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Control Loop Analysis: Measuring Feedback Loop Response in Power Systems

Every power supply has a feedback loop that monitors the output voltage or current. The average value of voltage (and current) fed to the load is controlled by fast switching between supply and load. The longer the switch is on compared to the off periods, the higher the power supplied to the load is.

Output regulation is achieved by modulating the amount of energy transferred in each cycle. The most common method is Pulse Width Modulation (PWM) but other methods, such as frequency modulation, are also used.  Modulation analysis can be used to characterize power supply stability under load changes, line changes, soft-starts, dropouts and short circuits.

Power analysis software

Power Analysis software, available in Teledyne LeCroy 12 bit-oscilloscopes, includes easy-to-use modulation analysis which can be used to view the information contained in the control circuit’s modulated signals.

Modulation analysis functions produce a time domain display that represents the modulated parameter (e.g. duty cycle value, width duration, frequency or period) in a time vs. time graphical plot.  Figure 1 illustrates, on a cycle-by-cycle basis, the behaviour of the entire control loop by demodulating the PWM signal and extracting the underlying modulation signal in order to assess the correct tracking and linearity in PWM regulator/controller.

Figure 1: Parameter Track plots measurement results versus time

Figure 1: Parameter Track plots measurement results versus time

The measurement setup will differ depending on the specific circuit topology and depending on the location of signal acquisition in the test circuit.  Examples shown here are based on a typical setup to acquire the modulated signal at the output of a Controller IC, as shown in the image below. The Teledyne LeCroy DA1855A Differential Amplifier is used to acquire the device’s gate drive signal.

Circuit diagram – control loop measurement

Circuit diagram – control loop measurement

In Figure 2 (below), the yellow trace, channel 1, contains 2ms of an acquired PWM waveform (gate-to-source drive signal).  The red trace, channel 2, shows a step load change.  This PWM signal is demodulated using a track function of width shown in the lower blue trace. The track function represents the change in pulse width (or other characteristic) as a function of elapsed time, which is time synchronous with the source waveform. Clearly, the control loop initially overshoots and then slowly recovers. Frequency, period, width and duty cycle are also measured with statistics.

Figure 2: Measuring the step load response in a switched mode power supply

Figure 2: Measuring the step load response in a switched mode power supply

The control loop analysis can be used to verify the soft-start performance.  In the example below, the oscilloscope acquires a 10ms record of every gate drive pulse from the time the power supply is turned on until it reaches steady state.  Figure 3 shows the Vds (Channel 1), the Current Id (Channel 2), the Gate Drive Signal (Channel 3) and the change in pulse width (lower trace) during the start-up of power supply.

Figure 3: Measuring the start-up in a switched mode power supply

Figure 3: Measuring the start-up in a switched mode power supply

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