Control Loop Analysis

Control Loop Analysis

Control Loop Analysis

 

Control loop response analysis (often called Bode plots, after the resulting curves) helps to characterize the frequency response of a power supply control loop. The Bode plot represents the gain and phase of the feedback loop computed over a range of frequency values, providing valuable information about the speed of the control loop and hence the stability of the power supply. It is important for the Power supply to be operating in stable region as it needs to work efficiently in varying load conditions. The control Loop Response helps to identify the margin the circuit has before it reaches unstable state. Designers work towards larger margins to ensure the power supply is in stable state despite worst-case load conditions. It is therefore an important measurement performed by power supply design engineers to ensure their designs operate in stable conditions.

Choosing the right test set-up

Choosing the right test set-up

To perform a closed loop response measurement, design teams need to perform these measurements very close to DC to a few MHz. However, most VNAs have start frequencies at 10s of MHz which limits it’s use for this test. Using a standalone frequency response analyzer is also limiting for this kind of test because they need to be controlled by an external computer. The advantage of an oscilloscope-based setup is that they also get to see the signals that is injected as well as the signal that comes out of the circuit which enabled designers to debug and analyze their circuit behavior effectively.

An Oscilloscope based frequency response analysis capability, with a built-in function generator, ensures the power design teams can perform the measurement starting from close to DC to a few 10s of MHz which is the window of interest. An oscilloscope with built-in function generator enables designers to view the signals that the design is handling thereby giving valuable insights to the circuit behavior.

Good probing techniques

Low capacitance, low attenuation passive probes such as the TPP0502 are recommended for the FRA measurements to not load the design during the measurement. This is very important as the signals that are being injected and measured are of very low voltage levels and should not be modified due to incorrect probe loading effects. This enables measurements with vertical sensitivity of 500 µV/div (with the MSO 6 Series).

Good probing techniques

Measuring stability margins

Measuring stability margins

First step is to find the Gain Cross over frequency, i.e. when the Gain plot crosses 0dB or when the circuit gain is. Cross over frequency is a measure of how quickly the power supply recovers from a step change in the load. The higher the cross over frequency, the faster the step response in time domain.

Next you track the Gain cross over frequency point on the plot and draw a line to intersect the phase plot. The delta between the phase value below 180deg gives the phase margin. In Figure1, we see this as -9 deg. As the phase margin reduces the system behavior becomes more and more oscillatory. If the phase margin ever becomes 0, then we would have a perfect oscillator, which in engineering world we would call unstable. Therefore, to be on the safe side we would like to have a certain amount of "safety margin" with our phase before hitting oscillations; this is called phase margin and we should aim for at least 45 under worse case conditions. We recommend designing for 55 to 60 degrees to be on the safe side.

Now we look at Phase cross over frequency; when the phase plot crossed 180 deg or Zero deg. From this Phase cross over frequency point a line is drawn to intersect the Gain plot. The delta between this Gain plot point to 0dB gives the GAIN MARGIN of the system which is -20dB in this example. Again, as gain margin approaches 0dB then the power supply will oscillate. For a robust power supply we would like to have about 10dB of gain margin; i.e. our gain needs to be 10dB below 0dB when the phase hits -180.

Control Loop Analysis Solution

To minimize the complexity of test set-up, Tektronix has put together a Control Loo Analysis kit will all the key piece of equipment and software needed to measure the stability of your power design. With the industry leading 5 and 6 Series MSO oscilloscopes, low attenuation passive probes, best-in-class 5/6 Power software with the FRA suite, power designers now have a single test set-up to design, verify, validate and test their designs effectively and faster.

Good probing techniques
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