# Design of compensator type 2 for current mode control of boost converter

I have designed a boost converter with peak current mode control in PLECS. I have used the method of k factor approach to design the compensator.

The Bode plot transfer function of the power stage is used to select the crossover frequency of 40 kHz as I am using a switching frequency of 2.2 MHz and from this, I defined my compensator which shows a good Bode plot:

The Bode plot of the complete system is perfectly matching my expectation as you can see in the figure.
However, when I run the same circuit in PLECS with the compensator I get a huge oscillation and a non-stable system:

The compensator is meant to stabilize my system.

How can the frequency domain analysis work perfectly, but when it comes to the time domain (circuit) the system is completely wrong?

I have used a crossover frequency of 5 kHz and it shows a good result which means that my control loop does not show good behavior when it comes to higher crossover frequency which goes against my understanding that says I can choose any crossover frequency as long as it is a tenth less than the switching frequency.

The Bode plot of the compensated circuit (circuit with the compensator):

The circuit:

The crossover freqquency where it is unstable is 40 kHz. I have chosen a phase margin of 60° and with the k factor approach I calculated the pole, the zeros, and the gain of the compensator.

• It's totally unclear from your bode plot what the compensated response is. Show the bode plot of only your compensated circuit. Also state what frequency the unstable system is oscillating at AND IMPORTANTLY show you circuit. Commented May 22, 2018 at 10:08
• Hi Andy, i just edited it. I guess you mean the compensated circuit is the circuit with the compensator.thanks Commented May 22, 2018 at 10:33
• For your bode plot where was your input and where was your output measured and how did you arrange the schematic so that the feedback was removed i.e. how did you measure the open-loop gain for your bode plot? Commented May 22, 2018 at 10:39
• for the bode plot, the input is the Verr (Vref-Vout) to the voltage output Vout, I have used the goto/from signals(Vout and I_ind) instead of wires. I used Vout/Verr transfer function(mathematical transfer function given by Basso) and I used as well the analysis tools of PLECS to get the open loop (Vout/Verr) Commented May 22, 2018 at 10:49
• You cannot pick any crossover frequency with a boost converter. In VM, you are stuck with the resonant frequency of the $LC$ filter and the RHP zero position. In CM, you no longer have the $LC$ peaking but the RHPZ occupies a similar position. So you have to select $f_c$ at a max of 20-30% of $f_c$ otherwise it won't be stable. Please check cbasso.pagesperso-orange.fr/Downloads/PPTs/… in which you have all the tools to tackle loop control. Commented Jun 2, 2018 at 12:47