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Posted on 09 November 2020

High Voltage Thyristors Adjusted for Usage in Series Assemblies and Stacks

 

Despite significant development of converters on the basis of fully controlled semiconductor stacks (IGBT, GTO, IGCT), today it is still technically legitimate and acceptable to use “traditional” high power thyristors in stacks of controlled rectifiers as well as in soft starters of electric motors. Usage of thyristors is especially relevant in the case of operating in AC network of 6/10 kV and higher because the devices produced on the basis of such thyristors have no competition in price and energy efficiency.

This is why development and production of high voltage thyristors is of current interest. During the last several years, some manufacturers developed and put into production high power devices with voltage up to 6,5-8,5 kV necessary for high voltage valves of electric converters operating in AC with 6 kV and higher.

The blocking voltage level is still low enough to allow using only one thyristor in such stacks. This is why the stack consists of several series-connected semiconductor devices which require operating synchronization of thyristors in such connection.

Unfortunately, along with the increase in maximum allowed blocking voltage, reverse recovery voltage increases as well, which is quite typical for high voltage devices. This is connected with the necessity to guarantee low voltage in off-state. For devices with 6,5-8,5kV voltage, values of reverse recovery voltage and maximum value of reverse recovery current reach a very high level, even with a low value of current rate of rise.

In Figure 1, typical values for high voltage thyristors manufactured by various companies are shown. These values for thyristors with 6,5-8,5 kV are reached when it is quite difficult to cross-match damping and conforming RC-circuits.

Calculations and experiments show that full power of losses in damping RC-circuit, limiting the pulse spike of reverse voltage at recovery of the typical high voltage thyristor on the level of 0,75-0,8VRRM in circuit with VDC~0,5VRRM voltage, is connected with reverse recovery charge by the semi-empirical formula:

ER~1,5Qrr*VDC,

where ER is the energy dissipated in the RC-circuit resistor in “turn on – turn off” cycle.

Considering the data shown in Figure 1, it is evident that power dissipated by damping circuits with high voltage thyristors can be compared with full power of losses of the thyristor, which is not at all optimistic considering the complexity of a stack cooling system as well as its efficiency coefficient.

Figure 1. Dependence of average current in on-state and reverse recovery charge for high voltage (6,5-8,5 kV) thyristors

Based on the information above, high voltage thyristors designed for series connection assemblies have been developed. These thyristors exhibit specific characteristics at reverse recovery which include the following:
- minimized values of reverse recovery charge and current (with the condition of low voltage in on-state);
- “soft” character of reverse recovery; usage of thyristors with soft reverse recovery allows for simplification of the requirements of the RC-circuit as long as acceptable levels of peak voltage are provided.

High voltage thyristors adjusted for usage in series assemblies

It is a well-known fact that the value of reverse recovery charge depends on the value of accumulated charge of excess electrons and holes in the n-base layer of the thyristor and also on the recombination rate of this accumulated charge. For high voltage thyristors, which recover at low rate of rise of anode current, the second factor is more crucial. Indeed, during rate of rise of anode current, the bigger part of excess carriers recombine. Thus, there is some optimum value of effective life time of carriers in n-base of the thyristor which helps it achieve low reverse recovery charge with a relatively low value of voltage rate of rise in the on-state.

To reach the optimum value of a carrier's life time in the n-base of the thyristor, accelerated electron and proton irradiation of silicon elements technology is used.

However, there are some additional options to lower the value of reverse recovery charge. If we lower the maximum concentration of atoms of acceptor dopant in the p-base of the thyristor, this will lower reverse recovery charge by means of transferring some of the excess electrons accumulated in the n-base into the n+-emitter, similar to the process that occurs in a diode. In a thyristor with a highly doped p-base, as a result of transistance, there are no electrons transfered from the n- base, but excess holes injected into the n- base, which leads to qa relative increase in reverse recovery charge.

Thyristors produced by Proton-Electrotex JSC have quite low doped p- base (usually maximum concentration of acceptors - (1 or 2)*1016 cm-3). This allows to lower reverse recovery charge without any effect on voltage rate of rise in the on-state.

To guarantee the required dU/dt  durability for thyristors with a low doped p- base, a special configuration of distributed cathodic diversion is used.

“Softness” of reverse recovery S is a very significant characteristic which can be shown as quotient of duration of rate of rise of reverse current (tf) and time of delay of reverse voltage applied (ts) in the process of reverse recovery of the thyristor:

S= tf/ ts.

It is known that increase of reverse recovery softness can be reached by lowering the concentration of excess carriers near the anode p-emitter.

This can be achieved in 2 ways:

  • lowering of injection efficiency of the anode p-emitter. This can be achieved by lowering the maximum concentration of acceptor dopant as well as carriers’ life time in the highly doped area of p-emitter layer;
  • local decrease in life time in the layers of n- base and low doped p-emitter joining the anode p-n junction.

Proton-Electrotex JSC uses both methods to achieve the desired results for thyristors.

First, relatively low doped p- emitter layers are used. This allows for reduction of reverse recovery surge current and increase in softness of reverse recovery. Furthermore, as proven by calculations and experiments, for such thyristors, low temperature dependence of time and reverse recovery charge is very characteristic.

Secondly, if it is necessary to reach soft reverse recovery, special technology of carrier lifetime regulation based on proton irradiation is used. This technology allows for a decrease in the carriers’ lifetime locally within the layers joining the p-n junction.

It is very crucial to have identical reverse recovery characteristics for thyristors and to have the same surge current and reverse recovery charge, as well as identical characteristic of current dependence on time. This makes it possible to avoid using RC-circuits when assembling thyristors.

In accordance with the above mentioned factors,  it is clear that in order to have identical characteristics of reverse recovery, it is necessary to provide a high precision of doping profile and lifetime distribution of carriers in semiconductor elements.

Identity of dopants distribution is provided by a high level of production of semiconductor elements technology and precise control of carriers’ lifetime becomes possible with help of special electron and/or proton irradiation technology.

In order to achieve low variation of reverse recovery parameters, the following process flowsheet is used:

Step 1. Pressumption of achieving low variation of reverse recovery characteristics – providing high identity of dopant profiles in produced silicon elements, which is achieved by thoroughly worked-out technology.

This step provides the repetition of reverse recovery current form and temperature dependences of reverse recovery characteristics.

Step 2. Precise control of reverse recovery parameters (reverse recovery time, reverse recovery current, reverse recovery charge, softness) with help of electron and proton irradiation.

This step provides additional correction of reverse recovery time and charge to lower the variation of these characteristics in groups.

Combination of electron and proton irradiation allows for softness to be adjusted simultaneously.

Step 3. Final presorting with equipment that provides the possibility to run tests of reverse recovery of two or more series connected thyristors in a mode close to operational.

The scheme of equipment necessary for such tests is shown in Fig. 2. Presorting is done during the test of each and every thyristor in series connection with a standard sample. A pulse power supply provides a positive voltage distribution to the switched-on thyristors, and current going through the inductive reactor L, linearly reaches the required value. Voltage polarity changes and two series connected thyristors reverse recover.

Figure 2. Basic diagram of equipment for final presorting of thyristors by reverse recovery characteristics

Fitting the criterion of reverse recovery characteristics of the device to the standard sample requires a voltage to the thyristors during the whole process of reverse recovery that is distributed equally between the test sample and the standard sample. Typical dependences of reverse recovery current and voltage of the test sample and the standard sample thyristors are shown in Figure 3.

Typical dependences of reverse recovery current

Typical dependences of reverse recovery voltage

Figure 3. Typical dependences of reverse recovery current and voltage of testee and standard sample thyristors using equipment for final presorting by reverse recovery characteristics

This equipment can be used for testing the assembled high voltage valves on basis of series connected thyristor stacks.

As a result of the above mentioned technologies in production, it is possible to have thyristors with relatively low reverse recovery charge, low temperature dependence, as well as high softness of reverse recovery. Typical characteristics of high voltage thyristors adjusted for usage in series assemblies are shown in Table 1.

Table 1. Characteristics of high voltage thyristors adjusted for usage in series assemblies

Series stacks for usage in soft starters of electric motors

New high voltage thyristors adjusted for usage in series connection are used for series connected stacks, for example, the КТ5.11-800 by Proton Electrolex, designed for usage in soft starters of electric motors operating in an AC network of 6 kV (Figure 4).

Figure 4. КТ5.11-800 stack for usage in soft starters of electric motors operating in AC network of 6 kV

The stack consists of thyristors with 6,5 kV blocking voltage and presents the complete unit – AC stack equipped with drivers, power units, conforming circuits and heat sinks. A basic diagram is shown in Figure 5. Thyristor groups, forming direct and reverse stacks are controlled separately by fiber optic line.

Figure 5. Basic diagram of КТ5.11-800

 

For more information, please read:

Thyristor Basics

Dynamic Properties of Thyristors

Proton Irradiation Technology

Power Devices Produced Using Proton Irradiation Technology

Modeling of Power Semiconductor Devices

 

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