Please check the actual amplitude of the gate voltage pulse applied, using an oscilloscope. If the amplitude is low, the MOSFET moves away from saturation and hence has higher heat generation. This is a problem on high side only since usually such power supplies are either separate or are derived through "bootstrap" techniques from the low-side. The pulse amplitude must be above 10volts.

Be carefull also not to turn on too slow.

Too slow generate heat and too fast generate EMI.

If the DC-link voltage is high a more adequate device is an IGBT. HV MOSFET usually present a high Rdson.

Agreed with above(Sujit K. Biswas), check the VGS voltage and charge-pump( assuming N-ch device for upper driver )

My friends, please note that Shiva stated that only the high-side MOSFETs get hot. Thus, it cannot be due to slow switching or due to HV MOSFETs, as then all MOSFETs would be hot !