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Posted on 29 June 2019

Features of High-Voltage Thyristors for Snubberless Stacks

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Influence of design and technology features of high-voltage n+-p-n--p thyristors (impurity doping of p-base and p-emitter, carrier life time in layers) on reverse recovery characteristics (reverse recovery time and charge, reverse recovery current form, temperature dependence of the mentioned characteristics).

By Anatoly Chernikov, Vitaly Gubarev, Alexander Stavtsev, Alexey Surma, and Igor Vetrov, Proton-Electrotex JSC

 

The requirements for thyristors’ design were formulated, which could synchronous operate in series connection without RC-circuits.

The possibility of safe operating of such thyristor stacks in wide range of electric and thermal conditions was experimentally proved.

During last few years many manufacturers of high-power semiconductors started production of thyristors and diodes with voltage up to 6.5kV – 8.5kV. These devices are mostly necessary for completing units of high voltage valves of electric converters in industry and power industry meant for application with voltage of AC current 6kV and higher. Each valve is consisted of several semiconductor devices in series connection, that’s why increase of voltage for each separate device is important since it allows to decrease the number of devices necessary for one valve, and as a result to simplify its design and increase its durability.

Unfortunately, increase of maximum allowed blocking voltage leads to increase of charge, characteristic of high-voltage device. This is connected with necessity to provide acceptable low voltage in onstate.

For devices with 6.5Kv-8.5kV the value of reverse recovery charge and maximum value of reverse recovery current are pretty high, even with relatively low speed of current drop.

Due to the above mentioned facts, the question of developing high voltage thyristors with specific characteristics at reverse recovery becomes very important.

These characteristics are:
• Minimized charge value and value of reverse recovery current (with low voltage in on-state).
• “Soft” character of reverse recovery; usage of thyristors with soft reverse recovery allows to simplify the requirements to RC-circuit in providing the acceptable level of peak voltage.
• Adequacy of reverse recovery charges, as well as reverse recovery current form for thyristors in series connection stack; this allows lowering the requirements to RC-circuits, and possibly in the long term to stop using them at all.

Our company undertakes all necessary steps in direction of development of high voltage thyristors with above mentioned characteristics.

Let’s consider the connection of reverse recovery characteristics with parameters of semiconductor layers in more details.

Well-known that the reverse recovery charge value, first and foremost, depends on the value of accumulated charge of excess electron and holes, as well as recombination rate of accumulated charge. For high voltage thyristors, which recover at low rate of anode current drop, the second factor is more important. During the anode current drop, the bigger part of excessive carriers recombinate. So, there is optimum value of effective life time of carriers in lightly doped n-base of thyristor (the bigger part of excessive carriers is located there), which allows to reach low value of reverse recovery charge at acceptably low value of voltage in on-state.

However there are some additional ways to lower the value of reverse recovery charge. Thus, if we lower the maximum concentration of atoms of acceptor dopant in p-base of thyristor, we can lower the value of reverse recovery charge at the expense of taking part of excessive electrons accumulated in p-base right into n+ emitter, similar to the process in diode. In thyristors with highly doped p-base, as a result of transistance, there is no taking part of excessive electrons out of n-base, but injection of excessive holes into n-base, which leads to relative increase of reverse recovery charge.

Thyristors produced by «Proton-Electrotex» have rather low-alloyed p-base (as a rule maximal concentration of acceptors doesn’t go beyond (1 or 2)* 1016cm-3. It allows to reduce recovered charge without influence on loss of voltage in running order.

To provide necessary dv/dt – stability special topology of distributed cathode diversion is used.

An important characteristic is soft reverse recovery.

It is known that increasing of soft reverse recovery can be achieved during impoverishment of charge excess carrier near anode p- emitter.

There are two ways of achieving this:
• Decrease of anode p-emitter injection efficiency; it can be achieved during decrease of maximal concentration of acceptor dopant and also of charge carrier operating time in high-alloyed part of pemitter layer.
• Local decrease of operating time in nbase layers and lightly doped p-emitter adjoining anode p-n transfer.

In thyristors of «Proton-Electrotex» production we use technologies which can realize both of these ways.

First, rather lightly doped p-emitter layers are applied. It allows reducing reverse recovery surge current. Besides as calculations and experiments show for such thyristors low temperature dependencies of time and reverse recovery charge are typical.

Secondary when it is necessary to get more flexible reverse recovery the special technology of charge carriers operating time regulation which is based on proton irradiation. This technology allows locally reduce the time of charge carriers operating time in the layers adjoining p-n transfer.

Of paramount importance is the opportunity to get thyristors with identical characteristics of reverse recovery. Herewith it is important to get not only the identical surge current and charges of reverse recovery but as well identical character of current dependence on time. It gives an opportunity in prospect to refuse from coordinating RC circuits while series assembly completing.

From written above we can make a conclusion that in order to get identical characteristics of reverse recovery it is necessary to provide high producibility for the dopant profile and as well for contribution of charge carriers life time in semiconductor devices.

Dopant species contribution identity is provided by the high level of production technology of semiconductor elements, precise control of charge carriers operating time is made with a help of special electrical or/and proton irradiation technology.

For achieving short variation of reverse recovery characteristics the following scheme is used:

Step 1: Presupposition for achieving short variation of reverse recovery characteristics is supplying of high dopant profile identity in producing silicon elements which can be achieved by well-worked technology. On this step repetition of current reverse recovery design is provided.

Step 2: Precision control of reverse recovery characteristics (reverse recovery time, reverse recovery current, reverse recovery charge, softness) with a help of electrical and proton irradiation technology. During this step time and reverse recovery charge values are additionally corrected in order to reduce the variation of these characteristics in series. Combination of electronic and proton irradiation allows correct softness simultaneously.

Step 3: Finish presorting while mounting, which allows examining reverse recovery of two or more consequently connected thyristors in conditions similar to operational.

Presorting is made during the examination of each thyristor in consequent connection to the etalon. Herewith the voltage applying to thyristors during the whole process of reverse recovery must be distributed between testing and etalon thyristor in equal parts.

On the same mounting it is possible to examine already mounted high voltage Sgates on the base of consequent thyristor stacks.

As a result of mentioned technologies application in production it was managed to achieve high voltage thyristors with rather low reverse recovery charge value, low temperature dependence and also with high soft reverse recovery.

In such a way for example a typical reverse recovery charge value for Txxx-xxx thyristor under the current fall speed XX A/µs and under the temperature 125°C makes YYYYYY µC under typical value S-factor about one. Typical increase of reverse recovery charge under temperature changes from 25°C to 125°C makes about 30-40%.

Identity of reverse recovery characteristics for thyristors, completing series assembly, allows on the whole getting equal distribution of reverse voltage even without matching RC-circuits.

 

 

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