Control of LLC Converter

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Martin Wattenberg
Martin Wattenberg on 23 Jul 2018
Commented: youji zhang on 4 Mar 2022
Hello,
I am currently trying to develop a control model for an LLC converter.
This is a topology in power electronics that relies heavily on the resonant behavior of an LC tank.
Please see the image attached for a simplified version
The output of the system is the amplitude of Vout. The input to the system is:
  1. the amplitude of Vin,
  2. the frequency of Vin.
Both parameters can be changed individually. The controller should later control the frequency of Vin to maintain Vout, while changes in amplitude of Vin are considered external disturbances.
My problem is how to model the frequency behavior in MATLAB? I have calculated the bode plot but how can I continue with the controller design?
R = 10;
L = 10e-6;
C = 10e-6;
G = tf([L*L*R*C,L*R*R*C,0,0],[L*L*R*C,L*L+L*R*R*C,2*R*L,R*R])
bode(G)
Thank you very much for your support!
  1 Comment
youji zhang
youji zhang on 4 Mar 2022
i'm focusing on the same problem recently, this might be helpful form matlab website:
Estimating Transfer Function Models for a Boost Converter
here is the .m function they present:
mdl = 'iddemo_boost_converter';%llc mdl name
open_system(mdl);
ios = [...
linio([mdl,'/DutyCycle'],1,'input'); ...
linio([mdl,'/PS-Simulink Converter'],1,'output')];
%change DutyCycle to your actual control input like frequency (its module name)
%PS-Simulink Converter,changing to the name of a module that actual measure output %voltage
%the frequency is ranged 200 to 20k rad/s
f = logspace(log10(200),log10(20000),10);
% perturb the frequency by same value
in = frest.Sinestream('Frequency',f,'Amplitude',0.03);
getSimulationTime(in)/0.02
%compute discrete points on the frequency response
[sysData,simlog] = frestimate(mdl,ios,in);
bopt = bodeoptions;
bopt.Grid = 'on';
bopt.PhaseMatching = 'on';
figure, bode(sysData,'*r',bopt)

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Answers (2)

Arkadiy Turevskiy
Arkadiy Turevskiy on 21 Aug 2018
What is G - is it transfer function from Fs(frequency) to Vout? Probably it is transfer function from Vin to Vout right?
If you know the transfer function Fs to Vout, then you can design your controller using Control System Designer or PID Tuner in Control System Toolbox.
But in general, as I understand, you cannot determine that transfer function and you have to measure it - see t his application note from TI with proposed approach.
You also might want to consider simulating the system in Simscape Power Systems and designing your controller in simulation, as shown here .

Martin Wattenberg
Martin Wattenberg on 22 Aug 2018
Dear Arkadiy Turevskiy,
thank you for taking the time and replaying to my question.
The simscape power system in combination with the system identification toolbox could be a possibility. This way I could realize the proposed approach by TI.
However, the converter is a white box to me. I was looking for a more straightforward (and numerically more efficient) solution to my problem.
I am still interested in controlling the frequency to maintain Vout.
  1 Comment
Arkadiy Turevskiy
Arkadiy Turevskiy on 22 Aug 2018
If it is white box, does it mean you can analytically derive a transfer function from Fs to Vout? If you can, then the problem becomes straightforward.

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