Transient voltage suppression thyristor is a new type of transient voltage absorption device, which is based on the four-layer PNPN semiconductor structure, which realizes the voltage switching protection against external lightning surges and grid operation overvoltages. Its external characteristics are similar to those of gas discharge tubes, both of which are energy transfer protection mechanisms, but their performance is better, such as the response speed can be as short as tens of nanoseconds, the voltage drop after turn-on is very low, which can be as low as about 3V, and the current capacity is large, up to 5kA. In addition, it has the advantages of stable operating voltage, long service life, and the ability to absorb transient voltages of positive and negative polarity in both directions. Therefore, in recent years, transient voltage suppression thyristors have been widely used in the anti-surge protection of low-voltage electrical appliances, telecommunication networks, and consumer electronics products at home and abroad.
First, the basic structure and working principle
A transient voltage suppression thyristor is essentially a special bidirectional SCR without a trigger pin. Figure 1(a) shows the structural schematic of a bidirectional transient voltage suppression thyristor. It is easy to see that the structure is composed of the PNPN thyristor structure on the left and the NPNP thyristor structure on the right side in parallel, which is symmetrical up and down. When the voltage between the electrodes is positive and negative, if the voltage is not high, the PNP of the middle three layers is in a reverse bias state, and the whole tube is in a blocked state, with only a few microamperes and a small leakage current. As the applied voltage increases, the leakage current also increases, and the PNPN structure on the left actually constitutes a complementary amplification connection between a PNP transistor and an NPN transistor. When the applied voltage reaches a certain value, the increased leakage current will increase the amplification of the two transistors, resulting in a positive feedback amplification effect, that is, the current amplification factor exceeds 1.
At this time, the two complementary transistors quickly enter the saturation state, so that the whole tube quickly enters the conduction state from the blocking state, the voltage at both ends of the tube drops rapidly, and the current increases rapidly, realizing the switching protection of overvoltage. When the external overvoltage energy is discharged, as the current flowing between the two poles of the tube drops to a certain value, the amplification factor of the complementary amplification transistor will also drop to less than 1, and the transistor will get out of the saturation state and resume the blocking state. The above is the basic working process of transient suppression thyristor. If the applied voltage is reversed, the same process is generated through the NPNP structure on the right.
Fig.1. Schematic diagram of the structure of a bidirectional transient voltage suppression thyristor
Figure 2(a) shows the circuit symbol of a bidirectional transient voltage suppression thyristor, and Figure 2(b) shows the actual picture. Similar to gas discharge tubes, some manufacturers use two or three devices connected in series or parallel to achieve the purpose of convenient application. Compared with the combined thyristor overvoltage switching protection circuit composed of traditional diodes and SCR, this structure has the advantages of small size, accurate protection and easy use, so it has been widely used.
Figure 2 Circuit symbol and physical diagram of a bidirectional transient voltage suppression thyristor
Second, the main performance parameters
Fig.3. Volt-ampere characteristic curve
Figure 3 shows the volt-ampere characteristic curve of a transient voltage suppression thyristor, which shows some similarity to that of a gas discharge tube. In the shutdown state, the leakage current of the transient voltage suppression thyristor can be less than 5μA, which is almost close to an open circuit, and when the transient voltage exceeds the VDRM, the transient voltage suppression thyristor is turned on like an avalanche diode, bypassing the current, and because the turn-on voltage can be as low as 3~4V, the device can withstand a large inrush current. Some parameters of thyristor suppression are also marked in the figure, and the main technical parameters are described below.
Maximum Maintenance Shutdown Voltage VDRM: The maximum voltage that can be applied to keep the thyristor in the shutdown state.
Turn-on voltage VS: The maximum voltage that can be applied before the thyristor enters the conduction state.
On-state voltage VT: The maximum voltage at both ends of the tube at a specified on-current.
Turn-on current IS: The maximum value of the current required for the thyristor to enter the conduction state.
Leakage Current IDRM: The maximum leakage current of the thyristor at the VDRM shutdown voltage.
Holding Current IH: The minimum current required to keep the thyristor in the conductive state.
Pulse Current Peak IPP: The maximum pulse current peak value of the rating.
Continuous on-state current IT: The maximum value of the continuous current that can be passed through the thyristor.
Single-cycle surge current peak ITSM: The maximum AC current that a thyristor is allowed to pass through in one cycle.
Parasitic capacitance Co: The capacitance between the two electrodes of the thyristor when it is turned off, usually measured at 1 MHz.
Comparatively speaking, this value is smaller than the parasitic capacitance of varistors and TVS tubes.
Current rise rate di/dt: The maximum rate of current change that can be accepted by the thyristor.
Voltage rise rate dv/dt: The maximum rate of voltage change that can be accepted by the thyristor.
Table 5-9 lists the models and parametric performance of several transient voltage suppression thyristors manufactured by Littelfuse.
It can be seen that it has certain advantages in on-state voltage, shutdown leakage current, etc.
Table 5-9 Electrical parameters of transient voltage suppression thyristors
3. Circuit application design of thyristor
Compared with gas discharge tubes, transient voltage suppression thyristors have the advantages of fast response speed, low on-state voltage, close trigger voltage and DC breakdown voltage in the pulse state, and small size, but also have the disadvantages that the current capacity is not yet large and the parasitic capacitance is large, so it is suitable for telecommunication networks and consumer electronics. For the transient voltage suppression thyristor with bidirectional protection characteristics, the specific protection application design scheme and parameter selection process are similar to those of gas discharge tubes.
Figure 4: Combination of transient voltage suppression thyristor and diode
Fig.5. Volt-ampere characteristic curves of parallel protection devices
Using the excellent characteristics of transient voltage suppression thyristor, it can be connected in series or parallel with the diode to achieve the unidirectional overvoltage protection function required for some special occasions. For example, in some network interface circuits, the protected circuit is sensitive to the polarity of the surge, for which a transient voltage suppression thyristor can be used to act in one direction. Figure 4(a) shows a design circuit in which the diode is connected in parallel with a transient voltage suppression thyristor, and Figure 4(b) shows a design circuit in which the diode is connected in series with a thyristor, and some manufacturers have combined them into a single package. It is easy to see that the parallel devices have the volt-ampere characteristic curve shown in Figure 5, which enables unidirectional overvoltage switching protection. For example, in the case of overvoltage protection in telecommunications user interface circuits (SLIC (SLIC), where a lower voltage limit is required for the negative polarity of the common mode overvoltage, a new combination of protection devices can be used to meet the requirements, as shown in Figure 6.
Figure 6 Design of an overvoltage protection circuit for SLIC
In addition, by taking advantage of the characteristics of large current capacity and low on-state voltage of thyristors, its response speed can also be improved and its parasitic capacitance can be reduced, which can be used for ESD pulse protection of integrated circuits or signal cables. Figure 7 shows a schematic diagram of a fast-response thyristor circuit, and the main measure is to add a start-up resistor R to the four-layer PNPN junction. It can be seen that due to the existence of R, when the applied voltage is positive and negative, the PNP transistor will have a large starting current, and the two transistors and NPN will immediately form an amplified conduction state, which will discharge the energy of the external overvoltage, so that the voltage at both ends of the electrode is only the saturation voltage of the transistor, and when the applied voltage is positive and negative, the two transistors are turned off in reverse, so they will not be turned on, and there is only a small leakage current.
Therefore, the structure has a one-way pressure-limiting protection effect. In addition, by optimizing the thickness and design of the PNPN layer, the response speed can reach the nanosecond level, the flow value is large, and the parasitic capacitance is greatly reduced. Taking advantage of the above characteristics, this structure can be combined with the power bus, and can be applied to ESD overvoltage protection of signal lines and IC pins. As shown in Figure 8, when the positive and negative ESD pulses arrive, the upper or lower thyristor is quickly turned on accordingly, and the V+ and V- voltage bus clamps are used to hold the peak value of the overvoltage pulse, dissipate the interference energy, and play a protective role. The thyristor has a stronger absorption capacity than diode ESD clamping schemes. Figure 9 shows an example design of this circuit for protection applications on multiple signal cables.
Fig.7. Schematic diagram of a fast-response thyristor circuit
Fig.8. Fast thyristor busbar clamping circuit
Fig.9. Design of fast thyristor ESD suppression circuit