Abstract:
Apparatus is provided to limit the current drawn from a power supply output connected to furnish power to a docking station or electronic device, when a fault connects the output to ground or other docking connection. The power supply is disconnected when such fault is present during power-up. In one embodiment, the apparatus controllably limits current delivered to a docking station and a mobile device connected thereto. The apparatus includes a switch device located along a current path extending between the power supply and docking station, to regulate current flow. A current limiting entity operates the switch device to prevent current flow, when the fault affects the circuit, and current through the path exceeds a pre-specified threshold level. The pre-specified current threshold level can be less than the current level provided to the docking station and mobile device without said fault. A disabling component de-activates the current limiting entity, when current is flowing to the charging station but the current is unaffected by the fault.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention disclosed and claimed herein generally pertains to an apparatus for providing fault protection, in a power supply or in a circuit that connects an electronic device to a power supply. More particularly, the invention pertains to apparatus of the above type wherein faults can occur that cause excessive levels of current to flow to the connection of the electronic device from the power supply. Even more particularly, the invention pertains to apparatus of the above type wherein one or more mobile or other electronic devices are removably insertable into a docking station, or otherwise connected to a power supply, in order to receive power. 
         [0003]    2. Description of the Related Art 
         [0004]    The use of electronic devices for computing and communications is becoming increasingly diverse, as the mobility of such devices increases and their costs diminish. As an example, mobile computing devices are currently being made available to customers at certain large discount stores. The mobile computing devices reside in a rack near a store entrance, and can easily be mounted on shopping carts by customers, to assist them in their shopping. When a device is returned to the rack, it is inserted into a charging or docking station, to allow the device battery to be recharged from a power supply. 
         [0005]    It is very common for a mobile device to be connected to a docking station, with the power turned on to both the docking station and the device. Because of this, power supplies used to provide current for recharging mobile devices are typically provided with over-current sensor and shutdown circuits. Thus, if excessive current is drawn from the power supply, so that a threshold is exceeded, the circuit will operate to shut the power supply down. The threshold for the amount of current drawn that will cause shutdown is usually selected to be significantly more than the amount of current required for normal operation of the docking station. This is done so that shutdown will not occur falsely or erroneously. The threshold can in fact be so high that the connector pins, or other elements used to connect the mobile device to the docking station, can become pitted or otherwise damaged by faults that cause excessive current. Moreover, typical shutdown circuits pulse the power supply back on, in order to sense if the fault is still present, and if so, act to shut the power supply back down. This causes the power supply to pulse on and off, possibly for a long period of time, until the fault has been corrected. 
         [0006]    Unfortunately, in highly public places such as stores of the above type, it is common for debris comprising conductive material, such as coins, tin foil or the like, to become scattered around the recharging rack that contains the mobile devices while not in use. It has been found that if debris of this sort comes into contact with a structure connecting a device to the charging station, the connecting structure may be short circuited to ground. Such a short circuit fault can cause an excessive amount of current to be drawn from the power supply. Moreover, a fault of this type tends to remain until corrected manually. Thus, if a short circuit fault occurs, a power supply provided with a current shutdown circuit as described above could continue to pulse on and off for a very long period of time, which is not desirable 
         [0007]    A further problem in circuits used with charging or docking stations relates to inrush current. When a mobile device is connected to a charging station and the battery charge of the device is low, a substantial inrush current will initially flow into the charging station and the device, from the power supply. The inrush current can cause the connectors to pit, and can also drag down the voltage on the host machine to the point where the power supply of the host machine goes out of regulation. Accordingly, it is common to provide an inrush control circuit that includes power transistors. However, the combination of an inrush control circuit, together with an over-current protection circuit of the type described above, can create further problems. For example, if the over-current shutoff threshold is high enough to avoid false shutdowns, the resulting current may be high enough to damage the power transistors of the inrush control circuit. This is especially likely to happen with power supplies that pulse on and off for long periods of time, when too much current is being drawn. Under this condition the inrush current control circuit acts repeatedly, to try and limit the large current, and will overheat unless large transistors with heat sinks are used. 
         [0008]    It would be desirable to provide an improved mechanism for protecting against excessive current in circuits that connect a power supply to a docking station. Such mechanism would be particularly useful in regard to excess currents that are caused by short circuit faults of the type described above. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    Embodiments of the invention generally limit the current that can be drawn from a power supply output, when the output is connected to supply power, for example, to a docking station, and a fault inadvertently connects the power supply output to ground. The power supply is shut off when a fault of this type is present during power-up, at a threshold current value that is less than the current required for normal device charging operations of the docking station. Such shutoff is usefully achieved by means of a current limiting mechanism or entity that is responsive to power supply output voltage. This entity functions to disconnect the docking station from the power supply, until the shorting fault is removed. Thus, the voltage on the docking connector is not allowed to pulse on and off. In another embodiment, apparatus controllably limits the current delivered to a charging station from a power supply output, wherein a mobile electronic device is detachably connectible to the charging station. A first switch device is located along a current path extending from the power supply to the charging station, to regulate current flow through the path. When a fault causes the current flowing through the path to exceed a pre-specified value while the power supply voltage is below a pre-specified value, a voltage sensitive current limiting entity operates the first switch device to prevent current flow through the path. The current limiting entity is disabled when the power supply voltage exceeds the pre-specified voltage. 
         [0010]    The invention is not, of course limited to use with docking stations, but generally can be used in any application where an external power source or power brick is connected to an electronic device, to provide power thereto. 
     
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0011]    The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
           [0012]      FIG. 1  is a circuit diagram showing an embodiment of the invention, wherein a fault is present. 
           [0013]      FIG. 2  is a circuit showing the embodiment of  FIG. 1 , wherein the fault shown in  FIG. 1  is not present. 
           [0014]      FIG. 3  is a circuit diagram showing a further embodiment of the invention, with the fault present. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Referring to  FIG. 1 , there is shown a current limiting circuit  100  for implementing an embodiment of the invention, wherein some of the components of circuit  100  have been simplified. Circuit  100  provides a path for routing DC current from a power supply  102  to a docking connector  104 , which is a component of a charging or docking station  106  (shown only in part). Docking connector  104  is configured to mate with a complementary connector  108  of a mobile electronic device  110  (shown only in part), such as a wireless telephone or computing device, in order to provide power to the device. Thus, DC power from power supply  102  is able to flow through circuit  100  to charge or recharge a battery of device  110  and also power the electronics in device  110 , the battery and electronics being represented in  FIG. 1  as device load  112 . Usefully, power supply  102  has a voltage of 16 volts DC, but the invention is not limited thereto. Power supply  102  may comprise a power supply device of a type commonly referred to in the art as a power brick. 
         [0016]    Connectors  104  and  108  usefully comprise JP2 and JP3 connectors, respectively.  FIG. 1  further shows connector elements  114 , which collectively represent pins or fingers on connector  104  that mate with complementary receptors (not shown) on connector  108 . Thus, mobile device  110  is connected through elements  114  to docking or charging station  106 , to receive power and signals therefrom. However, device  110  must be easily connectable to and disconnectable from station  106 . Accordingly, in many applications the connector elements  114  will not be sealed from the surrounding environment. As a result, debris that is formed of highly conductive material may inadvertently come into contact with elements  114 , and short circuit them to ground or short elements  114  together. A fault of this type can cause an excessive current to flow through the connector elements  114 , so that they become pitted. Arcing may also occur, which can remove gold plating or other conductive material from the elements  114 . In  FIG. 1 , a shorting fault of this type is depicted as resistance  116 , also referenced as R f , which is connected between one of the connector elements  114  and ground. More specifically, R f  represents the effect of the element  114  connected between terminal  3  of each connector, so that such element carries current from the power supply, wherein such element becomies connected to an element  114  that is connected to ground. It will be appreciated that if the conductivity of the debris causing the fault is high, the resistance of R f  will be very low. 
         [0017]    Referring further to  FIG. 1 , there is shown a connector  118 , such as a JK1 connector, disposed for connection to an output terminal  120  of power supply  102 . Connector  118  may serve as a jack for power supply  102 , and is connected to a current lead or trace  124  of circuit  100 . Thus, connector  118  acts to couple power from power supply output  120  to circuit  100 . An over current sensor and pulsing shut down mechanism of the type described above (not shown) may be included in power supply  102 .  FIG. 1  shows current flowing through lead  124  to a lead or trace  126 , through a large power transistor switch  128 , and through lead  126  to an input terminal of docking connector  104 . 
         [0018]    Switch  128  usefully comprises a comparatively large power FET  130 , such as a P channel FET FDS4435. An FET of this type has three source connections S 1 -S 3 , which receive an input through lead  124 , and four drain connections D 1 -D 4 . To control the operation of FET Switch  130 , a biasing circuit  122 , comprising a bias voltage DC_IN_R, resistors  132  and  134 , and a capacitor  136 , is connected to gate G of FET  130 . Bias voltage DC_IN_R can be the voltage supplied by power supply  102  or a voltage derived from it. Biasing circuit  122  also includes a transistor control switch  138 , connected between gate G of FET  130  and ground. Thus, switch  138  controls the gate of power FET  130 , to operate FET  130  as a switch. When switch  138  is closed, the FET switch  130  is turned on by biasing circuit  122 . This allows current to flow from power supply  102  through FET  130  to docking connector  104 , in order to charge and power the device  110 . On the other hand, opening switch  138  turns off FET  130 , to effectively prevent current provided by power supply  102  from reaching docking connector  104 . 
         [0019]    Control switch  138  usefully comprises an FET  140  that is substantially smaller than FET  130 . The drain D and source S of FET  140  are connected to components of biasing circuit  122 , and to ground, respectively. A biasing circuit  142  is connected to gate G of FET  140 , to control the operation thereof. Biasing circuit  142  comprises a bias voltage VCC, resistors  144  and  146 , and a capacitor  148 . Bias voltage VCC is a DC voltage derived (derivation not shown) from output of power supply  102 , typically 3.3 to 5.0 volts, but the circuit can be designed to use other voltages. In addition, biasing circuit  142  includes a switching node  152  to which a voltage is applied to turn on control FET  140 , so that FET  140  turns on FET switch  130 . In the absence of the voltage node  152  disables biasing circuit  142 , so that FET  140  becomes open. Thereupon, FET  140  acts to turn off FET switch  130 . 
         [0020]    In order to respond to the shorting fault represented by resistance R f , and to the excessive current resulting therefrom,  FIG. 1  shows a diode  160  connected between biasing circuit  142  and current lead  126 . Diode  160  is also referenced as D 1 . For a very low value of fault load R f , the short circuit caused thereby will result in very high current being drawn to docking connector  104  from the output of power supply  102 . At the same time, the output voltage which is supplied through lead  126  to terminal  3  of connector  104  will drop significantly. This occurs because power supply  102  has insufficient capability to maintain the voltage level. When the voltage reaches a low enough level, diode  160  becomes forward biased. This condition of diode  160  effectively pulls down the gate G of FET  140 , so that control FET  140  is turned off. This, in turn, allows pullup resistor  132  to pull up the gate G of FET switch  130 , so that FET  130  is likewise turned off. Current flow from power supply  102  to connector  104  is thereby prevented. 
         [0021]    It is to be appreciated that FET switch  130  will be shut off and remain so, whenever diode  160  is forward biased. Accordingly, even if the pulse shut down mechanism in power supply  102  pulses on, as described above, FET switch  130  will still block the flow of current, and thus protect the connectors  104  and  108 . 
         [0022]    The threshold value of current flowing from the power supply output, at which FET switch  130  is turned off, is controlled by the value of resistor  144  and by the forward voltage drop across diode  160 . This current threshold value can be made to be significantly less than the current level required by normal operation of circuit  100 , when the circuit is being used to charge device  110 . Diodes  158  and  162  in device  110  enable this to be done, as is explained below. 
         [0023]    Referring to  FIG. 2 , there is shown the embodiment of  FIG. 1  wherein the fault load R f , or resistance  116 , has been removed or is otherwise not present. 
         [0024]    Given the configuration of circuit  100  as described above, it is necessary to include diodes  158  and  162  in device  110 , so that the diodes are in series with the device load  112 . These two diodes are needed, because it could happen that device load  112  is so small that there is only a negligible voltage drop across it. In this circumstance, without further voltage drop diode  160  would be in a forward biased mode. As discussed above, forward biasing of the diode  160  has the effect of turning off control FET  140 , and thus turning off the power FET  130 . Accordingly, no current would flow to docking connector  104  or device  110 . 
         [0025]    The voltage drop across each diode  158  and  162 , required for the diodes to become forward biased and start conducting current, is typically about 0.7 volts. Thus, the series connection of diodes  158  and  162 , also referenced as diodes D 2  and D 3 , respectively, is on the order of 1.4 volts before current can flow through the series connection thereof. This voltage is sufficient to reverse bias diode  160 , so that such diode cannot conduct current. It is thus seen that diodes  158  and  162  collectively operate to deactivate diode  160 , in the absence of a fault R f , so that diode  160  cannot shut off FET switch  130  to prevent current flow through circuit  100 . Moreover, the amount of current flowing through circuit  100 , during the course of normal operation to charge and power device  110 , can be significantly greater than the current threshold level at which diode  160  acts to shut off FET switch  130 , in the presence of a fault R f  as described above. 
         [0026]    Referring to  FIG. 3 , there is shown a circuit  300 , comprising a more generic representation of an embodiment of the invention.  FIG. 3  shows a 16-volt DC power supply  302  connected to furnish power to a docking connector  308 , through a current path that includes a large power FET switch  304  and a conductive lead  306 . Docking connector  308  is a component of a docking or charging station  310 , which is configured to removably receive mobile electronic devices such as device  312 , in order to provide power thereto. Connector  308  can also be the power connector attachable to an electronic device.  FIG. 3  shows connector elements  314  of docking connector  308  adapted to mate with corresponding complementary connector elements  316  of device connector  318 .  FIG. 3  further shows a load  320  representing the resistive load R L  that device  312  places on docking station  310 , when device  312  is connected thereto to receive power. 
         [0027]    Referring further to  FIG. 3 , there is shown a resistor  338 , also referenced as R f , representing a fault that shorts the current path to ground, at a point on docking connector elements  314 . To limit excessive current resulting from the fault R f , a control FET  322  and a pull up resistor  324  are connected to the gate of power FET switch  304 , as shown in  FIG. 3 , to control the operation thereof. 
         [0028]    To further control the operation of power FET  304 , and thereby regulate current flow from power supply  302 ,  FIG. 3  shows a comparator  330 . The inverted input of comparator  330  is connected to a reference voltage V ref , and the non-inverted input is connected to a DC voltage source  326 , through a resistor  328 . Reference voltage V ref  can be derived from power supply  302 . DC voltage source  326  can be derived from power supply  302  and can be any commonly used DC voltage level such as 3.3 VDC or 5 VDC.  FIG. 3  also shows a diode  332  (D 4 ) connected between the non-inverted input of comparator  330  and current lead  306 . 
         [0029]    The values of V ref  and resistor  328  are respectively selected so that during normal operation of circuit  300 , and in the absence of fault R f , diode  332  will be reverse biased, and the voltage applied to the non-inverted input of the comparator will exceed voltage V ref  applied to the inverted input of the comparator. In this situation, the output of comparator  330 , which is connected to the gate of control FET  332 , is positive. Accordingly, control FET  332  is maintained in an on mode, and acts to similarly maintain power FET switch  304  in an on mode. Current is thus allowed to flow through switch  304  and current lead  306  to docking connector  308 . 
         [0030]    On the other hand, when a fault R f  occurs as shown in  FIG. 3 , to effectively connect the current path to ground, excessive current is pulled from the output of power supply  302 . The fault also results in very low voltage, so that diode  332  becomes forward biased, and the voltage applied to the non-inverted input of comparator  330  becomes less than V ref . The output of comparator  330  will therefore go negative and turn off control FET switch  332 . When FET switch  332  is turned off, pull up resistor  324  will pull up the voltage on the gate of power FET switch  304 , thereby turning off FET  304 . Voltage will then be removed from lead  306 . 
         [0031]      FIG. 3  further shows diodes  334  (D 5 ) and  336  (D 6 ) included in device  312 , and connected in series to the device load  320 . Diodes  334  and  336  function in like manner as diodes  158  and  162 , described above in connection with  FIG. 2 , to ensure that diode  332  is reverse biased in the absence of a fault  338 . 
         [0032]    The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.