Step increase in flyback converter output with ramp increase Duty Cycle

I am working on simulating controls for flyback converter . Once i was done modelling i gave the PWM generator a ramp input to understand how the output behaves . The output was in the form of steps and steadystates .

I tried using a ramp input on the SIMULINK Flyback Converter Example code and got a similar result

This is my the model I used for getting the above graph.

Modified example

Output from modified example model.

I used this example to revice a similar output , I believe there are no errors in the model construction and model parameters as i cross checked anything. Due to this behaviour i am unable to tune the PID controler as the outputwaveform is only stable at particular duty cycle ratios. If any one has experienced thios before or know why this is happening , or this is to be expected as the output, please do share. Thanks

Answer 1

[Sayan]

Hi Adithyan Mundayadu,

I understand from your query that the output voltages of the Flyback Converter in both the example model and your model are increasing in steps rather than ramp when ramp input is provided to the PWM Generator to provide PWM pulses to the Mosfet.

The possible reasons for the same are mentioned below:

• The PWM Generator contains a Carrier counter block that uses the Discrete-Time Integrator block (available at Simulink / Discrete) to generate the carrier signal with the "Sample time" provided in the PWM Generator block "Sample time" field. This Discrete-Time Integrator block computes the carrier signal in each step of the "Sample time". This makes the carrier signal stepped.
• The PWM Generator block changes the duty ratio when the ramp signal attains the step value of the carrier signal.
• During the time the ramp signal reaches from one step of the carrier signal to the next step, the duty ratio remains constant. This makes the output voltage constant within this period.
• When the ramp signal attains the value of the next step of the carrier signal, the duty ratio is increased, which results in a sudden increase in the output voltage, which again changes when the ramp signal attains the value of the next step of the carrier signal. This is shown in the following screenshot, which is taken at the junction, where the output voltage changes from one step to the next.(The PWM signal is multiplied with 50 to increase its "Y" range to plot in the same axis it has no other purpose)

Now I assume you need the PID Controller (available at Simulink / Continuous) to control the output voltage of the converter. In this case, you can use a PID Controller to drive the PWM Generator instead of the ramp signal. The PID Controller will reduce the error generated between the reference output voltage and the actual output voltage to meet the requirement. After this, you can tune the controller.

You can find more information on PWM Generator , PID Controller and Discrete-Time Integrator blocks in the following documentations:

• PWM Generator: https://www.mathworks.com/help/releases/R2023a/sps/ref/pwmgenerator.html
• PID Controller: https://www.mathworks.com/help/releases/R2023a/simulink/slref/pidcontroller.html
• Discrete-Time Integrator: https://www.mathworks.com/help/releases/R2023a/simulink/slref/discretetimeintegrator.html

Hope this helps in answering the query.