will need to choose a synchronous rectification type of solution, high performing switching MOSFETs a with good compromise between Rds_on resistance and gate charge to optimize switching losses versus conduction losses. At least he has to finetune the switching frequency to optimize efficiency/size. Home-appliances and consumer devices such as Set-top-boxes and TV-radio equipments will go towards a cost optimized solution.

Power supply experience- component selection

Once the system requirements are analyzed and the right topology has been selected that fits best with the application, the possible combination of external components should be investigated further. This is not a trivial task, since the engineer needs to disengage with several variables and many external components that influence the performance of the final solution. As already mentioned above, switching frequency is one of the main variables that engineers could arbitrarily change to find the right trade-off between size-cost-efficiency:

MOSFETs: as switching frequency increases, MOSFET losses, size and cost increase as well. - Output filter: higher switching frequency allows use of smaller inductance value and less output capacitors to meet load and line transient requirements. Inductor core loss increases with the frequency - Input filter: it’s easier to filter higher frequency noise, therefore an increase of switching frequency helps to reduce size and cost of the input filter.

Close loop performances: the bandwidth of the closed loop system is usually limited to a fraction of the switching frequency. Higher switching frequency allows faster load-line a transient response that translates in to a smaller output filter and a smaller size solution and lower cost.

Simplified hand computing calculations or trial-and-error type of approach would not lead toward an optimized design, but may tend to several costly and time consuming design iterations.

Design tools

All types of design tools such as Spice simulation software, Mathspreadsheets could be used to minimize design efforts, However they both have some limitations. Spice simulation software is helpful to evaluate a first circuit idea without the need of realizing and testing the real circuitry on the bench. Unfortunately it requires updated spice models, and the simulation is limited to an electrical behaviour of the circuit without deep investigation of the thermal-losses model. Furthermore spice simulation tools only reduce the time design constraints without providing an effective deep analysis of the real component selection. In other words, the designer still has to decide which components to choose, and then simulate the results.

Math-spreadsheets are also a valid support to component selections, and relieve the designer of complicated formula’s analysis processing. However they are complementary to Spice simulations, and suffer some limitations. The final results are just as good as the mathematical models, with heavy formulae computations of multiple orders systems and complex matrix solving. Math-spreadsheets usually analyze and identify some parameters of the external components, and the end-user still has to manually search real components from a catalogue.

The all in one WEBENCH® on-line design tool

National Semiconductor as a leader in power management regulators is the first company that recognized these needs by providing an online software tools that combines spice simulation benefits together with a sophisticated matrix process resolution to optimize complete systems.

The WEBENCH® online simulation tool was first introduced in the year 2001 as power supply design tool for SIMPLE SWITCHER® regulators. Ever since its introduction it has grown in popularity and strength. What was once a tool to facilitate power management development exclusively, is now supporting many different product areas, from SIMPLE SWITCHER products, to amplifiers, data converters, thermal simulations and high brightness LED systemsup to the latest addition of the WEBENCH Sensor Designer and the new version of the WEBENCH Power Designer.

http://www.national.com/analog/webench/sensors

http://www.national.com/analog/webench/power

LED WEBENCH design

The three main pieces of LED lighting design are:

- Select the proper LED and the number of LEDs needed to achieve the desired light condition. There are a wide variety of LEDs on the market today from many vendors. LED selection criteria include colour, LED current, luminous flux, angle and footprint.

- Select system requirements: input voltage, number of LEDs, maximum output current , parallel series connection.

- Choose the appropriate LED driver that fits with the system requirements.

The software provides the list of National Semiconductor IC solutions that full meet design requirements. Selection criteria include efficiency, price, maximum switching frequency and feature set.

LED selection and sorting by color, flux, power and angle

Optimize the bill of material to meet the trad-offs:

Here it comes to the real core of the tool which allows the user to experiment with different trade-offs between cost-size-efficiency without out several trial-and-error iterations or heavily mathematical calculations. The BOM of the LED design includes the voltage regulator, passive components and LEDs required for the design.

LED driver selections, quick view of possible solutions

The operating values for the design include: switching frequency, efficiency, footprint, peak to peak LED ripple current, dynamic resistance of the LED, power dissipation and junction temperature of the regulator. The design can be optimized in a number of ways, including the possibility of eliminating the output cap to save space and cost, at the sacrifice of higher LED ripple current. The user can also directly specify a peak to peak LED ripple current. In addition, trade-offs can be made between small footprint and high efficiency. For example, if the user trial to emphasize high efficiency, the design will typically have lower switching frequency to lower the AC switching losses and the focus will be on the selection of components with lower power dissipation. If the user chooses to optimize for small footprint, the design will tend to have higher switching frequency, resulting in a smaller inductance/inductor. Priority will be to choose components with small size.

For example, figure 4 shows two different designs with the same input and output requirements: the design on the left achieves high efficiency up to 91%, with a switching frequency reduced down to 100Khz. Low switching frequency forced the circuit optimization to select a large inductor of 270uH and output capacitor of 4.7uF to meet the LED output ripple requirement. The total estimated design area of the first design is up to 910 mm^2. The design on the right operates at a switching frequency up to 730 Khz, the output filter has been reduced down to 39uH inductor and 68nF capacitor. The estimated total circuit area is down to 170mm^2, about 5 times smaller of the first design. Smaller design has been achieved to the detriment of the total efficiency that went down to 85%.

A virtual knob to tune the design for size or efficiency

Simulate to verify electrical and thermal performances:

As mentioned above, Spice simulation is the right short-cut for a quick electrical verification of the performance of the design. The WEBENCH LED on-line tool includes a Spice based electrical simulation that allows steady state analysis, load and line transient responds and start-up behavior (figure 5). In particular, for an LED driver application it is important to verify the LED current ripple requirements, current tolerances at minimum and maximum input voltage range, and output current during PWM dimming.

Spice electrical simulation of a LED power supply design designed with LM3402 National Semiconductor LED driver

Figure 6 shows the steady state LED ripple current in three different scenarios: the original design with an output capacitor of 0.44uF, that shows an LED ripple current of 6% (Green sinusoidal waveform). A design without output capacitor, that reduces the component count and cost to the detriment of higher LED ripple current up to 15% ( triangular blue waveform). A third scenario (red sinusoidal waveform) aims to minimized LED ripple current, by using larger output capacitor of 1uF to reduce peak to peak ripple current down to 3% of nominal value.

LED ripple current in three different scenarios

The simulation in figure 7 shows a PWM dimming feature: PWM dimming consists of setting a desired maximum LED current through the sense resistor and changing the average LED current by turning on and off the LED driver at a speed faster than the human eye can detect. The main advantage of PWM dimming is that the LED brightness can be regulated over a wide range without influencing the dominant wavelength of light. So the color and the brightness can be well controlled. Fast output current rise time allows fast PWM dimming over a high PWM dimming ratio, as shown in figure 7.

PWM dimming simulation

Conclusions

The Webench on line design tool includes advanced external component optimization that allows the optimization of the bill of material, based on real trad-off requirements and it completes spice simulation for an easy bench verification.

 

The Webench toolset can be accessed at National Semiconductor webpage at www.national.com

National Semiconductor, Webench and PowerWise are registered trademarks of National Semiconductor Corporation.

 

 

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