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Posted on 16 November 2019

# High Voltage Power Thyristor with Built-in Protective Elements

Despite the possibility of “outer” protection, usage of thyristors with built-in elements for protection against excess voltage may also be attractive to customers if such a thyristor has the same quality as a the standard one at a lower price.

There are two types of protective elements that are different in principle of operation.

First, there is a voltage suppressor similar to a nonlinear resistor (varistor) or avalanche semiconductor voltage suppressor. Such elements can be characterized by nonlinear volt-ampere diagrams where, up to certain threshold voltage, current through the element has very low value, and upon exceeding this threshold value grows rapidly. Such protective elements can suppress surge discharges applied to the thyristor directly and in the blocking direction as well. However, admissible power of these pulses is low (up to several joules) because the larger part is dispersed in protective elements.

Second, there is an element with dynistor characteristics that switches at a level exceeding a certain threshold voltage value. The structure of such an element is as follows. Exceeding the given threshold voltage value initiates the switching of the protective thyristor. With the help of such an element, only direct excess voltage protection can be realized, though this protection is also effective for high power pulses. Moreover, such protective elements help to achieve a very important characteristic which improves the reliability of thyristors in series connection assemblies of high voltage valves . A thyristor in a series connection assembly can safely switch if the driver fails (absence of standard control signals) - i.e. the valve still works after failure of one of thyristor drivers.

A rather simple and relatively cheap way to produce protective elements of both types integrated in a semiconductor element of a thyristor is production of local areas with undervoltage of avalanche breakdown, shown in Figure 1.

Fig. 1. Integrated into semiconductor element of thyristor protective elements: a. –voltage suppressor of p-n-p type, b. – switching limiter of dinistor type.

Controlled voltage drop of avalanche breakdown in these areas is reached at the expense of n’-layers in the n-base with higher dopant concentration. To have protective elements of the first type p-n-p area, a semiconductor element with no n+ of emitter layer is used where the amplifying electrode is located. In this case, an avalanche current with a limited pulse of direct voltage doesn’t lead to switching of the thyristor element. To have protective elements of the second type p-n-p area, a semiconductor element located under the amplifying electrode. In this case, an avalanche current with limited pulse of direct voltage leads to thyristor switching, because this current is similar to the control current applied at this electrode. One of effective technologies, which may be used to create deep hidden n’layers in semiconductor elements of power high voltage thyristors, is proton irradiation. It is well known [1-4] that during proton irradiation of silicon, implanted atoms of hydrogen stimulate the appearance of connected dopant centers similar to traditional dopants in their characteristics (phosphorus, arsenium, stibium). Unlike the atoms of these chemical elements, hydrogen can be easily implanted into silicon at a depth of hundreds of microns. This supports the creation of n’-base hidden n’-layers with precise regulation of their depth and concentration of additional dopants. As a result, high precision of voltage regulation of avalanche breakdown is possible on the level of dozens volts with 4000-8000 V overall voltage.

Similarity of breakdown voltage is achieved as well, even in the relatively big areas. An example of temperature distribution with current of avalanche breakdown in an experimental semiconductor element is shown in Figure 2. This was taken with the help of an infrared image converter. The diameter of this element is 56 mm. With help of proton irradiation, in the center of it a ring-shaped area about 1 cm in diameter was created with reduced voltage of avalanche breakdown. It is clear that avalanche current causes relatively even overheat within nearly the entire area with reduced voltage breakdown, i.e. current distribution is close to even.

Fig. 2. Temperature distribution image during avalanche current through the experimental semiconductor element with area of reduced breakdown voltage in the center.

Thus the technology of proton irradiation allows creating local areas with controlled reduced voltage of avalanche breakdown within a big area, with rather evenly distributed power with avalanche current, i.e. implanted into the thyristor structure, protective elements can have quite high power capacity and admissible peak power.

The voltolt-amps diagram of a voltage suppressor based on a three-layer p-n-p semiconductor element (Fig. 1 a) has some peculiarities compared to  characteristics of an avalanche diode. As in a high voltage diode, generation of electron-hole pairs during avalanche breakdown happens in a relatively thin layer with maximum values of electric field intensity (Fig. 3).

Figure 3. Typical distribution profile of atoms of acceptor and donor dopants according to the thickness of high-voltage p-n-p element (a.); electric field intensity distribution (E), electrons (Jn) and holes (Jp) current densities during avalanche breakdown. I – area of volume charge of p-n junction, II – layer with maximum values of electric field intensity, where electron-hole pairs are generated

In the main part of the area of volume charge of the p–n junction located in a high resistivity n-layer, avalanche generation of electron-hole pairs does not happen, though in this layer a current of electrons is present, as well as a current of holes, within the second p-n junction of the p-n-p element as a result of transistance in the element. The electron current that results from avalanche generation for this p-n-p element is similar to the base current. The values electron and hole current transferred to the area of volume charge of the reverse-biased p-n junction are given by:

Jp = α . J
Jn = (1-α) J

Jp, Jn – hole and electron current density in the area of volume charge of the p-n junction located in the high resistivity n-layer (without layer of avalanche generation),

J – current density through limiter ,

α– current amplification factor (in the scheme with joint base) of transistored p-n-p element.

Supplementary charge appears with apparent density Qv during current of holes and electrons in the area of volume charge:

vps, vns – rich (maximum) speed of holes and electrons with high electric field intensity. Presence of this supplementary charge influences the electric-field gradient and as a result the voltage value. If

then Qv is positive and with increase of J, the voltage in the semiconductor element decreases, i.e. the volt-amps diagram has an area with negative dynamic stress. Otherwise the voltage of the semiconductor element evenly increases with increase of current density, as in an avalanche diode, though the value of the dynamic resistance of the volt-amps diagram can be much lower.

The value can be easily regulated by changing, for example, the lifetime of the carriers in the n-layer of the semiconductor element. This changes the appearance of the volt-amps diagram of the limiter. In Fig. 4, some typical volt-amps diagrams of high voltage suppressor are shown, which can be achieved by the above mentioned method (1). For comparison, there is a volt-amps diagram of diode structure with identical parameters of high-resistivity n-layer shown in the same figure.

Figure 4. Volt-amps diagram of p-n-p element (1-3) and diode p-n-n+ element (4) during avalanche breakdown. Flat area of semiconductor elements is 1 cm2, thickness of high-resistivity n-layer is 820 µm. 1 – a/(1-a)>vps/vns, 2 – a/(1-a)»vps/vns, 3 – a/(1-a)<vps/vns.

In case of usage of such elements as protective excess-voltage suppressors, it is better to have volt-amps diagram with low dynamic resistance but with no area of negative resistance, similar to one shown in figure 4b. It is clear that in comparison with characteristics of the diode protective element, the dynamic resistance of the volt-amps diagram can be significantly decreased.

Transfer of holes through the base layer of the p-n-p element has some persistence, which is why the question of voltage suppressor performance on the basis of such an element is quite important. Calculations and experiments prove that for limiters meant for voltage up to 8000 V during application of surge discharges with a rate of voltage rise up to 1000-2000 V/µs, voltage fluctuation, in comparison with “quasistatic” conditions, is hardly visible. Thus, for the example in fig. 5, “dynamic” volt-amps diagrams of a limiter with various rates of voltage compared to “quasistatic” volt-amps diagrams are shown.

Figure 5. Volt-amps diagram changes of p-n-p type voltage suppressor depending on rate of voltage rise in front of surge discharge. Flat area of semiconductor suppressing element is 1 cm2, thickness of high-resistivity n-layer is 820 µm. 1 – “quasistatic” characteristic, 2 - dV/dt = 1000 V/µs, 3 – dV/dt = 2000 V/µs, 4 – dV/dt = 4000 V/µs

For usage in protective elements of a dinistor type (fig. 1b), the volt-amps diagram of the p-n-p element with an area of negative dynamic resistance is the most acceptable. Such a characteristic allows the forming of the thyristor part of the protective element control current pulse with a necessary amplitude and rather high rate of rise, which is very important if the surge discharge applied at the protective element has a low rate of rise. In fig. 6, typical current changes of the p-n-p and thyristor elements, as well as the anode voltage at switching-on, of such a dinistor protective element with surge discharge with low rate of rise is shown.

Figure 6. Switching of dinistor protective element with surge discharge with low rate of rise in front. Volt-amps diagram of p-n-p element corresponds (1) in fig. 4, flat area of p-n-p element is 0.1 cm2

Presently, many companies have standard constructive solutions and technologies, which allow customized or mass production of devices with built-in elements that protect against surge discharge. Proton-Electrotex, using the above described standard structures and proton irradiation technologies [3], applies them optionally in accordance with special requirements of customers for all produced thyristors with voltage from 1200 V up to 6500 V. Typical characteristicss of a voltage suppressor of p-n-p type are shown in fig. 7, and an integrated dinistor element in fig. 8 and fig. 9.

Figure 7. Volt-amps diagram of element - excess-voltage suppressor for thyristors with voltage up to 1800 V

Figure 8. Swithching of thyristor with low rate of voltage rise

Figure 9. Swithching of thyristor with high rate of voltage rise

List of references

[1]      V.V. Kozlovski. Modification of semiconductors by proton beam. S.-Pb., Nauka, 1993.

[2]      E. P. Neustroev et al. Donor center formation in hydrogen implanted silicon. – Physica B. 1999, vol. 270, No. 1-2, p. 1-5.

[3]      V. N. Gubarev, A.Yu. Semenov, V. S. Stolbunov, A. M. Surma. Technology of proton irradiation and possibilities of applying it for performance improvement of power thyristors and diodes. – Power Electronics Europe, Issue 3, 2011, p. 35-38.

[4]J. N. Klug, J. Lutz, J. B. Meijer. N-type doping of silicon by proton implantation. - Conf. Proc. of EPE'2011, Birmingham, 2011

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