How to heat up MOSFET within seconds?
At low switching frequency, heating is mostly determined by RDSon (P=I2R). The STP4NK60ZFP is typically 1.7&ohm; while the IRF9530 is &lt;0.3&ohm;. Both FETs are in a TO220 package, so the STP4NK60ZFP will heat up much faster than the IRF9530 at the same current.

Thermal resistance of the STP4NK60ZFP is 62.5&deg;c/W junction to air, which at 20&deg;c ambient corresponds to 82.5&deg; die temperature at 1W dissipation. You probably want a considerably lower temperature at low PWM frequency, rising to a maximum of perhaps 80~100&deg;c at higher frequencies (for safety you don't want it to get too hot). 

Therefore I suggest making R1 = 18&ohm;, which sets the current to 0.5A and should raise the STP4NK60ZFP's temperature to about 45&deg;c when turned on continuously. At this current the IRF9530's temperature will only go up about 5&deg;c.