Abstract:
An apparatus includes an energy storage device, a driver circuit configured to receive energy from the energy storage device, and a bleed circuit configured to reduce an amount of the energy received by the driver circuit from the energy storage device. The bleed circuit is configured to reduce the amount of the energy received by the driver circuit during a startup period. The energy storage device may include a transformer, the driver circuit and bleed circuit being coupled to first and second windings of the transformer, respectively. A method includes receiving, by a driver circuit, energy from an energy storage device, and reducing, using a bleed circuit, the energy received by the driver circuit during a startup period. The energy storage device may include a transformer, the driver circuit and bleed circuit being coupled to first and second windings of the transformer, respectively.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This present disclosure claims the benefit of U.S. Provisional Application No. 61/939,408, filed on Feb. 13, 2014, which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     A dimmer circuit produces an output signal with a variable voltage or a variable current. The dimmer circuit receives a control signal and controls the current or voltage of the output signal according to a value of the control signal. The dimmer circuit may be used to control a brightness of a light source, such as a Light Emitting Diode (LED). 
     The dimmer circuit may produce an output signal having a higher current or a higher voltage than called for by the value of the control signal. The higher current or higher voltage is called an overshoot. In a lighting system wherein the dimmer circuit controls an LED, the overshoot may produce a brief interval during which the light emitted by the LED is undesirably bright, such as by producing a flash of light. 
     SUMMARY 
     In an embodiment, an apparatus includes an energy storage device, a driver circuit configured to receive energy from the energy storage device, and a bleed circuit configured to reduce an amount of the energy received by the driver circuit from the energy storage device. 
     In an embodiment, the bleed circuit is configured to reduce the amount of the energy received by the driver circuit during a startup period. 
     In an embodiment, the driver circuit is configured to receive the energy from the storage device during a discharge interval, and the bleed circuit is configured to reduce the amount of the energy received by the driver circuit during the discharge interval. 
     In an embodiment, the energy storage device includes a transformer. The driver circuit is coupled to a primary winding of the transformer, and the bleed circuit is coupled to an auxiliary winding of the transformer. 
     In an embodiment, the apparatus is a dimming system. 
     In an embodiment, the apparatus is provided in an integrated circuit. 
     In an embodiment, a method includes receiving, by a driver circuit, energy from an energy storage device, and reducing, using a bleed circuit, the energy received by the driver circuit during a startup period. 
     In an embodiment, the energy storage device is a transformer. The driver circuit is coupled to a first winding of the transformer, and the bleed circuit is coupled to a second winding of the transformer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  illustrate a dimmer system according to an embodiment. 
         FIG. 3  is a waveform diagram illustrating an operation of the dimmer system of  FIG. 2  according to an embodiment. 
         FIG. 4  shows waveforms illustrating an operation of a dimmer system. 
         FIG. 5  shows waveforms illustrating an operation of a dimmer system according to an embodiment. 
         FIG. 6  is a flowchart of a process for preventing an overshoot according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a dimmer system  1 - 100  according to an embodiment. The dimmer system  1 - 100  includes a voltage supply circuit  1 - 102 , a control circuit  1 - 104 , a bleed circuit  1 - 106 , a Light Emitting Diode (LED) driver circuit  1 - 116 , a transformer  1 - 110 , and an LED  1 - 120 . 
     The dimmer system  1 - 100  operates by periodically storing energy into the transformer  1 - 110  using a primary winding (or a first winding)  1 - 112  thereof, and then discharging the energy. The transformer  1 - 110  operates as an energy storage device. The control circuit  1 - 104  controls, using the LED driver circuit  1 - 116 , the sequencing of storing the energy during charge intervals and discharging the energy during discharge intervals. 
     In an embodiment, the charge intervals and the discharge intervals are disjoint, that is, the energy is not simultaneously stored into and discharged from the transformer  1 - 110 . In an embodiment, the control circuit  1 - 104 , transformer  1 - 110 , and LED driver circuit  1 - 116  form a Direct-Current-to-Direct-Current (DC-to-DC) converter such as a buck converter, a boost converter, a buck-boost converter, a boost-buck converter, a flyback converter, or the like. 
     A first portion of the energy stored in the transformer  1 - 110  may be discharged through the LED driver circuit  1 - 116  in order to provide a current to the LED  1 - 120 . The control circuit  1 - 104  controls an output of the LED  1 - 120 , that is, the control circuit  1 - 104  dims the LED  1 - 120 , by controlling a magnitude of the first portion of the energy. 
     The control circuit  1 - 104  controls the magnitude of the first portion of the energy according to a value of a control signal CTRL. In an embodiment, the control signal CTRL may include an Inter-Integrated Circuit (I 2 C) signal, a System Management Bus (SMBus) signal, a Pulse Width Modulated (PWM) signal, an analog signal, or the like. 
     Second and third portions of the energy may be discharged through the auxiliary winding (or a second winding)  1 - 114  of the transformer  1 - 110  into the voltage supply circuit  1 - 102  and the bleed circuit  1 - 106 , respectively. The voltage supply circuit  1 - 102  uses the second portion of the energy to provide a supply voltage VDD to the control circuit  1 - 104 . 
     The control circuit  1 - 104  controls the magnitude of the first portion of the energy discharged through the LED driver circuit  1 - 116  by one or more of controlling a magnitude of the energy stored in the primary winding  1 - 112  and controlling a magnitude of the third portion of the energy discharged through the auxiliary winding  1 - 114 . 
     The control circuit  1 - 104  controls the bleed circuit  1 - 106  to control the magnitude of the third portion of the energy. The control circuit  1 - 104  decreases the magnitude of the first portion of the energy discharged through the LED driver circuit  1 - 116  by controlling the bleed circuit  1 - 106  to increase the magnitude of the third portion of the energy discharged into the bleed circuit  1 - 106 . The magnitude of the third portion of the energy may be increased during a startup period to prevent an overshoot in the current supplied to the LED  1 - 120 . 
     A person of skill in the art in light of the teachings and disclosures herein would recognize that the dimmer system  1 - 100  may include a variety of other circuits known to the art, such as one or more high voltage startup circuits, communication circuits, synchronization circuits, reference voltage circuits, clock circuits, protection circuits, and the like, which are omitted from the present application in the interest of brevity. A person of skill in the art in light of the teachings and disclosures herein would also understand that the dimmer system  1 - 100  could be employed to control a magnitude of a voltage or current delivered to a plurality of LEDs  1 - 120  or to devices other than an LED. 
       FIG. 2  illustrates a dimmer system  2 - 100  suitable for use as the dimmer system  1 - 100  according to an embodiment, showing additional details of the embodiment. 
     The dimmer system  2 - 100  receives a line voltage VL at an anode of a first diode  202 . The line voltage VL may include an alternating current (AC) voltage, a pulsating direct current (DC) voltage, or a DC voltage. In an embodiment, a voltage value of the line voltage VL is a low voltage such as 1.5 volts. In another embodiment, the voltage value of the line voltage VL is a high voltage such as 120, 220, or 400 volts. 
     A cathode of the first diode  202  is connected to a first terminal of a first capacitor  204 . A second terminal of first capacitor  204  is connected to ground. The first diode  202  and the first capacitor  204  produce an input voltage VIN by rectifying and filtering the line voltage VL. The first diode  202  also prevents a flow of current back from the dimmer system  2 - 100  into a source of the line voltage VL. 
     The input voltage VIN is supplied to a first terminal of a primary winding  2 - 112  of a transformer  2 - 110 . The first terminal of the primary winding  2 - 112  and a second terminal of the primary winding  2 - 112  are connected to an LED driver circuit  2 - 116 . 
     The LED driver circuit  2 - 116  is a buck-boost converter suitable for use as an embodiment of the LED driver circuit  1 - 116  of  FIG. 1 . The LED driver circuit  2 - 116  includes a second diode  214 , a second capacitor  219 , a load resistor  212 , a driver n-channel Metal-Oxide-Semiconductor Field Effect Transistor (nMOSFET)  210 , a current sense resistor  218 , and positive and negative output terminals  222  and  220 . 
     The first terminal of the primary winding  2 - 112  is connected to a first terminal of the load resistor  212 , a first terminal of the second capacitor  219 , and the negative output terminal  220 . The second terminal of the primary winding  2 - 112  is connected to a drain of the driver re-channel nMOSFET  210  and to an anode of the second diode  214 . A cathode of the second diode  214  is connected to a second terminal of the load resistor  212 , a second terminal of the second capacitor  219 , and a positive output terminal  222 . 
     The second diode  214  and the second capacitor  219  are configured to receive energy discharged from the primary winding  2 - 112  and provide an output current I OUT  to a load connected to the positive and negative output terminals  222  and  220 . The output current I OUT  has a positive voltage at the positive output terminal  222  relative to a voltage at the negative output terminal  220 . The load resistor  212  operates to discharge energy stored in the second capacitor  219  when the output current I OUT  is not flowing through to the positive and negative output terminals  222  and  220 . 
     A source of the driver nMOSFET  210  is connected to a current sense signal I SNS  and a first terminal of the current sense resistor  218 . A second terminal of the current sense resistor  218  is connected to the ground. A voltage value of the current sense signal I SNS  is proportional to a current flowing through the primary winding  2 - 112  and the driver nMOSFET  210 . 
     A gate of the driver nMOSFET  210  is connected to a gate signal GATE. The driver nMOSFET  210  turns on when the gate signal GATE has a first value (i.e. an on value), and turns off when the gate signal GATE has a second value (i.e. an off value). 
     An anode of an LED  2 - 120  is connected to the positive output terminal  222 , and a cathode of the LED  2 - 120  is connected to the negative output terminal  220 . The LED  2 - 120  operates as the load of the LED driver circuit  2 - 116 . 
     A control circuit  2 - 104  receives the current sense signal I SNS  and a control signal CTRL, and produces the gate signal GATE according to values of the current sense signal I SNS  and the control signal CTRL. The control circuit  2 - 104  is configured to control the magnitude of the energy stored in the transformer  2 - 110  by generating the gate signal GATE having a sequence of on values and off values. In an embodiment, the control circuit  2 - 104  is configured to control the magnitude of the energy stored in the transformer  2 - 110  by one or more of controlling respective durations of a plurality of on values of the gate signal GATE and controlling respective intervals between consecutive on values of the gate signal GATE; that is, by one or more of Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM). 
     A person of skill in the art in light of the teachings and disclosures herein would understand how to configure the control circuit  2 - 104  to control the output current I OUT  by one or more of PWM and PFM of the gate signal GATE according to values of the control signal CTRL and the current sense signal I SNS , and therefore a description thereof is omitted from the present application in the interest of brevity. In an embodiment, the control circuit  2 - 104  includes a microcontroller or microprocessor that executes operations of the dimmer system  2 - 100  by executing computer programming instructions stored in a non-transient computer readable memory. 
     A first terminal of an auxiliary winding  2 - 114  of the transformer  2 - 110  is connected to a voltage supply circuit  2 - 102 , a voltage divider  224 , and a bleed circuit  2 - 106 . A second terminal of the auxiliary winding  2 - 114  is connected to the ground. The auxiliary winding  2 - 114  is configured to discharge energy stored into the transformer  2 - 110  into the voltage supply circuit  2 - 102  and the bleed circuit  2 - 106  as will be described below. 
     The voltage supply circuit  2 - 102  includes a third diode  208  and a third capacitor  206 . An anode of the third diode  208  is connected to the first terminal of the auxiliary winding  2 - 114  of the transformer  2 - 110 . A cathode the third diode  208  is connected to a first terminal of the third capacitor  206  and to an output of the voltage supply circuit  2 - 102  that provides a supply voltage VDD. A second terminal of the third capacitor  206  is connected to the ground. 
     A current flows through the third diode  208  and into the third capacitor  206  when a voltage value of the first terminal of the auxiliary winding  2 - 114  is greater than a voltage value of the first terminal of the third capacitor  206 . The current flowing into the third capacitor  206  increases the voltage value of the first terminal of the third capacitor  206 . Current flowing out of the third capacitor  206  through the output that provides the supply voltage VDD reduces the voltage value of the first terminal of the third capacitor  206 . 
     The voltage divider  224  includes an upper resistor  226  and a lower resistor  228 . A first terminal of the upper resistor  226  is connected to the first terminal of the auxiliary winding  2 - 114 . A second terminal of the upper resistor  226  is connected to an auxiliary voltage signal V SVR  and to a first terminal of the lower resistor  228 . A second terminal of the lower resistor  228  is connected to the ground. 
     The voltage divider  224  provides the auxiliary voltage signal V SVR  having a voltage value according to an auxiliary winding voltage V AW  of the auxiliary winding  2 - 114  and respective resistance values R U  and R L  of the upper and lower resistor  226  and  228 , according to Equation 1, below: 
                     V   SVR     =       V   AW     ·         R   L         R   U     +     R   L         .               (     Equation   ⁢           ⁢   1     )               
In an embodiment, the control circuit  2 - 104  senses the auxiliary winding voltage V AW  using the auxiliary voltage signal V SVR .
 
     The bleed circuit  2 - 106  is connected to the first terminal of the auxiliary winding  2 - 114  of the transformer  2 - 110 , and includes a bleed diode  230 , a bleed resistor  232 , and a bleed nMOSFET  234 . 
     The bleed diode  230  operates to prevent a backward flow of a bleed current I BLEED  from the bleed circuit  2 - 106  to the first terminal of the auxiliary winding  2 - 114 . An anode of the bleed diode  230  is connected to the first terminal of an auxiliary winding  2 - 114 . A cathode of the bleed diode  230  is connected to a first terminal of the bleed resistor  232 . 
     The bleed resistor  232  operates to determine a magnitude of a flow of the bleed current I BLEED  from the first terminal of the auxiliary winding  2 - 114  to the bleed circuit  2 - 106 . A second terminal of the bleed resistor  232  is connected to a drain of the bleed nMOSFET  234 . 
     A resistance value of the bleed resistor  232  is determined according to one or more of an amount of energy bled from the transformer  2 - 110  during a bleed operation, a magnitude of the supply voltage VDD, and a duration of the bleed operation T BLEED . 
     The bleed nMOSFET  234  is configured to control the bleed operation by turning on and off according to a value of the bleed signal BLEED. The bleed operation occurs when the bleed nMOSFET  234  is on and the energy is stored in the transformer  2 - 110 . 
     A source of the bleed nMOSFET  234  is connected to ground. A gate of the bleed nMOSFET  234  receives the bleed signal BLEED. When the bleed signal BLEED has a first value (i.e., an on value), the bleed nMOSFET  234  is on, and when the bleed signal BLEED has a second value (i.e., an off value), the bleed nMOSFET  234  is off. 
     The bleed signal BLEED is generated by the control circuit  2 - 104 . The control circuit  2 - 104  is configured to control an overshoot of the output current I OUT  by turning the bleed nMOSFET  234  on during a startup period of the dimmer system  2 - 100 . 
     The control circuit  2 - 104  is configured to provide a bleed signal BLEED having the on signal during intervals wherein the gate signal GATE has an off value and provide a bleed signal BLEED having the off signal during intervals wherein the gate signal GATE has an on value. In an embodiment, the control circuit  2 - 104  is configured to only turn the driver nMOSFET  210  on while the bleed nMOSFET  234  is off, and only turn the bleed nMOSFET  234  on while the driver nMOSFET  210  is off. 
     Although the embodiment described above includes nMOSFETs, embodiments are not limited thereto. A person of skill in the art would understand that any of a variety of three-terminal electronic devices or circuits able to amplify and switch electrical signals could be used instead of the nMOSFETs described above, including p-channel MOSFETs, Junction Field-Effect Transistors (JFETs), bipolar junction transistors (BJTs), and combinations thereof. 
       FIG. 3  is a waveform diagram illustrating an operation of the dimming system  2 - 100  of  FIG. 2  according to an embodiment. In an embodiment, the operations illustrated in  FIG. 3  occur during a startup period of the dimming system  2 - 100 . 
     A person of skill in the art in light of the teachings and disclosures herein would recognize that the waveforms of  FIG. 3  show one of a plurality of consecutive charge and discharge intervals of the dimming system  2 - 100 . That is, the operation of the dimming system  2 - 100  includes a plurality of charge intervals and a plurality of discharge intervals interleaved with the plurality of charge intervals, such that a discharge interval occurs between each consecutive pair of charge intervals. 
     At a charge start time T 1 , the control circuit  2 - 104  provides a gate signal GATE having an on value to the driver nMOSFET  210 . After a turn on delay time elapses after the charge start time T 1 , a drain-source current I DS  flows through the driver nMOSFET  210 , the primary winding  2 - 112  of the transformer  2 - 110 , and the current sense resistor  218 . The flow of the drain-source current I DS  stores energy into the transformer  2 - 110 . 
     While the drain-source current I DS  flows through the primary winding  2 - 112 , the magnetic coupling between primary and auxiliary windings  2 - 112  and  2 - 114  of the transformer  2 - 110  induces a value of the auxiliary winding voltage V AW  according to a value of the input voltage VIN, a number of turns N A  of the auxiliary winding  2 - 114 , and a number of turns N P  of the primary winding  2 - 112 , as shown in Equation 2: 
     
       
         
           
             
               
                 
                   
                     V 
                     AW 
                   
                   = 
                   
                     
                       - 
                       VIN 
                     
                     · 
                     
                       
                         
                           N 
                           A 
                         
                         
                           N 
                           P 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
     Because the induced value of the auxiliary winding voltage V AW  is negative while the drain-source current I DS  flows through the primary winding  2 - 112 , the third diode  208  and the bleed diode  230  are reverse biased, and no current flows from the auxiliary winding  2 - 114  into the voltage supply circuit  2 - 102  or into the bleed circuit  2 - 106 . When the auxiliary winding voltage V AW  is negative, because no current flows from the auxiliary winding  2 - 114 , no energy is discharged from the transformer  2 - 110 . 
     At a charge stop time T 2 , the control circuit  2 - 104  provides a gate signal GATE having an off value to the driver nMOSFET  210 . When a propagation delay time PD elapses after the charge stop time T 2 , the drain-source current I DS  stops flowing through the driver nMOSFET  210  and the current sense resistor  218 . 
     After the drain-source current I DS  stops flowing through the driver nMOSFET  210 , that is, after an end of the charge interval, at a discharge start time T 3  the energy stored in the transformer  2 - 110  begins to discharge by generating a primary winding voltage drop V PW  and the auxiliary winding voltage V AW  across the primary and auxiliary windings  2 - 112  and  2 - 114 , respectively. Respective values of the primary winding voltage drop V PW  and the auxiliary winding voltage V AW  are related according to Equation 3: 
     
       
         
           
             
               
                 
                   
                     V 
                     AW 
                   
                   = 
                   
                     
                       - 
                       
                         V 
                         PW 
                       
                     
                     · 
                     
                       
                         
                           N 
                           A 
                         
                         
                           N 
                           P 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
     The values of the primary winding voltage drop V PW  and the auxiliary winding voltage V AW  generated by the discharge of the energy stored in the transformer  2 - 110  have polarities opposite to their respective polarities when the drain-source current I DS  was flowing. Therefore, when the energy stored in the transformer is being discharged, a voltage at the second terminal of the primary windings  2 - 112  is greater than a voltage at the first terminal of the primary windings  2 - 112 , and the polarity of the value of the auxiliary winding voltage V AW  is positive. 
     When the value of the primary winding voltage drop V PW  exceeds a forward voltage drop of the second diode  214  and a voltage drop across the second capacitor  219  of the LED driver circuit  2 - 116 , the output current I OUT  flows through the second diode  214 . If the voltage drop across the second capacitor  219  exceeds a forward voltage drop of the LED  2 - 120 , some or all of the output current I OUT  flows through LED  2 - 120  and the LED  2 - 120  produces light. 
     At a bleed start time T 4 , the control circuit  2 - 104  turns the bleed nMOSFET  230  on by producing a bleed signal BLEED having the on value. In an embodiment, the bleed start time T 4  is determined by a delay function of the control circuit  2 - 104  according to the charge stop time T 2  and a propagation delay time PD. 
     In another embodiment, the bleed start time T 4  is determined according to a comparison of the auxiliary winding voltage V AW  to a reference voltage V REF . For example, the bleed start time T 4  may occur when the auxiliary winding voltage V AW  increases to a value greater than a value of a reference voltage V REF , such as occurs at a first crossover time T 5  shown in  FIG. 3 . 
     When the bleed nMOSFET  234  turns on, the bleed diode  230  conducts and the bleed current I BLEED  flows from the auxiliary winding  2 - 114  through the bleed circuit  2 - 106 . A value of the bleed current I BLEED  is determined according to a value of the auxiliary winding voltage V AW  and a resistance value of the bleed resistor  232 . 
     When the discharge of the energy stored in the transformer  2 - 110  begins, the sum of the value the output current I OUT  and the value of the bleed current I BLEED  will equal the value of the source-drain current I DS  at the time when the driver nMOSFET  210  was turned off Therefore, an increase in the value of the bleed current I BLEED  will cause a decrease in the value of the output current I OUT  and prevent an overshoot from occurring. 
     In an embodiment, the control circuit  2 - 104  continues to produce the bleed signal BLEED having the on value and the bleed nMOSFET  234  is turned on until, at the discharge stop time T 6 , the energy stored in the transformer  2 - 110  is substantially completely discharged. 
     At a bleed stop time T 8 , the control circuit  2 - 104  turns off the bleed nMOSFET  234  by producing the gate signal GATE having the off value. In an embodiment, the control circuit  2 - 104 , using a timing function of the control circuit  2 - 104 , determines the bleed stop time T 8  according to the bleed start time T 4  and a bleed duration T BLEED . In another embodiment, the control circuit  2 - 104  determines the bleed stop time T 8  according to the charge stop time T 2 , a propagation delay time PD, and the bleed duration T BLEED . In another embodiment, the control circuit  2 - 104  determines the bleed stop time T 8  according to a comparison of the value the auxiliary winding voltage V AW  and the voltage reference V REF , such as us shown at the second crossover time T 7 . 
       FIG. 4  shows waveforms illustrating an operation of a dimmer system of the related art.  FIG. 4  includes waveforms during a startup period of a current sense signal I SNS , an input voltage VIN, and a supply voltage VDD, and an output current I OUT . 
     At a startup time T STARTUP  a supply voltage VDD reaches a value sufficient for operation and generation of an output current I OUT  begins. At the overshoot time T OVER , an overshoot surge occurs in the output current I OUT . 
       FIG. 5  shows waveforms illustrating an operation of a dimmer system according to an embodiment.  FIG. 5  includes a waveform of an output current I OUT  during a startup period. As can be seen in the circled region, no overshoot occurs in the output current I OUT  during the startup period. 
       FIG. 6  is a flowchart of a process  600  for preventing overshoot during a startup period according to an embodiment. 
     At S 602 , an electronic system generates a charging pulse at the beginning of a charging cycle and provides the charging pulse to an energy storage device such as a transformer or an inductor. The charging pulse causes energy to be stored into the energy storage device and may be a pulse width modulated (PWM) pulse. When the charging pulse ends, the energy stored in the energy storage device begins to discharge. 
     At S 604 , whether the electronic system is in a startup period is determined. The startup period may be determined using one or more of a timer circuit, a timer function of a program executed on a processor of the electronic system, and a comparison of a supply voltage with a reference voltage. 
     When the electronic system is not in the startup period, the process  600  proceeds to S 612  to wait for the beginning of the next charging cycle. 
     When the electronic system is in the startup period, the process  600  proceeds to S 606  and a bleed circuit is turned on to begin a bleed operation. The bleed circuit is configured to receive energy discharged from the energy storage device. 
     In an embodiment, the bleed circuit is turned on immediately after the charging pulse ends. In another embodiment, the bleed circuit is turned on at a delayed time after the charging pulse ends. The interval between the end of the charging pulse and the turning on of the bleed circuit may include a propagation delay time. In another embodiment, the bleed circuit is turned on when a voltage associated with the discharge of the energy from the energy storage device reaches a threshold value. 
     At S 608 , the process  600  determines whether the bleed operation is complete. Whether the bleed operation is complete may be determined according to one or more of a time since the end of the previous charging pulse, a time since the start of the bleed operation, and a voltage associated with the discharge of the energy from the energy storage device. 
     When the bleed operation is complete, at S 610  the bleed circuit is turned off and the process  600  proceeds to S 612 . 
     At S 612 , the process  600  waits until the next charge cycle begins. The frequency of charging cycles may be fixed, or pulse frequency modulation (PFM) may be used to vary the frequency of the charging cycles. 
     Aspects of the present disclosure have been described in conjunction with the specific embodiments thereof that are proposed as examples. Numerous alternatives, modifications, and variations to the embodiments as set forth herein may be made without departing from the scope of the claims set forth below. Accordingly, embodiments as set forth herein are intended to be illustrative and not limiting.