Abstract:
In one embodiment, a startup circuit for a power supply is provided. The startup circuit comprises a resistance coupled between a voltage source and a first node. A first capacitor, coupled to the first node, is operable to be charged by current flowing through the resistance. A first transistor has an emitter, a base, and collector, wherein the collector is coupled to the voltage source and the base is coupled to the first node. A diac circuit. coupled to the emitter of the first transistor, is operable to fire to turn on the first transistor, thereby allowing discharge of the first capacitor through the base-emitter junction of the first transistor. A second capacitor is operable to be charged by current related to a discharge voltage resulting from the firing of the diac circuit. The second capacitor operable to store charge to provide VCC voltage to a controller of the power supply.

Description:
CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY 
       [0001]    The present application claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 61/372,793, filed Aug. 11, 2010, entitled, “High Voltage Startup Circuit and High Voltage Input Capacitor Balancing Circuit,” and to U.S. Provisional Patent Application Ser. No. 61/431,723, filed Jan. 11, 2011, entitled, “Startup Circuit and Input Capacitor Balancing Circuit,” the entirety of which is incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    1. Field of Invention 
         [0003]    Power converters are essential for many modern electronic devices. Among other capabilities, a power converter can adjust voltage level downward (buck converter and its derivatives) or adjust voltage level upward (boost converter and its derivatives). A power converter may also convert from alternating current (AC) power to direct current (DC) power, or vice versa. A power converter may also function to provide an output at a regulated level (e.g., 5.0V or 5 amps). Power converters are typically implemented using one or more switching devices, such as transistors, which are turned on and off to deliver power to the output of the converter. Control circuitry is provided to regulate the turning on and off of the switching devices, and thus, these converters are known as “switching regulators” or “switching converters.” Such a power converter may be incorporated into or used to implement a power supply—i.e., a switching mode power supply (SMPS). The power converter may also include one or more capacitors or inductors for alternately storing and delivering energy. 
         [0004]    Power supplies, and controllers for the same, are used in many applications. Off-line low voltage applications are in the range of 85 VAC to 265 VAC which correspond to standard line voltages, e.g., for the United States, Europe, Asia, etc. High voltage (HV) applications can be anything above 265V, such as, for example, industrial power supplies (requiring up to 600V AC input), bias supplies for windmills (requiring up to 1000V AC inputs), solar panels (requiring “string” converters that can operate to 800V to 1000V DC input), ballasts (which can operate from approximately 85V AC to more than 480V AC), power factor pre-regulators in industrial lighting applications, and “smart meters” (that can operate on inputs ranging from around 85V to 576V AC or approximately 120V to 820V DC). 
         [0005]    Meters for monitoring, e.g., electricity usage, can require power supplies ranging between 1 W and 15 W. Non-smart meters use power supplies of around 1 W. A “smart meter,” which can be a meter having a communications link to a central location to monitor and control electricity usage, typically use higher power levels, for example, ranging between 5 W and 7 W during transmission. 
         [0006]    Smart meters enable two-way communications between electric utilities and both consumer and business power users to attempt to improve delivery efficiency and the ability to control and regulate overall power consumption. They are part of the “Smart Grid” technology initiative. 
         [0007]    A smart meter can include a power supply, a controller for the power supply, an input for receiving power (e.g., AC source), a measurement section, and a communications section. The communications section typically includes an radio-frequency (RF) subsection which receives and/or transmits RF signals to/from a meter reader, other meters (e.g., gas or water), or electrical appliances (e.g., refrigerator). A smart meter should be able to withstand up to 6000V transients at AC input. Furthermore, a smart meter can be used in or exposed to a wide range of application (e.g., from 85V to 900V). For this, all components at the front-end of the smart meter typically must be rated for this wide range or arranged in a certain way. 
         [0008]    Specifications for smart meters allow continuous transmission so the power supplies need to be dimensioned for this higher power level. In addition, there are some special meters with power levels of over 15 W. Power supplies for meters can provide single outputs: 27 to 12 volts (or so), or dual outputs: 27 to 12V (or so) and 5V/3.3V. These power supplies are generally implemented using non-isolated flyback converters, but sometimes using buck or isolated flyback converters. 
         [0009]    All power supply controllers need to be started when a sufficient input voltage is present in order to drive the first pulses to energize the power supply controller. In low voltage applications, the controller can simply be connected to the rectified standard line voltage. For other applications such as those above 265V, a controller may typically include a high voltage (HV) startup pin or terminal and related circuitry which are connected to receive and convert the HV power to the levels required by the controller. 
       SUMMARY 
       [0010]    Briefly and generally, embodiments of the invention include a high voltage startup circuit. Embodiments of the invention also include a high voltage input capacitor balancing circuit. It is possible to use one or both of the inventive circuits in the same power supply. The startup and input capacitor balancing circuitry can be used in a wide range of applications, including low and high voltage applications. Embodiments of the invention also include power supplies having the start-up and input capacitor balancing circuitry. Further embodiments of the invention include a system (such as, a smart meter) incorporating such a power supply. 
         [0011]    In one embodiment, a startup circuit for a power supply is provided. The startup circuit comprises a resistance coupled between a voltage source and a first node. A first capacitor, coupled to the first node, is operable to be charged by current flowing through the resistance. A first transistor has an emitter, a base, and collector, wherein the collector is coupled to the voltage source and the base is coupled to the first node. A diac circuit. coupled to the emitter of the first transistor, is operable to fire to turn on the first transistor, thereby allowing discharge of the first capacitor through the base-emitter junction of the first transistor. A second capacitor is operable to be charged by current related to a discharge voltage resulting from the firing of the diac circuit. The second capacitor operable to store charge to provide VCC voltage to a controller of the power supply. 
         [0012]    In another embodiment, a startup circuit for providing a bias voltage comprises a resistance coupled between a voltage source and a first node. A first capacitor, coupled to the first node, is operable to be charged by current flowing through the resistance. A first transistor has a first terminal, a second terminal, and a control terminal, wherein the first terminal is coupled to the voltage source and the control terminal is coupled to the first node. A diac circuit, coupled to the second terminal of the first transistor, is operable to fire to turn on the first transistor, thereby allowing discharge of the first capacitor through the control-second terminal junction of the first transistor. A second capacitor is operable to be charged by current related to a discharge voltage resulting from the firing of the diac circuit. The second capacitor operable to store charge to provide the bias voltage. 
         [0013]    In yet another embodiment, a power supply comprises a resistance coupled between a voltage source and a first node. A first capacitor, coupled to the first node, is operable to be charged by current flowing through the resistance. A first transistor has a first terminal, a second terminal, and a control terminal, wherein the first terminal is coupled to the voltage source and the control terminal is coupled to the first node. A diac circuit, coupled to the second terminal of the first transistor, is operable to fire to turn on the first transistor, thereby allowing discharge of the first capacitor through the control-second terminal junction of the first transistor. A second capacitor is operable to be charged by current related to a discharge voltage resulting from the firing of the diac circuit. The second capacitor is operable to store charge to provide a supply voltage. 
         [0014]    Important technical advantages of the present invention are readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]    For a more complete understanding of the present invention and for further features and advantages, reference is now made to the following description taken in conjunction with the accompanying drawings. 
           [0016]      FIG. 1  is a schematic diagram of an exemplary implementation of a power supply with a start-up circuit. 
           [0017]      FIGS. 2A through 2C  are schematic diagrams of exemplary implementations of a start-up circuit. 
           [0018]      FIG. 2D  is a schematic diagram of exemplary implementations of a power supply circuit. 
           [0019]      FIG. 3  is a schematic diagram of an exemplary implementation of a power supply with an input capacitor balancing circuit. 
           [0020]      FIG. 4  is a schematic diagram of an exemplary implementation of a high voltage input capacitor balancing circuit for a power supply. 
           [0021]      FIGS. 5A and 5B  illustrate exemplary connection arrangements and configurations for the resistor divider and X 1  buffer. 
           [0022]      FIG. 6  is a schematic diagram of an exemplary implementation of an X 1  buffer. 
           [0023]      FIGS. 7A through 7G  are a schematic diagrams of exemplary implementations of a high voltage input capacitor balancing and startup circuit. 
           [0024]      FIG. 8  is schematic diagram of an exemplary implementation of a power supply. 
           [0025]      FIG. 9  is an exemplary equivalent circuit diagram and layout for capacitor balancing and startup circuits. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Embodiments of the present invention and their advantages are best understood by referring to  FIGS. 1 through 9  of the drawings. In general, like numerals are used for like and corresponding parts of the various drawings. 
       High Voltage Startup Circuit 
       [0027]    For high voltage supplies, embodiments of the invention can supply a properly scaled start-up voltage to many power supply controllers. All power supply controllers need to be biased via a start up supply so they can drive the first pulses to the output switch to get the supply running Once the power supply is running, the supply can provide the needed biasing to the controller via a Vcc or bias supply. 
         [0028]      FIG. 1  is schematic diagram of an exemplary implementation of a power supply  100  according to an embodiment of the invention. Such power supply could be used or incorporated into a device, such as, for example, a smart meter, that is subject to or used in a wide range of applications, from low voltage to high voltage. The power supply  100  can be connected to a power source at input terminal (CONS) and operate with inputs ranging from around, for example, approximately 150V to 1200V DC. The power supply  100  provides power for the device (e.g., smart meter) at output terminal (CON 2 ). 
         [0029]    As shown, the power supply can be a flyback design modified to operate over a very wide input voltage range and at low power. These modifications result in good efficiencies over the full input voltage range and operating loads. 
         [0030]    The power supply  100  can be a switching mode power supply (SMPS) having at least one switching device, which is turned on and off to deliver power to the output of the power supply  100 . In this embodiment, power supply  100  includes an emitter-switched, BJT/MOSFET cascode, which is made up of a BJT  112  and a switching device  114  (also labeled as Q 4  and Q 5 , respectively), in cascade so that a controller  110  drives the switching device  114  and the BJT  112  withstands the high-voltage. This makes the emitter-switched, BJT/MOSFET cascode easy to drive and delivers high performance switching at high voltages. 
         [0031]    The switching device  114  may be implemented as a metal-oxide-semiconductor field effect transistor (MOSFET), but it is understood that such switching device  114  can also be implemented with other suitable devices such as, for example, insulated gate bipolar transistors (IGBTs), insulated gate field effect transistors (IGFETs), bipolar junction transistors (BJTs), etc. 
         [0032]    The controller  110  generates a control signal which is provided to the control terminal of the switching device  114  for turning on and off the switching device  114  to provide AC drive to the transformer, output rectifier, and output regulator to produce a regulated voltage for current output. In some embodiments, for output voltage regulation, the control signal can be a pulse width modulation (PWM) control signal. In some embodiments, the controller  110  can be implemented with any suitable SMPS controller, such as, for example, as UC3845B or a FAN7601, both available from Fairchild Semiconductor Corp. The particular controller  110  in this illustrative embodiment does include a HV input terminal (VSTR). 
         [0033]    The power supply  100  includes a high voltage startup circuit  120  which is connected or coupled to and provides startup VCC voltage for the controller  110 . 
         [0034]    Details for the operation of a high voltage startup circuit are described with reference to  FIGS. 2A through 2C . As shown in  FIGS. 2A and 2B , in one embodiment, high voltage startup circuit  120  can include transistors  122 ,  124 , and a diac circuit  126  (also labeled as Q 2 , Q 3 , and DB 3 , respectively). The transistors  122 ,  124  can be implemented, for example, as BJTs, IGBTs, MOSFETS, or any other suitable device. The diac circuit  126  can be implemented with a diac or a similar circuit or device functioning in a similar manner to a diac (e.g., two transistors (BJTs or MOSFETs coupled in a compound arrangement with positive feedback). In one embodiment, transistor  122  can be implemented with a FJP5089 or FJP2222, transistor  124  can be implemented with a FJP5603, and the diac circuit  126  can be implemented with a DB 3 , all available as semiconductor integrated circuits (ICs) from Fairchild Semiconductor Corp. 
         [0035]    High voltage startup circuit  120  is a non-dissipative, active start-up circuit implemented to optimize converter efficiency for the power supply  100 . In contrast, a pure resistive start-up circuit would dissipate power and have extremely low total converter efficiency due to the resistive dissipation versus low output power (2 W). 
         [0036]    In some embodiments, the transistors  122  and  124 , and the diac circuit  126  can be provided in the same or separate integrated circuit (IC) packages. In one embodiment, these components for high voltage startup circuit  120  may be provided in a single module (“co-package” or “co-pack”). With such configuration, as shown, the module has terminals, for example, for power, input, ground, gate, and out ( FIG. 2A ). High voltage startup circuit  120  can be coupled to the controller to provide VCC start-up power (e.g., at the out terminal). In the power supply, the start up circuit  120  can be coupled to the power source (e.g., 120V to 1200V DC) through start-up resistance and capacitance. The start-up resistance comprises a series of resistors  128 - 140  (also labeled as R 1 -R 6 , respectively), and the capacitance includes capacitors  142 - 146  (also labeled as C 1 -C 3 , respectively). The startup resistors  128 - 140  can act as balance to ensure the same voltage drop across each input capacitor and to supply the current to the base of transistor  124  of the high voltage startup circuit  120 . Furthermore, startup resistors  128 - 140  form a current source from the power source used to charge a capacitor  148  (also labeled as C 4 ) of the power supply  100 . 
         [0037]    According to an embodiment of the invention, in operation for the high voltage start up circuit  120 , the current through resistors  128 - 140  charge capacitor  148  as long as switching device  114  ( FIG. 1 ) of the power supply  100  remains off Capacitor  148  charges to a sufficiently high voltage (e.g., approximately 32 volts) so that the voltage on the emitter of transistor  124  of start up circuit  120  reaches the trigger voltage for the diac circuit  126 . The diac circuit  126  fires and discharges capacitor  148  (about 10 volts) through the base-emitter junction of the transistor  124  and then turns off The discharge voltage divided by a resistor  152  (also labeled R 9 ) produces a pulse of current of around, for example, 0.3 A into the VCC storage capacitors  150  and  116  (also labeled as C 14  in  FIG. 1 ). Base current at transistor  124  turns it on. The gain of transistor  124  will multiply the discharge current to add to the current through the diac circuit  126 . Once the diac circuit  126  turns off, capacitor  148  begins to charge up again in another cycle. Each charge cycle adds to the voltage on VCC storage capacitors  150  and  116 . These cycles continue until the VCC voltage is sufficient for the controller  110  to start. Before VCC reaches the device threshold, the output of controller  110  is low and switching device  114  is off. 
         [0038]    The high voltage startup circuit  120  can be turned off after use to minimize current draw. Specifically, once the controller  110  starts, the output (e.g., Vref for a FAN7601 implementation) of the controller  110  goes high turning on switching device  112 . This in turn shunts the charging current for capacitor  148  to ground and turns off the high voltage startup circuit  120 . Thus, once startup for the controller  110  is complete, the high voltage startup circuit  120  shuts down and the circuit dissipation is reduced, for example, to the order of &lt;1 uW. If controller  110  stops, then high voltage startup circuit  120  will kick in again to bring VCC up to an appropriate level to allow the controller to restart. The high voltage startup circuit  120  can operate over a very wide input range—e.g., from 50V to 1500V DC. 
         [0039]      FIG. 2C  is a schematic diagram of another exemplary implementation of a start-up circuit  170  for a power supply, like that shown in  FIG. 1 . Startup circuit  170  is a non-dissipative, active start-up circuit implemented to optimize converter efficiency for a power supply. In one embodiment, high voltage startup circuit  170  can include transistors  172 ,  174 ,  175  and a diac circuit  176  (also labeled as Q 1 , Q 2 , Q 3 , and D 1 , respectively). As shown, the transistors  172 ,  174  are implemented as BJTs and transistor  175  may be implemented as a MOSFET, but it is understood that such transistors may also be implemented with other suitable devices such as, for example, insulated gate bipolar transistors (IGBTs), insulated gate field effect transistors (IGFETs), etc. 
         [0040]    In the power supply, the start up circuit  170  can be coupled to the power source through start-up resistance and capacitance. The start-up resistance comprises a series of resistors  178 ,  180  (also labeled as R 1  and R 2 , respectively), and the capacitance includes capacitors  184 ,  186  (also labeled as C 1  and C 2 , respectively). 
         [0041]    In operation for the start up circuit  170 , the current through resistors  178  and  180  charge capacitor  182  (also labeled as C 3 ) as long as switching device  114  ( FIG. 1 ) of the power supply remains off Capacitor  182  charges to a sufficiently high voltage so that the voltage on the emitter of transistor  172  of start up circuit  170  reaches the trigger voltage for the diac circuit  176 . The diac circuit  176  fires and discharges capacitor  182  through the base-emitter junction of the transistor  172  and then turns off The discharge charges VCC storage capacitor  188  (also labeled as C 4 ). Once the diac circuit  176  turns off, capacitor  182  begins to charge up again in another cycle. Each charge cycle adds to the voltage on VCC storage capacitor  188 . These cycles continue until the VCC voltage is sufficient for the controller  110  ( FIG. 1 ) to start. Before VCC reaches the device threshold, the output of controller  110  is low and switching device  114  is off The high voltage startup circuit  170  can be turned off after use to minimize current draw. Once the controller  110  starts, the switching device  114  is turned on lowering the voltage on the collector of transistor  172  to less than 2V typically. The collector of transistor  174  also turns on a diode  190  (also labeled as D 2 ), discharging the voltage on capacitor  182  so that the voltage never gets high enough on the base of transistor  172  to cause the diac circuit  176  to conduct. The pulses from the controller  110  operate at sufficiently high frequency so that the voltage on capacitor  182  stays discharged and the start up circuit  170  is effectively turned off 
         [0042]    The startup circuits (e.g.,  120  and  170 ) can be used for other applications besides providing VCC for a controller in a power supply. In some applications, for example, the high voltage startup circuits can used as a current source or to flash LEDs from a high voltage input. Furthermore, such circuits can be themselves be used as a power supply. And with a voltage regulator added, the circuits can be used as a regulated power supply.  FIG. 2D  illustrates how the start up circuit could be used as a regulated power supply. 
         [0043]      FIG. 2D  is a schematic diagram of an exemplary implementation of a power supply circuit  1000 . In one embodiment, power supply circuit  1000  can include transistors  1124 ,  1122 , and a diac circuit  1126  (also labeled as Q 1 , Q 2 , and D 1 , respectively). As shown, the transistors  1122 ,  1124  are implemented as BJTs, but it is understood that such transistors may also be implemented with other suitable devices such as, for example, a MOSFET, IGBTs, IGFETs, etc. Power supply circuit  1000  also includes resistance  1128 ,  1130  (also labeled as R 1  and R 2 , respectively), capacitors  1148 ,  1150  (also labeled as C 3  and C 4 , respectively), and Zener diode  1152  (also labeled as D 2 ). 
         [0044]    In operation, current flowing from input voltage Vin through resistance  1128  charges capacitor  1148  until the voltage reaches the firing voltage for diac circuit  1126 . When the diac circuit  1126  fires, it discharges capacitor  1148  through the base-emitter junction of transistor  1124 . The gain of transistor  1124  will multiply the discharge current to add to the current through the diac circuit  1126 . During discharge the voltage of diac circuit  1126  may drop from, for example, approximately 33V, to a few volts during conduction. Both diac circuit  1126  and transistor  1124  will remain on until the current through transistor  1124  drops to a point where the combined current of transistor  1124  and the capacitor  1148  discharge current can no longer keep the diac circuit  1126  conducting. Each current discharge cycle adds charge to capacitor  1150  which increases its voltage. Once the voltage reaches the breakdown voltage for zener diode  1152  and the base emitter voltage for transistor  1122 , transistor  112  discharges capacitor  1148  stopping the charge/discharge cycle and regulating the voltage on capacitor  1150  to provide a regulated voltage Vbias. 
       High Voltage Input Capacitor Balancing Circuit 
       [0045]    Power supplies (e.g., SMPSs) may have one or more capacitors to filter the input power. Power supplies with inputs greater than 450V DC typically use stacked electrolytic capacitors for the input filter. These capacitors can introduce problems. All capacitors have leakage current. The leakage of the capacitors can be significant and varies from capacitor to capacitor initially, and over time and temperature. The unbalanced capacitor leakage produces different voltages across the stacked capacitors which can lead to premature failure. 
         [0046]    The typical way to balance the voltage across stacked capacitors is to put balancing resistors across each capacitor. The voltage divides according to the ratio of the resistors instead of the capacitors. The resistor bleed currents are selected to be at least  10  times the expected worst-case capacitor leakage currents over time and temperature. This in itself is problematic because the bleed currents can be substantial, significantly increasing power consumption. More specifically, under normal working conditions, the balance resistances still dissipate a small amount of power; but this can be fairly low if the resistances are set fairly high. The typical value for such resistors is 200K ohm across each capacitor assuming the input capacitor values are below 33 uF at 450V. Two resistors are typically used to realize the 200K ohms to obtain adequate voltage breakdown. Thus, each resistor can be approximately 100K ohms The larger the capacitor value and the higher the operating temperatures, the higher the leakage currents through the capacitors and the lower the resistor values need to be to compensate. Unfortunately, the power lost via the balancing resistors is meaningful even under ideal conditions and maximum power output. This can be aggravated by the wide input operating range for the power supply. 
         [0047]    To address or reduce this problem, a high voltage input capacitor balancing circuit, according to some embodiments, functions to balance the input capacitors in a high voltage power supply. In some embodiments, the high voltage input capacitor balancing circuit may be used or combined with a high voltage startup circuit to provide startup voltage (e.g., startup VCC) to controllers with HV startup capabilities. The divider losses can be reduced by up to 90% by actively driving the capacitor balance using a buffer with a gain of approximately one. 
         [0048]    Smart meter supplies are sized based upon the worst-case power requirement. Smart meters draw most power during transmission/reception. Non-transmission power requirements (standby power) could be 10% or less of the maximum. During standby, fixed loads such as the input capacitor bleed resistors can be a substantial percentage of all power supply current. Thus, represent a significant loss during standby. Meters are typically run in standby most of their operating time. To maximize meter efficiency, a method needs to be employed to minimize capacitor balancing overhead power costs during standby. 
         [0049]      FIG. 3  is schematic diagram of an exemplary implementation of a power supply  200  according to an embodiment of the invention. Such power supply  200  could be used or incorporated in device, such as, for example, a smart meter, that is subject to or used in a wide range of applications, from low voltage to high voltage. The power supply  200  can be connected to a power source at an input terminal and operate with a wide range of input voltages, for example, approximately 120V to 850V DC. The power supply  200  provides power for the device (e.g., smart meter) at an output terminal 
         [0050]    Power supply  200  includes a switching controller  210  and a high voltage input capacitor balancing circuit  300 . In one embodiment, switching controller  210  can be implemented with a suitable controller having an HV input pin, such, for example, a FAN 7601 switching controller available from Fairchild Semiconductor Corp. The high voltage input capacitor balancing circuit  300  can be implemented in a number of ways. 
         [0051]      FIG. 4  is schematic diagram of an exemplary implementation of a high voltage input capacitor balancing circuit  300  according to an embodiment of the invention. High voltage input capacitor balancing circuit  300  can provide active balance for a two input capacitor configuration (capacitors C 1  and C 2 ) used as an input filter for a power supply. A resistor R 3  limits the drive current to capacitors C 1  and C 2  given worst-case capacitance value imbalance and input voltage ripple. 
         [0052]    As shown, high voltage input capacitor balancing circuit  300  includes voltage divider  310  and a X 1  (“times one”) buffer  312 . Voltage divider  310  can be set, for example, to be one-fifth to one-twentieth the normal required bleed current, given the values of capacitors C 1  and C 2  and their worst-case leakage current over time and temperature. In one embodiment, voltage divider  310  can be implemented with a plurality of resistors (e.g., R 1 , R 2  as shown) having values which provide the desired lower bleed current (e.g., one-fifth to one-twentieth the normal required bleed current).  FIGS. 5A and 5B  illustrate exemplary connection arrangements and configurations for the resistor divider  310  and X 1  buffer  312 . 
         [0053]      FIG. 6  is schematic diagram of an exemplary implementation of the X 1  buffer  312  according to an embodiment of the invention. As shown, the X 1  buffer  312  itself may include transistors  314 ,  316 , and  318 . In one embodiment, transistor  314  can be implemented with a MOSFET, such as a FQNIN60C, available from Fairchild Semiconductor Corp.; and each of transistors  316  and  318  can be implemented with a BJT, such as a KSP94, also available from Fairchild Semiconductor Corp. It should be understood, however, that transistors  314 ,  316 , and  318  can be implemented with other suitable devices. 
         [0054]    Referring to  FIGS. 4-6 , in operation, if capacitors C 1  and C 2  have the same leakage currents by chance, that is, are balanced the voltage on each capacitor would be the same, and no corrective current is required from high-voltage input capacitor balancing circuit  300  through resistor R 1 . Within the high voltage input capacitor balancing circuit  300 , some current flows through the resistors R 2 , R 3 , R 4 , and R 5  of voltage divider  310  to establish voltage reference levels, but the amount of current will be relatively small as compared to a typical bleed current that would be required for passive balancing (which does not include the X 1  buffer  312 ). In X 1  buffer  312 , the transistors  314 ,  316 , and  318  are turned off so that no current flows. Thus, the high voltage input capacitor balancing circuit  300  does not consume significant power when capacitors C 1  and C 2  are balanced. However, if capacitors C 1  and C 2  are not balanced, then the X 1  buffer  312  provides current through resistor R 1  to actively drive the capacitor balance. This current is sourced through transistor  314  or  316 ,  317 , which is turned on by the voltage difference between the input voltage to the buffer established by divider R 2 , R 3 , R 4 , R 5 , and the voltage on input filter capacitors common connection. If the buffer input is positive relative to the capacitor common voltage,  314  is tuned on. If the relative buffer input is negative,  316  and  318  are turned on and  314  remains off 
         [0055]    In addition, high voltage input capacitor balancing circuit  300  can also provide a buffered voltage equal to, for example, one-half the input voltage for a controller in the power supply that has a HV startup pin. The MOSFET implementation for transistor  314  can provide extra gain to drive the high-voltage startup controller pin of the controller. Note:  314  could also be implemented using a high gain BJT such as a Darlington transistor. Controllers with HV startup pins turn off the current draw after startup so are very efficient. This combined with the active balance circuit can greatly improve power supply efficiency. 
         [0056]      FIGS. 7A through 7G  are a schematic diagrams of exemplary implementations of a high voltage input capacitor balancing and startup circuit. 
         [0057]    Referring to  FIG. 7A , an exemplary implementation of a high voltage input capacitor balancing and startup circuit  400 , according to an embodiment of the invention, can provide active balance for two input capacitor configuration (capacitors C 1 , C 2 ) used as an input filter for a power supply. High voltage input capacitor balancing circuit  400  can also provide startup voltage for the controller in a power supply. 
         [0058]    As shown, high voltage input capacitor balancing circuit  400  includes voltage divider  410  and a X 1  (“times one”) buffer  412 . In one embodiment, the X 1  buffer  412  can be packaged in SOIC (small-outline integrated circuit) package with 8 pins. Voltage divider  410  can be implemented with resistors R 1 , R 2 , R 3 , and R 4 , having values which provide the desired lower bleed current (e.g., one-fifth to one-twentieth the normal required bleed current). 
         [0059]      FIG. 7B  is schematic diagram of an exemplary implementation of a high voltage input capacitor balancing and startup circuit  500  according to an embodiment of the invention. High voltage input capacitor balancing circuit  500  can provide active balance for a three input capacitor configuration (capacitors C 1 , C 2 , and C 3 ) used as an input filter for a power supply. High voltage input capacitor balancing circuit  500  can also provide startup voltage for the controller in a power supply. 
         [0060]      FIG. 7C  illustrates that the start up and balance circuits can be stacked in a similar manner to the input capacitors to provide capacitor balance for any number of input capacitors or input voltage. For example, in one embodiment, four input capacitors would require three start up and balance circuits in stacked arrangement. Six input capacitors would require five stacked start up and balance circuits. 
         [0061]    As shown, high voltage input capacitor balancing circuit  500  includes voltage divider  510  and two X 1  (“times one”) buffers  512 . In one embodiment, each X 1  buffer  512  can be packaged in a SOIC package with 8 pins. The X 1  buffers  512  can be stacked to balance capacitors C 1 , C 2 , and C 3 . Voltage divider  510  can be implemented with resistors R 1 , R 2 , R 3 , R 4 , R 5 , and R 6  having values which provide the desired lower bleed current (e.g., one-fifth to one-twentieth the normal required bleed current). 
         [0062]    From the foregoing, it would seem that active capacitor balancing would be very useful in high-voltage low-power supplies where standby power was important. However, as power output gets larger so does the input capacitor value and the resultant leakage. From this, it can be seen that virtually any power supply can benefit if minimum load efficiency is important. 
         [0063]      FIGS. 7D through 7G  are schematic diagrams of further exemplary implementation of input capacitor balancing and startup circuits, according to embodiments of the invention. 
       Low Voltage Capacitor Balancing Circuit 
       [0064]    Capacitive balancing may also be an issue at low voltages when using high capacitive value capacitors such as “Supercapacitors” or “Supercaps.” Supercaps typically have voltage ratings typically below 6 volts, and so must be stacked to increase the voltage at which they can function. Additionally, low voltage capacitors, such as Supercaps, are very sensitive to over-voltage and thus must be balanced carefully. Finally, the energy storage capacity of such capacitors is very high so the resistive balancing methodology may be inappropriate especially during charge and discharge. 
         [0065]    In one embodiment, low voltage capacitors are balanced without excessive bleed current, thereby increasing power supply efficiency. Such embodiment can reduce the traditional divider losses by up to 90% or more. 
         [0066]      FIG. 8  is schematic diagram of an exemplary implementation of a power supply  600  according to an embodiment of the invention. Such power supply  600  could be used or incorporated in device, such as, for example, smart meter output voltage storage, that is subject to or used in a wide range of applications. The power supply  600  can be connected to a power source at an input terminal and operate with a wide range of input voltages, for example, approximately 6 to 20 DC. The power supply  600  provides power for the device (e.g., smart meter) at an output terminal These balancing circuits can be stacked along with additional capacitors for higher voltage applications. 
         [0067]    Power supply  600  includes low voltage capacitors C 1 , C 2 , and C 3  in stacked arrangement. Each capacitor C 1 , C 2 , and C 3  may have high energy storage capacity and relatively low voltage rating. These capacitors can each be implemented with a Supercapacitor. Resistors R 1 , R 2 , R 2  form a voltage divider that evenly divides the input voltage among the three stacked capacitors C 1 , C 2 , and C 3 . Resistors R 4  and R 5  limit the drive current to capacitors C 1 , C 2 , and C 3 . Each of operational amplifiers U 1   a  and U 1   b  can be implemented as a buffer amplifier (e.g., with approximately x1 gain). With this arrangement, the low voltage capacitors C 1 , C 2 , and C 3  can be used at higher voltages and in many applications where they previously could not be used. 
         [0068]      FIG. 9  is an exemplary equivalent circuit diagram and layout (with  2  dies or “chips”) for capacitor balancing and startup circuits. 
         [0069]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention. That is, the discussion included in this application is intended to serve as a basic description. It should be understood that the specific discussion may not explicitly describe all embodiments possible; many alternatives are implicit. It also may not fully explain the generic nature of the invention and may not explicitly show how each feature or element can actually be representative of a broader function or of a great variety of alternative or equivalent elements. Again, these are implicitly included in this disclosure. Where the invention is described in device-oriented terminology, each element of the device implicitly performs a function.