Voltage vs Heating ?

Hi, 

Im testing VESC 4.12 and have some odd behavior with heating, does the battery voltage affect heating of the mosfet?
I have tried supply 20V and 40V.On 40 volts it get much hotter.
Test was done with 0% duty cycle.(FOC)

Device is working normally, just wonder why the voltage affects the temperature, especially when the duty is 0% and there is no current or is very small.

Can someone explain why is that?

Thanks