Patent Application
20120243136
This invention relates generally to surge protection arrangements in electrical circuits.
The electrical efficiency of switch-mode power electronics generally decreases as the voltage rating of the semiconductor devices increases, for given current ratings. Semiconductor devices, such as diodes and transistors, with increased voltage ratings also cost significantly more than lower voltage rated devices. Therefore, it is desirable to use lower voltage rated devices whenever possible. However, lower voltage rated devices are much more susceptible surges or overvoltage conditions. Simulated lightning surge tests, required by product safety certification standards, impose high voltage stresses on semiconductor-based power converters. The higher stresses tend to increase the required voltage rating of semiconductor devices, resulting in lower efficiency and higher costs.
Accordingly, there is a need for cost effective surge protection arrangements that limit surge stress on semiconductor devices, thereby permitting greater use of lower voltage rated devices and their associated efficiency/cost benefits.
The above-mentioned need is met by the present invention, one embodiment of which provides a surge protection arrangement for protecting against an overvoltage condition in an electrical circuit having a power source and an electrical load. The surge protection arrangement includes a reactive component (such as a capacitor or an inductor) and a switching circuit connected to the reactive component. The switching circuit causes surge energy to flow to the reactive component in response to an overvoltage condition in the electrical circuit so that the surge energy is stored in the reactive component.
Referring to the drawings wherein identical reference numerals denote like elements throughout the various views,
The switching circuit 18 includes a series enabling device 20 and a control circuit 22 that activates the series enabling device 20 in response to an overvoltage condition in the electrical circuit 10. The series enabling device 20 is connected in series with the capacitor 16, and the capacitor 16 and the series enabling device 20 are connected in parallel with the electrical load 14.
The series enabling device 20 can be any device that is capable of being operated or controlled so as to conduct current when activated and to block current flow when not activated. Suitable series enabling devices include, but are not necessarily limited to, forward-conducting, forward-blocking devices such as IGBTs, MOSFETs and other transistors, forward-conducting, reverse- and forward-blocking devices such as thyristors, and bi-directional devices such as TRIACs and SIDACs. These types of devices are advantageous because they turn off when the current falls below their turn-off threshold.
During normal operating condition of the electrical circuit 10, the series enabling device 20 is turned off and the capacitor 16 is completely discharged. When the control circuit 22 detects a sudden increase in the magnitude of the line voltage Vline (i.e., an overvoltage condition), the series enabling device 20 is turned on. When the series enabling device 20 is activated, current flows to the capacitor 16 so that the surge energy is stored in the capacitor 16. Once the line voltage Vline returns to its normal operating limits, the series enabling device 20 is turned off and normal operation of the electrical circuit 10 resumes. A resistor 24 is connected in parallel across the capacitor 16 to discharge the capacitor 16 when the series enabling device 20 is turned off.
Turning to
The control circuit 22 includes a sensing sub-circuit that senses the line voltage Vline and scales down the sensed line voltage Vline to an appropriate level that can be handled and processed by the other components of the control circuit 22. The sensing sub-circuit comprises first and second resistors 28, 30 and includes a filtering capacitor 32 connected in parallel with the second resistor 30. The filtering capacitor 32 functions as a low pass filter that filters out noise. This helps prevent nuisance activations of the IGBT 20 that could otherwise be triggered by the noise.
The filtered, scaled-down signal 34 produced by the sensing sub-circuit is fed to a comparator 36. The comparator 36 compares this signal 34 to a reference signal 38. The reference signal 38 is produced by a sub-circuit comprising a Zener diode 40 and a resistor 42 connected in series to a positive supply voltage VDD. When the comparator 36 determines that the scaled signal 34 exceeds the reference signal 38, the comparator 36 activates the IGBT 20 via a driver 44.
Referring to
Like the control circuit of
The filtered, scaled-down signal 34 produced by the sensing sub-circuit is fed to an amplifier sub-circuit, which comprises an inverting amplifier 46, a pair of resistors 48, 50 and shift signal generator 52. The shift signal generator 52 comprises a Zener diode 54 and a resistor 56 connected in series to a positive supply voltage VDD, and produces a shift signal 58 that is input to the amplifier 46. The filtered, scaled-down signal 34 is rescaled and shifted by the amplifier sub-circuit, and the output 60 of the amplifier sub-circuit is used by first and second comparators 62 and 64 to detect if the magnitude of the line voltage Vline has exceeded some positive maximum or negative minimum threshold.
The first comparator 62 compares the amplifier output signal 60 to a first reference signal 66, and the second comparator 64 compares the amplifier output signal 60 to a second reference signal 68. The first and second reference signals 66, 68 are produced by a sub-circuit comprising a first Zener diode 70, a second Zener diode 72 and a resistor 74 connected in series to a positive supply voltage VDD. The first comparator 62 produces an output when it determines that the amplifier output signal 60 exceeds the first reference signal 66. The second comparator 64 produces an output when it determines that the amplifier output signal 60 falls below the second reference signal 68. The outputs of the first and second comparators 62, 64 are used as a logical sum in an OR gate 76, which is connected to the TRIAC 20 via a driver 78. The TRIAC 20 is thus activated whenever the amplifier output signal 60 exceeds the first reference signal 66 or falls below the second reference signal 68.
The switching circuit 118 includes a transistor 120 and a diode 122 connected in series between the power source 112 and the electrical load 114. A resistor 124 is connected in series with the inductor 116, with the inductor 116 and the resistor 124 being connected between the power source 112 and the electrical load 114 and in parallel with the transistor 120 and the diode 122. In the illustrated embodiment, the transistor 120 is an IGBT having its collector connected to the power source 112 and its emitter connected to the diode 122 so as to conduct current from the power source 112 to the diode 122 when activated. The diode 122 is biased to conduct current from the transistor 120 to the electrical load 114.
During normal operating condition of the electrical circuit 110, the voltage V1 at node 1 (adjacent the power source 112) is greater than the voltage V2 at node 2 (adjacent the electrical load 114), and power transfer occurs from node 1 to node 2 through the transistor 120 and the diode 122. Current through the inductor 116 is maintained at or near zero by actively controlling the transistor 120. For instance, the transistor 120 can be controlled in a manner similar to how the IGBT 20 of
Because the electrical circuit 210 is an ac circuit, the switching circuit 218 comprises two parallel branches: a first branch 226 for conducting current from the power source 212 to the electrical load 214 and a second branch 228 for conducting current from the electrical load 214 to the power source 212. The first branch 226 includes a first transistor 220 and a first diode 222 connected in series, and the second branch 228 includes a second transistor 221 and a second diode 223 connected in series. A resistor 224 is connected in series with the inductor 216, with the inductor 216 and the resistor 224 being connected between the power source 212 and the electrical load 214 and in parallel with the first and second branches 226, 228.
In the illustrated embodiment, the first and second transistors 220, 221 are IGBTs. The first transistor 220 has its collector connected to the power source 212 and its emitter connected to the first diode 222 so as to conduct current from the power source 212 to the first diode 222 when activated. The first diode 222 is biased to conduct current from the first transistor 220 to the electrical load 214. The second diode 223 is biased to conduct current from the electrical load 214 to the second transistor 221. The second transistor 221 has its collector connected to the second diode 223 and its emitter connected to the power source 212 so as to conduct current from the second diode 223 to power source 212 when activated.
During normal operating condition of the electrical circuit 210, the first and second transistors 220, 221 are kept on so that ac current is able to flow between the power source 212 and the electrical load 214. When an overvoltage condition occurs at node 1 or node 2, both the first and second transistors 220, 221 are turned off. Current thus flows between the power source 212 and the electrical load 214 through the inductor-resister branch, allowing the inductor 216 to store the surge energy. When the electrical circuit 210 returns to its normal operation, the switching circuit 218 is turned on again, thus freewheeling energy stored in the inductor 216 while carrying normal load current between the power source 212 and the electrical load 214.
While specific embodiments of the present invention have been described, it should be noted that various modifications thereto can be made without departing from the spirit and scope of the invention as defined in the appended claims.
