Thyristor matrix having at least four columns

A thyristor matrix arrangement having four columns, each column being formed of n thyristors connected in series with each other. In one embodiment, the series connected thyristors in respective columns are coupled to one another via heat sinks. The thyristors in each column are poled such that forward conduction current cannot flow through only one of the columns. The heat sinks are cross-connected among the columns to form a current path which is formed of appropriately poled thyristors in different ones of the columns and the column cross-connections. The cross-connections are inductive so as to provide relatively large inductive voltage drops when the thyristors are fired, thereby enabling the firing of thyristors having relatively low firing and forward voltages.

BACKGROUND OF THE INVENTION 
This invention relates generally to thyristor maxtrix arrangements, and 
more particularly, to a thyristor matrix having at least four columns, 
each column having a plurality of thyristors connected in series, the 
series thyristors being connected to one another at junction points; 
selected ones of the junction points in different thyristor columns being 
connected to one another via cross-connections. The matrix conducts 
current in each of first and second conduction directions; equal numbers 
of thyristors being assigned to the conduction directions. 
Columns of thyristors are known, for example, from U.S. Pat. No. 3,943,426. 
In this known arrangement, disc type thyristors are stacked upon each 
other and held resiliently together. A heat sink is inserted between two 
disc type thyristors and serves for conducting electric current. In order 
to handle large currents, several such columns can be connected in 
parallel. Moreover, in arrangements where conduction is desired in two 
directions, such thyristor columns can be arranged antiparallel. In one 
known arrangement, two columns of thyristors are connected in parallel 
with one another, and poled for conduction in a first direction, and two 
further columns of thyristors are also connected in parallel with one 
another, but are poled for conduction in the opposite direction. In this 
manner, a matrix having a total of four thyristor columns is produced. In 
order to ensure a uniform distribution of current in the known, 
commercially available, four-column thyristor equipment, all of the 
thyristors are connected to one another. 
FIG. 1 shows a known thyristor matrix having four columns and two common 
terminals, A1 and A2. The known arrangement of FIG. 1 contains n 
thyristors in each column, the thyristors in the matrix being identified 
as T11 . . . , Tn1; T12 . . . , Tn2; T13 . . . , Tn3; and T14 . . . , Tn4. 
A plurality of heat sinks are each interconnected between two thyristors. 
The heat sinks are schematically indicated in FIG. 1, and designated with 
respective symbols K01 . . . , Kn4. 
In this known arrangement, thyristor columns S1 and S2 are connected in 
parallel. Thyristor columns S3 and S4 are also connected in parallel with 
one another, but antiparallel to the combination of columns S1 and S2. 
Corresponding heat sinks K11 . . . , K14 are connected to one another, as 
are heat sinks K21 . . . , K24; and Kn1 . . . , Kn4. The heat sinks are 
connected to each other by means of cross-connections which are 
respectively identified as Q11 . . . , Qn3. Generally, the heat sinks 
identified as Kp1 . . . , Kp4, where p assumes the values one to n, are 
connected to each other. In this manner, the thyristors within the groups 
T11 . . . , T14 through Tn1 . . . , Tn4 are also connected to one another. 
All interrelated thyristors in the generalized group Tp1 . . . , Tp4, with 
their associated heat sinks Kp1 . . . , Kp4, of the four thyristor columns 
S1 to S4, shall be generally designated in the following discussion as 
level Ep of the thyristor arrangement. This known thyristor arrangement 
therefore comprises n levels E1 . . . , En. 
Each of the levels E1 . . . , En of the thyristor arrangement is assigned 
one of common RC stages, R1, C1 . . . , Rn, Cn. The assigned RC stages are 
electrically disposed between the heat sinks of adjacent levels. 
FIG. 2 is a schematic representation of the physical arrangement of the 
four thyristor columns S1 . . . , S4. The four thyristor columns are 
represented in this figure in a rectangular shape, the cross-connections 
being shown as leads. As a result of the physical configuration of the 
matrix, the distance between columns S1 and S3, or S2 and S4, is larger 
than the distance between adjacent columns. In addition, the 
cross-connections Q12 . . . , Qn2 are longer than the other 
cross-connections. 
It is a problem with this known thyristor arrangement that the individual 
thyristors of the four columns, S1 . . . , S4, must be matched to each 
other in accordance with their respective dynamic forward characteristics. 
FIG. 3 shows a typical dynamic forward characteristic for heavy duty 
thyristors. As shown in FIG. 3, the curve of the thyristor current i.sub.t 
is a function of the anode-cathode voltage of the thyristor. Here, the 
firing voltage is designated as V.sub.Z, and the forward voltage for the 
thyristor current, i.sub.D, is designated with V.sub.D. Such a 
characteristic may result in failure of the thyristors to fire 
simultaneously, because one of the parallel connected thyristors may have 
a lower firing voltage than the other thyristors. The early firing 
thyristor would prevent the remaining thyristors from firing, and thereby 
conduct the entire current so as to be overloaded. Thus, in order to 
ensure substantially simultaneous firing of all thyristors, they must be 
sorted and matched with respect to their firing voltages V.sub.Z. In 
addition, the thyristors must also be sorted and matched with respect to 
their forward voltage V.sub.D because otherwise the parallel-connected 
thyristors would conduct unequal amounts of current. The requirement of 
sorting and matching thyristors with respect to two criteria presents 
major practical difficulties. Moreover, through aging or differences in 
the firing delay times, it may occur that only one of the thyristors fires 
while the remaining parallel-connected thyristors are below their firing 
voltages. In order to reliably exclude this possibility, transistors would 
have to be used for the parallel connections, because the firing voltages 
are lower than the forward voltages. The selection process, however, is 
quite expensive. 
It is a further problem with the known thyristor arrangement that an uneven 
current distribution caused by a deviation in the characteristics of a 
thyristor is continued over the entire column. Although the connections 
between the thyristors through the heat sinks have very small inductances 
because of the small lengths of the connections, the longer 
cross-connections have more inductance so that equalization via the 
cross-connections does not occur, at least not over a short period of 
time, and the current distribution becomes increasingly worse. 
It is known from BBC Silicon Converter Handbook, 1971, pages 95 and 96, 
that the current distribution among parallel-connected thyristors is 
improved by connecting an air-core choke in series with each individual 
thyristor of a parallel circuit. The effect of the air choke is that the 
thyristor which fires first will not take over the entire current 
immediately, and the voltage at the remaining thyristors increases because 
of the current change in the choke, so that the firing of the remaining 
thyristors is aided. However, such chokes for each individual thyristor 
not only increase the cost of the arrangement, but also the length because 
the chokes must be inserted between the thyristors and the heat sinks. 
It is, therefore, an object of this invention to provide a thyristor 
arrangement wherein equalization of the firing times of, and the current 
distribution among, parallel-connected thyristors is achieved without the 
use of separate chokes. 
SUMMARY OF THE INVENTION 
The foregoing and other objects are achieved by this invention which 
provides at least four columns of n thyristors connected in series. The 
thyristors are connected to each other at respective junction points. In 
each column, the n thyristors therein are divided into sequential 
subgroups of p thyristors each, where p is a predetermined number having a 
value greater than zero. The p thyristors in each subgroup are connected 
in series and poled for normal forward conduction in the same direction as 
each other. Thus, each subgroup of p thyristors has a normal direction of 
conduction. Sequential ones of the subgroups, however, are poled for 
normal conduction in directions opposite to their adjacent subgroups. Each 
column, therefore, is formed of a sequence of thyristors and junction 
point; every p.sup.th junction point being a junction point where 
subgroups of thyristors are joined. The columns of thyristors are 
cross-connected to each other at the p.sup.th junction points. In this 
manner, the current flowing through the thyristor matrix must change 
columns. The thyristors are arranged such that current flowing in either 
direction through the thyristor matrix will flow through equal numbers of 
thyristors. 
As indicated, current cannot flow only through an individual thyristor 
column, but must change from column to column. The current is therefore 
conducted through the cross-connections which have substantially more 
inductance than the connections within the thyristor columns. As each 
thyristor is switched on, an inductive voltage drop is produced at the 
cross-connections; the voltage drop maintaining the voltage across the 
thyristors which have not yet fired. The voltage is thus maintained for a 
certain period of time, thereby promoting the firing of the remaining 
thyristors. Moreover, a nonuniformity in the current distribution cannot 
continue itself over an entire thyristor column because of the change of 
the current flow between the thyristor columns. In addition, the static 
current distribution is improved, as in the known circuits where separate 
current distribution chokes are used. The advantages of the present 
invention are achieved without changing the physical design of the 
thyristor columns, and at no extra cost. Although the cross-connections 
must be designed to carry the full thyristor current, this will not 
require additional expense above the known arrangements because the known 
cross-connections carry the entire current when a thyristor fails. 
It is a feature of the present invention that each heat sink is arranged in 
a column between two thyristors, thereby permitting the cross-connections 
to be designed as connecting corresponding heat sinks. Such a connection 
between the thyristors of the individual thyristor columns is simple and 
can be realized without separate contact elements. 
The line length of the cross-connections is advantageously selected so that 
the inductive voltage drop at the cross-connections is larger, when the 
thyristor arrangement is switched on, than the maximum difference of the 
forward voltages and the firing voltages of the thyristors of the various 
thyristor columns connected to the cross-connections. This allows every 
thyristor to be fired reliably even if the firing voltage is higher than 
the forward voltage. 
A common RC stage is shunted across the thyristors of all thyristor columns 
connected together via a cross-connection. This RC stage is arranged 
between the p.sup.th and the (p+1).sup.th junction point of the thyristors 
of two different thyristor columns. As a result of such a connection, the 
discharge current of the RC stage flows, when the corresponding thyristors 
are switched on, along a relatively long connecting path so as to produce 
a relatively high inductive voltage drop which aids the firing of the 
unfired thyristors. The RC stage is advantageously connected across the 
contacts of the thyristors of the columns which have the longest line 
length of the cross-connections. This maximizes the inductive voltage 
drop. 
In a further embodiment of the invention, a current sensor may be arranged 
in a cross-connection. The current sensor is connected so as to conduct 
only the equalization currents of the parallel circuits, thereby 
permitting measurement of possible misdistribution of the currents. Thus, 
only one current sensor for both directions is required for each 
parallel-connected pair of thyristors.

DETAILED DESCRIPTION 
FIG. 4 shows a thyristor matrix arrangement according to the invention. The 
arrangement is formed of four thyristor columns, S1, S2, S3, and S4. Each 
column is provided with a plurality of disc-type thyristors T11 . . . , 
Tn1; T12 . . . , Tn2; T13 . . . , Tn3; and T14 . . . , Tn4. The thyristors 
are stacked on top of each other, and heat sinks K01 to Kn4 are inserted 
between respective thyristors. In addition to drawing off heat, the heat 
sinks form conductive connections between adjoining thyristors of a 
thyristor column. Thyristors T11, T12, T13, and T14, together with their 
associated heat sinks K11, K12, K13, and K14, form a level E1. Similarly, 
thyristors T21 . . . , T24, and their associated heat sinks K21 . . . , 
K24, form a second level, E2. In this specific illustrative embodiment, 
each column contains only one thyristor within each of levels E1 . . . , 
En. The respective heat sinks of each level are electrically coupled to 
each other by respective cross-connections Q11 . . . , Qn3. The first two 
heat sinks K01 and K02, of thyristor columns S1 and S2 are connected via 
cross-connection Q01; and the first two heat sinks K03 and K04 of 
thyristor columns S3 and S4, respectively, are connected by a 
cross-connection Q03. These two cross-connections, Q01 and Q03, are 
connected to a contact terminal A1. Similarly, the last heat sinks Kn1 . . 
. , Kn4 are connected to a terminal A2. 
The polarity of the thyristors T11 . . . , Tn4 is selected so that at each 
heat sink K11 . . . , Kn4, either the anodes or the cathodes of the two 
adjacent thyristors are connected. In this manner, the polarity of the 
thyristors change from level to level within each column S1 . . . , S4. In 
this embodiment, the first thyristors of columns S1 and S3 are poled in 
the positive conduction direction, and the first thyristors of columns S2 
and S4 are poled for conduction in the negative direction. Thus, each 
cross-connection Q11 . . . , Qn3 connects a heat sink with an anode 
terminal and a heat sink with a cathode terminal. No current flow is 
therefore possible within an individual thyristor column S1 . . . , S4. In 
addition, in this embodiment, the polarity of each thyristor T11 . . . , 
T14 alternates within each level E1 . . . , En from thyristor column S1 to 
thyristor column S4. In this thyristor arrangement, current flow is 
therefore possible only through the cross-connections Q11 . . . , Qn4. A 
current from terminal A1 to terminal A2 will therefore flow, for example, 
via the heat sink K02, the thyristor T12, the heat sink K12, the 
cross-connection Q11, the heat sink K11, the thyristor T21, etc. A 
parallel current also flows via heat sink K04, the thyristor T14, the heat 
sink K14, the cross-connection Q13, the heat sink K13, the thyristor T23, 
etc. A current in the reverse direction flows via the thyristor T22, the 
cross-connection Q11, and the thyristor T11, and a parallel current flows 
via thyristor T24, the cross-connection Q13, and the thyristor T13. 
If, for example, the current path should be fired first via thyristors T12 
and T21, an inductive voltage drop is produced at the connecting lines, 
specifically at the cross-connections Q01 and Q11. This voltage drop is 
added to the forward voltage of thyristors T12 and T21. Thus, a voltage 
higher than the forward voltage of the thyristors T12 and T21 is present 
at the yet to be fired parallel-connected thyristors T14 and T23. This 
higher voltage aids in the firing of thyristors T14 and T23. In this 
manner, the firing of all parallel-connected thyristors is assured, even 
thyristors which have a lower forward voltage than the firing voltage, if 
the inductive voltage drop at the cross-connections compensates for the 
difference between the forward voltage of the first-ignited thyristor and 
the possibly higher firing voltage of the unfired thyristors. 
The voltage distribution among the thyristors in the forward state is 
improved by the inductance of the connecting lines. The inductances of the 
cross-connections counteract an increase of the current flowing through 
the respective branches, while on the other hand, the voltages present at 
the remaining thyristors are increased by the inductive voltage drop in 
the event of a current increase. Thus, the current consumption for these 
thyristors is again aided. In sum, the inductances of the 
cross-connections have an effect which is similar to that of the known 
current distribution chokes in the sense of equalizing the current 
distribution. 
As noted, unlike the known arrangement, the change of current from one 
thyristor column to another prevents a misdistribution of current which is 
produced in one level from continuing through the entire thyristor column. 
A comparison of FIG. 1, which was discussed hereinabove, with FIG. 4, shows 
that the advantages of the present invention are achieved without changing 
the column design. In the embodiment of FIG. 4, merely the polarity of 
every other thyristor of a thyristor column is reversed. Thus, no further 
changes are necessary in the known thyristor columns, and no additional 
effort or expense is required. 
In another aspect of the inventive thyristor arrangement according to FIG. 
4, a respective RC stage of the group, C1, R1 . . . , CnRn, is assigned to 
each of the levels E1 . . . , En. Each RC stage is connected between a 
heat sink of thyristor column S1, and a heat sink of thyristor column S4 
on the following level. The discharge current from the capacitor, which 
flows when a thyristor of the level is switched-on, flows independently of 
the thyristor which is switched-on, and always through three 
cross-connections. This discharge current has a steep initial slope 
thereby causing a relatively large inductive voltage drop which aids in 
firing of the unfired thyristors. This voltage drop is independent of the 
initial rising slope of the main current. 
The current flowing through the cross-connections in the inventive 
thyristor arrangement enable a simple detection of a faulty current 
distribution of a current flowing through the thyristors. All equalization 
currents between the parallel-connected thyristors flow via the 
cross-connections Q12 . . . , Q(n-1)2 between the thyristor columns S2 and 
S3 for both current directions. As mentioned, these cross-connections are 
relatively long, and therefore the necessary current transformers W1 . . . 
, Wn-1 can be accommodated without problem. In the known thyristor 
arrangement according to FIG. 1, the current sensors for determining the 
faulty current distribution must be accommodated separately for each 
current direction between the thyristor columns S1 and S2, and 
respectively, between the thyristors columns S3 and S4. The sensors in the 
known arrangement can be accommodated only with difficulty because of the 
small spacing of the thyristor columns. 
Although the invention has been disclosed in terms of specific embodiments 
and applications, persons skilled in the art can produce additional 
embodiments, in light of this teaching, without departing from the spirit 
or exceeding the scope of the claimed invention. Accordingly, it is to be 
understood that the drawings and descriptions in this disclosure are 
proffered as illustrative to facilitate comprehension of the invention, 
and should not be construed to limit the scope thereof.