[26] Frequency Response Measurement Part 4: Loop Measurements

Measuring the open-loop gain of a system while the loop is kept closed for regulation. 

Introduction

In this article, Dr. Ridley continues the topic of frequency response of switching power supplies. This fourth article shows in detail how the AP300 analyzer is connected to measure the open loop gain of a power supply or any other feedback system, while keeping the loop closed and regulated.

Power Supply Loop Gain

The loop measurement of a power supply is something that should always be made. As pointed out in previous articles, specialized equipment is needed to isolate injected frequencies and measure them one at a time in the presence of large amounts of noise. Loop measurement requires some skill to implement but it provides powerful design guidance during the development phase of a power supply, and a very sensitive measure of a final production assembly.

Fig. 2 shows a switching power supply with feedback loop. The output voltage is compared to a reference, and the difference is amplified through the feedback error amplifier. The output signal of the error amplifier is used by the PWM modulator to set the duty cycle of the power switch. The loop gain measurement consists of the gain (in dB) of the power stage, plus the gain (in dB) of the feedback compensator.

Fig-1
Figure 1: Power Supply with Feedback Loop.

Fig. 2 shows how this could conceptually be measured with the loop physically opened and a signal with dc offset injected into the compensator.

Fig-2
Figure 2: Open Loop Gain Measurement with the Loop Physically Broken.

There are two problems with trying to measure the loop gain in this way. First, with a high gain feedback amplifier, it is impossible systems to apply exactly the right dc offset to the injected signal to prevent the error amplifier from saturating.

Second, the gain of the loop changes by many orders of magnitude over the full frequency range, and the size of the injected signal with this measurement technique would also need to change by the same amount to keep perturbation signals relatively constant. 

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