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

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A power-supply designer performs many chores. And in today's environment, the development time and cost are major constraints. Additionally, the loads have become very demanding in both power and response. The power-supply designer must judiciously study the various components that make up the power stage to meet the desired power transfer and cost target. After the designer finds the best compromise of components, the engineer must make them work in the intended application. To this end, many days are spent tuning the circuit across various operating conditions to arrive at, once again, the best compromise for load response.

With the advent of digitally controlled power, the engineer now has the opportunity to optimize loop compensation. The mechanism that compensates the loop is also available for analysis. The loop data converter is used to determine the disturbance in the power-supply output by slightly modifying the duty cycle that is controlled by the digital pulse width modulator (PWM). The loop control components provide both the stimulus and response needed to determine the power-supply frequency characteristics. The digital system helps in the analysis and also remembers the optimal compensation at various conditions of line and load. These compensation parameters are used based on the digital power solution either recognizing this same condition or when the system informs the power supply that a particular operating mode is pending.

To perform the power-supply loop analysis, an injection signal is constructed from a sine-wave sequence with a specified amplitude and frequency. This sine-wave sequence is injected into the feedback loop by adding the sequence to one of the control loop variables. At a different place in the loop, the response to the injected sequence is measured by performing a discrete fourier transform (DFT) at the injection frequency. If the DFT operation includes a sine and cosine component, the magnitude and phase of the response can be calculated from the orthogonal results of the DFT operation.

Loop Analysis Using Digital Power Controller

The techniques for measuring the frequency response for a feedback-controlled system are essentially the same, whether the system to be measured is a continuous-time analog system or a discrete-time digital system. Fig. 1 shows two possible locations to inject the excitation sine-wave signal, labeled as d1 and d2. The figure also shows the possible locations to measure the response to the excitation signal. They are labeled y, u, c, x and e.

In the case of d1, the sine wave is added digitally to the results of the error calculation. The error calculation is simply the difference between the digitized voltage output and the digital equivalent of the preferred voltage output. In a likewise manner, d2 is added to the digital value used to generate the pulse width in the digital PWM.

A more complete analysis of the transfer gains can be found in the many complete references listed at the end of this article.[1-6] However, the table lists the various gains for each of the designated measurement locations shown in Fig. 1.

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