Thyristor triggering mechanisms
The flow of current through a thyristor can result in several ways. Triggering mechanisms for thyristors include the following:
- Trigger current through the gate
- Reverse current as a result of exceeding the maximum blocking voltage (breakdown voltage) ("overhead" ignition, tilt)
- Capacitive displacement current due to a steep rise in anode voltage (dV / dt)
- Light irradiation in the space charge region (light ignition)
- High temperature (thermally generated leakage current)
Trigger current through the gate
The trigger current IGT is the minimum current between the gate and cathode which is sufficient to ignite the thyristor. The ignition current decreases with increasing temperature.
The minimum amount of gate-cathode current IGT (Gate Trigger Current) required to ignite a thyristor depends on the following:
- The geometry of the thyristor
- The doping of the p-base
- The temperature (the thyristor ignites more easily when it is hot)
- Anode-cathode voltage
A thyristor (without a shorted emitter) ignites at a few mA ignition current. Since the ignition current flows through the p-base/emitter junction, the ignition VGT voltage is generally greater than 0.7 V (lock-voltage of the base-emitter diode).
Thyristor breakover Triggering
If a sufficiently large forward voltage is applied accross the thyristor, the reversed biased PN-juction will experience breakdown and the thyristor will be fired into a conducting state. The minimum voltage that results in this type of thyristor firing is called the breakdown voltage.
Figure 2. Breakover triggering
Without a shorted emitter, the firing current is so low that the reverse current at low voltage is sufficient for firing. This is the reason why a thyristor without emitter metallization has a blocking voltage of only a few volts .
Thyristor dV/dt triggering
Each PN-junction has voltage dependent capacitance. This capacitance peaks when no voltage is applied and decreases as applied reverse voltage increases. A capacitive displacement current flows through the PN-junction when a change in voltage (high dV / dt) occurs. A steep increase in anode voltage can result in a gate trigger current at this p-base/emitter junction, causing the thyristor to fire.
Triggering using thermally generated reverse current
Since leakage current increases with temperature and the required triggering current decreases with rise in temperature, the breakover voltage also decreases with rise in temperature. As described above, when the breakover voltage is exceeded, the leakage current is sufficient to trigger the thyristor. Therefore, for a given voltage, a rise in temperature can induce the thyristor to fire.
Ways of increasing gate trigger current
Thyristor firing current can be increased by adding a portion of the undesired leakage current by bypassing the reversed biased PN-junction via short circuits between the p-base and the emitter (short-circuit emitter, shorted emitter). The short circuits are prepared using emitter metallization.
Figure 3. Shorted Emitter Principle
An important feature for emitter shorts is a uniform arrangement of the shorts on the surface of the emitter.
Thyristor turn-on behaviour
Figure 4. a) Current distribution in the thyristor upon turning on the gate current b) Current distribution in the thyristor right after ignition
The firing initially occors locally where the highest concentration of ignition current can be found. The spread of the firing area is relatively slowly (only 30 microns to 100 microns per microsecond depending on conditions). A thyristor whose diameter is 100 mm takes about 1000 microseconds until the whole thyristor surface becomes conductive.
Thyristor turn-on characteristics - di/dt limitation
A thyristor may be destroyed if the maximum allowed rate of current rise is exceeded. This is due to the fact that the turn-on losses are concentrated on the the small region of the thyristor already udergoing firing. This leads to very high local temperatures that cause the silicon to melt at the points that begin firing at the very beginning. Thyristor failure due to a high rate of current increase is made eviident by a small round melted channel at the edge of the cathode.
Figure 5. Current voltage and power loss characteristic upon turn on of thyristors with large surfaces
The primary firing area can be increased by applying an ignition current which is several times above the required minimum ignition current. This also increases the level of acceptable current rise di / dt.
Figure 6. Transverse field emitter
A transverse field emitter is an extension of the n+- emitter zone in the direction of the gate due to emitter metallization. The thyristor ignites at point 1, which means that this point assumes the n + zone anode potential, while point 2 has the cathode potential.
The anode-cathode voltage drops across the resistor R (transverse field) and accelerates the charge carriers towards the cathode. This increases the firing spread rate and leads to strong enlargement of the region fired during the first microseconds of firing.
An amplifying gate delivers the amplified trigger current to the pilot thyristor from the primary thyristor. Both thyristory may be integrated into the same chip just like in a darlington thyristor. The trigger current for the primary thyristor is taken from the primary circuit. The strong firing current ensures that the primary firing surface is much larger than a surface that lacks firing aid and also causes the firing to spread much faster. Thyristors are often used in conjunction with transverse field emitters and amplifying gates.
Improving thyristor switching characteristics using an amplifying gate
Switching characteristics of a thyristor can be improved by increasing length of the gate in order to even out the spread of the firing effect on the emitter surface. Gate structures are usually used in combination with ampliying gates in order to ensure that enough current for firing reaches the gate perimeter. Current rise values di/dt of a few 100A/μs are then permitted.
Thyristor turn-off behaviour and turn-off losses
Figure 7. Time evolution of primary current i, voltage v and switching power loss of a thyristor PRR with steeply decaying current
The fluctuations in voltage shown above are caused by the combined effect of inductance and capacitance within the circuit.
For many applications, thyristors can be brought back to the non conductive state using relatively tedious forced commutation. The circuit commutated turn-off time is the minimum time delay that must elapse before the anode can be positively biased again and return the thyristor to the off-state. The shorter the circuit commutated turn off time, the smaller and more cost effective is the forced commutation.
For more information, please read: