Why can't certain isolated DC/DC converters operate at over 50% duty cycle?
Image credit: Batarseh, I. (2015). Power Electronic Circuits (Student ed.). New Delhi: Wiley India Pvt.

As you can see from the above image, if the half-bridge DC-DC converter is operated at \$D\ge0.5\$ then it means that at one point of time, both the switches \$S_1\$ and \$S_2\$ will remain closed at one or some point of time. Due to this, the voltage on the upper half will have a negative polarity at the dot and positive polarity at the center tapping due to which induced polarity on the secondary winding has a negative polarity at the dot and positive polarity at its bottom. Since \$S_2\$ is also closed, the dot in the lower half will have a positive polarity and its bottom will be negative inducing a positive at the dot and a negative polarity at the bottom of the secondary as opposed to the previous condition due to \$S_1\$ thus short circuiting the transformer. So, normally, these type of DC-DC converters are used only with \$D&lt;0.5\$.