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Why does the topology(dc/dc) work with open control loop but with closed loop doesnt work?

0 votes


I would like to ask ask if anyone know why my dc/dc converter works with open loop control(Duty cycle) but with PI control(closed control loop) doesnt. My control circuit doesnt react for any change of Ki or Kp, My reference output voltage is 25V, my input voltage is 5-30V. It's buck-boost converter. If use duty cycle to control my MOSFET, it works perfectly.  I attach file with that topology. I checked hundreds times that topology of PI controler is okey, I wasted so many hours on that, maybe someone else can help me?



asked Jan 13, 2019 by Voyteck (28 points)

1 Answer

0 votes
Because your controller is not fitted to the controlled circuit.

For example, you have specified an upper limit of 1 for your controller output, but you also have configured the PWM block to expect input values from 0 to 2. This means your PWM duty cycle can vary between 0% and 50%. However, with a 50% PWM duty cycle your desired output voltage cannot be reached.
answered Jan 14, 2019 by Oliver Schwartz (622 points)
Yes, I solved this problem. The loop control of output voltage works. More problem is to control input current which upper limit is 68A(0-68A). It can be less but never more. My circuit has wide input voltage(5-30V) If it's 30-15 the value of input current it's okey but when the input voltage is from 15 to 5V the value of current is too high and I need more expensive inductors. Do u know how to connect this voltage control loop with current loop of input current? Do u have any idead?

I attached uptaded model

Regards, Wojciech

Maybe you want to take a look at the demo model Buck Converter with Peak Current Control which is shipped with PLECS.

Kind regards,

Oliver Schwartz