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I am building a model of a solar power system. In it, I connect a string of solar cells to a buck-boost converter. I use Simulink and Simscape Electrical to perform simulations. I am encountering an oscillation issue limited to a certain range of duty cycles.

I am modeling the cells using an ideal one-diode model for now, seen below. enter image description here

The entire system is built up like this; the switching frequency of the converter is 250 kHz. Solar power system

I have encountered a strange issue. For most duty cycles in the acceptable range, the current, power and voltage curves looks like this: Scope under normal conditions

However, for some duty cycles (around 14-15%), the input current begins to oscillate heavily: Scope with oscillations

By examining the oscillation closer, I can see that it has a frequency of about 125 kHz. However, I cannot see why the oscillation would be there in the first place. Looking at the input voltage trace, it can also be seen that it seems to be "capped" at a certain value.

I cannot explain why this happens; especially confusing is the fact that it only appears for a small range of duty cycles. For example, a duty cycle of 10% once again produces a clean trace. Therefore, I suspect that the model of the solar cells is at fault here. At the same time, it is extremely simple and does not include any parasitics.

What could be the cause of this oscillation? Is it a consequence of the simulation environment, or should I expect similar behavior in a real circuit?

I would appreciate any advice.

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    \$\begingroup\$ Check the current through the inductor, you're probably getting into DCM. Also, 1 mH is rather high for 250 KHz. Can you post the values for the load and the PV? \$\endgroup\$
    – a concerned citizen
    Feb 8, 2021 at 23:50
  • \$\begingroup\$ If you load resistance is too large, (output current too small) your circuit will definitely go into DCM. Try simulating with a lower load resistance. \$\endgroup\$
    – Math Keeps Me Busy
    Feb 9, 2021 at 0:30
  • \$\begingroup\$ @aconcernedcitizen The load resistance is 1 Ohm. I will check the inductor current, however, if I get DCm at say 14% duty cycle, shouldn't I observe the same with lower values as well? \$\endgroup\$
    – Mu3
    Feb 9, 2021 at 8:39
  • \$\begingroup\$ @EgorTamarin If it's DCM, it can't be subharmonic instabilities. Maybe you need to choose a different solver, maybe the models for the diode, transistor, etc, are idealized enough that they cause hiccups in certain conditions, the LC parts may also need some parasitics. I don't have Simulink, only LTspice, and I can't really duplicate this, but 1 Ohm load seems low. Given the input power (~3.5 W), the output voltage is around -1.8 V or less. Is that the load and voltage you expect to have? \$\endgroup\$
    – a concerned citizen
    Feb 9, 2021 at 10:41
  • \$\begingroup\$ @aconcernedcitizen Right now I am only using the converter to see what the response will be for a power point tracking algorithm I'm using, so I don't have specific output power requirements. It is a weird hiccup since it only seems to occur around the peak power. \$\endgroup\$
    – Mu3
    Feb 9, 2021 at 12:29

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