Hi Jaap,
I'm not an expert but I seem to remember reading that pwm to dim high power leds via mosfets can be tricky. A significant amount of charge has to be delivered to or from the mosfet gate in a very short time. If your circuit can't do this, the mosfets to not swtch completely on or off and the result is a lot of heat, which can shorten the life of the mosfets dramatically. There are special mosfet driver chips to overcome this problem.
Paul