Practical Testing Techniques For Modern Control Loops
New power supply designs are becoming harder to measure for gain margin and
phase margin. This measurement is important because step load testing does
not measure critical parameters such as conditional stability. Measuring
gain and phase of the feedback versus frequency requires finding a point
in the loop to inject a test signal. The signal at this point must be
confined to a single path, with a low source impedance and high load
impedance. One such point is the connection between the voltage error
amplifier and the pulse width modulator (PWM) comparator, but that point
has been moved inside most PWM integrated circuits and is no longer
accessible. A second point is in series with the output of single-output
supplies but this point is rapidly becoming inaccessible or invalid with
the inclusion of multiple output voltages feeding a single summing node.
A new technique has been developed which does not suffer from either of
these limitations and which can be implemented on virtually any PWM chip.
This paper describes this technique and gives examples of how to apply it
to various types of integrated circuits.
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