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    PID implementation for IGBT half bridge configuration

    Hello everyone,

    I want to design a PID controller for a closed-loop control system. Here I have IGBTs in Half-bridge configuration, I am sensing current output from a hall effect sensor, and taking it as feedback. By the current feedback, I am controlling the PWM to drive the IGBT. I have a set point and I have current feedback, which I can subtract and get the error. then I need to design a Prpotional control and integral control. So I do not know how to give proportional gain and integral gain. As in the control system, I have a transfer function which I do not know how to calculate for a non-linear system like in the switching circuit. So what are the other ways to calculate Proportional gain and integral gain?
    I am going to use SG3525 for the PWM controller and Op-amps for the PI implementation. Further, it will be replaced with a PID algorithm using a microcontroller.
    --Md Shahrukh

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    Re: PID implementation for IGBT half bridge configuration

    In the power supply world the controllers are commonly called TypeI (dominant pole), Type II (like PI), Type III (like PID).

    Search Type II controller and you'll easily find circuits and app notes to implement it with an opamp.

    Don't worry about switching non-linearities, at least at first. Assume Vout is Vin*duty

    As for your transfer function - what's your load? Load will determine the transfer function.



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    Re: PID implementation for IGBT half bridge configuration

    Thank you for suggesting this approach. I will study the application notes.

    As for your transfer function - what's your load? Load will determine the transfer function.
    I am designing a welding machine. I do not know the exact load. Taking it as a resistive load with low resistance.?
    --Md Shahrukh



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    Re: PID implementation for IGBT half bridge configuration

    Taking it as a resistive load with low resistance?
    I presume the converter has significant output inductance?



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    Re: PID implementation for IGBT half bridge configuration

    I presume the converter has significant output inductance?
    I guess it is a resistive load, as when the air resistance breaks, it is only the base metal that is conducting (considering a TIG/GTAW welding technology). If I consider inductance than it will be the filters that are connected after the rectification just before the torch out. Should I consider the wire inductance? And are you also talking about this? Or Is there something I am missing?
    --Md Shahrukh



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    Re: PID implementation for IGBT half bridge configuration

    You didn't even tell if it's an AC or DC welder. Why not provide a principle schematic of your welder circuit?

    Welding arc has negative dynamic impedance, there must be a series impedance, e.g. an inductor to achieve stable current control.



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    Re: PID implementation for IGBT half bridge configuration

    Quote Originally Posted by FvM View Post
    You didn't even tell if it's an AC or DC welder. Why not provide a principle schematic of your welder circuit?

    Welding arc has negative dynamic impedance, there must be a series impedance, e.g. an inductor to achieve stable current control.
    Hello FvM, thank you for the reply.

    It is a DC welder.

    Click image for larger version. 

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    I will use this circuit to drive a transformer. The transformer rating will be 14:5. The voltage to the IGBTs will be 310V DC.

    Click image for larger version. 

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    And this is the transformer where the IGBT out is connected.

    I don't know about negative dynamic impedance, I will study it. And I guess you are right, I am using an inductor in the circuit to achieve better stability.

    Now, with the help of the circuit above can you help me how to make a model out of it. I will replace the switching circuit with equivalent resistance of the channel and transconductance of the IGBTs.
    Or can you share me some sample example which can make me understand the theory behind solving this type of problems.

    Thank you.
    --Md Shahrukh



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    Re: PID implementation for IGBT half bridge configuration

    Is there any other way to design a controller for this problem, as I know there is a way with having a transfer function and do the analysis using the Matlab tool to observe various types of linear as well as non-linear control.
    How to design a type-2 or type-3 control for a system like this?

    Can you suggest me some technique to address this type to control design?

    Thank you.
    --Md Shahrukh



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    Re: PID implementation for IGBT half bridge configuration

    I think you are overcomplicating things. If you have difficulties to derive the plant transfer function from circuit analysis, you can record the system step response and calculate appropriate controller parameters. Or apply empirical tuning methods like Ziegler-Nicols.



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    Re: PID implementation for IGBT half bridge configuration

    Thank you FvM, I found the solution using MATLAB itself, it has the feature to linearise the model and give the transfer function. I got the open-loop transfer function and taking further analysis using the control system block using MATLAB. I have also found the Kp Ki and Kd value using the PID tuner in the MATLAB.

    I have some more doubts. I will ask it in the Microcontroller section.
    Thank you, everyone.
    --Md Shahrukh



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