Graphical System Design: A Bridge to Digital Power

Mar 1, 2007 12:00 PM
By Gustavo Castro, Staff Analog Engineer, and Luke Schreier, Product Marketing Manager, National Ins

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As the demand for higher-power and higher-precision instrumentation grows, the size requirements for these devices continue to shrink and the design complexity increases. The PCI eXtensions for Instrumentation (PXI) platform is a good example of a size requirement (3U Euro-card) that has already managed to accommodate digital multimeters, oscilloscopes and many other instruments, but has struggled to meet the traditional requirements of programmable power supplies. The reason is obvious: A volume measuring 2 cm × 13 cm × 22 cm can hardly support the transformers and heatsinks typically required in a system power supply.

Designing for PXI modules and other tightly constrained environments has been helped by two recent trends: higher-performance switch-mode power supplies and the use of digital control to regulate them. However, implementing these technologies without the combined efforts of digital, analog and software engineers can be daunting. Using an intuitive graphical system design process in National Instruments (NI) LabVIEW software can greatly improve a lone analog engineer's ability to implement the digital control necessary in these environments.

The PXI platform is a good environment in which to demonstrate design methods for digital power architectures. With limited power dissipation available to a PXI module (20 W) and tight requirements on output precision, noise, load regulation and input power, an FPGA-implemented control scheme is required. With the NI LabVIEW approach, an analog engineer can design this algorithm, prototype it in the loop and deploy it on silicon — all without VHDL programming knowledge. In addition to helping with the control scheme, LabVIEW is a common environment for developing test software for final verification or functional tests.

Planning the Control Scheme

To explain the graphical system design process with LabVIEW FPGA, a model of the digital power scheme is required. Consider the NI PXI-4110 triple-output programmable dc power supply shown in Fig. 1. The target specifications are three channels: 0 V to 6 V, 0 V to 20 V and 0 V to -20 V, all capable of delivering up to 1 A. Therefore, the total power output is 46 W. Remember that the maximum cooling capability is 20 W in that same space, so high efficiency is required in order to avoid thermal issues.

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© 2007 Penton Media, Inc.