How to develop a Closed loop current control for converter

I have developed a Buck-Boost converter to charge a SuperCap from a source, in Buck mode and supply a load in Boost mode, I have created a voltage control loop, to maintain the Voltage level within the limit 2.7V.

I also need to include a current controller for maintaing the current to the reference value, (ex.10A)

I need support/guidance with this part.

[Sam Chak]

Hi @Santhosh

Could you provide the differential equations for the Buck-Boost converter?

If it is common practice to linearize the Buck-Boost converter around a steady-state operating point for small-signal analysis, then the transfer function models of the Buck-Boost converter become valid tools for analyzing the frequency response and designing controllers to simultaneously regulate both output voltage and current in a dual-loop control architecture.

[Santhosh]

Dear @Sam Chak,

Please find the equations, the transfer function, Simulink Model and the behaviour of the simulink model below,

Thank you for your kind support

Equations:

1. a) Buck Mode: Q1– Closed, Q2 – Open

1. b) Buck Mode: Q1 – Open, Q2 – Open

Transfer Function

2. a) Boost Mode: Q2 – Open, Q1 – Open

2. b) Boost Mode: Q2– Closed, Q1 – Open

Inductor Voltage & Current

Capacitor Current

Transfer Function

Simulink Model

Behaviour of the Model

[Sam Chak]

Hi @Santhosh

Thank you for your update. Since the decoupled transfer functions are available and are of 2nd order, PID controllers can theoretically be designed to regulate the voltage and current outputs satisfactorily. In fact, the pidtune() tool can make the design process even easier when the performance requirements are properly specified. What is the issue with the controller design?

By the way, the voltage plot shows excessive oscillatory behavior upon reaching the target limit of 2.7V. However, it is very important to note that this behavior is not mathematically reflected in the differential equations and the transfer function models. Have you verified if the Simulink model behaves almost similarly to the transfer function model?

[Santhosh]

hi @Sam Chak

Thank you for your response.

I haven't yet compared the behavior of the transfer function model with the Simulink model. So far, I've only worked with the Simulink model. In this model, the output current reaches up to -40 A, and I haven't been able to identify the cause of this behavior. I'm also uncertain whether the model is functioning correctly. Therefore, I'm currently looking into implementing a current controller to optimize the system's performance and ensure more accurate and stable behavior.

[Sam Chak]

Hi @Santhosh

Don't mention it. I am sharing my experience. If the math model is available and reliable, as published by reputable researchers in journal articles or educators in textbooks, I will construct the Simulink model according to the math equations or functions. This approach is identical to writing the math equations or functions along with a list of parameters in MATLAB code and executing them using the ode45() solver.

I am unsure how you constructed the Simscape electrical model. Try to disable any extraneous dynamics and nonlinearities by turning off saturation or nonlinear element options (if any) in the Simscape blocks to align with the simplifications of the math models. After that, you should validate and verify that the Simscape model accurately implements the established math models.

[Santhosh]

Hi @Sam Chak

I would try to do the modifications as per your recommendations,

and will update you on the results.

Thank you

Answer 1

[Sabin]

We usually control a DC-DC converter using a cascade control structure with inductor current control in the inner loop and capacitor voltage in the outer loop. You must start with the inductor current as the fastest loop. You can then use the transfer functions you provided to tune the controller or use the PID Autotuner from Simulink Control Design.

Some relevant examples to start with:

1. Simscape Electrical Stand-Alone Solar PV AC Power System with Battery Backup example. Check the subsystem ‘SolarPVACWithBattery/PV Control/Constant Voltage’ for the cascade control structure of DC-DC converter.
2. Simulink Control Design Tune PID Controller in Real Time Using Closed-Loop PID Autotuner Block example.