Editing TRIAC (section)

===Commutating dv/dt and di/dt===
The commutating d''v''/d''t'' rating applies when a TRIAC has been conducting and attempts to turn off with a partially reactive load, such as an inductor. The current and voltage are out of phase, so when the current decreases below the holding value, the TRIAC attempts to turn off, but because of the phase shift between current and voltage, a sudden voltage step takes place between the two main terminals, which turns the device on again.

In datasheets, this parameter is usually indicated as <math> \left ( \frac{\operatorname{d}v}{\operatorname{d}t} \right ) _c </math> and is generally in the order of up to some volts per microsecond.

The reason why ''commutating dv/dt is less than static dv/dt'' is that, shortly before the device tries to turn off, there is still some excess minority charge in its internal layers as a result of the previous conduction. When the TRIAC starts to turn off, these charges alter the internal potential of the region near the gate and MT1, so it is easier for the capacitive current due to d''v''/d''t'' to turn on the device again.

Another important factor during a commutation from on-state to off-state is the d''i''/d''t'' of the current from MT1 to MT2. This is similar to the recovery in standard diodes: the higher the d''i''/d''t'', the greater the reverse current. Because in the TRIAC there are parasitic resistances, a high reverse current in the p-n junctions inside it can provoke a voltage drop between the gate region and the MT1 region which may make the TRIAC stay turned on.

In a datasheet, the commutating d''i''/d''t'' is usually indicated as <math> \left ( \frac{\operatorname{d}i}{\operatorname{d}t} \right ) _c </math> and is generally in the order of some amperes per microsecond.

The commutating d''v''/d''t'' is very important when the TRIAC is used to drive a load with a phase shift between current and voltage, such as an inductive load. Suppose one wants to turn the inductor off: when the current goes to zero, if the gate is not fed, the TRIAC attempts to turn off, but this causes a step in the voltage across it due to the aforementioned phase shift. If the commutating d''v''/d''t'' rating is exceeded, the device will not turn off.