Typical issues of Triac

There are some drawbacks one should know when using a TRIAC in a circuit. In this section, a few are summarized.
[edit] Gate threshold current, latching current and holding current

A TRIAC starts conducting when a current flowing into or out of its gate is sufficient to turn on the relevant junctions in the quadrant of operation. The minimum current able to do this is called gate threshold current and is generally indicated by IGT. In a typical TRIAC, the gate threshold current is generally few milliampères, but one has to take into account also that:

    * IGT depends on the temperature: indeed, the higher the temperature, the higher the reverse currents in the blocked junctions. This implies the presence of more free carriers in the gate region, which lowers the gate current needed.
    * IGT depends on the quadrant of operation, since a different quadrant implies a different way of triggering, as explained in the section "Physics of the device". As a rule, the first quadrant is the most sensitive (i.e. requires the least current to turn on), whereas the fourth quadrant is the least sensitive.
    * When turning on from an off-state, IGT depends on the voltage applied on the two main terminals MT1 and MT2. Higher voltage between MT1 and MT2 cause greater reverse currents in the blocked junctions requiring less gate current similar to high temperature operation. Generally, in datasheets, IGT is given for a specified voltage between MT1 and MT2.

When the gate current is discontinued, if the current flowing between the two main terminals is more than the so-called latching current the device keeps conducting, otherwise the device might turn off. Latching current is the minimum that can make up for the missing gate current in order to keep the device internal structure latched. The value of this parameter varies with:

    * gate current pulse (amplitude, shape and width)
    * temperature
    * control circuit (resistors or capacitors between the gate and MT1 increase the latching current because they steal some current from the gate before it can help the complete turn-on of the device)
    * quadrant of operation

In particular, if the pulse width of the gate current is sufficiently large (generally some tens of microseconds), the TRIAC has completed the triggering process when the gate signal is discontinued and the latching current reaches a minimum level called holding current. Holding current the minimum required current flowing between the two main terminals that keeps the device on after it has achieved commutation in every part of its internal structure.

In datasheets, the latching current is indicated as IL, while the holding current is indicated as IH. They are typically in the order of some milliampères.
[edit] Static dv/dt

A high dv/dt between A2/MT2 and A1/MT1 may turn on the TRIAC when it is off. Typical values of critical static dv/dt are in the tens of volts per microsecond.

The turn-on is due to a parasitic capacitive coupling of the gate terminal with the A2/MT2 terminal, which lets currents flow into the gate in response to a large rate of voltage change at A2/MT2. One way to cope with this limitation is to design a suitable RC or RCL snubber network. in many cases this is sufficient to lower the impedance of the gate towards A1/MT1. By putting a resistor or a small capacitor (or both in parallel) between these two terminals, the capacitive current generated during the transient, flows out of the device without activating it. A careful reading of the application notes provided by the manufacturer and testing of the particular device model to design the correct network is in order. Typical values for capacitors and resistors between the gate and A1/MT1 may be up to 100nF and up to 1kΩ.[3]

In datasheets, the static dv/dt is usually indicated as \left (\frac{\operatorname{d}v}{\operatorname{d}t}\right )_s and, as mentioned before, is in relation to the tendency of a TRIAC to turn on from the off state after a large voltage rate of rise even without applying any current in the gate.
[edit] Critical di/dt

A high rate of rise of the current flowing between A1/MT1 and A2/MT2 (in either direction) when the device is turning on can damage or destroy the TRIAC even if the pulse duration is very short. The reason is that during the commutation, the power dissipation is not uniformly distributed across the device. When switching on, the device starts to conduct current before the conduction finishes to spread accross the entire junction. The device typically starts to conduct the current imposed by the external circuitry after some nanoseconds or microseconds but the complete switch on of the whole junction takes a much longer time, so too swift a current rise may cause local hot spots that can permanently damage the TRIAC.

In datasheets, this parameter is usually indicated as \frac{\operatorname{d}i}{\operatorname{d}t} and is typically in the order of the tens of ampère per microsecond.[1]
[edit] Commutating dv/dt and di/dt

The commutating dv/dt 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 \left ( \frac{\operatorname{d}v}{\operatorname{d}t} \right ) _c 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 A1/MT1, so it is easier for the capacitive current due to dv/dt to turn on the device again.

Another important factor during a commutation from on-state to off-state is the di/dt of the current from A1/MT1 to A2/MT2. This is similar to the recovery in standard diodes: the higher the di/dt, 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 A1/MT1 region which may make the TRIAC stay turned on.

In a datasheet, the commutating di/dt is usually indicated as \left ( \frac{\operatorname{d}i}{\operatorname{d}t} \right ) _c and is generally in the order of some ampères per microsecond.

The commutating dv/dt 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 afore-mentioned phase shift. If the commutating dv/dt rating is exceeded, the device will not turn off.