  We demonstrate numerically novel mechanism providing generation of the
flat-top solitonic pulses, platicons, in optical microresonators at normal GVD
via negative thermal effects. We found that platicon excitation is possible if
the ratio of the photon lifetime to the thermal relaxation time is large
enough. We show that there are two regimes of the platicon generation depending
on the pump amplitude: the smooth one and the oscillatory one. Parameter ranges
providing platicon excitation are found and analysed for different values of
the thermal relaxation time, frequency-scan rate and GVD coefficient.
Possibility of the turn-key generation regime is also shown.
