+ Post New Thread
Results 1 to 15 of 15
  1. #1
    Newbie level 4
    Points: 86, Level: 1

    Join Date
    May 2019
    Posts
    7
    Helped
    0 / 0
    Points
    86
    Level
    1

    Simulation Buck Converter

    Greetings Experts! I am trying to solve a homework problem on the subject of Power Electronics (DC-DC Converters--Buck Converter). The following problem statement has been provided:

    1. Ideal source battery = 100Vdc

    2. Load = Series combination of resistor (0.25 ohm), inductor (1mH), battery (10Vdc)

    3. The total period (control time) of the chopper is 2.5ms and the ON time is 1ms.

    My question is how to properly set-up this circuit in Pspice because my waveforms look incorrect. I have provided the link to the Pspice files and the image of the desired waveforms from the book. I believe my problem is in the Vpulse parameters, however, I do not have enough experience to understand what exactly to change. Please help.

    buck.zip

    Click image for larger version. 

Name:	IMG_1788.jpg 
Views:	6 
Size:	254.2 KB 
ID:	154426

    Click image for larger version. 

Name:	2019-07-14_031813.jpg 
Views:	9 
Size:	64.6 KB 
ID:	154425

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 5
    Points: 10,137, Level: 24

    Join Date
    Aug 2015
    Posts
    1,652
    Helped
    599 / 599
    Points
    10,137
    Level
    24

    Re: Simulation Buck Converter

    you can't turn an SCR off once fired - so your ckt needs a xtor or mosfet - which you can turn off

    the SCR will stay on forever once triggered ...



  3. #3
    Advanced Member level 1
    Points: 2,659, Level: 12

    Join Date
    Aug 2016
    Posts
    470
    Helped
    72 / 72
    Points
    2,659
    Level
    12

    Re: Simulation Buck Converter

    Also look at the switching frequency. It's about 400Hz and I believe that's too low.

    When you change your main switch to MOSFET, ensure you have sufficient gate drive voltage.

    You mentioned that the 1mH inductance is part of your load. This suggests that it cannot be changed. But really, 1mH inductance as part of a battery is...

    Well, ignoring the 1mH-load connection, by visual inspection 1mH seems to me a high value already for your spec. Umm, you may need to back calculate for switching frequency.

    This is a charger so you have to give info on the charging voltage (and current).

    Also, I'd recommend you change your rectifier diode to transistor too.

    With these, i believe we can start from here.
    Last edited by Akanimo; 16th July 2019 at 06:38.
    -------------
    --Akanimo.



  4. #4
    Advanced Member level 5
    Points: 10,137, Level: 24

    Join Date
    Aug 2015
    Posts
    1,652
    Helped
    599 / 599
    Points
    10,137
    Level
    24

    Re: Simulation Buck Converter

    a good idea to leave the diode as it is - keep things simple initially, it is fairly obvious the 1mH choke is part of the buck ckt.



  5. #5
    Newbie level 4
    Points: 86, Level: 1

    Join Date
    May 2019
    Posts
    7
    Helped
    0 / 0
    Points
    86
    Level
    1

    Re: Simulation Buck Converter

    Greetings. I have replaced the SCR with an NMOS. The 1mH inductor is part of the 'motor' armature which is represented by resistor, inductor, and back-emf (10Vdc) combined. Correct me if I'm wrong, but the back emf is only applicable during steady-state operation of a motor, such that it has gained its rated rpm. At start-up, back-emf is very small. So, this analysis is for a motor which is in steady-state operation?
    Can anyone provide the correct settings because the waveforms are still incorrect with swapping the SCR with an NMOS.

    Click image for larger version. 

Name:	2019-07-16_223330.jpg 
Views:	5 
Size:	79.4 KB 
ID:	154441



    •   AltAdvertisement

        
       

  6. #6
    Advanced Member level 5
    Points: 10,137, Level: 24

    Join Date
    Aug 2015
    Posts
    1,652
    Helped
    599 / 599
    Points
    10,137
    Level
    24

    Re: Simulation Buck Converter

    for 1mH and say 25% ripple the required sw freq can be found from: V/L = di/dt let us assume you want 5A ave into the batt, so +/- 1.25A peak ripple ( 25% peak )

    We know the duty cycle will settle at 10V/100 = 10-12% hence Ton = 12%, Toff = 88%, (100-10)V / 0.001H = 2.5 / dt therefore Ton = 28uS ( 12% say )

    and therefore Toff = 205uS ( 88%) Tperiod = 205+28 = 233uS, thus F = 4.291kHz for the assumed ripple current

    At higher freq's the ripple current will be less but the duty cycle will stay largely unchanged for the required Vout ( for 100VDC in )



  7. #7
    Advanced Member level 1
    Points: 2,659, Level: 12

    Join Date
    Aug 2016
    Posts
    470
    Helped
    72 / 72
    Points
    2,659
    Level
    12

    Re: Simulation Buck Converter

    Quote Originally Posted by Oranje View Post
    Greetings. I have replaced the SCR with an NMOS. The 1mH inductor is part of the 'motor' armature which is represented by resistor, inductor, and back-emf (10Vdc)
    Can anyone provide the correct settings because the waveforms are still incorrect with swapping the SCR with an NMOS.

    Click image for larger version. 

Name:	2019-07-16_223330.jpg 
Views:	5 
Size:	79.4 KB 
ID:	154441
    Please provide your simulation waveform along with the schematic. That way it can be seen quickly what's happening. There's really not much time to simulate your circuit.
    Last edited by Akanimo; 17th July 2019 at 04:14.
    -------------
    --Akanimo.



    •   AltAdvertisement

        
       

  8. #8
    Advanced Member level 5
    Points: 10,137, Level: 24

    Join Date
    Aug 2015
    Posts
    1,652
    Helped
    599 / 599
    Points
    10,137
    Level
    24

    Re: Simulation Buck Converter

    5V might not be enough to turn on the mosfet



  9. #9
    Newbie level 4
    Points: 86, Level: 1

    Join Date
    May 2019
    Posts
    7
    Helped
    0 / 0
    Points
    86
    Level
    1

    Re: Simulation Buck Converter

    Easy peasy,
    Please provide the definition of sw frequency, what does it mean, and its purpose. Also, what is the source of ripple current? I know that when converting AC to DC there is a small ac component which is desired to be small and that is why a smoothing capacitor is used for such purpose; however, where does ripple come from in case of DC to DC conversion? Also, what exactly do you mean by dury cycle will settle at 10V/100V, why you divide these numbers? Duty cycle is defined as ton/T. Please explain my confusions.
    I will post my simulation images after work today.



  10. #10
    Newbie level 4
    Points: 86, Level: 1

    Join Date
    May 2019
    Posts
    7
    Helped
    0 / 0
    Points
    86
    Level
    1

    Re: Simulation Buck Converter

    Greetings Pspice Experts,

    I am attaching the results of simulation. The results do not look anything like that of the book. The way I set the Vpulse model is by double-clicking on each of the properties and setting the value. Please confirm if this is correct. What values should I be setting these properties to? Please make any suggestions to resolve my current situation. Thank you.

    Click image for larger version. 

Name:	2019-07-17_184441.jpg 
Views:	4 
Size:	145.6 KB 
ID:	154457
    Click image for larger version. 

Name:	2019-07-17_184405.jpg 
Views:	7 
Size:	123.6 KB 
ID:	154456



  11. #11
    Advanced Member level 5
    Points: 10,137, Level: 24

    Join Date
    Aug 2015
    Posts
    1,652
    Helped
    599 / 599
    Points
    10,137
    Level
    24

    Re: Simulation Buck Converter

    A 10V source will always read 10V regardless of the current in or out of it - you need to read up more on the sim program



  12. #12
    Newbie level 4
    Points: 86, Level: 1

    Join Date
    May 2019
    Posts
    7
    Helped
    0 / 0
    Points
    86
    Level
    1

    Re: Simulation Buck Converter

    Easy peasy,

    Well, if I knew the software I wouldn't post questions here. So, please provide constructive guidance and/or suggestions, otherwise don't post anything. Tell me exactly what I need to change about the circuit.

    Also, I would like to add that the 10Vdc is the back-emf of the motor.



  13. #13
    Advanced Member level 5
    Points: 10,137, Level: 24

    Join Date
    Aug 2015
    Posts
    1,652
    Helped
    599 / 599
    Points
    10,137
    Level
    24

    Re: Simulation Buck Converter

    It's very hard to advise when you don't know the level the OP is at, if you look at a 10V source you will see 10V regardless of all else, this is what you seem to have done - again hard to tell as information supplied is limited - you need to look at the gate drive and the output side of the mosfet ( volts ) and the amps in the choke.

    then you can increase the pulse voltage to the gate to say 12V for ON, and see the mosfet turn on....

    regards, EP.



    •   AltAdvertisement

        
       

  14. #14
    Newbie level 4
    Points: 86, Level: 1

    Join Date
    May 2019
    Posts
    7
    Helped
    0 / 0
    Points
    86
    Level
    1

    Re: Simulation Buck Converter

    Easy peasy,

    All I know is what was included in the problem statement (attached). All else I cannot provide because I do not have enough experience in whatever you are talking about. I know that the waveforms provided as a solution look like the ones attached. My question is how can we model the circuit, such that we get similar waveforms?
    As I know you are very experienced and this takes time on your end, we can discuss mentoring in private (write me in private) with an incentive from my side.

    Click image for larger version. 

Name:	IMG_1799.jpg 
Views:	2 
Size:	222.0 KB 
ID:	154459
    Click image for larger version. 

Name:	IMG_1800.jpg 
Views:	2 
Size:	465.3 KB 
ID:	154460
    Click image for larger version. 

Name:	IMG_1788.jpg 
Views:	0 
Size:	254.2 KB 
ID:	154461



  15. #15
    Super Moderator
    Points: 52,365, Level: 55

    Join Date
    Apr 2011
    Location
    Minneapolis, Minnesota, USA
    Posts
    12,838
    Helped
    2558 / 2558
    Points
    52,365
    Level
    55

    Re: Simulation Buck Converter

    For a tutorial, here is a simulation containing the chief components of a buck converter. It is animated and user-interactive (you press the momentary switch to operate the converter).

    Electron flow (or rather current bundle flow) is portrayed according to direction and intensity. Scope traces tell the story.

    It takes a minute of experimentation, to find the right tempo of switch-clicking, so that you create waveforms that are similar to your diagrams. You'll see how duty cycle affect output voltage. Below the diode waveform you might spot a frequency readout.

    By clicking the link below:

    1) Navigates to website falstad.com/circuit

    2) Loads the schematic.

    3) Runs it on your computer.

    tinyurl.com/y5xp27vj

    It takes a minute of experimentation, to find the right tempo of switch-clicking, so that you create waveforms that are similar to your diagrams.

    You can save the schematic to your computer.

    You can edit it, change values, add components, etc.


    1 members found this post helpful.

--[[ ]]--