When applying load step to a DCDC converter we can approximate the phase margin from the presence/absence of oscillation and the crossover frequency as well from the response time.

The question is: as the transfer function of the open loop is load dependent, which crossover frequency and phase margin can we see from the step load? To make a big difference let's step from no load to heavy load. Which crossover frequency dominates the response, the no load transfer function's or the heavy load's?

  • \$\begingroup\$ There are nuances to each and every DC/DC converter design; do you have a specific issue you are trying to solve? \$\endgroup\$ Oct 17 '18 at 9:44
  • \$\begingroup\$ Of course, the estimation of the phase margin applies only to the open loop under these specific conditons. A modification of the loop gain (caused by a load modification) changes the phase margin. \$\endgroup\$
    – LvW
    Oct 17 '18 at 9:49
  • \$\begingroup\$ Whichever comes nearer to the point of instability. \$\endgroup\$ Oct 17 '18 at 9:55
  • \$\begingroup\$ Yes, I have designed the feedback loop for full load to get a high crossover frequency with lot of phase margin. However, step load from zero to full load shows worse results than expected from the PM. So if I understand correctly, I have to improve the transfer function to get enough PM for low load as well. \$\endgroup\$
    – U.L.
    Oct 17 '18 at 10:15
  • \$\begingroup\$ Apparently the load affects the overall loop gain. With load the gain is decreasing, so when you do a step response at no load it is unstable. You should somehow compensate the gain with regard to the load. \$\endgroup\$ Oct 17 '18 at 10:55

With switching regulators, you store energy in an inductor and then release that energy to an output capacitor. That output capacitor is across the output terminals and in parallel with the load. However, if you analyzed a linear regulator (with an added inductor in the output terminal) you would have the same problem and you would be forced to realize that the LCR circuit is resonant and fairly abruptly changes the phase of the output signal by 180 degrees over a small range of frequencies: -

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You can analyze the effect of the LCR circuit above and at light loads (high values of R) you would have high resonance and a significant change in phase shift around resonance: -

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This is with 100 uH, 10 uF and a load of 316 ohms. Note that there is significant gain at 5 kHz and that phase angle has shifted by 180 degrees between about 4.8 kHz and 5.2 kHz. If you made the load resistor smaller in value (representing a high load current): -

enter image description here

... You can see that the phase change is much more gradual and there is barely any resonant peak to be seen. This is likely to be much stabler. Note also the step responses between the light and heavy load scenarios.

Interactive RLC bode plot generator used above.

  • \$\begingroup\$ Good explanation: "...fairly abruptly changes the phase of the output signal by 180 degrees..." \$\endgroup\$ Oct 17 '18 at 16:14

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