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

SciebouSciebou Posts: 10
edited December 2015 in Saber Model Exchange
Hello,

      I would like to simulate a buck converter in closed-loop with SABER. Here is the schematic:

      Classically, 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 = 1.8V
fsw = 400k
time step = 0.5u

      However, I obtained the following strange results:
Output voltage:

The two op amp are both saturated. (Vcc and Vee are increased to ±30V, which is very big, but if not Vout is very small).

The MOSFET's and diode's current are strange, the PWM signals as well:

Finally, the diode cathode voltage is not as expected. Its maximum is normally Vin = 270V, but is limited here to 28V:

      PI parameters are carefully calculated with these two transfert functions:

Do you have some ideas to troubleshoot my problem?

Many thanks for your help.
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