Simulation problems of a classical PI controllers using operational amplifiers for buck converters

SciebouSciebou Posts: 10
edited April 2018 in Saber Model Exchange
Hello,
I would like to simulate a buck converter in closed-loop with SABER. Here is the schematic:imageClassically, there are two loops with PI controllers: external voltage loop, and internal current loop. The output of the current loop's PI controller is compared to a sawtooth signal in order to generate PWM signals for gate drive. Vin = 270V
Vout_setpoint = 30V
Rload = 1.8ohms
fsw = 400k
time step = 0.5u       However, I obtained the following strange results:
Output voltage:imageThe two op amp are both saturated. (Vcc and Vee are increased to ±30V, which is very big, but if not Vout is very small).
image
The MOSFET's and diode's current are strange, the PWM signals as well:image
image
Finally, the diode cathode voltage is not as expected. Its maximum is normally Vin = 270V, but is limited here to 28V:image      PI parameters are carefully calculated with these two transfert functions:image
Do you have some ideas to troubleshoot my problem?

Many thanks for your help.
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Comments

  • Datsen Davies TharakanDatsen Davies Tharakan Posts: 24
    The pwm signal is having 400Hz as frequency which is completely wrong and Vd is also wrong as you have mentioned. To start investigation, you can try tstep = 1n and also check the feedback loop. I wonder if you could attach all design related
    files in a zip file for further investigation. Also let me know if you are
    using commercial version or student version of SaberRD. 

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