Power Supply Design: Information, dimensioning and simulation

Published:2011/8/17 1:44:00 Author:Li xiao na From:SeekIC

Harry Baggen ＆ Karel Walraven

Designing a power supply for one of your circuits can be plain sailing but also turn out to be a surprisingly complex job. Switch-mode power supplies in particular may present a real challenge before useful results are obtained. Fortunately, several semiconductor manufacturers come to our rescue.



A small power supply consisting of o mains transformer, a rectifier, electrolytics and a voltage regulator can be designed and put together by most electronics enthusiast with excellent results. However, switch-mode power supplies (SMPSUs) see increasing use thanks to being smaller, lighter and more efficient than a conventional, linear supply.

Semiconductor manufacturers today offer a wide selection of dedicated, switching ICs for use at the heart of an SMPSU. Dimensioning the circuitry around such ICs is, however, more complex than with a linear counterpart.

In addition to many datasheets and application notes, Fairchild offer a Power Supply Design Toolkit that allows users to quickly design a switch-mode power supply. The toolkit con be downloaded in its entirety, but the program may also be used on-line.

With Linear Technology we saw clearly laid out tables for switch-mode as well as linear regulators. When a certain component is selected, a compact page appears showing essential data and an application example, which we found very useful. The software available here includes SwitcherCAD III, which is a Spice-3 simulator comprising the greater part of Linear’s switching ICs.

National Semiconductor supplies WeBENCH for PSU designers. Once a component has been selected, the program assists in creating the design and then starts simulating it. Next the utility WebTHERM may be launched to run a thermal simulation.

ON Semiconductor also supplies an impressive range of special ICs for switching power supplies. This manufacturer provides software called Power 4-5-6 Plus for PSU design and simulation.

Another big player in the arena, STMicroelectronics has designed special software called VIPer Design Software specially for flyback converters employing ICs from the VIPer series.

On the Texas Instruments website we found, among others, SWIFT Designer Software [6]. SWIFT is Tl’s acronym for Switched With Integrated FET Technology; a family of synchronous buck PWM converters that go by type codes like TPS5461 x.

In nearly all cases the use or downloading of the above mentioned software requires registering your contact details with the relevant manufacturer. Fortunately, the process is invariably free of charge and obligation. Besides the IC, when designing a power supply you should also pay attention to the surrounding components like inductors, capacitors and not forgetting the PCB layout (some programs do this for you).

Coils and coil formers are often seen as stumbling blocks when designing a switch-mode power supply. Several brands are currently available like Coilcraft, Fair-rite, Sumida, Microrwetals, Ferroxcube, Magnetics, TDK and Wurth. The availability of most of these products is reasonable from mail order companies like Digikey. The company Wurth even supplies design kits with an assortment of coils in direct support of the designs of different semiconductor manufacturers.

The choice of the capacitors in a switch-mode power supply is also a critical factor. Special electrolytics are now available marked by a low ESR (equivalent series resistance) which is maintained at higher (switching) frequencies. As compared with these low-ESR caps, ’ordinary’ capacitors have relatively poor filtering abilities hence run much hotter in actual use! Some names you should know in this area include Panasonic, Wima, Vishay Sprague and Epcos.

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