Abstract:
A power supply includes a DC-DC converter, a boost converter, an energy storage element; and a voltage clamping circuit. The DC-DC converter is connected to a power source with an output voltage in a first voltage range. The voltage clamper circuit is configured to discharge at least a portion of energy of the energy storage element and to produce current at a clamped output voltage range that is substantially equal to the first voltage range. The discharged energy provides hold-up time for the power supply.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to maintaining power output from a DC-DC converter system during an interruption of input power. More particularly, the invention relates to storing energy within the converter system and releasing the stored energy to maintain output power during interruption of input power. 
         [0002]    In modern power supplies, two critical parameters which play a major role are the efficiency and size of the power supply. Power supply engineers have to optimize the power supply such that it achieves maximum efficiency in minimum area on a circuit board. One major parameter which affects the area of the power supply is the size of an energy storage element for a holdup feature. 
         [0003]    Hold up time or transparency time is a critical factor in the power supply designs for aero applications. Hold up time refers to the time or interval during which there is a loss of input power to the power supply. The power supply performance should not be degraded when the equipment is subjected to power cuts shorter than its transparency time. 
         [0004]    A typical hold up time for power supplies, used in Aerospace applications, could vary from 3 milliseconds (mS) to 200 mS based on the application or the circuitry to which the DC-DC converter may be connected. Typically, the holdup time of 200 mS would call for huge energy storage capacitors in the range of hundreds-thousands of micro farads if the capacitor were charged at a voltage equivalent to the nominal input voltage of the power supply. In some prior art power supplies, filtered input voltage is boosted to a level higher than the nominal input voltage and then fed to charge a capacitor bank. Due to the boosted charging voltage, the amount of capacitance required to provide the suitable holdup time is reduced. 
         [0005]    While such systems may reduce the amount of capacitance required, they may subject a main converter to the high boosted voltage at its input terminals. As a consequence, such a converter may need to be constructed with components that can tolerate exposure to the high boosted voltage. 
         [0006]    As can be seen, there is a need for an efficient system and method to provide extended hold-up time for a DC-DC converter of a power supply. More particularly, there is a need for such a system and method that provides such hold-up time without subjecting the main converter to voltages higher than nominal converter input voltages. 
       SUMMARY OF THE INVENTION 
       [0007]    In one aspect of the present invention, a power supply comprises a DC-DC converter; a boost converter; an energy storage element; and a voltage clamping circuit; wherein the DC-DC converter is connected to a power source with an output voltage in a first voltage range; wherein the boost converter, the energy storage element and the voltage clamper circuit are positioned on an electrical pathway that is parallel to an input of the DC-DC converter; wherein the boost converter is connected in series with the energy storage element for charging the energy storage element to a second charge voltage range, higher than the first voltage range; wherein the voltage clamper circuit is interposed between the energy storage element and the input of the DC-DC converter; and wherein the voltage clamper circuit is configured to discharge at least a portion of energy of the energy storage element and to produce current at a clamped output voltage range that is substantially equal to the first voltage range. 
         [0008]    In another aspect of the present invention, an energy storage block for a power supply comprises a boost converter with an input connected to be provided with power at a voltage within a first voltage range; a chargeable energy storage element connected in series with the boost converter to be provided with power at a voltage within a second voltage range, the second voltage range being higher than the first voltage range; and a voltage clamping circuit connected in series with the chargeable energy storage element and having an output clamped at a voltage within the first voltage range. 
         [0009]    In still another aspect of the present invention, a voltage clamping circuit for a power supply comprises: an input connection; an output connection; and two semiconductor clamping switches connected in a series pass configuration and interposed between the input connection and the output connection. 
         [0010]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a block diagram of a power supply in accordance with an exemplary embodiment of the invention; and 
           [0012]      FIG. 2  is block diagram of a voltage clamping circuit of the power supply of  FIG. 1  in accordance with an exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0014]    Various inventive features are described below that can each be used independently of one another or in combination with other features. 
         [0015]    The present invention generally may provide a system and method for hold-up time for a DC-DC converter. More particularly, the invention may provide a system and method for providing such hold-up time by delivering power at a desired voltage from an internal capacitor bank that is charged at a voltage greater than a nominal input voltage of the converter. 
         [0016]    Referring now to  FIG. 1 , an exemplary DC-DC converter  102  is shown in a power supply  100  in which input power to the DC-DC converter  102  may be temporarily interrupted. In many applications, it is important to maintain output power from the DC-DC converter  102  during interruption of input power. In the exemplary embodiment of  FIG. 1 , the DC-DC converter  102  may be provided with input power through a hotswap circuit  104 . In the case of a hot swap, interruption of power from the hotswap circuit  104  may extend for a period of time ranging from about 3 milliseconds (mS) to 200 mS. Thus, it is desirable to provide the power supply  100  with hold-up time capability (i.e., continuity of input power) for the DC-DC converter  102  during interruption of power from the hotswap circuit  104 . 
         [0017]    In an exemplary embodiment of power supply  100  hold-up capability may be provided by an energy storage block  101 . The energy storage block  101  may include a boost converter  106  and a chargeable energy storage element  108 . The energy storage block  101  may be connected in parallel with the DC-DC converter  102 . In one embodiment the energy storage element  108  may be one or more supercapacitors. In another exemplary embodiment, shown in  FIG. 1 , the chargeable energy storage element  108  may be a capacitor bank  108 . The capacitor bank  108  may be charged to a boost voltage Vb. The boost voltage Vb may be higher than an output range of voltage Vo of the hotswap circuit  104 . A voltage clamper circuit  110  may be interposed between the capacitor bank  108  and an input  112  of the DC-DC converter  102 . The voltage clamper circuit  110  may clamp the boosted voltage Vb to a range substantially equal to the voltage input range of the DC-DC converter  102  (i.e., the range output voltage Vo of the hotswap circuit  104 ). As explained in detail hereinbelow, the voltage clamper circuit  110  may be in operation only during a condition that requires hold-up. 
         [0018]    Referring now to  FIG. 2 , an exemplary embodiment of the voltage clamping circuit  110  is illustrated in detail. The voltage clamping circuit may include an input connection  111  and an output connection  113 . Two or more clamping switches,  114  and  116 , may be connected as series pass MOSFETs and may implement voltage clamping to the voltage Vo. The clamping circuit  110  may consume power only during voltage clamping activity. Such limited power consumption may be achieved through employment of a combination of under-voltage detection switches  122  and  124  and a resistor feedback circuit comprised of resistor  136  and resistor  137  along with an operational amplifier  118  with a built-in reference  120 . If external feedback voltage, Vo, from the converter input  112  is less than a reference voltage Vr of the reference  120 , the under-voltage detection switches  122  and  124  may enable the switches  114  and  116  to turn on and voltage clamping may occur. Conversely, if Vo equals or exceeds Vr, the switches  114  and  116  may turn off and power consumption of the clamping switches  114  and  116  may cease. 
         [0019]    The diodes  126  and  128  may reduce temperature dependence of forward voltage drop of the switch  124 . In an exemplary embodiment, the diodes  126  and  128  may be Schottky diodes and the switch  124  may be a bipolar junction transistor (BJT). Whenever Vo is less than the reference Vr, the under voltage detection switches  122  and  124  may be enabled and boosted voltage may be current limited through a current limit circuit  130  comprised of resistor  132 , resistor  134 , diode  126 , diode  128  and switch  124 . Thus, the voltage clamper circuit  110  may operate such that its output voltage is within a hysteresis band whose limits are set by the under-voltage detection switches  122  and  124  and the current limit circuit  130 . 
         [0020]    In the embodiment of the power supply shown in  FIG. 1 , an input voltage range may be 18V to 32V, as an example. The boost converter  106  may charge the capacitor bank  108  to a voltage Vb of about 70V, as an example. During normal operating conditions, the voltage clamper circuit  110  may cut off the boost converter  106  from the DC-DC converter  102 . The DC-DC converter  102  may operate on the conditioned power coming from the hot swap circuit  104  and EMI filter. During a power interrupt, under voltage detection may result in turn on of the voltage clamp circuit  110  which may linearly clamp the boosted 70V to a nominal voltage of 27V. The DC-DC converter  102  would thus see only an input operating voltage range Vo voltage of about 18V to 32V and not the boosted voltage of 70V. 
         [0021]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.