CA1132656A

CA1132656A - Circuit for protecting anti-parallel-connected switches against overvoltages

Info

Publication number: CA1132656A
Application number: CA312,832A
Authority: CA
Inventors: Bernd Amelunxen
Original assignee: BBC Brown Boveri AG Switzerland
Current assignee: BBC Brown Boveri AG Switzerland
Priority date: 1977-10-06
Filing date: 1978-10-06
Publication date: 1982-09-28
Also published as: DE2744961C2; SE7810308L; GB2005493B; DE2744961A1; SE440713B; CH632358A5; GB2005493A

Abstract

TITLE OF THE INVENTION:

A CIRCUIT FOR PROTECTING ANTI-PARALLEL-CONNECTED
SWITCHES AGAINST OVERVOLTAGES

ABSTRACT OF THE DISCLOSURE

A circuit for protecting anti-parallel-connected thyristors against overvoltages in the forward blocking direction and the reverse blocking direction including a Graetz bridge connected between the control electrodes of the thyristors to provide relief firing thereof.

Description

~ z~

~::
BACKGROUND OF THE INVENTION

Field of the Invention:

The invention relates to a circuit for the protection of two anti-parallel-connected controllable semiconductor switches against overvoltages both in the forward and the reverse directions by means of a symmetric relief-triggering device, containing at least one auxiliary semiconductor switch, connected between the two control electrodes of these switches, which device, when a presettable threshold voltage of a value below the maximum permissible overvoltage is exceeded, fires the appropriate switch having positive anode voltage applied thereto. The circuit of the invention can find application in AC controllers for special technologies, e.g. for welding ~- purposes.
':~
Description of the Prior Art:

, ~
In a known such circuit, there is located between the control electrodes of two anti-parallel-connec~ed thyristors a single symmetric auxiliary semiconductor switch which exhibits a voltage polarity-independent, but voltage magnitude-dependent current voltage characteristic, which, when a presettable threshold voltage of the order of magnitude of the highest permissible overvoltage is exceeded, fires the appropriate thyristor with positive anode voltage applied to it (German patent 2 258 112, May 30, 197~, Brenneisen, Jorg). Between each 25 point of connection of this auxiliary semiconductor switch with the control electrode of a tyristor and the corresponding A~

.,~
.: --1--. : ~

~ ~Z~5~

line is provided a diode in the blocking direction, for preventing an impermissible current or voltage load on the : control electrode cathode barrier layer of that thyristor. This known relief-triggering device is advantageous insofar as it relieves -the high overvoltages with large energy content to be expected, e.g. in an AC controller for welding purposes, by firing and thus short-circuiting the appropriate (main) thyristor. The semiconductor switch must, however, have a precisely symmetric characteristic, i.e~ exactly equal breakover and turn-on voltages.

It is also known, both from the cited German patent

2 258 112 as well as from the BBC publication "BBC Components .
for Thyristor Protection (Publication Order No. DHS 60590D), to connect each thyristor to be protected separately to an auxiliary semiconductor switch matched -to a threshold voltage in its forward blocking direction, and a series diode in the blocking direction between the anode and control electrode of the thyristor. In utilizing such circuits with discrete components for anti-parallel-connected thyristors, where again for relief firing of the concerned thyristor there can be , connected between anode and control electrodes an auxiliary thyristor having a control electrode connected to the auxiliary semiconductor switch with definite threshold voltage, it is ~ necessary in analogy with -the foregoing that two auxiliary 25 semiconductor switches with exactly equal breakover voltages under both stationary and dynamic loading be used.

., '~

, ~ ~

1~3;~6~

The use of the AC controller with a following-connected transformer for welding purposes therefore allows in both known cases no difference in breakover voltages or turn-on valves, because this would lead to a DC loading of the welding transformer and thus to short-circuiting of the main power lines, with overvoltages over several cycles thereby experienced.
.~
SUMMARY OF THE INVENTION
`:~
Accordingly, this invention then, on the basis of the above-described circuit, seeks to produce a relief-triggering device with equal turn-on values for both polarities of overvoltages.

~A?

Broadly this and other aspects are achieved according to the invention by providing a relief-triggering device which consists of a full-wave rectifier bridge, known as a Graetz ` bridge for short~ with a semiconductor breakover switch matched to the threshold voltage connected in its DC path. The diodes of the Graetz bridge and a breakover switch, preferably a breakover diode, are thus employed as auxiliary~switches. A
' 20 semiconductor component usable as a breakover switch also is described in the cited BBC publication.

~ .
'~
The invention as broadly claimed pertains to a circuit for ~ ..
controlling the application of alternating current and voltage ; from an AC source to an inductive load including a transformer including a pair of anti-parallel connected semiconductor thyristors connected in series between the AC source and the , ~ ~

'~
transformer, the thyristors having respective control electrodes. Means for protecting the thyristors against overvoltages in both the forward and reverse directions and for protecting the thyristors and the transformer against a direct : 5 current load are provided and comprise an emergency ignition circuit connected between the control electrodes of the thyristors and including a full wave rectifier bridge having a semiconductor breakdown switch inserted in a DC path of the bridge, the bridge defining an ~C path connected in series ; 10 between the control electrodes of the semiconductor thyristors, and the breakdown switch exhibiting a breakdown voltage lower than a maximum permissible voltage. When the breakdown voltage -is exceeded, the full wave rectifier bridge enables switching of the breakdown switch always in the same direction and always at the same breakdown voltage irrespective of the polarity of the source voltage to fire the thyristor having a positive voltage applied thereto, whereby the thyristors are symmetrically switched in the event of overvoltages in the forward and reverse directions.
., .`, , The circuit of the invention has the advantage that :
regardless of the overvoltage polarity, the breakover switch is matched to only one threshold voltage and acts in each case. The protection of both the controllable (main) ., :
~j 25 , .
,, .~ . .

, . ., 4~-ll 6witches sgainst overvolt~ges in the reverse a~ well as the forward direction i~ en6ured, because blocking voltage, i.e. barrier potential in the forward direction, for the one controllsble semiconductor ~witch results at the same time in negative barrier potential for the other controllable semiconductor switch, the two are matched, and 80 the described relief-triggerin~ device works indepen-dently of voltage polarity when the threshold voltage is exceeded.

The circuit arrangement is improved further by a protective resistor in serie6 with the Graetz bridge on the AC cormection side thereof. This limits capacitive displacement currents in the barrier layer capacitie6, and the capacitie~ of the different overall circuit constructions dependent upon the particular applications.
' . ' .
Damage resulting from defective components, e.g. the diodes in the Graetz bridge, i8 auitably preverlted by installation of a fuse. Failure of thisfuse is indic~ted by actuation of a pilot switeh.
.1 . . :

- All components of the circuit are advantageously mounted on a common printed circuit bo~rd 60 RS to con titute a module.
,'' .

BRIEF DESCRIPTION OF T~IE DRAWINGS
~ . , , A more complete appreciation of the invention and m~ny oî the attendant advantage~ thereof will be readily obtained as the 6ame becomes better under-stood by reference to the following detailed description when considered in . .
::

.. _ .
. ...... ,~ - ~

-'`'`

connection with the lone accompanying drawing, which iB a circuit diagram of the circuit of the invention.
` ., DETAlLED D~SC~IP~ION OF T}IE ~EI ~ EI) E~1~30011~1~NTS

Referring now to the drawing, between AC terminals 1 and 2 in tlle line~
leading to the user are anti-parallel-connected thyri6tors 3 and 4, the control electrodes 5 and 6 of which are connected to a circuit for protection against overvoltages. This circuit consi6ts of a Graetz bridge made up of four diodes 8, 9, 10 and 11, in the DC path of which is a breakover switch 12, preferably a breakover diode, matched to a threshold voltage of a value below the highest permissible overvoltage of the thyri6tors 3 and 4. Connected in serie6 with the Graetz bridge are a protective resi3tor 13 and a fuse 14 having a pilot switch 15.
: The pilot switch can be in the circuit o an indication light, not shown.
.,~
The operation of the circuit arrangement is now explained as follows: ¦
' When thyristor 3 is loaded with poæitive barrier potential (blocking voltage), it i8 cut off a8 long as there is no signai from the normal control device (not shown) . In a pnpn-thyristor, nearly the entire positive anode voltage thus- lie6 as barrier potential across the ~econd pn-junction (lookir~ from the anode), which region act~ as barrier layer. On the other hand, in thyristor 4 with the assumed applied voltage (da~hed sign~) the two outer pn-junctions are raised to reverse blocking voltage, because its anocle hs6 a negative voltag~ with respect ., . 1, ~ ' . .

__ . . .. .. . . ,. , .. , .. __ _ . _. -- _,, ... . .. . _ _..... . . . ... ..

~32~
,~: '' , to its cathode. If now the voltage across thyristor 3 exceeds a certain value in the forward blocking direction, then via the cathode-control eIectrode path of thyristor 4, the corresponding matched threshold voltage of the breakover diode 12 is reached. Diode 12 conducts, i.e. there flows through diode 9, the break-over diod~ 12, the diode 10, the protective resistor 13 and the fuse 14 a control current IG which in contrast to the previously flowing leakage current turns on thyristor 3 . One can also imAgine and provide that between the cathode and control electrode of thyristor 4 there is A resistance 16, the voltage drop acros~ whlch is negligible. Positive overvoltages at terminal 1 thus effect by a kind of Zener char~cteristic in thyristor 4 a firing of the breakover diode 12 and thereby turn on thyristor 3. ln the same way thyristor 4 is turned on, in case positive overvoltages arise at terminal 2. The transmission losses of the diodes 8 to 11 play no significant role. It is stated here merely for orientation that a circuit with thyristors 3 and 4 with peak inYerse voltages of 1600 volts, with a BOD element, known from the cited BBC publicAtion, as a breakover switch 12 with a threshold voltage of 1200 volts,and without resi6tors 16 works satisfactorily.
.' ' Obviousiy, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that i~ within the scope of the appended clsims, the invention may be practiced other-~0 wi6e thsn e l~eoi~icelly described herein.

.- . .
' -: ~

Claims

The embodiments of the invention in which an exclusive property of privilege is claimed are defined as follows:

1. A circuit for controlling the application of alternating current and voltage from an AC source to an inductive load including a transformer, comprising:
a pair of anti-parallel connected semiconductor thyristors connected in series between said AC source and said transformer, said thyristors having respective control elect-rodes; and means for protecting said thyristors against over-voltages in both the forward and reverse directions and for protecting said thyristors and said transformer against a direct current load, comprising:
an emergency ignition circuit connected between the control electrodes of said thyristors and including a full wave rectifier bridge having a semiconductor breakdown switch inserted in a DC path of said bridge, said bridge defining an AC path connected in series between the control electrodes of said semiconductor thyristors, said breakdown switch exhibiting a breakdown voltage lower than a maximum permissible voltage;
wherein when said breakdown voltage is exceeded, the full wave rectifier bridge enables switching of said break-down switch always in the same direction and always at the same breakdown voltage irrespective of the polarity of the source voltage to fire the thyristor having a positive voltage applied thereto, whereby said thyristors are symmetrically switched in the event of overvoltages in the forward and reverse directions.

2. A circuit as in Claim 1, further comprising a resistor inserted in series with the full wave rectifier bridge.

3. A circuit as in Claim 2, further comprising a fuse having an indicator switch inserted in series with the full wave rectifier bridge and said resistor.

4. A circuit as in Claim 3, wherein said full wave rectifier bridge, said resistor, and said fuse are mounted on a common circuit board to form a module.