Abstract:
A power supply ( 200 ) is provided having a plurality of independent current limiting circuits. A first current limiting circuit ( 6 ) provides protection against a short circuit or other extraneous load conditions, while a second current limiting circuit, using a trace on a PCB as a sensing element, is programmable on the basis of a time constant and a current level.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a power supply having a power limit of less than 100 VA, yet still capable of supplying for a short duration an instantaneous load demand that is larger than its power limit. 
         [0003]    2. Description of Related Art 
         [0004]    Power supplies are available as off-the-shelf electronic components meeting the “Limited Power Source” requirement of applicable UL specifications, more particularly, of UL 1950, aimed at eliminating the possibility of the power supply causing hazardous fires when overloaded or malfunctioning. “Limited power” is generally defined as power not exceeding 100 VA. 
         [0005]    To meet such specifications, “fuses” have been employed in the prior art that blow open when the load element exceeds 100 VA. This technology is however undesirable, because a fuse needs to be replaced every time a fault occurs, and because the fuse reaction time to an overload condition is relatively long, possibly causing unwanted damage to the equipment to which the power supply is supplying power. 
         [0006]    Another type of power supply in the prior art that meets the “Limited Power Source” requirement uses an internal electronic current limiting the total power output so that it never exceed 100 VA. This technology provides operating disadvantages, because some types of electronic equipment demand a high momentary current of a relative short duration, with a peak power demand as high as 400-500 VA, while the average power demand in a longer time span is always less than 100 VA. This causes this type of power supply, rated less than 100 VA, useless for such applications. 
         [0007]    Further, while UL 1950 requires a reaction time of 30 seconds, a revision of UL 1950 presently under review will decrease the reaction time to five seconds. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    It is an advantage of the present invention to provide to a power supply with current limiting circuits that can supply a load with an average power demand that is substantially less than 100 VA, but with pulse power demand that is substantially higher than 100 VA. 
         [0009]    It is another advantage of the present invention to provide a power supply with current limiting circuits wherein the reaction time to a pulse power demand can be set to predetermined limits. 
         [0010]    Briefly, a power supply is provided having a plurality of independent current limiting circuits. A first current limiting circuit provides protection against a short circuit or other extraneous load conditions, while a second current limiting circuit, using a trace link on the printed wiring board as a sensing element, is programmable on the basis of a time constant and a current limit level. As a result, the power supply can provide high pulse current demand from the load and can at the same time limit the power output from the power supply, complying, on an average basis, with the “Limited Power Source” requirement imposed by applicable regulations. 
         [0011]    The power supply design according to one embodiment of the present invention comprises two electronic power limit circuits, each having a deferent limit level and time delay. Such a power supply is capable of supplying power at a predetermined peak level that is substantially higher than 100 VA and that has a predetermined duration from zero seconds to five seconds. At the same time, the power supply is prevented from supplying power in the event that the drawn load is more than 100 VA for more than five seconds. 
         [0012]    In one variant of this embodiment, the power supply can either latch-off or automatically restart if the over power condition is reached. In the latch-off mode, the input power must be removed and re-applied, in order to cause the power supply to restart. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0013]    The enclosed  FIG. 1  constitutes a part of this specification and includes an exemplary embodiment of the invention, which may be embodied in various forms. 
           [0014]    More specifically,  FIG. 1  is a diagrammatic view of one embodiment of the power supply with current limiting circuits. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    A detailed description of an embodiment of the invention is provided herein. It is to be understood, however, that the present invention may be embodied in various forms. Therefore, the specific details disclosed herein are not to be interpreted as limiting, but rather as a representative basis for teaching one skilled in the art how to employ the present invention in virtually any detailed system, structure, or manner. 
         [0016]    Turning to  FIG. 1 , there is shown an embodiment 200 of a power supply with current limiting circuits. An AC-DC converter circuit inputs a 90 V to 264 V AC voltage into connections  75  and  36 , charging bulk capacitor  13  and establishing a DC voltage of 120 V DC-370 V DC across bulk capacitor  13 , according to the AC input voltage. Such high voltage carries through voltage dropping resistors  32 ,  33 , and  34 , thereby charging capacitor  21  with a voltage up to 16 V. Pulse Width Modulator (PWM) control  18  becomes then active and begins producing a pulse width modulated gate drive to MOSFET (metal-oxide field-effect transistor)  58 , causing energy to be stored into transformer  63  and be released to output capacitor  106 , while at the same time charging storage capacitor  60 . 
         [0017]    While these events occur, the voltage at pin  8  of PWM control  18  rises to 5 V, providing current through resistor  19  that turns transistor  25  to an active state, which causes transistor  24  to become active and transistors  24  and  25  to become in a latching-on state, thereby connecting resistor  27  to capacitor  21 . The values of resistors  27  and  23  are carefully chosen, so that resistors  32 ,  33 , and  34  are prevented from supplying sufficient current to maintain a 10 V voltage across resistors  27  and  23 , while transistors  24  and  25  remain in the latching-on state. 
         [0018]    The DC voltage that supports PWM control  18  in operation is generated by main transformer  63 , for which the voltage of winding  63 - 2  is rectified by diode  66  and filtered by capacitor  60 . Such a voltage is connected, by transistor  42  and diode  20 , to PWM control  18 . 
         [0019]    When the output voltage is regulated to reach a predetermined value, shunt-regulator  77  will conduct current through options-coupler  53 . With a primary photo diode, this current will be coupled to its secondary photo transistor and the current flow in this transistor will provide the base current for transistor  42 , keeping it in fully turn-on condition. 
         [0020]    A first current limiting function is accomplished by sensing the voltage drop across resistor  57 . When PWM control  18  provides gate drive voltage to MOSFET  58 , the current level will ramp up into resistor  57 , and this current will develop voltage on resistor  57 . This sensed voltage is then fed to pin  3  of PWM control  18 , and when this voltage level reaches a predetermined level set by PWM control  18 , then PWM control  18  will turn off the gate drive to MOSFET  58  and stop the current flow. Such an action will limit the energy transfer process of the power supply, and, therefore, the output voltage will drop below the regulated voltage setting, shunt-regulator  77  will no longer conduct current, and the base current for transistor  42  will cease, causing transistor  42  to turn-off. Consequently, PWM control  18  will not receive a voltage high enough to operate, and the power supply will no longer provide power to the output, remaining in such a latch-off condition indefinitely. This will complete the current limiting function of the power supply. 
         [0021]    This current limiting level is designed to be set high enough to ensure proper output voltage during a short high current pulse condition demanded by the load. To reset this latching condition, the AC input voltage must be turned off for an adequate time, and re-applied to enable the power supply to start and operate continuously again. This turn-off and latch-off function is of relevance, in order to provide proper protection of the power supply and load combination and to prevent a fire hazard. 
         [0022]    A second current limiting circuit is in the secondary side of the power supply circuit. Resistor  76  and zener diode  99  provide a regulated voltage that enables operational amplifiers (OP)  100  and  111  to operate. As shown, OP  100  derives its reference voltage from the thermistor  96 , resistor  112  and resistor  93  voltage divider network. 
         [0023]    Such a voltage is compared with the voltage sensed from printed circuit board (PCB) copper trace link  72 , wherein trace link  72  is formed by a PCB trace that has only a few milliohm resistance. Using a PCB trace link as current sensing element is important, because this will prevent the circuit from generating any more unwanted heat due to the extra voltage drop across the resistive sensing element. A PCB trace link is also inherently more reliable than having one more component. 
         [0024]    Further, PCB copper traces have a positive temperature coefficient, which can increase the voltage drop across it at a higher temperature, making the current sensing threshold drift with temperature. To eliminate such a temperature drift effect, a negative temperature coefficient thermistor  96  is inserted in the voltage reference divider network. By choosing the thermistor  96  value carefully, the temperature drift problem can be substantially removed. 
         [0025]    The output load current develops a voltage drop across PCB trace link  72 , creating a negative voltage drop on the left hand side of link  72 , and adding to the positive voltage set-up on resistor  93 , which is eventually fed to OP  100 &#39;s negative input port. In the presence of an output over-current condition, the magnitude of the negative voltage across link  72  is high enough to offset the positive voltage across resistor  93 , and the input voltage to OP  100 &#39;s negative input will be negative. This forces the output of OP  100  to be high, with OP  100  working as a comparator. Current is then caused to flow through resistor  89 , charging capacitor  87 . 
         [0026]    If the time of an output over-current condition is long enough, the voltage developed on capacitor  87  will rise high enough that its voltage level will go beyond the voltage levels set by resistor  84  and resistor  86  voltage divider networks. This will change the output state of OP  111  from low to high, with resistor  84  and capacitor  87  determining the time constant of the delay functions. 
         [0027]    The direction of diode  90  and the value of resistor  91  can provide for a different behavior of the time delay circuit when the time delay circuit reacts to a different output over-current condition. 
         [0028]    When the output of OP  111  increases to a high level, current travels through resistor  83  and diode  82 , forcing shunt regulator  77  into fall shunt mode and further forcing current to flow though the primary light emitting diode (LED) of an OPTO-coupler  53 . Secondary transistor of  53  will then turn on fully. Such an action will command PWM control  18  to stop the gate drive duty cycle to MOSFET  58  and, with MOSFET  58  not switching, both the output voltage, as well as the voltage on capacitor  60 , will drop. When voltage of capacitor  60  drops below about 12V, PWM control  18  shuts down, preventing any more gate drive voltage to MOSFET  58 . The power supply then initiates a shut down sequence and ends up in a latch-off condition. Subsequently, the only way to restart the power supply is to reset the AC input voltage as described before. 
         [0029]    When transistor  24  is not installed, transistor  25  cannot remain in a latched on state. Once the PWM control  18  shuts down, no more base drive is supplied to transistor  25 . In this situation, the only current drain to capacitor  21  is only the about two hundred microamperes of current that PWM control  18  is drawing. Therefore, a start sequence as described above will initiate, and the power supply will start and try regulating again. If the over-current condition persists, the same shutdown sequence will commence again and repeat itself. This is a circuit option enabling the power supply to have an automatic restarting capability. 
         [0030]    While the invention has been described in connection with the above described embodiments, it is not intended to limit the scope of the invention to the particular forms set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the scope of the invention. 
       INDUSTRIAL APPLICABILITY 
       [0031]    The present invention has industrial application in circuits within information technology equipment where it is important or essential to meet or exceed the UL 1950 requirements of a limited current circuit, and to comply with the limited power source safety requirements wherein a limited power source shall incorporate an. isolating transformer as per UL 1950, section 2.11.