How to improve your dynamic loop response?
A DC-DC converter converts a varying input voltage to a (usually) fixed output voltage via a feedback control system. The feedback control system should be as stable as possible to avoid oscillations or, in the worst case, an unregulated output voltage. The control system should be as fast as possible to respond to dynamic changes, such as fast input voltage changes or load transients at the output, and to minimize deviations from the regulated output voltage. To represent the behavior of a control loop, a typical Bode plot can be used to show how the phase shift and gain of the loop vary with frequency. This control loop can be implemented using either analog or digital techniques.
Some digital power supplies offer control loop optimization, which allows for very fast responses to dynamic influences. Figure 1 shows an example of a circuit using the ADP1055 controller IC with digital control loop optimization. Digital controllers offer designers many control features, some of which can even be controlled dynamically during operation. Figure 2 shows the various ADP1055 features that can be controlled through the ADP1055 evaluation software.
Figure 1. ADP1055 digital switching regulator in a full-bridge application.
Figure 2. Digital power allows designers to easily manage power supply parameters through a graphical user interface.
A particularly interesting setting option related to the control loop is the nonlinear gain/response function, which is accessed through the filter button. Nonlinear gain/response allows dynamic adjustments to the control loop, for example, immediately after a load transient. After a large load transient, the output voltage of a power supply will often fluctuate around the ideal rectified voltage. In an analog-only control loop, the control loop and power supply power stage components are selected to minimize the voltage fluctuations under the most predictable conditions. The advantage of a digitally adjustable control loop, such as the one featured in the ADP1055, is that the response of the loop can be adjusted instantly to compensate for widely varying conditions.
Figure 3 shows the interface for controlling this function. The blue curve in the figure shows the typical behavior of the output voltage after a high-to-low load transient. It can be seen that the voltage response at the output of the regulator typically overshoots. When the output voltage exceeds certain thresholds, the overshoot can be minimized by simply increasing the control loop gain.
Figure 3. Setting the control loop gain based on the output voltage state.
In the example of Figure 3, the nominal output voltage is set to 12 V. The adjustable control loop gain can be set to multiple values, depending on the output voltage. For example, if the voltage rises above 12.12 V due to the increased gain of the error amplifier, the control loop can be set in the corresponding drop-down menu. There are three other voltage thresholds above 12.12 V, with independent gain settings. Note that these gain settings are completely independent of the poles and zeros set when designing the regulation loop.
Adjustable, voltage-based gain settings allow you to find a control loop setting that responds faster to voltage overshoots, thereby optimizing the quality of the output voltage feedback control. Note that the optimized control loop characteristics are not affected during normal operation. Dynamically adjusting the control loop under certain conditions (such as after a load transient) can be done with a digital controller such as the ADP1055, but this is difficult to achieve with a traditional analog control loop.
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