Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/241,173 filed Sep. 10, 2009, which is incorporated herein by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     The disclosure is directed generally at a power supply conversion system and more specifically at a bootstrap startup and assist circuit for a power supply and the like. 
     BACKGROUND OF THE DISCLOSURE 
     With the rapid increase in Light Emitting Diode (LED) efficacies for high powered LEDs compared to common light sources, the latest LED technologies have exceeded the capabilities of incandescent and halogen sources and are now starting to compete with fluorescent, mercury vapour, metal halide and sodium lighting sources. In addition to better energy usage, LEDs also have considerable advantages over traditional light sources such as longer working life and better durability. 
     Solid State Lighting (SSL) systems that incorporate LEDs have the potential to generate energy savings if the power sources used to power them are energy efficient as well. 
     Various energy efficiency standards have been developed for consumer products including external power supplies and lighting fixtures. The Energy Star Program created by the Environmental Protection Agency (EPA) has recently established industry wide requirements for Solid State Lighting (SSL) products. The principle energy efficiency metric used is the luminaire efficacy whereby luminaire efficacy is defined as net light output from the fixture divided by input power. 
     Therefore, there is provided a method and apparatus for a bootstrap startup and assist circuit. 
     SUMMARY OF DISCLOSURE 
     The present disclosure relates generally to a high efficiency power converter for HB LED (High Brightness Light Emitting Diodes) lighting systems and in particular to a non isolated power converter with a constant current output. Applications for the converter include, but are not limited to, being used in LED street lighting and LED industrial and commercial lighting applications such as high bay or low bay lighting systems. 
     The disclosure is also directed at a bootstrap assist and start up circuit for coupling with a power factor correction (PFC) transition mode (TM) control and high side, low side MOSFET gate drive to control a two-switch buck-boost non-isolated converter. 
     Such embodiments of the present disclosure also include a unique start up circuit and “bootstrap” method to control turn on of the high side switch and an optional dimming interface and optional enable/disable input function. 
     In another embodiment, there is provided a bootstrap technique which is a method used to continuously switch a floating high side switch such as a MOSFET by means of continuously charging a capacitor and then “level shifting” this capacitor voltage across the gate and source of the high side switch to turn the switch on. 
     Other aspects and features of the present application will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the attached Figures, wherein: 
         FIG. 1  is a block diagram of a power factor corrected buck-boost converter with a constant current output; 
         FIG. 2  is a block diagram of a power factor corrected buck-boost convertor with multiple constant current outputs; 
         FIG. 3  is a schematic diagram of a non-isolated, power factor corrected buck boost converter with constant current output; 
         FIG. 4   a  is a schematic diagram of a bootstrap assist and start up circuit with a bootstrap capacitor charge with low side switch on; 
         FIG. 4   b  is a schematic diagram of a bootstrap assist and start up circuit with a bootstrap capacitor charge with a low side switch off; 
         FIG. 5  is a schematic diagram of bootstrap assist and start up circuit waveforms; 
         FIG. 6  is a flowchart outlining a method of non-isolated power factor corrected buck-boost power converting with constant current output; 
         FIG. 7  is a flowchart outlining a method of bootstrap assisting; and 
         FIG. 8  is a schematic diagram of a bootstrap assist and start up circuit. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure is directed at a method and apparatus for a bootstrap startup and assist circuit. 
     Turning to  FIG. 1 , a block diagram of a power factor corrected buck-boost converter with constant current output is shown. The converter  10  receives an input from a power supply such as an AC mains voltage input  12  and supplies an output to a load  14 . Some examples of loads include, but are not limited to, a set of light emitting diodes (LEDs), or any other component or apparatus requiring a constant current such as a rechargeable battery requiring a constant current for recharging. The converter  10  includes an electromagnetic interference (EMI) filter  16  which is connected to a bridge rectifier  18  which, in turn, is connected to a two-switch non-isolated buck boost power stage  20  including a current sense  22  which may be a resistor, as disclosed in more detail below. The converter  10  also includes a bootstrap assist and start up means, such as a circuit,  24  which is connected to a high side and low side gate drive  26  and a power factor correction (PFC) transition mode (TM) control  28 . The PFC transition mode control  28  is also connected to an error amplifier  30 . An output of the high side and low side gate drive  26  is connected to the power stage  20 . As will be understood, the arrows in  FIG. 1  represent the direction in which data or signals are travelling, however it will be understood that some of the connections may be bi-directional. In one embodiment, the bootstrap startup and assist circuit  24  or component, the PFC TM control  28  and the error amplifier  30  may be seen as a controller  31  as shown in  FIG. 3 . An alternate embodiment may include a transient suppression circuit connected between the AC mains voltage input  12  and the EMI filter  16 . 
     Turning to  FIG. 3 , a more detailed schematic diagram of the power factor corrected buck-boost converter  10  is shown. The AC mains voltage  12  provides an input voltage (seen as V 500 ) to the EMI filter  16  which is connected to the bridge rectifier  18 . The output of the bridge rectifier  18  is connected to the gate drive  26  and to the power stage  20 . 
     The power stage  20  includes a first switch  32 , an inductor  34  and a diode  36  which are connected in series between a positive output of the bridge rectifier  18  and the load  14 . Between the bridge rectifier  18  and the load  14 , there is also a set of components which are placed in parallel with each other. In the current embodiment, this parallel circuitry includes a diode  38 , in parallel with a second switch  40  and a first sense resistor  42 , in parallel with an output storage capacitor  44 . A second sense resistor  46  is located in series between the parallel circuitry and the load  14 . A high side output  48  of the gate drive  26  is connected to the first switch  32  and a low side output  50  of the gate drive  26  is connected to the second switch  40 . An output of the controller  31  is connected to the gate drive  26 . The controller  31  is also connected at various other contact points within the power factor converter as shown in  FIG. 3  such as, but not limited to, a disable/enable signal  52  and an external reference  54 . 
     In operation, the single-phase AC line voltage V 500  is applied to the input of the power converter  10 . The EMI filter  16  attenuates any EMI which is generated by the converter  10  when the voltage V 500  is received. In the current embodiment, the bridge rectifier  18  rectifies the output of the EMI filter  16 , however, it will be understood that any discrete rectifier component, such as a rectifier diode, may perform this function. 
     In the current embodiment, to begin a power conversion cycle, the two switches  32  and  40  are turned on by a gate drive signal from the controller  31 , simultaneously causing current to flow through inductor  34  thereby increasing energy stored in the inductor  34 . Diodes  38  and  36  are reversed biased during this state thereby decoupling the output storage capacitor  44  and the LED load  14  from the rectified line output of bridge rectifier  18 . 
     The PFC TM control  28  controls the amount of energy delivered to the load  14  by controlling energy stored and corresponding current through inductor  34 . When an appropriate or predetermined amount of energy is stored in inductor  34 , switches  32  and  40  are simultaneously switched off. When the switches are turned off, the polarity of inductor  34  reverses and freewheeling diode  38  and rectifying diode  36  conduct and transfer the stored energy in the inductor  34  to the output storage capacitor  44  and LED load  14 . In other embodiments the diodes  38  and  36  may be replaced by switches such as MOSFETs with a gate drive to allow the switches to be turned on and off at appropriate intervals. 
     The controller  31  comprises at least three components to assist in implementing the required control functions for the two-switch buck boost converter  20  with constant current output. In the preferred embodiment, the controller  31  includes PFC TM control  28 , such as a ST Microelectronics L6562A, to implement power factor correction with an error amplifier  30  to regulate output current. Regulation of the output current is accomplished by the second sense resistor  46  connected in series with the LED load  14 . The sensed voltage drop across the second sense resistor  46 , which may be the current sense  22 , is coupled back to the PFC TM control  28  via error amplifier  30  as part of a feedback loop. The bootstrap startup and assist circuit  24  comprises a unique means to facilitate the bootstrap technique for the high side switch  32  during initial power up of the converter  10 . 
     The controller  31  may also receive the enable/disable function  52  to interrupt power to the output LED load  14  by terminating the switching action of the converter  10 . The external reference  54  or external reference input, typically a reference voltage generated by the end user may be provided to program the required output current to a fixed value such as 350 mA or the external reference voltage can be varied to provide a simple means to dim the LED load. 
     Other embodiments may incorporate various control block functions, such as the bootstrap startup and assist circuit  24 , the PFC TM control  28  and the error amplifier  30  as a monolithic integrated circuit. Alternate embodiments may have the external reference internal to the two-switch buck boost converter  10  where a fixed output current with no dimming and end user accessibility is required. 
     A schematic diagram of a bootstrap startup and assist circuit  24  is shown in  FIG. 8 . The bootstrap startup and assist circuit  24  includes a voltage controlled switch  320  which is connected to a start up ramp voltage generator  340 . As will be described in more detail below, in one embodiment, the switch  320  comprises a diode and a transistor while the ramp up voltage generator  340  comprises a set of resistors and a capacitor. The voltage controlled switch  320  is connected to a PFC TM gate drive (shown in  FIG. 4   a ) to receive an input from the gate drive and to couple the signal via the voltage controlled switch  320  to a PFC TM current sense signal (shown in  FIG. 4   a ). The start up ramp voltage generator  340  is connected to Vauxiliary  104  and also to ground  118 . In one embodiment, control of the start up ramp voltage generator  340  may be via the enable/disable signal  52 . Alternate embodiments may include a means to couple the cathode of diode  80  as shown in  FIGS. 4   a  and  4   b  to ground  118  in order to provide the enable/disable function. 
     Turning to  FIG. 2 , a schematic diagram of another embodiment of a power factor corrected buck-boost converter with multiple constant current outputs is shown. In this embodiment, the converter  60  receives an input from a power supply, such as an AC mains voltage input  62  and supplies an output to a plurality of loads  64 . The converter  60  includes an EMI filter  66  which is connected to a bridge rectifier  68  which is connected to a plurality power conversion stages  70 . In this embodiment, the number of power conversion stages  70  equals the number of loads  64 . 
     Each power conversion stage  70  includes a two-switch non-isolated buck boost power stage  72 , a current sense  74  and a controller  76 . Although not shown, the converter also includes a controller  31  such as the one shown in  FIG. 1  or  3 . 
     Turning to  FIGS. 4   a  and  4   b , more detailed schematics of the bootstrap startup and assist circuit  24  interacting with other components of the converter are shown. In  FIG. 4   a , the bootstrap startup and assist circuit  24  with a bootstrap capacitor charge is shown with a low side switch on  40  (arrow  108 ), and in  FIG. 4   b  the bootstrap assist and start up circuit with a bootstrap capacitor charge is shown with a low side switch  40  off. In one embodiment, this circuit  24  is intended to overcome the shortcoming(s) of unpredictable and unguaranteed turn on of the high side switch, or MOSFET  32 , during initial power up of the buck boost two-switch converter  10 .  FIGS. 4   a  and  4   b  show how the bootstrap startup and assist circuit  24  is coupled to the PFC TM control  28  and the high side, low side gate drive  26 . The circuit  24  comprises a series of components such as diodes  80  and  82 , resistors  84 ,  86 ,  88  and  90 , transistor  92 , capacitor  94  as well as the auxiliary winding of inductor  34  identified as  34   b . In one embodiment, the voltage controlled switch  320  as shown in  FIG. 8  comprises diode  82  and transistor  92  while the start up ramp voltage generator  340  comprises capacitor  94  and resistors  84  and  88 . While  FIG. 4   a  shows a specific circuitry and set up of the components for implementing the circuit  24 , it will be understood that other components providing the equivalent functionality and properties of components  80  to  94  and  34   b  may be contemplated. 
     During continuous steady state operation, the bootstrap technique as applied to a buck-boost two switch converter requires a bootstrap capacitor  96  to be charged when both high side  32  and low side  40  switches are simultaneously turned off Subsequently, when the gate drive of both switches is turned on, the bootstrap capacitor voltage is level shifted across the high side switch in order to turn it on. 
     If the bootstrap startup and assist circuit  24  is not implemented during start up of the power converter, charging of the bootstrap capacitor  96  may be unpredictable and not guaranteed. Referencing  FIGS. 4   a  and  4   b , the PFC TM control  28  turns its gate drive signal  100  (via gate drive  98 ) on or active high, after detecting a demagnetization of inductor  34  and then compares a voltage across first sense resistor  42  (which represents the current through a primary winding  34   a  of inductor  34 ) to a current demand signal to terminate the gate drive signal  100 . Due to the lack of charge on bootstrap capacitor  96  and resultant lack of charge to turn the high side switch  32  on, no current flows through the primary winding  34   a , the second switch  40  or the first sense resistor  42 . The resulting lack of voltage across the first sense resistor  42  results in a corresponding lack of voltage at the current sense input  102  of the PFC TM control  28  causing the gate drive signal  100  to stay high. This operating characteristic of the PFC TM control  28  is effectively overcome when utilized in conjunction with the bootstrap startup and assist circuit  24  to provide a method to charge the bootstrap capacitor  96  during startup. 
     In the preferred embodiment, the bootstrap startup and assist circuit  24  facilitates the charging of the bootstrap capacitor  96  during the initial power up of the converter by causing the PFC TM control  28  to generate narrow gate pulses even though no current is detected across the first sense resistor  42 . 
     During the initial startup of the converter, the bootstrap startup and assist circuit  24  terminates the gate drive signal, or pulse,  100  by forcing the current sense pin  102  of the PFC TM control  28  above a desired current demand signal. Initially, very narrow gate drive pulses are generated over the startup period to pulse the low side switch  40  on and off several hundred times causing energy to build up in inductor  34  and bootstrap capacitor  96 . 
     As shown in  FIG. 5 , which are a series of waveforms during an initial start up period, an under voltage lockout function within the high side and low side drive  26  reduces the likelihood of or prevents the high side switch  32  from being driven until bootstrap capacitor  96  has charged to a sufficient voltage. This allows for continued charging of bootstrap capacitor  96  as described below. 
     As shown in  FIG. 4   a , when low side switch  40  turns on, current flows from Vauxiliary (an auxiliary voltage source)  104  through diode  106 , and bootstrap capacitor  96  via the primary winding  34   a  of the inductor  34  and low side switch  40  and first sense resistor  42  to ground  118  (as reflected by arrow  108 ). 
     As shown in  FIG. 4   b , when the low side switch  40  turns off, the polarity of the primary winding  34   a  of the inductor  34  reverses, freewheeling diode  38  conducts and energy is transferred to the output filter capacitor  44 . Note that no energy is initially transferred to the load  14  until the voltage across capacitor  44  is greater than the forward voltage drops of the LED load  14 . As freewheeling diode  38  conducts, it clamps one end of inductor  34   a  at node  110  to ground  118  allowing bootstrap capacitor  96  to continue to charge via Vauxiliary  104  and diode  106  (as shown by  109 ). 
     Eventually, the energy in inductor  34   a  is depleted as current decreases to zero and the voltage across the primary winding  34   a  collapses. The inductor auxiliary winding  34   b  is coupled to a zero current detector (ZCD) pin  112  of the PFC TM control  28  which detects zero current and switches on the gate drive signal of  100  which in turn switches low side switch  40  on. 
     As further shown in  FIG. 5 , this repetitive pulsing of the low side switch  40  continues to charge bootstrap capacitor  96  while the under voltage lockout function of the high/low side drive  26  disables the high side gate drive to switch  32 . When the voltage across capacitor  96  reaches a lockout enable threshold, the high/low side drive  26  enables the high side drive to level shift the bootstrap capacitor  96  voltage across high side switch  32 . At this stage, the two switch buck boost converter has reached its on state and the freewheeling action of diode  38  is able to continually recharge the bootstrap capacitor  96  when both high side and low side switches  32  and  40  are turned off. 
     The bootstrap startup and assist circuit  24  includes an RC time constant provided by resistor  84 , resistor  88  and capacitor  94  which determines the time period that narrow gate drive pulses are applied to the low side switch  40  during start up. This RC time constant is sufficient to allow many hundreds of gate pulses to be applied to the current sense pin  102 . 
     At initial power up of the buck boost converter  10 , the capacitor  94  within the start up circuit  24  is at zero volts. PFC TM control  28  starts when Vauxiliary  104  is applied to it and asserts the gate drive signal  100  active high which forward biases transistor  92 . Collector current from transistor  92  flows through resistor  86 , resistor  130  and capacitor  120  causing the voltage across the capacitor  120  to ramp up at a time constant (RC) of resistor  86  times capacitor  120 . 
     The voltage divider ratio of resistor  86  and resistor  130  is selected to have an appropriate voltage developed at the current sense pin  102  of PFC TM control  28  and is used to terminate the gate drive pulse  100 . The pulse width of the gate is thus narrow and its duration is a function of the leading edge blanking circuit within PFC TM control  28  typically greater than 200 ns. After the gate drive pulse  100  is terminated, the ZCD pin  112  waits for a negative leading edge which is developed due to the demagnetizing of inductor  34 . Once this signal is detected, the gate drive signal  100  is asserted once again. This cycle repeats several hundred times as the capacitor  94  charges toward a value of Vauxiliary  104  via resistor  84 . As the capacitor  94  charges, the voltage across it increases until it reverse biases transistor  92 . When capacitor  94  is charged to the Vauxiliary rail, the transistor is completely reversed biased decoupling the gate drive pulses  100  from the current signal sensing performed by the current sense pin  102  of the control  28 . 
     To reset the bootstrap startup and assist circuit  24 , diode  80  discharges capacitor  94  when the Vauxiliary  104  has been removed. In other embodiments, the enable/disable signal  52  could be implemented to reset the bootstrap startup and assist circuit  24 . 
     Turning to  FIG. 6 , a method of providing a constant current output to an LED load is shown. In operation, an AC voltage is applied to, or received by  1000 , the converter. After receiving the AC voltage from an AC mains voltage input, the EMI filter filters  1002  the signal. In one embodiment, the filtering attenuates the unwanted differential and common mode noise that is generated by the converter and coupled or fed back to the voltage input. The output of the EMI filter is then rectified  1004  by the bridge rectifier. The output of the bridge rectifier is then processed  1006  to produce a regulated constant current output. The current output is the supplied  1008  to a load, such as a high brightness LED, which requires a constant current output. 
     Turning to  FIG. 7 , a flowchart outlining a method of bootstrap startup and assist during initial startup of power converter is shown. Initially, after receiving an AC mains voltage  1100 , the PFC TM control starts up  1110  and provides  1112  an active high state, such as a positive pulse, on its gate drive pin. The voltage ramp circuit begins charging by means of the circuit comprising resistor  88 , resistor  84  and capacitor  94 . 
     The gate drive signal is then applied  1116  to the voltage controlled switch  92  of bootstrap startup and assist circuit  24 . The voltage controlled switch is where the transistor  92  is in a common base configuration and where the transistor base voltage exponentially rises, via the charging of the capacitor  94  within the bootstrap startup and circuit  24  from base current delivered via the forward biased junction of the transistor  92  from the gate drive signal  100 . 
     An artificial current ramp sense signal is then produced  1120 . In one embodiment, this is accomplished by the current through the collector of transistor  92  which creates the artificial ramp current sense signal from an RC constant between resistor  86  and capacitor  120  connected to the current sense pin  102  of the PFC TM control  28 . When a voltage is then detected  1122  at the current sense pin  102 , the gate drive signal pulse  100  is terminated  1124 . 
     When a zero current signal is sensed  1126  by the PFC TM control  28 , via the zero current detector sense pin  112 , further monitoring is performed to determine if the transistor  92  in the bootstrap startup and assist circuit  24  is reverse biased  1128 . If the transistor  92  is not reverse biased, the artificial ramp current sense signal is produced  1120  again. However, if the transistor  92  is reverse biased, the bootstrap startup and assist circuit  24  is disabled  1130 . 
     In the preceding description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments. However, it will be apparent to one skilled in the art that other arrangements and embodiments would be feasible. 
     The above-described embodiments are intended to be examples only. Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art without departing from the scope of the application, which is defined solely by the claims appended hereto.

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