Using ADP1055 as an example to improve dynamic loop response

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Using ADP1055 as an example to improve dynamic loop response [Copy link]

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 the voltage difference between the regulated output voltages. To illustrate the behavior of the control loop, a typical Bode plot can be used to show the phase shift and loop gain over frequency. This control loop can be implemented using analog or digital techniques.

Figure 1. ADP1055 digital switching regulator in a full-bridge application

Some digital power supplies offer control loop optimization that can respond very quickly to dynamic images. Figure 1 shows an example of a circuit with an ADP1055 controller IC that has been optimized with a digital control loop. Digital controllers offer designers many control functions, some even capable of dynamic control during operation. Figure 2 shows the various functions of the ADP1055 that can be controlled through the ADP1055 evaluation software.

Figure 2. Digital power allows designers to easily control power supply parameters through a graphical user interface

One particularly interesting setting option related to the control loop is the nonlinear gain/response function, which is accessed via the filter button. Nonlinear gain/response allows dynamic adjustments to the control loop, for example, after a load transient. After a large load transient, the output voltage of a power supply will often be above or below the ideal rectified voltage. In an analog-only control loop, components in the control loop and the power stage of the power supply are used to minimize voltage fluctuations under the most predictable conditions. The advantage of a dynamically adjustable control loop, such as the one in the Analog Devices ADP1055, is that the response of the loop can be adjusted instantly to compensate for widely varying conditions.

Figure 3. Setting the control loop gain based on the output voltage state

Figure 3 shows the interface for controlling this function. The typical behavior of the output voltage after a high-to-low load transient is shown in blue. It can be seen that the voltage response at the output of the regulator typically exhibits an overshoot. When the output voltage exceeds certain thresholds, the overshoot can be minimized by simply increasing the control loop gain.

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 using a digital controller (such as the ADP1055 from Analog Devices), but this is difficult to implement using a traditional analog control loop.

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