Hello Yasser,
you will need to design a variable frequency oscillator. Here is an example of a triangle wave genertor implemented in simscape, https://www.mathworks.com/help/physmod/sps/examples/triangle-wave-generator.html This isn't actually necessarily the best design for you to use, just an example. I highly recommend researching the various methods for implementing variable frequency oscillators and picking one that suits your particular needs, especially if you will actually be building this circuit. Alternatively, you could implement a digital system with variable switching frequency with simulink. In that case, I would recommend using a resetable integrator and feed the error value into it.
Regards,
Joel
