Matlab Function block changes input values by order of magnitude

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Lillebror Sagmen-Andersson
Commented: Adarsh on 27 Mar 2025
Hello!
I'm trying to implement a basic MPPT P&O Algorithm in Simulink. For this I have a Matlab Function block that takes in the current, voltage, and power. It then applies the P&O algorithm and outputs a desired dutycycle which is feed into a PWM generator.
The system was not behaving as I expected so I added a output form the Function Block
StateVector = [Ppv, Pprev, Vpv, Vprev, Dprev];
When then viewing the output from the StateVector with a scope I found that Ppv (INPUT) =/= Ppv (OUTPUT)
The full code (Incorrect Algorithm) is as follows:
function [D,DeltaP,StateVector] = fcn(Vpv,Ipv,Ppv)
persistent Dprev Pprev Vprev
if isempty(Dprev)
Dprev = 0.1;
Vprev = 48;
Pprev = 380;
end
%Ppv = Vpv*Ipv;
DeltaP = Ppv-Pprev;
deltaD = 0.1;
if (Ppv-Vprev) > 0
if (Vpv-Vprev) > 0
D = Dprev-deltaD;
else
D = Dprev+deltaD;
end
else
if (Vpv-Vprev) > 0
D = Dprev + deltaD;
else
D = Dprev - deltaD;
end
end
Dprev = D;
Vprev = Vpv;
Pprev = Ppv;
StateVector = [Ppv, Pprev, Vpv, Vprev, Dprev];
As you can see at no point is Ppv changed. At first it was calculated internally, but when I found this error I moved the computation outside making use of a product block.
The input Ppv can be seen below:
And the output:
I do not understand what could cause this. These signals are pulled directly at the input and output.
I appreciate any suggestions :)
EDIT: Ops the algorithm was very wrong. But ignoring the incorrectly formulated P&O Algorithm, why would the input-output relation be so strange?
  2 Comments
Yifeng Tang
Yifeng Tang on 19 Mar 2025
Attaching the model will be helpful for the community to diagnose the issues.
Adarsh
Adarsh on 27 Mar 2025
Hi @Lillebror Sagmen-Andersson,
I have tried to reproduce the issue that you are facing using an example model, in which i have used the given code in the "MATLABFcn" block and provided the input ports with data generated by different signal generators, each having different frequencies (so that i can verify where exactly the issue is occuring). when I inspected the Input and the output "Ppv" signals in a "scope" block I see that both the signals are same, pointing no issue with the given function.
Here is an image of the example model that I have tested:
Here is the output of the "scope" block:
Attaching the exact model will be helpful to find out the exact issue.

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