TRIAC Dimming for LED Buck controller circuits
Triacs are typically used in mains powered devices because they rely on the zero-crossing of the mains current to switch off after each half cycle.  

Driving the triac with a PWM signal doesn't work. The pulse will switch the triac on, but the zero current condition to switch off is never met, so it will stay on forever.  

Controlling brightness of a constant current driven LED with PWM, for instance using a FET, is also a bad idea: current sources should never be unloaded, as they will build up a high output voltage in an attempt to keep the current flowing. Maybe not a problem for linear devices, as their maximum voltage is the input voltage, but a switcher's inductor can create voltages high enough to destroy the FET.