April 10, 1997

Handy tool troubleshoots feedback loops

Robert Buono, DRS Medical Systems, Mahwah, NJ

One of the more difficult aspects of power-supply design is stabilizing the feedback control loop. Practical shortcuts and empirical tips come in handy when you evaluate a supply’s stability or when you attempt to stabilize a supply that oscillates. In such an attempt, it’s occasionally helpful to reduce the open-loop gain at frequencies from dc to the 0-dB feedback-loop crossover point to see if an oscillating loop becomes stable. This technique increases the loop’s gain margin. Unfortunately, you have limited ways to try the method, given that some ICs have few accessible nodes to manipulate. The technique presented here stabilizes a feedback loop by manipulating the output-voltage feedback divider.

The resistive divider is a component of almost every regulated supply. It represents one of the gain blocks that determine the supply’s open-loop gain. In the circuit in Figure 1a, which uses a generic power-supply controller IC, the feedback voltage present at the controller’s feedback pin is nominally 1.24V when the loop is regulating. The R₁-R₂ divider sets the output voltage, V_OUT. The power supply’s feedback loop causes V_OUT to rise to the point at which V_FB=1.24V. The gain of the resistor divider is uppercase deltaV_FB/uppercase deltaV_OUT. In Figure 1a, this gain is R₂/(R₁+R₂). For the values shown, the gain is –19.67 dB. Figure 1b gives more circuit detail for using the method with an LM2577 controller.

Figure 2 shows a technique whereby you can reduce the gain of the output-divider circuit without changing the regulated dc output voltage and without introducing any pole or zero frequencies that would complicate the feedback-loop response. By introducing the voltage reference, V_Z, V_FB becomes a function of both V_OUT and V_Z. By adjusting the relative values of R₁ and R₃, you can reduce the sensitivity (or gain) of uppercase deltaV_FB to uppercase deltaV_OUT and still maintain dc regulation at the same absolute V_OUT (in this case, 12V dc). The gain of the configuration in Figure 2 is now

For the values in Figure 2, the output feedback divider’s gain decreases to –26.53 dB, a gain reduction of more than 6 dB. Further gain reduction is possible; you need only be mindful of the values and tolerances of the components involved to keep V_OUT at the desired value. You can use this technique as both a design method for achieving stability and as a handy diagnostic tool to use in the lab during design, development, and debug phases to evaluate and stabilize power-supply feedback loops. (DI #2009)

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