User:Vitaltrust/TRIAC

 

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TRIAC, from Triode for Alternating Current, is a genericized tradename for an electronic component which can conduct current in either direction when it is triggered (turned on), and is formally called a bidirectional triode thyristor or bilateral triode thyristor.



A TRIAC is approximately equivalent to two complementary unilateral thyristors (one is anode triggered and another is cathode triggered SCR) joined in inverse parallel (paralleled but with the polarity reversed) and with their gates connected together. It can be triggered by either a positive or a negative voltage being applied to its gate electrode (with respect to A1, otherwise known as MT1). Once triggered, the device continues to conduct until the current through it drops below a certain threshold value, the holding current, such as at the end of a half-cycle of alternating current (AC) mains power. This makes the TRIAC a very convenient switch for AC circuits, allowing the control of very large power flows with milliampere-scale control currents. In addition, applying a trigger pulse at a controllable point in an AC cycle allows one to control the percentage of current that flows through the TRIAC to the load (phase control).

Physics of the device
To expain the way TRIACs work, one has to individually analyze the triggering in each one of the four quadrants. The four quadrants are defined as in the Figure 1 below, according to the voltage of the gate and the A2 terminals with respect to the A1 terminal. The A1 and A2 terminals are sometimes called also MT1 and MT2, respectively.



The relative sensitivity depends on the physical structure of a particular triac, but as a rule, sensitivity is highest in quadrant I and quadrant IV is generally considerably less sensitive than the others. A triac is said to be "sensitive" if a low gate current is needed to turn it on.



Triggering in Quadrant I
This triggering mode happens when the gate and MT2 (or A2) are positive with respect to MT1 (or A1).



The precise mechanism is explained in Figure 3. In this case, the injection of holes in the p-silicon makes the stacked n, p and n layers beneath MT1 behave like a npn transistor, which turns on due to the presence of a current in its base. This, in turn, causes the p, n and p layers over MT2 to behave like a pnp transistor, which turns on because its n-type base becomes forward-biased with the emitter (MT2). Thus, the triggering scheme is the same as a SCR and the equivalent circuit is outlined in Figure 4.



However, the structure is different from SCRs. In particular, in TRIACs there is always a small current flowing directly from the gate to the MT1 through the p-silicon without passing through the p-n junction between the base and the emitter of the equivalent npn transistor. This current is indicated in Figure 3 by a dotted red line and it is the reason why a TRIAC needs more gate current to turn on than a comparable-rating SCR.

Generally, this quadrant is the most sensitive one of the four.

Triggering in Quadrant II
According to Figure 1, Q-II is defined as the operation mode which has a negative gate and a positive MT2 with respect to MT1.



Figure 5 gives a graphical explanation of the triggering process. The turn-on of the device is three-fold and starts when the current from MT1 flows into the gate through the p-n junction under the gate. This switches on a structure composed by a npn transistor and a pnp transistor, which has the gate as cathode (the turn-on of this structure is indicated by a "1" in the figure). As current into the gate increases, the potential of the left side of the p-silicon under the gate rises towards MT1, since the difference in potential between the gate and MT2 tends to lower: this causes the estabilishment of a current between the left side and the right side of the p-silicon (indicated by a "2" in the figure), which in turn switches on the npn transistor under the MT1 terminal and as a consequence also the pnp transistor between MT2 and the right side of the upper p-silicon. So, in the end, the structure which is crossed by the major portion of the current is the same as in the case of Q-I and is indicated by a "3" in Figure 5.

Triggering in Quadrant III
According to Figure 1, triggering is achieved in the Q-III when both the gate and the A2 (or MT2) are negative with respect to the A1 (or MT1) terminal.



The whole process is outlined in Figure 6. As one can see in the figure, the process happens in different steps here too. In the first phase, the pn junction between the MT1 terminal and the gate becomes forward-biased (step 1). As forward-biasing implies the injection of minority carriers in the two layers joining the junction, electrons are injected in the p-layer under the gate. Some of these electrons do not recombinate and escape to the underlaying n-region (step 2). This in turn lowers the potential of the n-region, which acts as the base of a pnp transistor which switches on (the fact of turning the transistor on without directly lowering the base potential is called remote gate control). The lower p-layer works as the collector of this pnp transistor and has its voltage heightened: actually, this p-layer also acts as the base of a npn transistor made up by the last three layers just over the MT2 terminal, which, in turn, gets activated. Therefore, the red arrow labeled with a "3" in Figure 6 shows the final conduction path of the current.

Triggering in Quadrant IV
According to Figure 1, triggering is achieved in the Q-IV when the gate is positive and the A2 (or MT2) is negative with respect to A1 (or MT1).



Triggering in this quadrant is similar to triggering in Q-III. The process uses a remote gate control also in this case and is explained in Figure 7. As current flows from the p-layer under the gate into the n-layer under the MT1 (or A1) terminal, minority carriers in the form of free electrons are injected into the p-region and some of them manage are collected by the underlaying np-junction and pass into the adjoining n-region without recombinating. As in the case of a triggering in the Q-III, this lowers the potential of such n-layer and turns on the pnp transistor formed by the n-layer and the two p-layers next to it. The lower p-layer works as the collector of this pnp transistor and has its voltage heightened: actually, this p-layer also acts as the base of a npn transistor made up by the last three layers just over the MT2 terminal, which, in turn, gets activated. Therefore, the red arrow labeled with a "3" in Figure 6 shows the final conduction path of the current.

Generally, this quadrant is the less sensitive one of the four.

Typical issues
There are some drawbacks one should know when using a TRIAC in a circuit. In this section, a few are summarized.

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 is, the higher the reverse currents in the blocked junctions are. 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 "Phisics 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. Indeed, the higher the voltage is, the higher the reverse currents are in the blocked junctions, the less current is needed, like in the case of a high temperature. Generally, in datasheets IGT is therefore given for a specified voltage between MT1 and MT2.

When the gate current is discontinued, if the current flowing accross the two main terminals is more than the so-called latching current the device keeps conducting, otherwise the device might turn off: this current is indeed the minimum one which can make up for the missing gate current in order to keeping 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 drive some current out of 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, which is therefore the minimum required current flowing across the two main terminals which 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.

Static dv/dt
A high dv/dt between A2 and A1 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 terminal, which lets currents flow into the gate in response to a large rate of change of A2. Therefore, one way to cope with this limitation consists in snubbering the A2 terminal designing a suitable RC or RCL snubber accross its terminal, but in many cases it is sufficient to lower the impedance of the gate towards A1, for example by putting a resistor or a small capacitor (or both in parallel) between these two terminals: this lets the capacitive current which is generated during the transient flow out of the device without activating it. One should carefully read the application notes provided by the manufacturer and make tests on the particular device model to design the correct network, but typical values for resistors between gate and A2 may be up to 1kΩ and typical values for capacitors between gate and A2 may be up to 100nF.

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.

Critical di/dt
A high rate of rise of the current flowing from A2 to A1 (or viceversa) 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 accross the device: at switching on, the device starts to conduct current before the conductions 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 which can permanently make the TRIAC fail.

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.

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 steps takes place between the two main terminals, which turns on the device 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 turns off following a conduction, 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, 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 passing from A1 (MT1) to A2 (MT2). This is similar to the recovery in standard diodes: the higher di/dt, the higher reverse current one gets. Because in the TRIAC, like in every other device, there are parasitic resistances, a high reverse current in the pn junctions inside it can provoke a voltage drop between the gate region and the A1 region which may make the TRIAC stay turned on.

In 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 ampère 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, indeed, we want 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 accross it due to the afore-mentioned phase shift. If the commutating dv/dt rating is exceeded, the device will not turn off.

Typical Applications
Low power TRIACs are used in many applications such as light dimmers, speed controls for electric fans and other electric motors, and in the modern computerized control circuits of many household small and major appliances.

However, when used with inductive loads such as electric fans, care must be taken to assure that the TRIAC will turn off correctly at the end of each half-cycle of the AC power. Indeed, TRIACs can be very sensitive to high values of dv/dt between A1 and A2, so a phase shift between current and voltage (as in the case of an inductive load) leads to sudden voltage step which can make the device turn on in an unwanted manner.

Unwanted turn-ons can be avoided by using a snubber circuit (usually of the RC or RCL type) between A1 and A2. Snubber circuits are also used to prevent premature triggering, caused for example by voltage spikes in the mains supply.

Because turn-ons are caused by internal capacitive currents flowing into the gate as a consequence of a high voltage dv/dt, a gate resistor or capacitor (or both in parallel) may be connected between gate and A1 to provide a low-impedance path to A1 and further prevent false triggering. This, however, increases the required trigger current or adds latency due to capacitor charging. On the other hand, a resistor between the gate and A1 helps dragging leakage currents out of the device, thus improving the performance of the TRIAC at high temperature, where the maximum allowed dv/dt is lower. Values of resistors less than 1kΩ and capacitors of 100pF are generally suitable for this purpose, although the fine-tuning should be done on the particular device model.

For higher-powered, more-demanding loads, two SCRs in inverse parallel may be used instead of one TRIAC. Because each SCR will have an entire half-cycle of reverse polarity voltage applied to it, turn-off of the SCRs is assured, no matter what the character of the load. However, due to the separate gates, proper triggering of the SCRs is more complex than triggering a TRIAC.

In addition to commutation, a TRIAC may also not turn on reliably with non-resistive loads if the phase shift of the current prevents achieving holding current at trigger time. To overcome that, pulse trains may be used to repeatedly try to trigger the TRIAC until it finally turns on. The advantage is that the gate current does not need to be maintained throughout the entire conduction angle, which can be beneficial when there is only limited drive capability available.

Alternistor
Alternistor is a trade name for a proprietary class of TRIAC with an improved turn-off (commutation) characteristic. The term "Alternistor" has been used for the first time by Thomson Semiconductors (now named ST Microelectronics).

These devices are made specifically for improved commutation when controlling a highly-inductive load, such as a motor, an application which causes problems for "normal" Triacs due to high voltage/current angles. Most Triacs' commutation with inductive loads can be improved by use of a "snubber network", but Alternistors are made specifically for this purpose and they dispose of the snubber requirement altogether. This improvement is achieved at the expense of the ability to trigger the device in the 4th quadrant (negative voltage and positive gate current). However, this is usually no problem, because this trigger mode is seldom used since even normal TRIACs are least sensitive there. ST Microelectronics has another version of improved commutation Triac, but they are not marketing them under the proprietary "Alternistor" moniker, but uses the trademark "SNUBBERLESS".