CA1278340C - Dc chopper circuit fed by a constant current source - Google Patents
Dc chopper circuit fed by a constant current sourceInfo
- Publication number
- CA1278340C CA1278340C CA000506428A CA506428A CA1278340C CA 1278340 C CA1278340 C CA 1278340C CA 000506428 A CA000506428 A CA 000506428A CA 506428 A CA506428 A CA 506428A CA 1278340 C CA1278340 C CA 1278340C
- Authority
- CA
- Canada
- Prior art keywords
- capacitor
- current
- load
- scr
- semiconductor switch
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/51—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used
- H03K17/56—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used by the use, as active elements, of semiconductor devices
- H03K17/72—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used by the use, as active elements, of semiconductor devices having more than two PN junctions; having more than three electrodes; having more than one electrode connected to the same conductivity region
- H03K17/73—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used by the use, as active elements, of semiconductor devices having more than two PN junctions; having more than three electrodes; having more than one electrode connected to the same conductivity region for dc voltages or currents
- H03K17/731—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used by the use, as active elements, of semiconductor devices having more than two PN junctions; having more than three electrodes; having more than one electrode connected to the same conductivity region for dc voltages or currents with inductive load
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/04—Modifications for accelerating switching
- H03K17/0403—Modifications for accelerating switching in thyristor switches
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K3/00—Circuits for generating electric pulses; Monostable, bistable or multistable circuits
- H03K3/02—Generators characterised by the type of circuit or by the means used for producing pulses
- H03K3/357—Generators characterised by the type of circuit or by the means used for producing pulses by the use, as active elements, of bulk negative resistance devices, e.g. Gunn-effect devices
Landscapes
- Dc-Dc Converters (AREA)
Abstract
DC CHOPPER CIRCUIT FED BY A
CONSTANT CURRENT SOURCE
Abstract of the Disclosure An apparatus for periodically interrupting a large direct current flow through a load is able to maintain a substantially constant level of current from the source both when it flows through the load and when it is interrupted, with minimum effect on the direct current source. A first power semiconductor switch is connected in parallel with the load and is switched on briefly to transfer current flow from the load to the first semiconductor switch. A second power semiconductor switch is in series with a precharged capacitor across the load. When the second power semiconductor switch is gated on a large influx of current through the second semiconductor switch into the capacitor transfers the current flow from the first semiconductor switch to the second semiconductor switch When the charge on the capacitor reaches a level sufficient to turn off the second semiconductor switch, the current flow transfers back to the load.
Any minor fluctuations in current level during the transfers are smoothed by an inductor in series with the output from the direct current source.
CONSTANT CURRENT SOURCE
Abstract of the Disclosure An apparatus for periodically interrupting a large direct current flow through a load is able to maintain a substantially constant level of current from the source both when it flows through the load and when it is interrupted, with minimum effect on the direct current source. A first power semiconductor switch is connected in parallel with the load and is switched on briefly to transfer current flow from the load to the first semiconductor switch. A second power semiconductor switch is in series with a precharged capacitor across the load. When the second power semiconductor switch is gated on a large influx of current through the second semiconductor switch into the capacitor transfers the current flow from the first semiconductor switch to the second semiconductor switch When the charge on the capacitor reaches a level sufficient to turn off the second semiconductor switch, the current flow transfers back to the load.
Any minor fluctuations in current level during the transfers are smoothed by an inductor in series with the output from the direct current source.
Description
~;~7~ t3 CAS~ 295~
DC CHOPPER CIRCUIT FED BY A
CONSI'ANI' C~RRENT SO~RCE
Background of the Invention This invention relates to an apparatus for periodically interrupting a large load current with a minimum effect Oll the current supply both when the current is flowing through the load and when it is interrupted, and with minimum degradation of a constant level of load current during the time it is flowing.
It is known to have a direct current source connected to a load through a series connected switch which opens periodically to interrupt the flow of current through a load. rrhe interrupting circuit is often referred to as a chopper circuit. When a current is interrupted periodically, the current must change quickly from its normal or design value to ~ero and back again to its design value. It is difficult to provide a current supply which can maintain a constant controlled current output through the periodic interruptions and also ~naintain a constant current through the load at a predetermined desiyn value between the interruptions. This is particularly so when the direct current design value is large, for example greater than 1000 or 2000 amperes whicl~ might , ~ AS~ 2954 be re~uired in a metal coating process.
SumMary of the Invention The present invention provides apparatus for interrupting a flow of direct current, that is it provides a D~ chopper circuit, which periodically directs current from the load through a low resistance branch circuit parallel to the load circuit to interrupt the flow of current through the load circuit for a short period. This permits the current flowing from the current source to remain substantially constant.
A solid state power switch is used in the parallel or branch circuit, and at larger currents it is difficult to commutate this solid state switch or semiconductor switch. According to the invention there is a second solid state switch connected in series with a pre-charged capacitor. Duriny the period when the load current is interrupted, current flows tbrough the first solid state power switch. To redirect the current so that it again flows through the load, the second solid state switch is turned on and, because the capacitor in series with it is precharged with a reverse or opposite polarity, tllere is initially a large flow of current into the capacitor which permits the first solid state switch to turn off. Thus the current is switched from the first solid state switch to the second solid state switch. The second solid state switch is then turned off when the capacitor becomes charged by the inflowing current. The current is thereby switched back to flow through the load. A circuit is provided to charge the capacitor.
It is therefore an object of the invention to provide an improved apparatus for periodically interruptlng the flow of current through a load wilile , ' , 34~
C~SE z954 maintaining a substantially constant current throuyn the load between the interruptions.
It is another object of the invention to provide an improved apparatus which maintains a substantially constant current flow frorn a DC source both when current is flowing through a load and when the current through the load is interrupted.
It is yet another object of the inverltion to provide an improved DC cnopper circuit arrangement which interrupts current throug~l a load by directing the load current periodically through a semiconductor power switch arrangement parallel to the load.
Accordingly there is provided apparatus for periodically interrupting the flow of direct current from a source of direct current to a load, comprising a first power semiconductor switch connected in parallel with the load and responsive to a first control signal for switching to a conducting condition to transfer the flow of current through the load substantially to flow through said first power semiconductor switch, a second power semiconductor switch connected in series with a pre-charged capacitor having a charge at a first polarity, said switch and capacitor being in parallel with said load, and said second semiconductor switch being respansive to a second control signal following said first control signal for switching to a conducting condition to initiate current flow through said second power semiconductor switch to charge said capacitor, the flow of current being transferred from said first power semiconductor switch to said second power semiconductor switch permitting said first power semiconductor switch to change to a non-conductiny condition, the current flow through said second power semiconductor switch ceasing when sufficient charge of :a~'7~ 4~
a second polarity opposite said first polarity is Oll said capacitor to stop conduction through said ~econd power semiconductor switch to restore current flow througll said load.
Also according to the inventioll there is provided apparatus for periodically interruptiny a large flow of direct current from a source of direct current to a load, said source of direct current having positive and negative terminals connected by a positive and a negative bus to said load, cornprising a first SCR having anode, cathode and gate electrodes, said anode electrode being connected to said positive bus and said cathode electrode being connected to said negative bus, a second SCR having anode, cathode and gate electrodes, a capacitor having first and second terminals, said first terminal of said capacitor being connected to said cathode slectrode of said second SCR
and said second terminal of said capacitor being connected to said negative bUS, said anode electrode of said second SCR being connected to said positive bus, a diode having an anode connected to said negative bus and a cathode connected to said positive bus, means connected to said first and second terminals of said capacitor for charging said capacitor with its first terminal negative and its second terminal positive, a control circuit connected to said gate electrodes of said fir~t and second SCXs for providing a gating signal to said gate electrode of said first SCR at a first predetermined time to cause it to conduct and transfer the flow of current substantially from the load to said first SCX, and a gating signal to said gate electrode of said second SCR at a second predeterrnined time a predetermined interval after said first predetermined tirne causing said second SCR to conduct and charge said capacitor ~.~7~33~ CA S E 2 g ~ ~
with an opposite charge, the conduction of said second SCR transferring the flow of current from said first SCR to said second SCR, said first SCR becominy non-conducting, conduction through said second SCR
ceasing when sufficient charge flows to said capacitor, said second S~R then becoMing non-conducting and restoring current ti~rough said load.
Brief Description of the Drawings The invention will be descri~ed Wit reference to the drawings, in which Figure 1 is a siMplified schematic diayram showing the invention, Figure 2 is an idealiæed graph of current plotted against time, useful in describing the invention, and Figure 3 is an idealized graph showing capacitor voltage plotted against time.
Description of the Preferred Embodiments ~ eferring to Figure 1 there is shown a schematic diagram of a form of DC chopper circuit where 10 is a source of direct current such as a rectifier system or a power converter intended to provide a substantially constant source of direct current. The source 10 has positive and negative terminals, the positive terminal being connected to a posi~ive bus 28 and the negative terminal being connected to a negative bus 29. A load is indicated by the dashed line 11 and is represented by an in~uctance 12 and a resistance 14. The load is connected between bus 28 and bus 29. The load is such that it requires the direct current to be a series of pulses of direct current. One example of such a load is metal coatiny apparatus which might require a direct current of 1000 amperes or more to be switche~
on for perhaps 100 to 150 milliseconds and off for 7~
CAS~ 2954 perhaps 10 to 30 milliseconds. T~e direct current must be rnaintained at a substantially constant value during the period it is flowing through load 11. It is difficult to ac~lieve a substantially constant current and a short off cycle with a series connected power semiconductor switch.
In Figure 1, to interrupt load current through load 11, a power semiconductor SWitCil 16, which may conveniently be a silicon controlled recti~ier or SCR, and is s~lown as an SCR, having anode, cathode and gate electrodes. It is connected across load 11, and when it conducts it diverts current from load 11 to interrupt the flow through load 11. Because the current from source 10 is not turned on and off as it would be by a series connected switch, the output from source 10 is not affected to nearly as great an extent. A relatively large inductor 15 is connected to the positive output frorn DC source 10 that is, it is in positive bus 28, to take care of minor fluctuations which, ~or example, occur when current flows through the very low resistance of SCR 16. The SCR 16 is turned on by a signal applied to its gate over conductor 17 frorn a control circuit 18.
A power semiconductor switch 20, shown as SCX 20 with anode, cathode and gate electrodes, is connected in series with a pre-charged capacitor 22 across load 11. The SCR 20 is triggered by a siynal on conductor 21 from control circuit 18. An SCX 23 and an inductor 24 are connected in series, and the series connection is in parallel with capacitor 22. The SCR 23 is triggered by a signal from control circuit 28 applied to its gate electrode over conductor 25. A power supply 26 is connected across capacitor 22 to provide an initial pre-charge to capacitor 22 and to replenish charge lost due to the previous commutation. A diode 27 is also connected across load 11 to permit the flow of excess current as will be explained hereinafter.
The operation of the circuit of Figure 1 will be explained with reference to both Figures 1, 2, and 3. Assuming the entire current from ~C
source 10 is passing through load 11, the level o~
current flow is indicated by 30 (Figure 2) and the time is prior to tl. Time tl represents t~le time at which the current through the load is interrupted.
At time tl control circuit 18 provides a signal on conductor 17 which gates on SCR 16. rrhe current through SCX 16 begins to increase from zero as is shown by small dashed line 31 (Figure 2). ~t the same time the current through load 11 begins to decrease as is shown by large dashed line 32 (Figure 2). ~t time t2 the current through the load becomes substantially zero and the current from DC source 10 is substantially all flowing through SCR 16 which has a very low resistance. ~owever choke 15 or inductor 15 prevents the current frorn increasing for the time SCR 16 is on and the resistance to current flow is smaller.
At time t3 SCR 20 is gated on by control circuit 18. The current continues to flow through SCR 16 but decreases as the current flow transfers to SCR 20. The small dashed line 33 (Figure 2) indicates the decreasing flow of current through SCR 16~ and the curve 34 (Figure 2) represented by small crosses indicates the current flow through SCR 20. Note that the current from source 10 represented by line 3U
(Yigure 2) is substantially constant. The current flowing throuyh SCR 20 and into capacitor 22, which previously had a negative pre-charge, increases until ~;~7~33~(~
CAS~ 2954 it exceeds the current which was flowing through SCR 16 and the current flow is thus transferre~ to SCR 20. The SCR 16 no longer conducts now that the current flow has ~een transferred to SC~ 20. ~rhi6 occurs at time t4 (Figure 2). The excess current causes a flow throuyh feedback diode 27. ~rhis excess flow is represented by that portion of curve 34 ~Figure 2) which extends above current line 3U
(Figure 2).
As the curve 34 (Figure 2) passes line 30, that is at time t5, current flow is decreasing throuyh SCR 20 and is beginnirlg to flow through load 11. The increasing flow throug~l load 11 is indicated by large dashed line 35 (Figure 2).
Capacitor 22 charges positively through sC~ 20 until the charye is sufficient to turn off SC~ 20. At time t6 current flow through load 11 is restored, and SCR 20 is off. To prepare capacitor 22 for the next commutation it must be charged negatively, that is, it must be charged with a polarity opposite to the polarity of the charge existing just after flow through load 11 is restored. Control circuit 18 provides a triggering pulse to SCR 23 over conductor 25 gating SC~ 23 on. This is at time t7 (Figure 3).
The inductor 24 and capacitor 22 form a resonant circuit, The charge on capacitor 22 flows into this circuit and a reverse polarity builds up on capacitor 22, that is a negative charge is left on capacitor 22. There will be some loss of charge during commutation and the reversal of the char~e on capacitor 22. The negative charge is brought wp to its predetermined value indicated ~y -V in Figure 3, by power supply 26.
Referring for the rnoment to Figure 3, there is a negative pre-charge of -V on capacitor 22 prior ~ " , .
. ~
.
- . - : . , :
~7~;~40 _ g _ to a commutation~ This pre-charye is on the capacitor 22 until time t3 when SCR 20 is yated on.
There is an inrush of current into capacitor 22 which is initially quite large for transferring current flow 5 from SCR 16 to SC~ 20. ~his inrush of current charyes capacitor 22 to a positive voltage (that is to a voltage polarity opposite to the pre charge voltaye polarity). This is shown by curve portion 38 in Fiyure 3 between times t3 and about t5. ~hen SCR 23 is gated on at time t7, SCR 20 is in a non-conducting condition. Capacitor 22 begins to recharge in its other direction (negative polarity is shown). The capacitor 22 is brouyht up to its full pre-charged condition (-V) by power supply 26.
It is believed that the operation of the circuitry of Figure 1 will now be clear.
The description refers to SC~s 16, 20 and 23.
Any semiconductor power switching devices suitable for the currents involved could be used in place of these SCRs.
, ' '
DC CHOPPER CIRCUIT FED BY A
CONSI'ANI' C~RRENT SO~RCE
Background of the Invention This invention relates to an apparatus for periodically interrupting a large load current with a minimum effect Oll the current supply both when the current is flowing through the load and when it is interrupted, and with minimum degradation of a constant level of load current during the time it is flowing.
It is known to have a direct current source connected to a load through a series connected switch which opens periodically to interrupt the flow of current through a load. rrhe interrupting circuit is often referred to as a chopper circuit. When a current is interrupted periodically, the current must change quickly from its normal or design value to ~ero and back again to its design value. It is difficult to provide a current supply which can maintain a constant controlled current output through the periodic interruptions and also ~naintain a constant current through the load at a predetermined desiyn value between the interruptions. This is particularly so when the direct current design value is large, for example greater than 1000 or 2000 amperes whicl~ might , ~ AS~ 2954 be re~uired in a metal coating process.
SumMary of the Invention The present invention provides apparatus for interrupting a flow of direct current, that is it provides a D~ chopper circuit, which periodically directs current from the load through a low resistance branch circuit parallel to the load circuit to interrupt the flow of current through the load circuit for a short period. This permits the current flowing from the current source to remain substantially constant.
A solid state power switch is used in the parallel or branch circuit, and at larger currents it is difficult to commutate this solid state switch or semiconductor switch. According to the invention there is a second solid state switch connected in series with a pre-charged capacitor. Duriny the period when the load current is interrupted, current flows tbrough the first solid state power switch. To redirect the current so that it again flows through the load, the second solid state switch is turned on and, because the capacitor in series with it is precharged with a reverse or opposite polarity, tllere is initially a large flow of current into the capacitor which permits the first solid state switch to turn off. Thus the current is switched from the first solid state switch to the second solid state switch. The second solid state switch is then turned off when the capacitor becomes charged by the inflowing current. The current is thereby switched back to flow through the load. A circuit is provided to charge the capacitor.
It is therefore an object of the invention to provide an improved apparatus for periodically interruptlng the flow of current through a load wilile , ' , 34~
C~SE z954 maintaining a substantially constant current throuyn the load between the interruptions.
It is another object of the invention to provide an improved apparatus which maintains a substantially constant current flow frorn a DC source both when current is flowing through a load and when the current through the load is interrupted.
It is yet another object of the inverltion to provide an improved DC cnopper circuit arrangement which interrupts current throug~l a load by directing the load current periodically through a semiconductor power switch arrangement parallel to the load.
Accordingly there is provided apparatus for periodically interrupting the flow of direct current from a source of direct current to a load, comprising a first power semiconductor switch connected in parallel with the load and responsive to a first control signal for switching to a conducting condition to transfer the flow of current through the load substantially to flow through said first power semiconductor switch, a second power semiconductor switch connected in series with a pre-charged capacitor having a charge at a first polarity, said switch and capacitor being in parallel with said load, and said second semiconductor switch being respansive to a second control signal following said first control signal for switching to a conducting condition to initiate current flow through said second power semiconductor switch to charge said capacitor, the flow of current being transferred from said first power semiconductor switch to said second power semiconductor switch permitting said first power semiconductor switch to change to a non-conductiny condition, the current flow through said second power semiconductor switch ceasing when sufficient charge of :a~'7~ 4~
a second polarity opposite said first polarity is Oll said capacitor to stop conduction through said ~econd power semiconductor switch to restore current flow througll said load.
Also according to the inventioll there is provided apparatus for periodically interruptiny a large flow of direct current from a source of direct current to a load, said source of direct current having positive and negative terminals connected by a positive and a negative bus to said load, cornprising a first SCR having anode, cathode and gate electrodes, said anode electrode being connected to said positive bus and said cathode electrode being connected to said negative bus, a second SCR having anode, cathode and gate electrodes, a capacitor having first and second terminals, said first terminal of said capacitor being connected to said cathode slectrode of said second SCR
and said second terminal of said capacitor being connected to said negative bUS, said anode electrode of said second SCR being connected to said positive bus, a diode having an anode connected to said negative bus and a cathode connected to said positive bus, means connected to said first and second terminals of said capacitor for charging said capacitor with its first terminal negative and its second terminal positive, a control circuit connected to said gate electrodes of said fir~t and second SCXs for providing a gating signal to said gate electrode of said first SCR at a first predetermined time to cause it to conduct and transfer the flow of current substantially from the load to said first SCX, and a gating signal to said gate electrode of said second SCR at a second predeterrnined time a predetermined interval after said first predetermined tirne causing said second SCR to conduct and charge said capacitor ~.~7~33~ CA S E 2 g ~ ~
with an opposite charge, the conduction of said second SCR transferring the flow of current from said first SCR to said second SCR, said first SCR becominy non-conducting, conduction through said second SCR
ceasing when sufficient charge flows to said capacitor, said second S~R then becoMing non-conducting and restoring current ti~rough said load.
Brief Description of the Drawings The invention will be descri~ed Wit reference to the drawings, in which Figure 1 is a siMplified schematic diayram showing the invention, Figure 2 is an idealiæed graph of current plotted against time, useful in describing the invention, and Figure 3 is an idealized graph showing capacitor voltage plotted against time.
Description of the Preferred Embodiments ~ eferring to Figure 1 there is shown a schematic diagram of a form of DC chopper circuit where 10 is a source of direct current such as a rectifier system or a power converter intended to provide a substantially constant source of direct current. The source 10 has positive and negative terminals, the positive terminal being connected to a posi~ive bus 28 and the negative terminal being connected to a negative bus 29. A load is indicated by the dashed line 11 and is represented by an in~uctance 12 and a resistance 14. The load is connected between bus 28 and bus 29. The load is such that it requires the direct current to be a series of pulses of direct current. One example of such a load is metal coatiny apparatus which might require a direct current of 1000 amperes or more to be switche~
on for perhaps 100 to 150 milliseconds and off for 7~
CAS~ 2954 perhaps 10 to 30 milliseconds. T~e direct current must be rnaintained at a substantially constant value during the period it is flowing through load 11. It is difficult to ac~lieve a substantially constant current and a short off cycle with a series connected power semiconductor switch.
In Figure 1, to interrupt load current through load 11, a power semiconductor SWitCil 16, which may conveniently be a silicon controlled recti~ier or SCR, and is s~lown as an SCR, having anode, cathode and gate electrodes. It is connected across load 11, and when it conducts it diverts current from load 11 to interrupt the flow through load 11. Because the current from source 10 is not turned on and off as it would be by a series connected switch, the output from source 10 is not affected to nearly as great an extent. A relatively large inductor 15 is connected to the positive output frorn DC source 10 that is, it is in positive bus 28, to take care of minor fluctuations which, ~or example, occur when current flows through the very low resistance of SCR 16. The SCR 16 is turned on by a signal applied to its gate over conductor 17 frorn a control circuit 18.
A power semiconductor switch 20, shown as SCX 20 with anode, cathode and gate electrodes, is connected in series with a pre-charged capacitor 22 across load 11. The SCR 20 is triggered by a siynal on conductor 21 from control circuit 18. An SCX 23 and an inductor 24 are connected in series, and the series connection is in parallel with capacitor 22. The SCR 23 is triggered by a signal from control circuit 28 applied to its gate electrode over conductor 25. A power supply 26 is connected across capacitor 22 to provide an initial pre-charge to capacitor 22 and to replenish charge lost due to the previous commutation. A diode 27 is also connected across load 11 to permit the flow of excess current as will be explained hereinafter.
The operation of the circuit of Figure 1 will be explained with reference to both Figures 1, 2, and 3. Assuming the entire current from ~C
source 10 is passing through load 11, the level o~
current flow is indicated by 30 (Figure 2) and the time is prior to tl. Time tl represents t~le time at which the current through the load is interrupted.
At time tl control circuit 18 provides a signal on conductor 17 which gates on SCR 16. rrhe current through SCX 16 begins to increase from zero as is shown by small dashed line 31 (Figure 2). ~t the same time the current through load 11 begins to decrease as is shown by large dashed line 32 (Figure 2). ~t time t2 the current through the load becomes substantially zero and the current from DC source 10 is substantially all flowing through SCR 16 which has a very low resistance. ~owever choke 15 or inductor 15 prevents the current frorn increasing for the time SCR 16 is on and the resistance to current flow is smaller.
At time t3 SCR 20 is gated on by control circuit 18. The current continues to flow through SCR 16 but decreases as the current flow transfers to SCR 20. The small dashed line 33 (Figure 2) indicates the decreasing flow of current through SCR 16~ and the curve 34 (Figure 2) represented by small crosses indicates the current flow through SCR 20. Note that the current from source 10 represented by line 3U
(Yigure 2) is substantially constant. The current flowing throuyh SCR 20 and into capacitor 22, which previously had a negative pre-charge, increases until ~;~7~33~(~
CAS~ 2954 it exceeds the current which was flowing through SCR 16 and the current flow is thus transferre~ to SCR 20. The SCR 16 no longer conducts now that the current flow has ~een transferred to SC~ 20. ~rhi6 occurs at time t4 (Figure 2). The excess current causes a flow throuyh feedback diode 27. ~rhis excess flow is represented by that portion of curve 34 ~Figure 2) which extends above current line 3U
(Figure 2).
As the curve 34 (Figure 2) passes line 30, that is at time t5, current flow is decreasing throuyh SCR 20 and is beginnirlg to flow through load 11. The increasing flow throug~l load 11 is indicated by large dashed line 35 (Figure 2).
Capacitor 22 charges positively through sC~ 20 until the charye is sufficient to turn off SC~ 20. At time t6 current flow through load 11 is restored, and SCR 20 is off. To prepare capacitor 22 for the next commutation it must be charged negatively, that is, it must be charged with a polarity opposite to the polarity of the charge existing just after flow through load 11 is restored. Control circuit 18 provides a triggering pulse to SCR 23 over conductor 25 gating SC~ 23 on. This is at time t7 (Figure 3).
The inductor 24 and capacitor 22 form a resonant circuit, The charge on capacitor 22 flows into this circuit and a reverse polarity builds up on capacitor 22, that is a negative charge is left on capacitor 22. There will be some loss of charge during commutation and the reversal of the char~e on capacitor 22. The negative charge is brought wp to its predetermined value indicated ~y -V in Figure 3, by power supply 26.
Referring for the rnoment to Figure 3, there is a negative pre-charge of -V on capacitor 22 prior ~ " , .
. ~
.
- . - : . , :
~7~;~40 _ g _ to a commutation~ This pre-charye is on the capacitor 22 until time t3 when SCR 20 is yated on.
There is an inrush of current into capacitor 22 which is initially quite large for transferring current flow 5 from SCR 16 to SC~ 20. ~his inrush of current charyes capacitor 22 to a positive voltage (that is to a voltage polarity opposite to the pre charge voltaye polarity). This is shown by curve portion 38 in Fiyure 3 between times t3 and about t5. ~hen SCR 23 is gated on at time t7, SCR 20 is in a non-conducting condition. Capacitor 22 begins to recharge in its other direction (negative polarity is shown). The capacitor 22 is brouyht up to its full pre-charged condition (-V) by power supply 26.
It is believed that the operation of the circuitry of Figure 1 will now be clear.
The description refers to SC~s 16, 20 and 23.
Any semiconductor power switching devices suitable for the currents involved could be used in place of these SCRs.
, ' '
Claims (8)
1. Apparatus for periodically interrupting the flow of direct current from a source of direct current to a load, comprising:
a first power semiconductor switch connected in parallel with the load and responsive to a first control signal for switching to a conducting condition to transfer the flow of current through the load substantially to flow through said first power semiconductor switch, a second power semiconductor switch connected in series with a pre-charged capacitor having a charge of a first polarity, said switch and capacitor being in parallel with said load and responsive to a second control signal following said first control signal for switching to a conducting condition to initiate current flow through said second power semiconductor switch to charge said capacitor, the flow of current being transferred from said first power semiconductor switch to said second power semiconductor switch permitting said first power semiconductor switch to change to a non-conducting condition, the current flow through said second power semiconductor switch ceasing when sufficient charge of a second polarity opposite said first polarity is on said capacitor to stop conduction through said second power semiconductor switch to restore current flow through said load.
a first power semiconductor switch connected in parallel with the load and responsive to a first control signal for switching to a conducting condition to transfer the flow of current through the load substantially to flow through said first power semiconductor switch, a second power semiconductor switch connected in series with a pre-charged capacitor having a charge of a first polarity, said switch and capacitor being in parallel with said load and responsive to a second control signal following said first control signal for switching to a conducting condition to initiate current flow through said second power semiconductor switch to charge said capacitor, the flow of current being transferred from said first power semiconductor switch to said second power semiconductor switch permitting said first power semiconductor switch to change to a non-conducting condition, the current flow through said second power semiconductor switch ceasing when sufficient charge of a second polarity opposite said first polarity is on said capacitor to stop conduction through said second power semiconductor switch to restore current flow through said load.
2. Apparatus as defined in Claim 1 and further comprising an inductor connected in the current path to said load and said first and second power semiconductor switches to compensate for fluctuations when the current flow is transferred from the load to a path having a lower resistance provided at least by said first power semiconductor switch.
3. Apparatus as defined in Claim 2 and further comprising means for charging said capacitor with a charge opposite to the polarity across said source of direct current for increasing the initial flow of current through said second power semiconductor switch when it changes to its conducting condition.
4. Apparatus as defined in Claim 3 and further comprising a diode connected in parallel with said load, and in a reverse direction for carrying current due to the discharge of said capacitor.
5. Apparatus as defined in Claim 3 in which the means for charging said capacitor includes a power supply.
6. Apparatus for periodically interrupting a large flow of direct current from a source of direct current to a load, said source of direct current having positive and negative terminals connected by a positive and a negative bus to said load, comprising:
a first SCR having anode, cathode and gate electrodes, said anode electrode being connected to said positive bus and said cathode electrode being connected to said negative bus, a second SCR having anode, cathode and gate electrodes, a capacitor having first and second terminals, said first terminal of said capacitor being connected to said cathode electrode of said second SCR
and said second terminal of said capacitor being connected to said negative bus, said anode electrode of said second SCR
being connected to said positive bus, a diode having an anode connected to said negative bus and a cathode connected to said positive bus, means connected to said first and second terminals of said capacitor for charging said capacitor with its first terminal negative and its second terminal positive, a control circuit connected to said gate electrodes of said first and second SCRs for providing a gating signal to said gate electrode of said first SCR at a first predetermined time to cause it to conduct and transfer the flow of current substantially from the load to said first SCR, and a gating signal to said gate electrode of said second SCR at a second predetermined time a predetermined interval after said first predetermined time causing said second SCR to conduct and charge said capacitor with an opposite charge, the conduction of said second SCR transferring the flow of current from said first SCR to said second SCR, said first SCR becoming non-conducting, and conduction through said second SCR ceasing and becoming non-conducting when sufficient charge flows to said capacitor thereby restoring current through said load.
a first SCR having anode, cathode and gate electrodes, said anode electrode being connected to said positive bus and said cathode electrode being connected to said negative bus, a second SCR having anode, cathode and gate electrodes, a capacitor having first and second terminals, said first terminal of said capacitor being connected to said cathode electrode of said second SCR
and said second terminal of said capacitor being connected to said negative bus, said anode electrode of said second SCR
being connected to said positive bus, a diode having an anode connected to said negative bus and a cathode connected to said positive bus, means connected to said first and second terminals of said capacitor for charging said capacitor with its first terminal negative and its second terminal positive, a control circuit connected to said gate electrodes of said first and second SCRs for providing a gating signal to said gate electrode of said first SCR at a first predetermined time to cause it to conduct and transfer the flow of current substantially from the load to said first SCR, and a gating signal to said gate electrode of said second SCR at a second predetermined time a predetermined interval after said first predetermined time causing said second SCR to conduct and charge said capacitor with an opposite charge, the conduction of said second SCR transferring the flow of current from said first SCR to said second SCR, said first SCR becoming non-conducting, and conduction through said second SCR ceasing and becoming non-conducting when sufficient charge flows to said capacitor thereby restoring current through said load.
7. Apparatus as defined in Claim 6 and further comprising an inductor connected between said positive terminal of said source of direct current and said positive bus to reduce fluctuations caused by the transfer of current from said load to a lower resistance path through said first SCR.
8. Apparatus as defined in Claim 7 in which said means connected to said first and second terminals of said capacitor comprises:
- 13 - Case 2954 Claim 8 continued:
a third SCR having anode, cathode and gate electrodes, said anode electrode being connected to said first terminal of said capacitor, an inductor having one end connected to said cathode electrode of said third SCR and the other end connected to said second terminal of said capacitor for forming with said capacitor a resonant circuit, said gate electrode of said third SCR being connected to said control circuit for receiving a gating signal from said control circuit at a time when said capacitor is charged with its first terminal positive and its second terminal negative and said second SCR is in a non-conducting condition, and responsive to said gating signal to conduct permitting a degree of oscillation in the said resonant circuit tending to change said capacitor with its first terminal negative and its second terminal positive, and a power supply having a negative terminal connected to said first terminal of said capacitor and a positive terminal connected to said second terminal of said capacitor for increasing the charge on said capacitor to a predetermined level.
- 13 - Case 2954 Claim 8 continued:
a third SCR having anode, cathode and gate electrodes, said anode electrode being connected to said first terminal of said capacitor, an inductor having one end connected to said cathode electrode of said third SCR and the other end connected to said second terminal of said capacitor for forming with said capacitor a resonant circuit, said gate electrode of said third SCR being connected to said control circuit for receiving a gating signal from said control circuit at a time when said capacitor is charged with its first terminal positive and its second terminal negative and said second SCR is in a non-conducting condition, and responsive to said gating signal to conduct permitting a degree of oscillation in the said resonant circuit tending to change said capacitor with its first terminal negative and its second terminal positive, and a power supply having a negative terminal connected to said first terminal of said capacitor and a positive terminal connected to said second terminal of said capacitor for increasing the charge on said capacitor to a predetermined level.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000506428A CA1278340C (en) | 1986-04-11 | 1986-04-11 | Dc chopper circuit fed by a constant current source |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000506428A CA1278340C (en) | 1986-04-11 | 1986-04-11 | Dc chopper circuit fed by a constant current source |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1278340C true CA1278340C (en) | 1990-12-27 |
Family
ID=4132863
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000506428A Expired - Lifetime CA1278340C (en) | 1986-04-11 | 1986-04-11 | Dc chopper circuit fed by a constant current source |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1278340C (en) |
-
1986
- 1986-04-11 CA CA000506428A patent/CA1278340C/en not_active Expired - Lifetime
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| Date | Code | Title | Description |
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| MKEX | Expiry |