CA2314323A1 - Ac power backfeed protection for inverter circuits - Google Patents
Ac power backfeed protection for inverter circuits Download PDFInfo
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- CA2314323A1 CA2314323A1 CA002314323A CA2314323A CA2314323A1 CA 2314323 A1 CA2314323 A1 CA 2314323A1 CA 002314323 A CA002314323 A CA 002314323A CA 2314323 A CA2314323 A CA 2314323A CA 2314323 A1 CA2314323 A1 CA 2314323A1
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- current
- output path
- switching stage
- condition
- gate drive
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Classifications
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/32—Means for protecting converters other than automatic disconnection
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/5387—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
Abstract
An apparatus for preventing power backfeed into an inverter circuit having a switching stage for supplying AC power to an output path of the inverter circuit is disclosed. The apparatus comprises a current sensor which senses a current condition in the output path and a switching stage current limiter operable to limit current in the switching stage when said current condition meets a criterion. The current sensor device senses current in the output path and compares it with a reference value to determine whether or not it is within a range. If it is not within this range, a signal is communicated to a gate drive controller which prevents gate drive signals produced by the inverter, from reaching the switching devices of the switching stage. This prevention of gate drive signals from reaching the switching devices occurs very quickly, before the damaging AC backfeed power can reach a level sufficiently high to damage the switching devices because the rate of flow of AC backfeed power is limited to permit disclosed AC backfeed power protection circuitry to operate quickly to prevent AC backfeed power from reaching damaging levels.
Description
AC POWER BACKFEED PROTECTION FOR INVERTER CIRCUITS
BACKGROUND OF THE INVENTION
1. Field of Invention This invention relates to protection circuits for inverters and more particularly to methods and apparatus for preventing power backfeed in inverter circuits having a switching stage for supplying AC power to an output path of the inverter circuit.
BACKGROUND OF THE INVENTION
1. Field of Invention This invention relates to protection circuits for inverters and more particularly to methods and apparatus for preventing power backfeed in inverter circuits having a switching stage for supplying AC power to an output path of the inverter circuit.
2. Description of Related Art Electrical inverters are often used for providing alternating current power to electrical appliances, in places where alternating current is not available from a utility supply. Common places of use include recreational vehicles and powerboats, for example. Most inverters are powered by batteries and the output of these inverters are often connected to an alternating current (AC) bus. Alternate connection between the AC utility and the inverter is usually required, so that the user may use the same appliances in the same outlets, by selectively supplying power from the inverter, or from the utility.
Alternate connection between these two sources is usually provided through a relay or transfer switch.
The use of a relay or transfer switch, however, creates problems, especially if a reactive load is being supplied by the utility and then the transfer switch is thrown, to alternatively supply power from the inverter. Relay contacts or transfer switch contacts experiencing this condition may draw an electrical arc for a short period of time while the transfer contacts settle in favour of the inverter. If arcing occurs during this period, power from the AC utility can be backfed into the inverter, possibly damaging the inverter and particularly the switching devices in the inverter.
What would be desirable therefore, is a device which limits AC power backfeed into an inverter, especially during source switching by a relay or transfer switch.
SUMMARY OF THE INVENTION
The present invention addresses the above need by providing an apparatus and method for preventing power backfeed into an inverter circuit having a switching stage for supplying AC power to an output path of the inverter circuit. In accordance with one aspect of the invention, the method may involve sensing a current condition in the output path and limiting current in the switching stage when the current condition meets a criterion. This may be achieved in one embodiment through the use of a current sensor which senses the current condition in the output path and through the use of a switching stage current limiter operable to limit current in the switching stage when the current condition meets a criterion.
The current sensor may sense an instantaneous value of alternating current in the output path and may comprise a current transformer in the output path. A
resistor may be connected to the current transformer to produce a voltage representative of current in the output path.
The switching stage current limiter may comprise switching devices in the switching stage of the inverter circuit. It may also comprise a gate drive controller for controlling the transmission of gate drive signals from the inverter circuit to the switching devices in the switching stage, in response to the current condition. Preferably, the gate drive controller is operable to control the gate drive signals to cause at least some of the switching devices to be placed in a non-conducting mode.
The switching stage current limiter may comprise a condition determiner for determining the condition sensed by the current sensor. The condition determiner may comprise a condition comparator in communication with the gate drive controller for comparing an output of the current sensor with a
Alternate connection between these two sources is usually provided through a relay or transfer switch.
The use of a relay or transfer switch, however, creates problems, especially if a reactive load is being supplied by the utility and then the transfer switch is thrown, to alternatively supply power from the inverter. Relay contacts or transfer switch contacts experiencing this condition may draw an electrical arc for a short period of time while the transfer contacts settle in favour of the inverter. If arcing occurs during this period, power from the AC utility can be backfed into the inverter, possibly damaging the inverter and particularly the switching devices in the inverter.
What would be desirable therefore, is a device which limits AC power backfeed into an inverter, especially during source switching by a relay or transfer switch.
SUMMARY OF THE INVENTION
The present invention addresses the above need by providing an apparatus and method for preventing power backfeed into an inverter circuit having a switching stage for supplying AC power to an output path of the inverter circuit. In accordance with one aspect of the invention, the method may involve sensing a current condition in the output path and limiting current in the switching stage when the current condition meets a criterion. This may be achieved in one embodiment through the use of a current sensor which senses the current condition in the output path and through the use of a switching stage current limiter operable to limit current in the switching stage when the current condition meets a criterion.
The current sensor may sense an instantaneous value of alternating current in the output path and may comprise a current transformer in the output path. A
resistor may be connected to the current transformer to produce a voltage representative of current in the output path.
The switching stage current limiter may comprise switching devices in the switching stage of the inverter circuit. It may also comprise a gate drive controller for controlling the transmission of gate drive signals from the inverter circuit to the switching devices in the switching stage, in response to the current condition. Preferably, the gate drive controller is operable to control the gate drive signals to cause at least some of the switching devices to be placed in a non-conducting mode.
The switching stage current limiter may comprise a condition determiner for determining the condition sensed by the current sensor. The condition determiner may comprise a condition comparator in communication with the gate drive controller for comparing an output of the current sensor with a
-3-reference to cause the gate drive controller to cause at least some of the switching devices to be placed in the non-conducting mode when the output of the current sensor is within a range.
The current sensor may include a current to voltage converter operable to develop a sense voltage in response to instantaneous current in the output path of the inverter circuit.
The condition comparator may have a voltage summing circuit for adding a common mode voltage to the sense voltage and may further include first and second voltage comparators connected to cause the gate drive controller to place at least some of the switching devices into the non-conducting mode when positive or negative excursions of the sense voltage exceed high and low reference voltages respectively.
The condition determiner may include a latch activated by the condition comparator to cause the gate drive controller to hold at least some of the switching devices in the non-conducting mode.
Preferably, a current time rate limiter is provided in the output path, for limiting alternating current therein. The current time rate limiter may be operable to time rate limit alternating current in the output path to a value sufficiently high to permit the inverter circuit to respond to changing load conditions and sufficiently low to permit the switching stage current limiter to operate, before damaging alternating current is conducted in the output path, and in particular the switching devices. A current time rate limiter may be provided by an inductor.
In one embodiment, the current sensor may be comprised of resistors in series with switching devices of the switching stage of the inverter circuit.
In accordance with another aspect of the invention, there is provided an apparatus for preventing power backfeed into an inverter circuit having a switching stage for supplying AC power to an output path of the inverter circuit.
The apparatus may comprise provisions for sensing a current condition in the
The current sensor may include a current to voltage converter operable to develop a sense voltage in response to instantaneous current in the output path of the inverter circuit.
The condition comparator may have a voltage summing circuit for adding a common mode voltage to the sense voltage and may further include first and second voltage comparators connected to cause the gate drive controller to place at least some of the switching devices into the non-conducting mode when positive or negative excursions of the sense voltage exceed high and low reference voltages respectively.
The condition determiner may include a latch activated by the condition comparator to cause the gate drive controller to hold at least some of the switching devices in the non-conducting mode.
Preferably, a current time rate limiter is provided in the output path, for limiting alternating current therein. The current time rate limiter may be operable to time rate limit alternating current in the output path to a value sufficiently high to permit the inverter circuit to respond to changing load conditions and sufficiently low to permit the switching stage current limiter to operate, before damaging alternating current is conducted in the output path, and in particular the switching devices. A current time rate limiter may be provided by an inductor.
In one embodiment, the current sensor may be comprised of resistors in series with switching devices of the switching stage of the inverter circuit.
In accordance with another aspect of the invention, there is provided an apparatus for preventing power backfeed into an inverter circuit having a switching stage for supplying AC power to an output path of the inverter circuit.
The apparatus may comprise provisions for sensing a current condition in the
-4-output path and provisions for limiting current in the switching stage when the current condition meets a criterion.
In one embodiment, backfed AC power is limited by an inductor in the rate of rise of current fed back into the switching stage of the inverter circuit. A
current sensing device senses current in the output path and compares it with a reference value to determine whether or not it is within a range. If it is not within this range, a signal is communicated to the gate drive controller which prevents gate drive signals produced by the inverter, from reaching the switching devices of the switching stage. This prevention of gate drive signals from reaching the switching devices occurs very quickly, before the damaging AC backfeed power can reach a level sufficiently high to damage the switching devices. Thus, the rate of flow of AC backfeed power is limited to permit AC backfeed power protection circuitry to operate quickly to prevent AC backfeed power from reaching damaging levels.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
In drawings which illustrate embodiments of the invention, Figure 1 is a block diagram of an apparatus according to a first embodiment of the invention;
Figure 2 is a schematic diagram of a comparator circuit according to the first embodiment of the invention;
Figure 3 is a logic diagram of a gate drive controller circuit shown in Figure 1; and
In one embodiment, backfed AC power is limited by an inductor in the rate of rise of current fed back into the switching stage of the inverter circuit. A
current sensing device senses current in the output path and compares it with a reference value to determine whether or not it is within a range. If it is not within this range, a signal is communicated to the gate drive controller which prevents gate drive signals produced by the inverter, from reaching the switching devices of the switching stage. This prevention of gate drive signals from reaching the switching devices occurs very quickly, before the damaging AC backfeed power can reach a level sufficiently high to damage the switching devices. Thus, the rate of flow of AC backfeed power is limited to permit AC backfeed power protection circuitry to operate quickly to prevent AC backfeed power from reaching damaging levels.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
In drawings which illustrate embodiments of the invention, Figure 1 is a block diagram of an apparatus according to a first embodiment of the invention;
Figure 2 is a schematic diagram of a comparator circuit according to the first embodiment of the invention;
Figure 3 is a logic diagram of a gate drive controller circuit shown in Figure 1; and
-5-Figure 4 is a schematic diagram of current sensing devices in a switching stage of an inverter, according to a second embodiment of the invention.
DETAILED DESCRIPTION
Referring to Figure 1, an apparatus for preventing alternating current backfeed into an inverter circuit having a switching stage for supplying AC
power to an output path of the inverter circuit according to a first embodiment of the invention, is shown generally at 10. The inverter circuit itself is represented schematically at 12 and it will be appreciated that in this embodiment the apparatus 10 is incorporated within the inverter circuit 12.
Generally the inverter circuit has a power supply circuit which is usually a DC
supply circuit 14 which supplies power to the switching stage 16 of the inverter circuit 12. The switching stage 16 includes a plurality of switching devices 18, 20, 22 and 24 which may be Metallic Oxide Semiconductor Field Effect Transistors (MOSFETs), for example, which receive power from the power supply 14 and provide alternating current at a switching stage output shown generally at 26. The inverter produces gate drive signals on signal lines 28, 30, 32 and 34 and these gate drive signals control the operation of the switching devices 18 to 24, to cause AC power to be produced at the switching stage output 26. The switching stage output 26 and the switching devices 18, 20, 22 and 24 connected to the output 26 as an output path 36 of the inverter circuit 12. The output path 36 is terminated in an output port 38, from which AC power may be supplied to an AC bus 40 or AC appliance, etc.
Generally, the apparatus 10, according to the first embodiment of the invention includes a current sensor 42 which senses a current condition in the output path 36, and further includes a switching stage current limiter shown generally at 44, operable to limit current in the switching stage 16 when the current condition sensed by the current sensor 42 meets a criterion.
In this embodiment, the current sensor 42 includes a current transformer 46 having a primary winding in series with one leg 48 of the output path 36. A
DETAILED DESCRIPTION
Referring to Figure 1, an apparatus for preventing alternating current backfeed into an inverter circuit having a switching stage for supplying AC
power to an output path of the inverter circuit according to a first embodiment of the invention, is shown generally at 10. The inverter circuit itself is represented schematically at 12 and it will be appreciated that in this embodiment the apparatus 10 is incorporated within the inverter circuit 12.
Generally the inverter circuit has a power supply circuit which is usually a DC
supply circuit 14 which supplies power to the switching stage 16 of the inverter circuit 12. The switching stage 16 includes a plurality of switching devices 18, 20, 22 and 24 which may be Metallic Oxide Semiconductor Field Effect Transistors (MOSFETs), for example, which receive power from the power supply 14 and provide alternating current at a switching stage output shown generally at 26. The inverter produces gate drive signals on signal lines 28, 30, 32 and 34 and these gate drive signals control the operation of the switching devices 18 to 24, to cause AC power to be produced at the switching stage output 26. The switching stage output 26 and the switching devices 18, 20, 22 and 24 connected to the output 26 as an output path 36 of the inverter circuit 12. The output path 36 is terminated in an output port 38, from which AC power may be supplied to an AC bus 40 or AC appliance, etc.
Generally, the apparatus 10, according to the first embodiment of the invention includes a current sensor 42 which senses a current condition in the output path 36, and further includes a switching stage current limiter shown generally at 44, operable to limit current in the switching stage 16 when the current condition sensed by the current sensor 42 meets a criterion.
In this embodiment, the current sensor 42 includes a current transformer 46 having a primary winding in series with one leg 48 of the output path 36. A
-6-suitable current transformer has a 1:1000 turns ratio. A suitable device is provided under model No. LD50 by Amecon Inc. of California, U.S.A. A
secondary winding 50 of the current transformer is connected in parallel with a resistor 52 such that a sense voltage is produced on a signal line 54, representing an instantaneous value of alternating current in the output path 36. This sense voltage is provided to a current condition determiner 56 which determines a condition sensed by the current sensor 42 and communicates the sensed condition to a gate drive controller 58 which is operable to control the gate drive signals on signal lines 28, 30, 32 and 34 produced by the inverter circuit 12 to cause at least some of the switching devices 18, 20, 22 and 24 to be placed in a non-conducting mode. Effectively, the current condition determiner 56 controls the operation of the gate drive controller 58 in response to the current condition sensed by the current sensor 42.
The current condition determiner 56 generally includes a comparator 60 ultimately in communication with the gate drive controller 58, for comparing an output of the current condition sensor 42 with a reference to cause the gate drive controller 58 to cause at least some of the switching devices 18, 20, 22 and 24 to be placed in the non-conducting mode when the output of the current condition sensor 42 is within a range. This range may be about 75 amperes or above, in an 800 watt inverter having a destruction threshold of approximately 140 amperes, for example.
Preferably, the current condition determiner 56 includes a latch 62 which is activated by the condition comparator 60 to cause the gate drive controller 58 to hold at least some of the switching devices 18, 20, 22, and 24 in the non-conducting mode.
Referring to Figure 2, in this embodiment, the current condition determiner 56 includes a voltage summing circuit shown generally at 70 for adding a common mode voltage to the sense voltage produced on signal line 54. The common mode voltage is provided by a reference source 72. Effectively, the voltage summing circuit 70 serves to shift or add a DC offset to the sense
secondary winding 50 of the current transformer is connected in parallel with a resistor 52 such that a sense voltage is produced on a signal line 54, representing an instantaneous value of alternating current in the output path 36. This sense voltage is provided to a current condition determiner 56 which determines a condition sensed by the current sensor 42 and communicates the sensed condition to a gate drive controller 58 which is operable to control the gate drive signals on signal lines 28, 30, 32 and 34 produced by the inverter circuit 12 to cause at least some of the switching devices 18, 20, 22 and 24 to be placed in a non-conducting mode. Effectively, the current condition determiner 56 controls the operation of the gate drive controller 58 in response to the current condition sensed by the current sensor 42.
The current condition determiner 56 generally includes a comparator 60 ultimately in communication with the gate drive controller 58, for comparing an output of the current condition sensor 42 with a reference to cause the gate drive controller 58 to cause at least some of the switching devices 18, 20, 22 and 24 to be placed in the non-conducting mode when the output of the current condition sensor 42 is within a range. This range may be about 75 amperes or above, in an 800 watt inverter having a destruction threshold of approximately 140 amperes, for example.
Preferably, the current condition determiner 56 includes a latch 62 which is activated by the condition comparator 60 to cause the gate drive controller 58 to hold at least some of the switching devices 18, 20, 22, and 24 in the non-conducting mode.
Referring to Figure 2, in this embodiment, the current condition determiner 56 includes a voltage summing circuit shown generally at 70 for adding a common mode voltage to the sense voltage produced on signal line 54. The common mode voltage is provided by a reference source 72. Effectively, the voltage summing circuit 70 serves to shift or add a DC offset to the sense
-7-voltage on signal line 54, to allow positive and negative excursions of the sense voltage to be sensed using first and second voltage comparators 74 and 76 operating off of a single polarity power supply. The summing circuit 70 could be omitted if a dual polarity power supply is provided.
The first and second voltage comparators 74 and 76 are connected to cause the gate drive control circuit 58 shown in Figure 1 to place at least some of the switching devices into the non-conducting modes if positive or negative excursions of the sense voltage exceed high and low reference voltages produced by reference sources 78 and 80, respectively. The high and low reference voltages may represent +75 amperes and -75 amperes respectively, for example. When such conditions occur, the output 82 or 84 of the respective comparator 74 or 76 causes an active low danger signal to become active, and this danger signal is ultimately communicated to the gate drive controller 58, through the latch 62 shown in Figure 1.
Referring back to Figure 1, when the danger signal becomes active, the latch 62 holds its output signal active to produce a shut down signal on a signal line 86 for receipt by the gate drive controller 58.
The gate drive controller 58 is shown generally at 90 in Figure 3 and includes transistor-implemented logical "AND" circuits 92, 94, 96 and 98, each respectively controlled by the shut down signal for producing new gate drive control signals 100, 102, 104 and 106, by effectively gating, whether or not the gate drive control signals 28, 30, 32 and 34 are communicated to respective switching devices, It will be appreciated that it takes time for the effect of current in the primary winding 46 of the current transformer to be noticed at the gate drive signals 100, 102, 104 and 106 due to delays in the operation of the intervening circuitry including the comparator 60, the latch 62 and the gate drive controller 58. Consequently, an inductor 110 is placed in series in the output path 36, to time rate limit current in the output path. The inductance of the inductor 110 is chosen such that the rate of change of current in the output path 36 is _$_ sufficiently high to permit the inverter circuit to respond to changing load conditions, and sufficiently low to permit the switching stage current limiter to operate, before damaging alternating current is conducted in the output path 36 and ultimately conducted through the switching devices 18, 20, 22 and 24. In this embodiment, for an 800 to 1500 watt inverter circuit, an inductor having an inductance of approximately 25 microhenries is used to provide current limiting to approximately 10 amperes per microsecond.
Typically, the time for the intervening circuitry of the current condition determiner 56 and the gate drive controller 58 is approximately 1 or 2 microseconds.
Referring to Figure 4, in an alternative embodiment, the current sensor 42 may be replaced by resistors 120, 122, 124 and 126 in series between respective switching devices 18, 20, 22 and 24 and the switching stage output 26. In this case, individual currents through respective individual resistors 120, 122, 124 and 126 may be monitored and compared with four separate comparators of the type shown at 60 in Figure 3, wired in a logical OR
relation, to produce the active low danger signal which is supplied to the latch 62.
It will be appreciated that with an embodiment described herein the power drawn from the output 38 of the inverter circuit is typically limited by the power supply capacity of the inverter circuit and is usually limited to a significantly low value, which will not burn out the switching devices 18, 20, 22 and 24.
However, if the output 38 is connected to an AC bus 40, the AC bus 40 may attempt to supply current into the inverter circuit, i.e., in a direction from the output port 38 to the switching stage 16. Since an AC bus 40, if connected to a utility supply, effectively acts as an infinite current source with zero impedance, extremely high currents can be received at the switching stage 16, burning out the switching components 18, 20, 22 and 24, especially during relay, transfer switch operation. However, using the embodiments indicated above, the inductor 110 limits the rate of rise of current flow from the output port 38 to the switching stage output port 26 and the current sensing device _g_ 42 senses backfeed current which is outside a predefined range such as provided by the comparator 60, and causes the gate drive controller 58 to place the switching devices 18, 20, 22 and 24 in the non-conducting mode, whereby they are all placed in a high impedance state and inoperable to conduct current. Thus, AC power backfeed is prevented from damaging the switching devices 18, 20, 22 and 24.
While specific embodiments of the invention have been described and illustrated, such embodiments should be considered illustrative of the invention only and not as limiting the invention as construed in accordance with the accompanying claims.
The first and second voltage comparators 74 and 76 are connected to cause the gate drive control circuit 58 shown in Figure 1 to place at least some of the switching devices into the non-conducting modes if positive or negative excursions of the sense voltage exceed high and low reference voltages produced by reference sources 78 and 80, respectively. The high and low reference voltages may represent +75 amperes and -75 amperes respectively, for example. When such conditions occur, the output 82 or 84 of the respective comparator 74 or 76 causes an active low danger signal to become active, and this danger signal is ultimately communicated to the gate drive controller 58, through the latch 62 shown in Figure 1.
Referring back to Figure 1, when the danger signal becomes active, the latch 62 holds its output signal active to produce a shut down signal on a signal line 86 for receipt by the gate drive controller 58.
The gate drive controller 58 is shown generally at 90 in Figure 3 and includes transistor-implemented logical "AND" circuits 92, 94, 96 and 98, each respectively controlled by the shut down signal for producing new gate drive control signals 100, 102, 104 and 106, by effectively gating, whether or not the gate drive control signals 28, 30, 32 and 34 are communicated to respective switching devices, It will be appreciated that it takes time for the effect of current in the primary winding 46 of the current transformer to be noticed at the gate drive signals 100, 102, 104 and 106 due to delays in the operation of the intervening circuitry including the comparator 60, the latch 62 and the gate drive controller 58. Consequently, an inductor 110 is placed in series in the output path 36, to time rate limit current in the output path. The inductance of the inductor 110 is chosen such that the rate of change of current in the output path 36 is _$_ sufficiently high to permit the inverter circuit to respond to changing load conditions, and sufficiently low to permit the switching stage current limiter to operate, before damaging alternating current is conducted in the output path 36 and ultimately conducted through the switching devices 18, 20, 22 and 24. In this embodiment, for an 800 to 1500 watt inverter circuit, an inductor having an inductance of approximately 25 microhenries is used to provide current limiting to approximately 10 amperes per microsecond.
Typically, the time for the intervening circuitry of the current condition determiner 56 and the gate drive controller 58 is approximately 1 or 2 microseconds.
Referring to Figure 4, in an alternative embodiment, the current sensor 42 may be replaced by resistors 120, 122, 124 and 126 in series between respective switching devices 18, 20, 22 and 24 and the switching stage output 26. In this case, individual currents through respective individual resistors 120, 122, 124 and 126 may be monitored and compared with four separate comparators of the type shown at 60 in Figure 3, wired in a logical OR
relation, to produce the active low danger signal which is supplied to the latch 62.
It will be appreciated that with an embodiment described herein the power drawn from the output 38 of the inverter circuit is typically limited by the power supply capacity of the inverter circuit and is usually limited to a significantly low value, which will not burn out the switching devices 18, 20, 22 and 24.
However, if the output 38 is connected to an AC bus 40, the AC bus 40 may attempt to supply current into the inverter circuit, i.e., in a direction from the output port 38 to the switching stage 16. Since an AC bus 40, if connected to a utility supply, effectively acts as an infinite current source with zero impedance, extremely high currents can be received at the switching stage 16, burning out the switching components 18, 20, 22 and 24, especially during relay, transfer switch operation. However, using the embodiments indicated above, the inductor 110 limits the rate of rise of current flow from the output port 38 to the switching stage output port 26 and the current sensing device _g_ 42 senses backfeed current which is outside a predefined range such as provided by the comparator 60, and causes the gate drive controller 58 to place the switching devices 18, 20, 22 and 24 in the non-conducting mode, whereby they are all placed in a high impedance state and inoperable to conduct current. Thus, AC power backfeed is prevented from damaging the switching devices 18, 20, 22 and 24.
While specific embodiments of the invention have been described and illustrated, such embodiments should be considered illustrative of the invention only and not as limiting the invention as construed in accordance with the accompanying claims.
Claims (33)
1. A method of preventing power backfeed into an inverter circuit having a switching stage for supplying AC power to an output path of the inverter circuit, the method comprising:
a) sensing a current condition in the output path; and b) limiting current in said switching stage when said current condition meets a criterion.
a) sensing a current condition in the output path; and b) limiting current in said switching stage when said current condition meets a criterion.
2. The method of claim 1 wherein sensing comprises sensing an instantaneous value of alternating current in the output path.
3. The method of claim 1 wherein sensing comprises inductively sensing current in the output path.
4. The method of claim 1 wherein sensing comprises developing a voltage representative of current in the output path.
5. The method of claim 1 wherein limiting comprises isolating said switching stage from the output path.
6. The method of claim 5 wherein isolating comprises placing switching devices in said switching stage in a non-conducting mode.
7. The method of claim 6 wherein placing switches in the switching stage in said non-conducting mode comprises disabling a gate drive signal from being received at said switching devices in said switching stage.
8. The method of claim 5 wherein limiting comprises comparing a sensed current condition with a reference to cause at least some of the switching devices to be placed in a non-conducting mode when an output of the current condition sensor is within a range.
9. The method of claim 8 further comprising developing a sense voltage in response to instantaneous current in the output path.
10. The method of claim 9 wherein developing further comprises adding a common mode voltage to said sense voltage.
11. The method of claim 10 further comprising causing at least some of the switching devices to be placed into the non-conducting mode when positive or negative excursions of the sense voltage exceed high and low reference voltages respectively.
12. The method of claim 11 further comprising holding said at least some of said switching devices in the non-conducting mode.
13. The method of claim 1 further comprising time rate limiting current in the output path.
14. The method of claim 13 wherein time rate limiting comprises passing current in the output path through an inductor.
15. The method of claim 13 wherein time rate limiting comprises time rate limiting said current in the output path to a value sufficiently high to permit the inverter circuit to respond to changing load conditions, while insufficiently low to permit AC backfeed protection circuitry to operate, before damaging alternating current is conducted by the switching stage.
16. An apparatus for preventing power backfeed into an inverter circuit having a switching stage for supplying AC power to an output path of the inverter circuit, the apparatus comprising:
a) a current sensor which senses a current condition in the output path; and b) a switching stage current limiter operable to limit current in the switching stage when said current condition meets a criterion.
a) a current sensor which senses a current condition in the output path; and b) a switching stage current limiter operable to limit current in the switching stage when said current condition meets a criterion.
17. The apparatus of claim 16 wherein said current sensor is operable to sense an instantaneous value of alternating current in the output path.
18. The apparatus of claim 16 wherein said current sensor comprises a current transformer in the output path.
19. The apparatus of claim 18 further comprising a resistor connected to said current transformer to produce a voltage representative of current in the output path.
20. The apparatus of claim 16 wherein said switching stage current limiter comprises switching devices in the switching stage.
21. The apparatus of claim 16 wherein said switching stage current limiter comprises a gate drive controller for controlling the transmission of gate drive signals from the inverter circuit to the switching devices in the switching stage, in response to said current condition.
22. The apparatus of claim 21 wherein said gate drive controller is operable to control said gate drive signals to cause at least some of the switching devices to be placed in a non-conducting mode.
23. The apparatus of claim 21 wherein said switching stage current limiter comprises a condition determiner for determining said current condition sensed by said current sensor.
24. The apparatus of claim 23 wherein said condition determiner comprises a condition comparator in communication with said gate drive controller for comparing an output of said current sensor with a reference to cause said gate drive controller to cause at least some of said switching devices to be placed in a non-conducting mode when said output of said current sensor is within a range.
25. The apparatus of claim 24 wherein said current sensor comprises a current to voltage converter operable to develop a sense voltage in response to instantaneous current in the output path.
26. The apparatus of claim 25 wherein said condition comparator further comprises a voltage summing circuit for adding a common mode voltage to said sense voltage.
27. The apparatus of claim 26 wherein said condition comparator further comprises first and second voltage comparators connected to cause said gate drive controller to place at least some of the switching devices into said non-conducting mode when positive or negative excursions of said sense voltage exceed high and low reference voltages respectively.
28. The apparatus of claim 27 wherein said condition determiner further comprises a latch activated by said condition comparator to cause said gate drive controller to hold said at least some of said switching devices in said non-conducting mode.
29. The apparatus of claim 16 further comprising a current time rate limiter for limiting alternating current in the output path.
30. The apparatus of claim 29 wherein said current time rate limiter is operable to time rate limit alternating current in the output path to a value sufficiently high to permit the inverter circuit to respond to changing load conditions, and sufficiently low to permit said switching stage current limiter to operate, before damaging alternating current is conducted in the output path.
31. The apparatus of claim 29 wherein said time rate limiter comprises an inductor.
32. The apparatus of claim 16 wherein said current sensor comprises resistors in series with respective switching devices of said switching stage.
33. An apparatus for preventing alternating current backfeed into an inverter circuit having a switching stage for supplying AC power to an output path of the inverter circuit, the apparatus comprising:
a) means for sensing a current condition in the output path; and b) means for limiting current in said switching stage when said current condition meets a criterion.
a) means for sensing a current condition in the output path; and b) means for limiting current in said switching stage when said current condition meets a criterion.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002314323A CA2314323A1 (en) | 2000-07-21 | 2000-07-21 | Ac power backfeed protection for inverter circuits |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002314323A CA2314323A1 (en) | 2000-07-21 | 2000-07-21 | Ac power backfeed protection for inverter circuits |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2314323A1 true CA2314323A1 (en) | 2002-01-21 |
Family
ID=4166756
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002314323A Abandoned CA2314323A1 (en) | 2000-07-21 | 2000-07-21 | Ac power backfeed protection for inverter circuits |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2314323A1 (en) |
-
2000
- 2000-07-21 CA CA002314323A patent/CA2314323A1/en not_active Abandoned
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