Abstract:
In a DC power supply or a battery charger, plural output filter capacitors remain highly charged after the load is removed and the converter is turned off. A transistor connected across the capacitors is non-conductive during normal power supply operation and a bleed resistor connecting the transistor to the output capacitors is not in the circuit during normal power supply operation. When the power supply is turned off and the load is removed, the transistor is automatically rendered conductive with the removal of the pulsed output of the power supply&#39;s power transformer to the transistor, with the energy stored in the capacitors safely and quickly discharged to the output return via the bleed resistor which is placed in circuit by the conducting transistor.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to power supplies and battery chargers and is particularly directed to an arrangement for automatically and safely discharging a high voltage charge stored in the output filter capacitors of a power supply to essentially zero potential when the load is removed or the power supply is turned off. 
   BACKGROUND OF THE INVENTION 
   Power supplies of various types are widely used in electronics and can be found in literally any electronic device. Many of these power supplies produce a high voltage output and are capable of driving hazardous voltages and current. Unless special provision is made, these power supplies can retain a large voltage in their output filter capacitors even when the power supply is turned off or the output load is removed. The energy stored in the output filter capacitors is given by the expression 
   
     
       
         
           
             
               
                 
                   Energy 
                   = 
                   
                     
                       CV 
                       2 
                     
                     2 
                   
                 
                 , 
                 
                     
                 
                 ⁢ 
                 where 
               
             
           
           
             
               
                 
                   C 
                   = 
                   
                     the 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     capacitance 
                   
                 
                 , 
                 
                     
                 
                 ⁢ 
                 and 
               
             
           
           
             
               
                 V 
                 = 
                 
                   the 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   output 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     voltage 
                     . 
                   
                 
               
             
           
         
       
     
   
   This energy is typically measured in tens of joules and can reside on unloaded electrolytic filter capacitors for hours or even days. This large residual charge at high voltage poses a significant hazard to service and operating personnel, as well as to the power supply itself and associated equipment. 
   One approach to resolving this problem is shown in the schematic diagram of  FIG. 1 , where a 120V AC input is provided via a switch  72  and power transformer  74  to a rectifying bridge  76  in a DC power supply  70 . The output filter capacitors are shown in simplified form as capacitor  78  connected across the power supply&#39;s output terminals  82   a  and  82   b . In this approach, a bleed resistor  80  is also connected across the output terminals  82   a ,  82   b  for dissipating the residual charge on the output capacitors. The primary problem with this approach is the presence of the bleed resistor  80  in the circuit during operation of the DC power supply  70 , resulting in substantial energy dissipation via the bleed resistor. 
   Another approach to discharging the output filter capacitors of a DC power supply  90  is shown in  FIG. 2 . In this approach, the otherwise-unused off-throw contacts  94   a  and  94   b  of a double pole double throw (DPDT) on/off power switch  92  are used to discharge the output filter capacitors, which are shown in  FIG. 2  as a single capacitor  100  for simplicity. An input current is provided via the DPDT on/off power switch  92  to a power transformer  102 , the output of which is rectified by a bridge  104 . When the DPDT on/off power switch  92  is moved to the off position, it establishes a discharge path  96  through first and second resistors  98   a ,  98   b  and the primary winding  102   a  of the power transformer  102 . The energy stored in the output filter capacitors  100  is rapidly dumped. In this approach, the first and second resistors  98   a ,  98   b  in the turn-off discharge path are only in the circuit when the DC power supply  90  is turned off, and thus do not reduce the efficiency of the power supply during operation. However, this approach requires a complicated switching arrangement at the input of the DC power supply. The approaches to discharging the power supply output filter capacitors of  FIGS. 1 and 2  are described in the Jul. 5, 2001 edition of Electronic Design News, in an article entitled “Quickly Discharge Power-Supply Capacitors”, by Stephen Woodward, page 132. 
   Other approaches to dissipating the charge on the output filter capacitors employ a manually operated switch for discharging this energy when the converter is turned off or the output load is removed. This latter approach is, of course, not automatic. Other approaches are automatic in operation, but require additional circuitry in the power supply, resulting in a more complicated arrangement and require circuitry for interfacing the power supply with the energy discharge circuit. Moreover, modern high power supply modules are hot unpluggable and therefore have no mechanical power switches. 
   The present invention addresses the aforementioned limitations of the prior art by providing a power supply with a device, which rapidly and automatically provides for the full discharge of energy stored in the power supply&#39;s output filter capacitors. The device includes a combination of a switching transistor and bleed resistor which are not in circuit during normal operation of the power supply, but are automatically switched in circuit when the power supply input is turned off and the output load is removed from the converter to fully discharge the output filter capacitors. The discharge circuit is integral with the power supply and does not itself reduce the efficiency of the converter during normal power supply operation. While disclosed primarily in terms of use in a soft switching power supply, i.e., where switching occurs at essentially zero voltage, the present invention is applicable for use in any type of switching and linear power supply. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to avoid hazardous operating conditions in a high voltage power supply or battery charger by quickly and automatically discharging residual high voltage charge. 
   It is another object of the present invention to provide for the automatic, safe and rapid discharge of output filter capacitors in a power supply when the load is removed and the power supply is turned off without increasing the load during normal power supply operation. 
   Yet another object of the present invention is to provide for the high voltage discharge of post rectification filter capacitors in a power supply at turn-off for improved human safety and equipment protection. 
   The present invention contemplates apparatus for converting a first DC or AC input voltage to a second DC output voltage. The apparatus comprises: a power transformer having primary and secondary windings, wherein an input alternating current is provided to the primary winding and an output alternating voltage is induced in the secondary winding; a rectifier circuit coupled to the secondary winding for converting the output alternating voltage to a DC output voltage waveform; output capacitors and a bleed resistor forming an output filter coupled between the rectifier circuit and output terminals of the apparatus for filtering the DC output voltage pulses prior to providing the DC output voltage pulses via the output terminals to a DC load, wherein the output capacitors are charged to a high voltage by the DC output voltage pulses; and a transistor switch connecting the output capacitors and bleed resistor to an output return, wherein the transistor switch is non-conductive during normal operation of the apparatus and the transistor switch is rendered conductive for automatically discharging the output capacitors via the bleed resistor to the output return when the apparatus is turned off and the DC load is removed from the apparatus. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The appended claims set forth those novel features which characterize the invention. However, the invention itself, as well as further objects and advantages thereof, will best be understood by reference to the following detailed description of a preferred embodiment taken in conjunction with the accompanying drawings, where like reference characters identify like elements throughout the various figures, in which: 
       FIG. 1  is a simplified schematic diagram of a prior art approach incorporating a bleed resistor for discharging output filter capacitors in a DC power supply; 
       FIG. 2  is another prior art approach for discharging the energy stored in output filter capacitors of a DC power supply employing a power supply turn-off discharge path for rapidly dumping the energy stored in the output filter capacitors; 
       FIG. 3  is a simplified combined schematic and block diagram of a power supply incorporating an arrangement for the automatic, quick discharge of energy stored in the converter&#39;s output filter capacitors in accordance with the present invention; 
       FIG. 4  is a graphic illustration of the voltage waveform at the power supply&#39;s rectifier output which is shown as a rectangular waveform, but also may be in the form of a sine wave; and 
       FIG. 5  is a graphic illustration of the change in voltage over time on the terminals of a switching transistor in a power supply incorporating automatic capacitor discharge in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 3 , there is shown a schematic diagram of a power supply  10  with automatic output filter capacitor discharge in accordance with the principles of the present invention. The inventive power supply  10  is intended for use in a DC power supply where a DC or AC input voltage is converted to a DC output voltage. These types of DC power supplies are used in various applications such as in battery chargers, telecommunications systems, motor drives, etc. However, the inventive capacitor discharge arrangement is not limited to the specific power supply arrangement disclosed herein, but is applicable to virtually any type of DC output device incorporating output filter capacitors which remain charged after the converter is turned off and the output load is removed. 
   Power supply  10  includes an output transformer  12  which may be either a line transformer of a linear power supply or an output transformer of a switching power supply. Power supply  10  further includes a rectifier circuit  20  and a peak detector circuit  40 . Transformer  12  includes a secondary winding (not shown for simplicity) connected to both the rectifier circuit  20  and the peak detector circuit  40 . The secondary side voltage of transformer  12  is rectified by either a center tap or full wave bridge configuration in the rectifier circuit  20 . The output of rectifier circuit  20  is a sequence of unipolar DC pulses. 
   Once power supply  10  is turned off and the output load is disconnected, its output filter capacitors  54   a  and  54   b  remain highly charged. It should be noted that both output capacitors  54   a  and  54   b  may be within the power supply&#39;s output filter  50 , or only one of these capacitors may be within the output filter as shown for the case of capacitor  54   a  in  FIG. 3 . Output capacitors  54   a ,  54   b  are capable of storing the output voltage for an extended period of time. Any person touching or any object contacting the output terminals  58  and  60  of power supply  10  would receive a large electrical shock when the output load is no longer present. In accordance with the present invention, a PNP switching transistor  52  is coupled across the output lines for providing the filter capacitors  54   a  and  54   b  with a discharge path to an output return  30  via a bleed resistor  53  for safety reasons when the power supply  10  is turned off and the output load is removed from the power supply. This safety feature of power supply  10  operates in the following manner. 
   During normal power supply  10  operation, voltage generated by the secondary winding of transformer  12  is provided to rectifier circuit  20  and peak detector circuit  40 . Peak detector circuit  40  includes resistors  64  and  66 , diode  46  and capacitor  48 . Resistor  66  in combination with capacitor  48  also forms a filter for the rectified output of diode  46 . This rectified, filtered output voltage has a value equal to the maximum value of the voltage waveform at the output of transformer  12  and is provided to the base of PNP transistor  52  via resistor  68 . The emitter of transistor  52  is maintained at the root-mean-square (RMS) value of the rectified output voltage of power supply  10 , while the base of the transistor is maintained at the peak output voltage of diode  46  during normal power supply operation. With the base of transistor  52  maintained at a higher voltage than its emitter during normal operation of the power supply  10 , the transistor is off and thus not providing a bleeding path to the circuit during normal power supply operation. 
   In the event the power supply  10  is turned off and its output load is removed, the high voltage on the base of transistor  52  is first removed and the output filter capacitors  54   a  and  54   b  maintain a high voltage on the transistor&#39;s emitter. Under these conditions, with the base of transistor  52  having a lower voltage than its emitter, the transistor is rendered conductive. With the combination of transistor  52  and resistor  53  connected across the output filter capacitors  54   a  and  54   b , the charge on the capacitors is directed to the output return  30  via bleed resistor  53 . The RC time constant of this discharge circuit is preferably selected to provide a maximum discharge time of on the order of a few seconds, and preferably less than five seconds. Because transistor  52  is non-conductive during normal power supply  10  operation and bleed resistor  53  is then not connected in circuit, the overall energy efficiency of power supply  10  is not reduced because of the presence of resistor  53 . Diode  51  connected between the base and emitter of transistor  52  protects the transistor by limiting the reverse voltage across the base-emitter junction of the transistor to approximately 0.6 V. Resistor  68  functions to limit current flow and thus protects diode  51  from excessive currents. 
   During normal operation, peak detector circuit  40  produces a voltage equal to the amplitude of the pulses at the output of rectifier circuit  20 . The output voltage of peak detector circuit  40  is greater, i.e., more positive, than the output voltage of an output filter circuit  50  coupled to rectifier circuit  20 , where the output filter derives the root-mean-square (RMS) value from the pulses at the output of the rectifier circuit. The output of the peak detector  40  is provided via resistor  68  to the base of transistor  52 , while the output of the output filter  50  is provided to the transistor&#39;s emitter. Thus, as discussed above, during normal operation transistor  52  is always off and bleed resistor  53  is not employed in the operation of power supply  10 . This can be seen in  FIG. 4  which illustrates the series of pulses provided to the input of output filter  50 , where the voltage of the pulses represents the voltage between the base of transistor  52  and the output return  30  produced by the peak detector circuit  40 . The lower horizontal dotted line shown in  FIG. 4  passing through upper portions of each of the pulses represents the voltage between the emitter of transistor  52  and the output return  30  produced by output filter  50 . Output capacitors  54   a  and  54   b  are charged up to the output voltage. Output filter  50  includes not only capacitor  54   a , but also an inductor  59 . 
   When power supply  10  is unplugged and no load is connected to its output terminals  58   a  and  60 , rectifier circuit  20  no longer produces output pulses. In addition, the voltage at the output peak detector circuit  40  decays abruptly because the capacitance of capacitor  48  is very small, but the output capacitors  54   a  and  54   b  maintain a charge because there is no means for bleeding a charge from these capacitors. As a result, the base of transistor  52  goes lower than the emitter of the transistor, rendering the transistor conductive. When transistor  52  is turned on and rendered conductive, bleed resistor  53  is connected to the power supply output and bleeds charge away from capacitors  54   a  and  54   b  to the output return  30 . This is shown graphically in  FIG. 5 , where the upper curve represents the decay of the output voltage at no load with bleed resistor  53  performing no function in power supply  10 . The lower curve in  FIG. 5  represents the voltage decay at the base of transistor  52  under a no load condition. Diode  51  clamps the base-emitter junction of transistor  52  to prevent reverse biasing of the transistor&#39;s base-to-emitter junction and maintain the transistor conductive. 
   While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the relevant arts that changes and modifications may be made without departing from the invention in its broader aspects. Therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. The actual scope of the invention is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.