Digital Control Measures In-System Response

Nov 1, 2006 12:00 PM
By Mark Hagen, Systems Application Engineer, and Dave Freeman, System Engineering Manager, Texas Ins

Digitally Controlled Power Analytical Benefits

The digitally controlled power-supply loop analysis is beneficial during the power-supply design, in manufacturing and during system operation. The power-supply designer determines the compensation for the desired operating conditions as they do today for analog supplies. Where today the designer uses a network analyzer for this analysis and adjusts the resistors and capacitors of the compensation network, the digital power designer can work in virtual space adjusting the compensation terms to achieve the best results. At the same time, the designer can be assured that the compensator will be very deterministic and without the circuit tolerances of the analog components.

During manufacturing, each supply can be optimized for frequency response based on the characteristics on the power-stage components rather than compromised by the variances that are expected. This allows for a broader range of acceptability of the power-stage component values without compromising the supply's frequency response.

One of the major nemeses of the power-supply designer is the system designer. The system designer may place large amounts of capacitors around various components to help bypass or provide local energy storage. In many cases, the actual results of such liberal use of capacitance can actually reduce the power-supply frequency performance.

Fig. 5 shows examples of adding various capacitances to the output of a 1-kW telecom rectifier without adjusting the compensation. As the capacitance is added, the gain of the system is reduced and, therefore, the frequency response may not meet the system needs. With the ability of in-system frequency analysis, the power supply can be recompensated to adjust for this unexpected capacitance. If this operation is not done, then at a minimum, the host can be notified that the power-supply frequency response may not meet the requirement.

Exploiting Predictability

The technique described for measuring the transfer function of the power stage is efficient in its use of memory and mixed-integer programming. This technique also has very good signal-to-noise ratio characteristics if the right nodes are chosen for injecting the excitation signal and for measuring the response. Ultimately, this measurement capability opens up the possibility of moving the measurement and calculation of loop compensation from the designer's lab bench to the factory floor, or to the end customer's application.

References

1. Miftakhutdinov, Rais. “Compensating DC/DC Converters with Ceramic Output Capacitors.” Texas Instruments Power-Supply Design Seminar (SEM1600), 2004.

2. Figoli, David. “Creating a Two Channel Sine Wave Generator,” Texas Instruments application note (spra412.htm, 1 KB), January 1999.

3. TMS320x280x DSP Boot ROM Reference Guide Rev. B (spru722b.htm, 9 KB), Aug. 26, 2005.

4. Zwillinger, Daniel (editor), Standard Mathematical Tables and Formula, 30th Edition, CRC Press, 1996.

5. Wikipedia, http://en.wikipedia.org/wiki/Window_function.

6. Hagen, Mark. “In-situ Bode Measurement in a Digitally Controlled Power Supply,” Darnell Digital Power Forum, 2006.

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