Abstract:
A direct current (dc) overvoltage, pre-regulation circuit regulates dc voltage supplied to a cable television line amplifier. The circuit provides overvoltage protection while permitting continuous operation of the line amplifier. The circuit opens the input to the downstream continuous voltage regulation circuit and cyclically charges a filter storage capacitor by periodic applications of the un-clipped voltage. The repeated switching regulates the dc voltage such that operation is sustained during periods of overvoltage that would normally shut down conventional circuits. No overall feedback is required to control the active device.

Description:
This application is a continuation, of application Ser. No. 08/392,362, filed Feb. 22, 1995, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to devices for regulating voltage. In particular, the present invention pertains to a device which pre-regulates voltage from a dc voltage source before a first stage filter. More particularly, the present invention is directed to a device which pre-regulates voltage to the power supply of a cable television radio frequency (RF) line amplifier to permit uninterrupted operation during mains ac overvoltage conditions. 
     BACKGROUND OF THE INVENTION 
     Electric utility companies have generally provided consumers with a reliable source of electrical power to meet their demands. However, utilities cannot guarantee that the voltage of the power supplied will remain constant as it is distributed over the electrical distribution network. The line voltage may exhibit variations due to a variety of causes. Consumer demand may degrade the voltage across the entire electrical grid, as experienced during a brownout. Energization and deenergization of electrical equipment may also cause fluctuations in voltage. Portions of the grid are frequently subject to electrical transients caused by lightening strikes, fallen power lines and other electrical faults. 
     Electricity output from utility generating stations is high-voltage, three-phase alternating current, where a 120° angular relationship is maintained between each phase. The electrical distribution system maintains the three-phase configuration until lower voltage single-phase power is required. The voltage is reduced by transformers placed throughout the electrical distribution system. 
     One method employed to reduce three-phase voltage levels is by using a Delta to Y (A-Y) transformer which creates a common neutral and ground between all three phases. Electrical loads placed on a three-phase system must be balanced with regard to inductive, capacitive, and resistive characteristics for each individual phase. When the respective loads are balanced, ground path currents are low. If one or more phases of a three-phase system are open or short circuited, or degraded, the result is a phase-to-phase imbalance which elevates currents in the ground path. The current-resistance (IR) drop through the ground conductor will manifest itself as an increase in the potential difference between the normal ground potential and the supply voltage, thus appearing as an overvoltage condition. 
     A ground conductor experiencing fault currents tied to a system neutral will impress the resulting overvoltage condition on the neutral conductor. The overvoltage condition will be experienced by devices connected to the neutral conductor in close proximity to the fault. 
     Cable television line amplifiers are suspended by the signal carrying coaxial cable support strand between telephone poles and are powered from the signal coax. Typically, the common ground path used by the utility is tied to the outer cable sheath that also serves as the neutral conductor for the cable television company. A ground fault in close proximity to the ground-neutral common connection elevates the neutral conductor potential for a distance from that fault location until the energy sufficiently dissipates. The overvoltage is manifest between the center conductor and shield of the coaxial cable. This overvoltage can persist up to a ten pole distance on either side of the fault location. 
     Overvoltage protection devices currently utilized within line amplifier power supplies isolate the power supply during the overvoltage condition to prevent damage to the amplifiers. Prior art overvoltage protection circuits either open the circuit, clamp the output of the power supply to a safe level, or crowbar the ac input by placing a low-voltage short circuit across the input of the power supply while the overvoltage persists thereby providing protection. During the operation of overvoltage protection devices, downstream circuitry within an electronic device is removed from the current path or shunted, thereby interrupting operation of the electronic device. 
     FIG. 2, shows a prior art switching voltage regulator. A voltage regulator delivers a constant output voltage even though the input voltage to the circuit and current drawn from the regulator may vary. A N-channel depletion MOSFET (metal-oxide semiconductor field-effect transistor) 135 provides the current switching action. Resistors 150, 155 and comparator 145 provide the feedback signal from the output of the voltage regulator. A reference voltage is compared to the feedback voltage and an error signal is outputted to oscillator 140, which adjusts the switching rate or duty cycle of the regulator to conform to the voltage reference signal. The circuit continuously regulates the input voltage to that of the reference, however, no overvoltage protection is provided. 
     FIG. 3 is an overvoltage clamping circuit which is well known in the prior art. The active element is a Zener diode 160 in series with current limiting resistor 165. This combination determines the overvoltage at which the circuit activates. As the potential difference across terminals 170 and 180 increases above the Zener breakdown voltage of Zener diode 160, current will flow and turn-on npn pass transistor 175, thereby shunting and dissipating the energy between terminals 170 and 180. Although the &#34;clamping&#34; action provides the overvoltage protection, the downstream electronic device will be inoperable for the duration of the overvoltage condition. 
     Although brief interruptions may be acceptable for cable television systems which provide only entertainment services, cable television systems have been increasingly used for life-saving services and critical information exchanges. Cable television system interruptions, therefore, are no longer tolerable. Accordingly, there is a need for an overvoltage protection circuit which permits continuous operation of the downstream electronic device while providing adequate protection during an overvoltage event. 
     SUMMARY OF THE INVENTION 
     The present invention provides a direct current (dc) overvoltage, pre-regulation circuit that regulates dc voltage supplied to a cable television line amplifier. The invention utilizes an overvoltage regulation means in combination with a switching regulator means to provide overvoltage protection at considerably higher voltage levels while permitting continuous operation of the line amplifiers. The circuit operates by opening the input to the downstream continuous voltage regulation circuit and cyclically charging a filter storage capacitor by periodic applications of the un-clipped voltage during an overvoltage event. The filter capacitor is part of the continuous voltage regulation circuit and becomes the voltage source to the downstream circuitry between full-wave rectification peaks. Due to full-wave rectification, the cyclic charging rate is double the line frequency during the overvoltage event. No overall feedback is required to control the active device. The repeated switching of the current regulates the dc voltage such that operation is sustained during periods of overvoltage that would normally shut down conventional circuits. 
     Accordingly, it is an object of the present invention to provide means for pre-regulating a power supply during an overvoltage condition to allow continuous operation of the line amplifier. 
     It is a further object of the invention to provide an inexpensive and simple means for pre-regulating the dc voltage of a power supply during extreme and continuous overvoltage durations. 
     Further objects and advantages of the invention will become apparent to those of ordinary skill in the art after reading the detailed description of the preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the telephone pole mounted cable television components; 
     FIG. 2 is a simplified electrical schematic of a prior art switching regulator; 
     FIG. 3 is a simplified electrical schematic of a prior art overvoltage clamp circuit; 
     FIG. 4A is a graph of the single-phase voltage supplied from the utility; 
     FIG. 4B is a graph of the quasi-square wave voltage output from a ferroresonant transformer; 
     FIG. 4C is a graph of the voltage output from the full-wave rectifier; 
     FIG. 4D is a graph of the voltage across the capacitor during normal voltage operation; 
     FIG. 4E is a graph of the voltage across the capacitor during overvoltage conditions; 
     FIG. 5 is a simplified electrical schematic of a prior art direct current power supply; 
     FIG. 6 is a block diagram of the present invention used in a typical application; and 
     FIG. 7 is an electrical plan of the adaptive power direct current pre-regulator. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A cable television (CATV) communication system 1 utilizing the present invention is shown in FIG. 1. Three high tension conductors 111, 113, 117 carry three-phase high-voltage power from the electric utility to remote consumers. Line conductor 110 supplies single-phase 120 Vac line voltage to local consumers. Neutral conductor 112 provides the return path and connection to the utility ground. The 120 Vac line voltage 110, as shown in FIG. 4A, is a 60 cycle sinusoid. The voltage is reduced and regulated by means of a pole-mounted, ferroresonant voltage regulating transformer 115, which outputs 60 Vac 60 cycle quasi-square wave and can source up to 15 Amperes of current as shown in FIG. 4B. Referring again to FIG. 1, the reduced and regulated ac voltage is inserted in the cable television signal carrying coaxial cable 125 via cable television power inserter 120. The single-phase line conductor 110 in conjunction with neutral conductor 112 supply power to the CATV communication system 100. 
     The coaxial cable 125 supports communications between the headend of the CATV communication system 100 and a plurality of subscribers by transmitting the RF signals. Since the RF signals within the coaxial cable 125 become attenuated over long distances, CATV line amplifiers 130 must be inserted at specific locations within the CATV communication system 100 to maintain minimum signal levels. 
     Referring to FIG. 6, a 60 Vac 60 cycle quasi-square wave is imposed on the RF signal conductor 10. Line amplifier 130 first separates the RF signal and 60 Vac with the ac power combiner 15. With the ac voltage component removed, the RF signal 35 can be amplified by the line amplifier. A suitable line amplifier for this application is Model Number BLE-750 series manufactured by General Instrument Corporation. 
     The 60 Vac is full-wave rectified by rectifier 20 and is then pre-regulated by the pre-regulator 25 of the present invention. After pre-regulation, the voltage is applied to the filter storage capacitor 30 for further voltage regulation and reduction by the line amplifier 130. 
     A typical cable television line amplifier dc power supply is shown in FIG. 5. The ac voltage, as shown in FIG. 4B, is applied to the terminals of a full-wave bridge rectifier 20 comprised of four rectifiers. The output is full-wave rectified dc as shown in FIG. 4C. 
     The unfiltered output voltage fluctuates about an average value as the successive pulses of energy determined by the line frequency are delivered to the load. The output of the rectifier is composed of a direct voltage component and an alternating or ripple voltage component. The frequency of the main component of the ripple for the full-wave rectifier shown in FIG. 4C, is twice the frequency of the voltage that is being rectified, in this case 120 cycles. This pulsating voltage is applied to a filter storage capacitor which is charged to the peak voltage of the rectifier within a few cycles. The charge on the capacitor represents a storage of energy, and consequently the amplitude of the ripple is greatly reduced. At this point, the voltage across capacitor 30 is stabilized, shown in FIG. 4D. Although the power supply of FIG. 5 is full-wave rectified, it does not provide overvoltage protection. 
     Referring to FIG. 7, the preferred embodiment of the adaptive power pre-regulator 25 is shown. The pre-regulator 25 is located within a power supply with an input from a full-wave bridge rectifier and an output to a filter storage capacitor. The pre-regulator 25 includes two transistors, Q1 and Q2. Transistor Q2 is an N-channel enhancement power MOSFET with the source 105 connected to the negative leg of the full wave rectifier 20 and the drain 100 connected to the negative terminal of filter storage capacitor 30. An LED (light emitting diode) D4 is driven by a high input impedance voltage comparator 43 connected across the source 105 and drain 100 of transistor Q2. Under normal voltage conditions, the transistor Q1 is held in a state of conduction by a bias circuit comprised of a current limiting resistor 75 and a Zener diode D2 in a shunt regulator configuration. Resistor 75 and diode D2 are connected in series, with one side of resistor 75 connected to the positive leg of the full wave rectifier 20 and the other side of resistor 75 connected to the cathode 85 of diode D2. The anode 90 of diode D2 is connected to the negative leg of the full-wave rectifier 20. The common electrical node 80 between resistor 75 and diode D2 is connected to the gate 95 of transistor Q2. This combination allows a constant voltage to be impressed on the gate 95 of transistor Q2. 
     Transistor Q2 is controlled by a small signal, npn transistor Q1. Transistor Q1 is controlled by Zener diode D1 and a voltage divider comprising two resistors 40, 45 that monitor the voltage across storage capacitor 30. The resistors 40, 45 are connected in series across the output of the full-wave rectifier 20. The cathode 50 of diode D1 is connected to the common electrical node between resistors 40, 45. The anode 55 of diode D1 is connected to one side of a base bias voltage divider comprising resistors 42. Resistors 41 and 42 are connected in series between anode 55 of diode D1 and the negative leg of full-wave rectifier 20. The base 60 of transistor Q1 and the cathode of protection diode D3 are connected to the common electrical node between resistors 41, 42. The anode of protection diode D3 and emitter 70 of transistor Q1 are connected to the negative leg of full-wave rectifier 20. The collector 65 of transistor Q1 is connected to the common electrical node 80 of resistor 75, diode D2 and gate 95 of transistor Q2. The component values of the preferred embodiment are shown in Table 1. 
     
                       TABLE 1______________________________________COMPONENT       SPECIFICATIONS______________________________________D1              5.1 Volt, 1 Watt ZenerD2              18 Volt, 1 Watt ZenerD3              1n4148D4              2mA HLMP-3750Q1              IRF840 N-channel MOSFETQ2              2n3904 npn switching transistor40              160 kΩ, 2 Watt41              1 kΩ, 1/2 Watt42              10 kΩ, 1/2 Watt45              6.8 kΩ, 1/2 Watt75              150 kΩ, 2 Watt______________________________________ 
    
     Under normal voltage conditions, as shown in FIG. 4D, the voltage drop across resistor 45 is not enough to allow current to flow through diode D1 and across the base 60 emitter 65 junction of transistor Q1. Therefore, transistor Q1 remains turned-off. The voltage at node 80 is sufficient to keep Q2 turned-on. Since the potential difference across source 105 and drain 100 is near zero when transistor Q2 is turned-on, voltage comparator 43 does not illuminate LED D4. 
     During an overvoltage event, as shown in FIG. 4E, the overvoltage threshold value as determined by voltage divider resistors 40 and 45, and diode D1 is exceeded. When 6 Volts are dropped across resistor 45 as set by the Zener breakdown voltage value of diode D1, current flows through diode D1, through voltage divider resistor 41 turning on transistor Q2. The current flowing across the collector 65 emitter 70 junction thereby shunts diode D2 and turns-off transistor Q2. When transistor Q2 is turned-off, the overvoltage impressed on the input of the pre-regulator 25 is isolated from the output of the pre-regulator 25. Voltage comparator 43 senses the potential difference across source 105 and drain 100 when transistor Q2 is turned-off and in turn illuminates LED D4. The input to the pre-regulator 25 experiences a full-wave rectification waveform greater than the overvoltage threshold value. The pre-regulator 25 &#34;switches&#34;, and thereby limits, the voltage as shown in FIG. 4E, which is output to storage capacitor 30 and the remainder of the electronic device. When the input voltage decreases in magnitude below the threshold value, transistor Q1 is turned-off and normal voltage operation of the circuit resumes. As the voltage increases again during the next cycle, the pre-regulation circuit is activated. When the pre-regulation circuit is active, the LED D4 illuminates, indicating that the line amplifier is experiencing an overvoltage condition. It should be apparent to those skilled in the art that the adaptive power direct current pre-regulator of the present invention provides a simple and inexpensive pre-regulating circuit. The pre-regulator performs both voltage regulation and over-voltage protection to permit continuous operation of the downstream electronic device, thereby providing distinct advantages over prior art devices. 
     The function of voltage comparator 43 and the LED D4 is to indicate that potentially lethal voltages exist at the input to the pre-regulator. Both components are not needed for the pre-regulator circuit to operate. Alternative embodiments of the present invention can have the overvoltage indicator placed at the input side of the circuit. 
     It should also be apparent to those skilled in the art that the adaptive power pre-regulator of the present invention is not limited to applications within the CATV industry. The invention may be utilized in any dc circuit to provide voltage regulation and overvoltage protection for downstream electronics. For example, the pre-regulator may be used in television sets, computer monitors, video tape recorders and other sensitive electronic equipment that would be damaged by extreme overvoltage conditions. 
     Although the invention has been described in part by making detailed reference to certain specific embodiments, such detail is intended to be instructive rather than restrictive. It will be appreciated by those skilled in the art that many variations may be made in the structure and mode of operation without departing from the spirit and scope of the invention as disclosed in the teachings herein.