Simulation of open-loop Buck converter in Simulink

In Ref. [1], we built a simple open-loop Buck converter in Xscheme. In this tutorial, we are going to create a simple open-loop Buck converter in Matlab.

Equations

In order to create the model in Matlab, we have to derive equations first. For a Buck converter inductor, we have the following:

For the output capacitor, we have:

Eq. 1 can be rearranged as:

Eq. 2 can be rearranged as:

Simulink modeling

• Next we are going to use Simulink to implement Eq. 1 and Eq. 2. First, let us import to integrator into the simulink cavans as shown in Fig. 1.

Fig. 1Import integrator and mark the signal

• To mimic the switching behavior on the VSW net, we will use a pulse generator. Fig. 2 shows the settings of the pulse generator. The amplitude is set to be 3.8, which represents Vin=3.8V. The duty cycle is set to be 0.5, which represents 50% duty cycle of the Buck converter control D.

Fig. 2Import pulse generator

• Add product, divide and subtract block. Basically, we are preparing the model as required by Eq. 3 and Eq. 4.

Fig. 3Add product, divide, subtract block

• Finished model is as shown in Fig. 4.

Fig. 4Completed simulink model for open-loop Buck converter

Simulation

Finally, we can run the model. Before we do so, let us change the simulation time to 1ms as shown in Fig. 5.

Fig. 5Simulation results

Also, we need to constrain the max step size as well. In my setting, I set the max step size to be 25ns, which means within a switching period, simulink has to have at least 40 simulation points.

Fig. 6Simulink model settings

The simulation results are as shown in Fig. 7.

Fig. 7Simulation results

Zoomed-in waveform is as shown in Fig. 8.

Fig. 8Simulation results - zoom in

References and materials

[1] Simulation of open-loop switching regulators in Xschem

[2] Matlab self-paced online course

[3] Open-loop Buck converter simulink model - pdf

[4] Open-loop Buck converter simulink model - download