boost converter is not boosting to desired value

Discussion in 'General Electronics Chat' started by dhevkumar, Sep 15, 2015.

  1. dhevkumar

    Thread Starter New Member

    Sep 11, 2015
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    i have designed the boost converter as circuit model in matlab simulink. given the input as 5v and output as 15v with average load current 0.5A with L=150 microfard and C=220microfarad load resistance as 30ohm freq=25khz and duty cycle found 66.67%.

    But my converter boost only to 13.89v but not 15v. what could be the problem. please help to rectify

    Please find my attachment for the default diode value
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Have you tried changing the duty cycle?
    What values have you assumed for the series resistance of L and the ESR of C?
     
  3. crutschow

    Expert

    Mar 14, 2008
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    You calculated the output voltage for an ideal circuit but yours is not. There will be losses in the circuit due to any resistance in the circuit including Ron, and the free-wheeling diode forward voltage.
    You need to include those in your calculations.
    The result of that will show you need a slightly greater duty cycle to get 5V output.
    To minimize the losses in the diode a Schottky type is usually used.
     
  4. dhevkumar

    Thread Starter New Member

    Sep 11, 2015
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    Thank you so much for ur response to my queries sir. Yes sir i have taken this from a book problem taken from rashid for 5 input to 15 with D=0.6667. so i have just used the values to my boost converter circuit i have not calculated the ESR for L and C. If so should i include the inductor (with R) and Capacitor(with R) in the circuit itself. What values can be assumed sir..Ur suggestion
     
  5. dhevkumar

    Thread Starter New Member

    Sep 11, 2015
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    Yes sir..i have taken this from a book problem(input 5 output 15 and D=0.6667) and i have used it in my circuit (matlab/simulink).how can i take the losses into my calculation to get accurate output.please guide me sir
     
  6. crutschow

    Expert

    Mar 14, 2008
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    You need to look ar representative inductors rated for the inductance and current you need for their series resistance.
    The capacitor ESR generally will have only a small effect on the efficiency.
    There will then be a voltage loss in the output due to the series resistance of the inductor due to it's current, the voltage drop of the MOSFET when it is ON, and the forward drop of the diode when it is conducting. These drops can be used to approximate the increase in duty-cycle needed to get a 15V output.

    The easiest way is just to incrementally increase the duty cycle in the simulation until you get the output you want. ;)
     
  7. dhevkumar

    Thread Starter New Member

    Sep 11, 2015
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    Thank u so much for ur valuable suggestion...One more clarification sir, I have desined the pv and boost converter(open loop). i have implemented the lookup table mppt for maximum power(ie maximum voltage ) and i have found the duty cycle to maintain contant bus voltage(60v) for those mpp voltage(for various irradiation condition). These values i have assigned for lookup table and used as mppt which gives duty cycle for boost converter.
    When i change the input voltage of boost converter(stepchange30v to 32v),the corresponding duty cycle given to boost converter(from lookup table) to achieve 60v output. the steady value is achieved in 0.60s without pi controller.
    1. Now how should i implement the pi controller so that i achieve the constant voltage of 60v at output of boost converter so that it settles much faster than 0.60s when input voltage is changed. Please help.. i got struck..

    eg. for irradiation 600 of pv i get maximum power with maximum voltage to 30v then to achieve 60v at output of boost convert the duty cyle to be given is 0.5104. with the 800 irradiation voltage as 30.4(Vmp) the dutycyle is 0.504 to achieve 60v. now where should i implement the pi controller to achieve fast response for change in input voltage and to maintain constant 60v at output
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Since I have no knowledge of how your control loop is designed, it's difficult to give good advice.
    A block diagram of the loop would be a big help.
    Why does it take 0.6s to change duty-cycle without PI?
     
  9. dhevkumar

    Thread Starter New Member

    Sep 11, 2015
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    Thank u for ur response sir. its not 0.6s to change duty cycle. It is the response take to settle for step change in input voltage. the duty cycle control i have designed using then LOOK up table MPPT. My question is how to put the PI controller to achieve the response of settling time much faster for step change in input voltage. Please find my circuit diagram, in that how should i implement PI controller
     
  10. dhevkumar

    Thread Starter New Member

    Sep 11, 2015
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    sorry i forgot for upload the main block file
     
  11. dhevkumar

    Thread Starter New Member

    Sep 11, 2015
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    Thank u sir for ur response to my doubts clarification. Sir its not the change duty cycle it takes 0.06sec, it the settling time taken for the step change for an input voltage. now i should use the PI controller to make this response quicker. i have already implemented the lookup table mppt for the duty cycle to achieve a constant dc bus voltage of 60v. pleasle find the attachment
     
  12. crutschow

    Expert

    Mar 14, 2008
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    A PI loop will reduce he response time but I can't tell you how to design one in 100 words or less.
    You would need to study how a PI loop works and how they are designed.
    If this feedback loop is going to be computer controlled then I suggest you look at Fuzzy Logic since I think it's easier to understand and easier to implement with a computer than PI (or PID).
    Here's some info on that.
     
  13. dhevkumar

    Thread Starter New Member

    Sep 11, 2015
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    Thank u sir.. for the above circuit of boost converter how should i model in terms of transfer function that is replacing the whole cirucit by means of a single block(Transfer function). how should i start with my work. i got struck pl guide me...
     
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