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How to neutralize the derivative filter coefficient (Kf) in a continuous PID controller?

0 votes
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Hello,

Based on the PID continuous controller block, there is always a derivative filter coefficient (Kf) when using the PID mode. The question is how to choose a value for this Kf without affecting the PID controller dynamics (Knowing that I am controlling a DC-DC buck converter operating at 1MHz). 

Another way to ask the question is how to neutralize this Kf term? According to the equation provided in the PLECS manual (as in the screenshot below), I assume that Kf should be a very high value, how much high? 
When I used 106 for Kf, I got problems using a relational operator (>=) when Kd was on the order of 0.01 or 0.001. If Kd is below 10-5, there are no porblems.

Thanks. 

 

asked Feb 1 by Mostafa Amer (17 points)

1 Answer

0 votes

The appropriate value of Kf will depend on your model.  

I would generally not recommend neutralizing the damping term.  Derivative action has poor numerical properties and is quite susceptible to noise.

The highlighted portion of the transfer function has a derivative-like behavior (Gd = Kd*s) up until Kf.  Kf defines the corner frequency for the real-pole that neutralizes the derivative action.  Well above Kf one sees Gd = Kd*Kf for a constant gain and zero phase shift.

Setting Kf = 10results in a -3 dB point at Kf/(2*pi), which will probably have your 1MHz switching frequency interfering in your controls. The actual answer depends on where exactly you would need phase lead in your system and your overall loop response.  Furthermore, it may be necessary to add another high-frequency pole, but these are general control design questions and not related specifically to PLECS.

answered Feb 6 by Bryan Lieblick (1,795 points)
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