I was not able to implement a step size of 5e-6 seconds (200 kHz) either, I think it is too fast for this controller. However a sample step size of 12.5e-6 seconds (80 kHz) seems to work.
I implemented the attached model on a 280049C launchpad. There are two subsystems. The subsystem labeled "280049_pwm" is using a PWM block instead of the digital outs. The advantage of this implementation is that irrespective of the duty cycle value, you can have a sample step-size same as the switching period.
The subsystem labeled "280049_digital" is using a pulse generator block. The disadvantage here is that you need to consider the duty cycle value in the sample step-size. For example, for a switching frequency of 40e3 and duty cycle of 0.5, the sample step size must be an integer multiple of 0.5/40e3.
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