Oct. 24, 1961 E. Salzer 3,005,945
SEMICONDUCTOR DIODE PROTECTION Filed Oct. 27, 1958 2 Sheets-Sheet 1
United States Patent Office
3,005,945
Patented Oct. 24, 1961
3,005,945
SEMICONDUCTOR DIODE PROTECTION
Envin Salzer, Waban, Mass., assignor to The Chase-
Shawmut Company, Newburyport, Mass.
Filed Oct. 27,1958, Ser. No. 769,691
10 Claims. (CI. 321—11)
This invention relates to semiconductor rectifiers as, for instance, semiconductor rectifiers comprising silicon
It is, therefore, another object of the invention to provide semiconductor rectifiers for relatively high voltages the cell fuses of which are cooled by systems of cooling fins designed to achieve a match between the time-current curve of the fuses and the danger characteristic, or damage characteristic, of the rectifier cells.
Prior art systems of fuse cooling fins, though well serving the ends for which they are intended, are not sufficiently effective for use in semiconductor rectifiers. The
diodes, germanium diodes, or other semiconductor diodes 10 relative ineffectiveness of prior art systems of fuse cool
which are likely to be damaged or destroyed by relatively small overcurrents.
It is a general object of this invention to provide improved self-protected semiconductor rectifiers.
It is possible to design current-limiting fuses which may 15 beconsidered to be fair thermal images of semiconductor diodes, or semiconductor cells, in combination with which the particular fuses are intended to be applied. Such
fuses may either be designed to preclude damage to a „ , . , - , . .. . „. •„„„
diode, or may be designed to interrupt any fault or short- 20 aPP^r from the particular description thereof, as urnscircuit current resulting from breakdown of a diode. It trated m th.e drawings, wherein
is possible to achieve a relatively close match between the FIG. 1 is a wiring diagram of a self-protected senu
time-current curves, or blowing characteristics, of current- conductor rectifier;
limiting fuses and of the danger characteristics, or dam- FKJ- 2 15 a diagram illustrating matching of cell and
age characteristics, of semiconductor rectifier cells, as 25 ^^^^^[^^.^^.J^0^
ing fins is due to the relatively high thermal impedance between the axially outer ends of the fusible elements and the heat dissipating fin structure, which impedance limits the flow of heat from the former to the latter.
It is, therefore, another object of the invention to provide fin-cooled fuse structures of drastically reduced thermal impedance.
The foregoing and other general and special objects of the invention and advantages thereof will more clearly
long as the voltage for which the particular rectifier and
its constituent parts are designed is relatively low. It has,
however, not been possible heretofore to achieve the afore-
mentioned match where the particular rectifier is designed
for a relatively high voltage. 30
It is, therefore, another object of the invention to pro-
vide self-protected semiconductor rectifiers wherein a
better match between the time-current-curve of current-
limiting fuses and the danger or damage characteristic of o.
FIG. 3 is a diagrammatic representation of a fault current;
FIG. 3a is a diagrammatic representation of data derived from those plotted in FIG. 3;
FIG. 4 shows a fin-cooled cell fuse structure embodying the invention and is a section along 4—4 of FIG. 5;
FIG. 5 shows the same structure as FIG. 4 and is a front view seen in the direction of the arrow 5 in FIG. 4;
FIG. 6 shows a modification of the structure of FIGS.
semiconductor cells is being achieved than could be 35 4 and 5 and is a section along 6—6 of FIG. 7;
achieved heretofore in semiconductor rectifiers designed for relatively high circuit voltages, say circuit voltages in the order of, or exceeding, many hundred volts.
Current-limiting fuses in semiconductor rectifiers may either be required to blow in order to preclude an impending cell failure, or to blow in response to failure of a cell. Such fuses may be referred to as cell fuses in order to distinguish this application of fuses in semiconductor rectifiers from their application as a means of protection against over-currents resulting from external
FIG. 7 shows the same structure as FIG. 6 and is a front view seen in the direction of the arrow 7 in FIG. 6;
FIG. 8 shows another embodiment of the invention and is a section along 8—8 of FIG. 9, and M FIG. 9 shows the same structure as FIG. 8 and is a section along 9—9 of FIG. 8.
Referring now to the drawings, and more particularly to FIG. 1 thereof illustrating a three phase semiconductor bridge rectifier, numeral 1 has been applied to indicate the semiconductor rectifier cells and numeral 2 to in
I i v-i wuiiw±*u n/juiuug i J Will tALGlIltU A fZ '— —
faults. This invention is more particularly concerned dicate current-limiting cell fuses of which each is asso
with the application of cell fuses
Any increase of the circuit voltage calls for an increase of the length of the fusible elements in the cell
ciated, and serially connected, with one of cells 1. Reference characters R, S, T have been applied to indicate three A.-C. leads and — have been applied to
fuses. Any increase in length of the fusible elements in -n indicate the two D.-C. bus bars to which any desired
t1 11 C _1 :_ . . _. _. OU T-V 1„„J m„„V.a
the cell fuses changes, in turn, the time-current curves thereof. This upsets the match which may have been present between the time-current curve of the cell fuses and the danger characteristic, or damage characteristic, of the semiconductor cells or diodes.
It is, therefore, another object of the invention to provide semiconductor rectifiers for relatively high voltages comprising cell fuses having fusible elements of adequate length, i.e. fusible elements which are relatively long, in which rectifiers the time-current curve of the cell fuses matches well with the danger characteristic, or damage characteristic, of the rectifier cells.
Another object of the invention is to provide improved semiconductor rectifiers having current-limiting cell fuses which are being cooled by systems of cooling fins.
Heretofore cooling of fuse structures by systems of cooling fins has not been fully understood. To be more specific, heretofore cooling fins have been provided on the knife blades of fuses in an endeavor to achieve cool
D.-C. load may be connected.
In FIG. 2 currents have been plotted as abscissae in percent of cell rating and times have been plotted as ordinates in terms of cycles at 60 per sec. Both the gg abscissae, and the ordinates are drawn on logarithmic scales. Reference character C has been applied to indicate the danger characteristic, or damage characteristic, of one of cells 1 of FIG. 1. A danger characteristic is based on currents which cause a permanent incurable change to a cell structure, and a damage characteristic is based on currents which cause immediate damage to a cell structure. Reference character Fi has been applied to indicate a time-current curve of a curi'ent-limiting cell fuse considered to fairly match curve C, except in the range of relatively small currents.
Assuming now that the system voltage is being increased requiring substitution of a plurality of serially related cells for each cell 1 of FIG. 1. This calls for substitution of the current-limiting fuses by cell fuses
running fuse structures rather than for the purpose of _n having fusible elements of increased length. Increase of
x: * ~e c *l~~ , a,~ i i.i* ~e +i,„ c *ui„ ~l +_ 1*_ • _ „i _r
matching the time-current curve of fuses with the danger characteristic, or damage characteristic, or other devices,
the length of the fusible elements results in a change of the time-current curve relating to currents causing fusing
of the fusible element or blowing of the fuse in times more than one cycle of a current wave of 60 c.p.s. or, in other words, relating to currents in the range of small overcurrents, say overcurrents of less than 300-500 percent of the rated current of cells 1. This changed char- 5 acteristic has been indicated by a curve to which reference character F2 has been applied. It is thus apparent that something must be done to the fuse to bring the shape of its time-current curve F2 substantially back to its original shape, i.e. the shape of curve Fj without, 10 however, shortening the length of the link. Such a change of characteristic can be achieved by intense convection codling, preferably forced convection cooling.
For a better understanding of the degree of convection cooling required for current-limiting cell fuses in a semi- 15 conductor rectifier reference may be had to FIGS. 3 and 3a. In FIG. 3 reference numeral I has been applied to indicate the current trace of a fault current caused by a cell failure which would develop in the absence of fuses 2, or if fuses 2 were shunted by shunts whose imped- 20 ance were zero. In the presence of fuses 2 and the absence of such shunts the fusible element or elements of the fuse 2 serially connected with a faulted cell 1 fuses or melts at the time Tx and the current begins to decay at the time Tj and becomes zero at the time T2. 25 The time interval T0-Tx or tf is known as the fusing or pre-arcing time, and the time interval Tj-T2 or t-s, is known as the arcing time. The total time T0-T2 or tt plus ta is referred to as the clearing time. The current which flows actually through the current-limiting fuse is consider- 30 ably smaller than the available current I and is referred-to as the let-through current. The let-through current is much shorter than V2 cycle of a current wave in the 60 c.p.s. A.-G. system R, S, T (see FIG. 1), and its peak value indicated by reference letter i is much less than the peak 35 value of the available current I.
In FIG. 3a reference letter I2 has been applied to indicate a curve arrived at by plotting the squares of the momentary values of curves I. In the same figure
40
j> dt
has been applied to indicate a curve obtained by integrating the momentary values of curve P. FIG. 3a shows also the squares of the let-through current. The scale in which FIG. 3a has been drawn is too small to show 45 therein the curve which may be obtained by integrating the momentary values of the squares of the let-through current. The integral of the squares of the let-through current for the interval Tq-tj is known as the fusing
SP-dt 50
Considering short fusing times, say times less than .01 sec, the fusing fp-dt is a constant for any given fuse structure. There is a critical short-time damage Or danger JP'dt for each type of rectifier cell which is likewise 55 a constant considering short times, say times less than approximately .01 sec. (See F. E. Gentry, "Forward Current Surge Failure in Semiconductor Rectifiers," AIEE Transaction Paper No. 58-927.) The integral of the squares of the let-through current for the interval T0-T2 60 is known as the clearing SP-dt. For practical purposes the clearing $P-dt may be approximated by multiplying the short-time fusing fp-dt by three. (See F. W. Gntzwiller "The Current-Limiting Fuse as Fault Protection for Semiconductor Rectifiers," AIEE Transaction Paper 65 No. 58-928.)
In an arrangement such as the bridge circuit shown in FIG. 1 a predetermined current rating is assigned to each rectifier cell. The particular current rating depends not only upon the particular design of the cell but also 70 upon the design of the rectifier, i.e. on both its electrical and its thermal parameters. Each cell 1 has also a predetermined danger or damage JP'dt which, at short times, say of less than .01 sec, is a design constant, as mentioned above. The cell fuses 2 which are arranged in 75
series with cells 1 have, according to this invention, normally a smaller current rating than that assigned to cells 1 in the particular rectifier. The time-current curve F2 in FIG. 2 is that of a fuse having normally a much smaller current rating than that of the rectifier cells whose danger or damage characteristic has been indicated by C.
For reasons of selectivity fuses 2 are designed to have a smaller short-time fusing fi2 ■ dt than the aforementioned short-time danger $P-dt of cells 1. Preferably the shorttime clearing fp-dt of fuses 2 should be less than the danger J72-dt of cells 1.
Fuses 2 are associated with systems of cooling fins having a sufficiently large heat dissipating ability to cause an increase of the normal current rating of fuses 2 to approximately the current rating of cells 1 in the particular rectifier. In order to make it possible to significantly up-rate cell fuses 2, as required if these fuses are relatively long and designed for relatively high voltages, the thermal impedance of the fuses, i.e. their impedance to heat flow from the fusible element to the cooling fins, must be minimized. FIGS. 4-9 show several structures designed to minimize the aforementioned thermal impedance.
Referring now to FIGS. 4 and 5, numeral 2 has been applied to generally indicate a current-limiting cell fuse. Fuse 2 comprises a tubular casing 20 of insulating material closed on both ends by copper plugs 21. Each plug 21 projects with its axially outer surface 22 slightly beyond casing 20. The axially inner surfaces of plugs 21 are each provided with a groove receiving a fusible element 23 in form of a silver ribbon. Fusible element 23 is provided with a pair of lateral incisions defining therebetween a point 24 of reduced cross-section. The point 24 of reduced cross-section is sandwiched between a pair of plates 25 of a heat resistant synthetic-resin-glass-cloth laminate. Casing 20 is filled with a pulverulent filler 26 of silicon dioxide, e.g., quartz sand. Plates 25 form a fulgurite suppressing arc-chute separating filler 26 from the point of reduced cross-section 24. The mode of operation of such fulgurite suppressing or inhibiting arc chutes and desirable structural features thereof have been more fully disclosed in the patent application of Frederick J. Kozacka Ser. No. 658,162 filed May 9, 1957, for Current Limiting Fuses With Increased Interrupting Capacity, now United States Patent 2,866,038, and reference may be had to that patent for further details in regard to structure 25.
Current-limiting fuse 2 is cooled by two systems of cooling fins generally indicated by numeral 27. Each system 27 of cooling fins is formed by a casting comprising a substantially fin-shaped base plate 28 and spaced fins 29 projecting therefrom at right angles. Base plates 23 are each provided with a lug 28a forming a cable connector for inserting current-limiting fuse 2 into the circuit of one of cells 1. (See FIG. 1.) The fin 29 situated in the center of each plate 28 defines a gap 30 adapted to accommodate a screw 31. Screws 31 project transversely across plates 28 into plugs 21 and establish firm electrical contacts between plates 28 and plugs 21. Plugs 21 and plates 28 form means for conducting heat from fuse link 23 to the system of cooling fins 27. These heat conducting means have a cross-sectional area being at no point less than the cross-sectional area of the inside of casing 20. Because of that large area of heat exchange between fuse link or fusible element 23 and the cooling fins 29 the latter are highly effective and allow the use, in series with semiconductor cells 1, of cell fuses 2 which have normally a much smaller current-rating than cells 1 and which have normally a time-current characteristic which differs in the range of currents less than 300-500% the current rating of cell - drastically from the danger or damage characteristic of cell 1.
The time current curve of cell fuses 2 in the range of more than 300-500% the current rating of cells 1, i.e., the right portion of the time-current curve referring to blowing in about 1 cycle or less of a current wave of 60
|
