Since it's half-bridge, for short-circuit protection, you should use a current transformer in the path of the power transformer.
For overvoltage protection, step down the DC bus using a voltage divider, eg, by a factor of 100. So, 300VDC gives 3V at the output of the voltage divider. Say, you realize that overvoltage is defined for voltages greater than 360VDC. So, set up a comparator circuit, which turns off the PWM controller when the voltage at the output of the voltage divider is greater than 3.6V.
Since you're going to be using a current transformer for short circuit protection, you can also use that for overload protection. Monitor the output voltage from the transformer and set up a comparator circuit there as well.
Hope this helps.
Tahmid.