Patent Publication Number: US-2007114981-A1

Title: Switching power supply system with pre-regulator for circuit or personnel protection devices

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to a power supply system, and more particularly to a switching power supply system with a pre-regulator for a circuit or personnel protection device.  
      2. Background of the Invention  
      Current power supply systems for circuit and personnel protection devices use pre-regulator/regulator IC combinations that include multiple inductors, or large value/package inductors, transformers or capacitors. Such systems have relatively low power efficiencies, e.g., more than 1 Watt @40 mA, and a relatively narrow range of voltage inputs. The current systems also require a relatively large amount of printed circuit board space and relatively high parts count, trace runs, etc.  
     SUMMARY OF THE INVENTION  
      The present invention provides a switching power supply system for a circuit or personnel protection device. The system comprises an input stage for outputting a rectified signal, a switching regulator for outputting a DC output signal, a pre-regulator in cascade between the input stage and the switching regulator, and an output stage for filtering noise from the DC output signal. The pre-regulator includes a transistor for providing variable resistance, and a control part for maintaining a substantially fixed voltage at a gate of the transistor.  
      In one embodiment, the rectified signal from the input stage is linearly pre-regulated with a series variable resistance that decreases in response to increases in current drawn by the switching regulator and increases in response to decreases in the current drawn.  
      In a preferred embodiment, the transistor has a drain connected to a storage component such as a capacitor that stores energy to be used during the transistor&#39;s off-to-on transition, so as to reduce surge current switching noise effects on the AC input voltage. A gate control element such as a zener diode may be used to prevent the transistor&#39;s gate-to-source voltage from exceeding its maximum peak voltage during the transistor&#39;s switched-on transitions.  
      It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an exemplary block diagram representation of a switching power supply system according to an embodiment of the present invention.  
       FIG. 2  is a schematic diagram of the switching power supply system shown in  FIG. 1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 1  is an exemplary block diagram of a switching power supply system according to an embodiment of the present invention. Referring to  FIG. 1 , the switching power supply system  300  includes an input stage  320 , a linear pre-regulator  340 , a switching regulator  360 , and an output stage  380 . An alternating current (AC) power source  310  can be electrically connected to the input stage  320  for providing an input voltage to the switching power supply system  300 . For example, the AC power source  310  may provide a standard electrical signal having a 60 Hz frequency and a 120V AC voltage. A load  390  can be connected to an output of the output stage  380  to be powered by the switching power supply system  300 . For example, the load  390  can have a resistance of 165Ω and draw a current of about 40 mA. The input stage  320  provides a rectified voltage to an input of the linear pre-regulator  340 . Thus, the input stage  320  provides power to the following stages of the switching power supply system  300 , for example, the linear pre-regulator  340 , the switching regulator  360 , and the output stage  380 . In addition, the input stage  320  can control the linear pre-regulator  340 .  
       FIG. 2  is an exemplary schematic circuit diagram of the switching power supply system of  FIG. 1 . Referring to  FIG. 2 , the AC voltage received at the input of the input stage  320  can be rectified by a bridge rectifier CR 4 . The rectified voltage can be a pulsating direct current (DC) waveform having a frequency of 120 Hz and a peak voltage of 170V. The input stage  320  can include a trip solenoid L 1  in series with the bridge rectifier CR 4 . A varistor RV 1  can be electrically connected between input terminals of the bridge rectifier CR 4 . A capacitor C 21  can be provided at the output of the bridge rectifier CR 4  to suppress voltage transients at the output of the input stage  320 .  
      In an embodiment of the present invention, the bridge rectifier CR 4  is selected to provide a maximum peak reverse voltage of 187V, a continuous reverse current of 25 mA, a 3.5 A rms forward current during a trip operation, and a peak surge of 350V. The selected trip solenoid L 1  can have an internal inductance of 9 mH and an internal resistance of 48 ohm. The varistor RV 1  can have a standoff rating of 150V rms and a voltage clamping of 350V. The capacitor C 21  can have a capacitance of 0.01 μF and is selected is to provide a frequency rolloff at 17 kHz in conjunction with the trip solenoid L 1 .  
      The linear pre-regulator  340  provides linear step-down regulation of the voltage to be applied at the input of the switching regulator  360 . The pre-regulator  340  is connected in cascade between the input stage  320  and the switching regulator  360 . The pre-regulator  340  can include, for example, an N-channel enhancement MOSFET series-pass transistor Q 2 . A resistor R 26  and a zener diode CR 5  can provide gate control for the transistor Q 2 . A current limiting resistor R 27  and a storage capacitor C 23  can also be provided. The linear pre-regulator can also include a second zener diode CR 7 .  
      As shown in  FIG. 2 , the transistor Q 2  can be connected in a source follower configuration. The zener diode CR 5  and the resistor R 26 , which are connected in series with each other, provide gate control for the transistor Q 2 . The zener diode CR 5  provides an electrical path between a gate of the transistor Q 2  and a ground of the switching power supply system  300 . The resistor R 26  provides an electrical path between the gate G of the transistor Q 2  and the output of the input stage  320 . Thus, the zener diode CR 5  and the resistor R 6  maintain the transistor Q 2  in a partially-on linear mode. Hence, the transistor Q 2  operates more efficiently.  
      The transistor Q 2  acts as a variable resistor controlled by the gate controller, which includes the zener diode CR 5  and the resistor R 26 . When a current from the switching power source  300  into the load  390  increases, the source voltage at the input of the pre-regulator  340  decreases. However, the zener diode CR 5  maintains a constant voltage at the gate G of the transistor Q 2 . Hence, the drop in the source voltage at the input of the pre-regulator  340  causes an increase in a forward bias current of the transistor Q 2 . Thus, a drain-to-source resistance of the transistor Q 2  also decreases. The decrease in drain-to-source resistance causes a corresponding increase in the source voltage at the input of the pre-regulator  340  back to its original value. When the current from the switching power source  300  into the load  390  decreases, the source voltage at the input of the pre-regulator  340  increases. However, the zener diode CR 5  maintains a constant voltage at the gate G of the transistor Q 2 . Hence, the increase in the source voltage at the input of the pre-regulator  340  causes a decrease in a forward bias current of the transistor Q 2 . Thus, a drain-to-source resistance of the transistor Q 2  also increases. The increase in drain-to-source resistance causes a corresponding decrease in the source voltage at the input of the pre-regulator  340  down to its original value.  
      As shown in  FIG. 2 , the current limiting resistor R 27  electrically connects a drain D of the transistor Q 2  to the output of the input stage  320 . Resistor R 27  limits the source-to-drain current within the transistor Q 2  and the current drawn by the switching regulator  360  at the output S of the pre-regulator  340 .  
      The storage capacitor C 23  electrically connects the drain D of the transistor Q 2  to ground. The capacitor C 23  provides energy storage capability to the pre-regulator  340 . For example, the capacitor C 23  stores energy to be used by the transistor Q 2  during an initial off-to-on transition. The initial OFF-to-ON transition of the transistor Q 2  can occur once every 120 Hz cycle, for example. The capacitor C 23  also provides filtering capability to reduce a surge current switching noise caused by the power supply system  300 . Thus, the power supply system can quickly start operating after the initial OFF-to-ON transition.  
      A second zener diode CR 7  can be provided between the source S and the gate G of the transistor Q 2 . The zener diode CR 7  limits the gate-to-source voltage of the transistor Q 2  when the transistor Q 2  is switched on. For example, the zener diode CR 7  can prevent the OFF-to-ON transition gate-to-source voltage from exceeding a maximum peak voltage of the transistor Q 2 .  
      In one embodiment, the MOSFET transistor Q 2  is selected to provide a continuous drain current of about 140 mA, a maximum drain current of about 600 mA, a drain-to-source voltage of about 450V, and a gate-to-source voltage of about +/−20V max. The maximum power output by the MOSFET transistor Q 2  is about 2 W. The gate voltage limiting diode CR 5  provides a zener voltage in a range of about 78 V to about 86 V. The zener diode CR 5  can provide a voltage of about 82V at the gate of the transistor Q 2 , and the resistor R 26  can have a resistance of about 249 kilohms. The current-limiting resistor R 27  can have a resistance of about 1.5 Kilohms. The capacitor C 23  can have a capacitance of about 0.056° F. for effective high frequency filtering. The zener diode CR 7  provides voltage limiting capability to a value of about 18V, well below the 20V peak gate-to-source voltage of the transistor Q 2 . Hence, the pre-regulator  340  maintains a voltage of about 80V at its output S.  
      The switching regulator  360  takes as input the regulated voltage generated by the pre-regulator  340  and outputs a desired voltage to power the load  390 . For example, the switching regulator converts the 80VDC supplied by the pre-regulator  340  and outputs a 5VDC voltage. In one embodiment, the switching regulator  360  includes an IC U 2 , which can be a step-down DC/DC buck bias switching regulator, such as the prepackaged National Semiconductor LM5008. The IC U 2  converts the regulated input voltage to a low output voltage. The switching regulator  360  includes additional circuitry to interface the IC U 2  with the pre-regulator  340  and the output stage  380 .  
      Referring to  FIG. 2 , the output of the pre-regulator  340  is electrically connected to an input Vin of the IC U 2 . A capacitor C 14  is provided at an input Vin of the IC U 2  to attenuate voltage transients and noise. Another capacitor C 18  is provided at the input Vin for supplying a final output switched current during an ON time of the IC U 2  and to limit a voltage ripple at the input Vin. An on-time resistor R 20  is provided between inputs Vin and Ron of the IC U 2  to set a switching on-time of the IC U 2  to an appropriate value. The on-time is inversely proportional to the input voltage and provides hysteretic control for the IC U 2 . A current-limiting resistor R 24  is provided between an output Rcl of the IC U 2  and the ground to set a minimum forced OFF time.  
      A capacitor C 16  is provided to filter and stabilize an internal power supply Vcc of the IC U 2 . A capacitor C 25  is electrically connected between outputs BST and SW of the IC U 2  to provide a surge current for charging a switch gate at turn-on. A re-circulating or catch diode CR 9  provides a path from the output SW of the IC U 2  to the ground.  
      An inductor L 2  electrically connects the output of the IC U 2  to the output of the switching regulator  360 . A filter capacitor C 20  is provided at the output of the switching regulator  340  to filter noise from the DC output. The capacitance and equivalent series resistance (ESR) of the capacitor C 20  generate a feedback ripple voltage for the switching IC U 2 . The feedback ripple voltage can have a peak-to-peak value of about 50 mV, for example.  
      Voltage-divider resistors R 21  and R 3  are provided to regulate the low voltage output of the switching regulator  360 . The resistors R 21  and R 23  are electrically connected in series with each other to provide a path from the output of the switching regulator  360  to ground. The voltage across resistor R 23  is applied as a feedback signal to an input FB of the IC U 2 .  
      During the on-time of the IC U 2 , an inductor current ramps up linearly to charge the inductor L 2 . Thus, the inductor L 2  stores energy during the on-time of the IC U 2 . During the OFF time of the IC U 2 , the catch diode CR 9  becomes forward-biased and directs a local off-time current loop to the load  390  through an inductor L 2 . Thus, the inductor current is drained during the off-time of the IC U 2 .  
      The IC U 2  can sustain a maximum input voltage of about 100V and a maximum input current of 610 mA and can operate in a range of about −40 to 125° C. The on-time resistor R 20  can be selected to set the switching on-time of the IC U 2  to about 400 ns with a time-off value of about 4.6 μs. The minimum forced OFF time of the IC U 2  is set to about 500 ns with an on-time of 4.5 ns. The internal power supply Vcc can have a DC voltage output of about 7V. The inductor L 2  and the capacitor C 20  are selected to provide a discontinuous switching frequency in a range of about 150 KHz to about 220 KHz. Thus, the switching regulator  360  converts the DC voltage of about 80V supplied by the pre-regulator  340  to a DC output of about 5V by supplying a 200 KHz/80V-peak pulse to the 220 μH inductor, which stores and releases energy during ON and OFF times of the IC U 2 , respectively.  
      In one embodiment, the inductor L 2  can have an inductance of about 220 μH and the capacitor C 20  can have a capacitance of about 10 μF. The filtering capacitor C 14  can have a capacitance of about 0.1 μF. The capacitor C 18  can have a capacitance of about 1.0 μF. The resistor R 20  can have a resistance of about 255 KΩ for setting the switching on-time of the IC U 2 . The ON time is inversely proportional to the input voltage and provides hysteretic control for the IC U 2 . The current-limiting resistor R 24  can have a resistance of about 20 KΩ for setting the minimum forced OFF time. The filtering capacitor C 16  can have capacitance of about 0.1 μF. The capacitor C 25  can have a capacitance of about 0.01 μF. Voltage-divider resistors R 21  and R 23  can have values of 1.62 KΩ and 1 KΩ, respectively.  
      Still referring to  FIG. 2 , the output stage  380  includes a resistor R 18  and a capacitor C 24  to provide RC-filtering of any noise frequencies caused by the switching regulator  360 . The capacitor C 24  provides a path from the output of the output stage  380  to ground. The capacitor C 24  can have a capacitance value of 0.001 μF. The resistor R 18  electrically connects the switching regulator  360  to the output of the output stage  380 . The RC filter formed by resistor R 18  and capacitor C 24  filters noise frequencies above a specified frequency, for example above 5 MHz. The resistor R 18  can have a resistance of about 1.31Ω. A zener diode CR 3  can be provided in parallel with the capacitor C 24  at the output of the output stage  380 . The zener diode CR 3  limits the output voltage to a maximum voltage, for example 6V peak.  
      In one particular application, the load  390  in the system of  FIG. 2  is an application specific integrated circuit (ASIC) that is described in copending U.S. application Ser. No. 09/026,556, filed Feb. 19, 1998, entitled “Electrical Fault Detection System,” owned by the assignee of the present application and incorporated herein by reference. The output from the switching power supply system  300  is electrically connected to the high positive ASIC supply voltage input pin VSUP of the ASIC. A capacitor can be provided between the input terminal VSUP and ground to filter out unwanted signals, such as a noise signal.  
      When a trip decision is reached by the ASIC, a trip signal buffer latches and drives the gate of a silicon controlled rectifier (SCR  98  in the copending application) having its anode connected to the output of the diode bridge CR 4 . In the ON state, the SCR causes the coil L 1  (coil  100  in the copending application) to be momentarily shorted across the line to mechanically de-latch the contacts of the host device and to subsequently interrupt flow of current.  
      According to embodiments of the present invention, a wide range switching power supply system having high efficiency and a quick start capability is provided by combining a linear pre-regulator having a gate control together with a switching regulator. Embodiments of the present invention can be implemented as an ASIC that would require limited external components. The DC output voltage provided by the linear pre-regulator can be adjusted to a desired value by appropriate selection of the zener diode in the gate control part. Other components, like resistors and capacitors, can be selected to adjust the final DC output voltage, the switching characteristics, and the load current capability of the switching power supply system. In contrast to the related art power supplies, the switching power supply system according to embodiments of the present invention does not require or use multiple inductors or large value inductors, transformers or capacitors.  
      It will be apparent to those skilled in the art that various modifications and variations can be made in the switching power supply system with pre-regulator for circuit or personnel protection devices of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.