Reverse phase control
The commercially-practical implementation of reverse phase dimming uses IGBT transistors. A transistor differs from an SCR in that it can be controlled to gradually vary the current, not just to switch it on. By gradually turning off the current rather than turning it on, a reverse phase angle dimmer reduces the filament noise in a similar fashion as a forward phase SCR dimmer without the need of a choke.
Using a transistor as a switch (on or off) produces minimal heat, while using a transistor to directly control the voltage to a load creates a lot of heat. The more time spent by the transistor in the analog mode, the more heat it produces. Specifications for 800µS rise or fall time require the transistor to operate in an analog mode the entire 800µS. This can raise the operating temperature past the limits set by the transistor manufacturer. Some designs prevent failure of the IGBT by turning it off early, thereby producing less heat, and importantly, less fall time. Reduced fall time means increased lamp noise!
The reverse phase dimmer is still chopping the waveform at the line frequency, so harmonic currents and electrical interference are still present. Acoustic noise for forward and reverse phase dimmers has been evaluated and is quantitatively the same for equivalent rise and fall times.
More importantly, reverse phase dimmers that modify their behavior to become forward phase dimmers at will, in order to handle certain loads, are capable of creating harmonic neutral currents well in excess of the design specifications for either a completely forward phase or reverse phase system. A sine wave dimmer creates no harmonic currents.