Abstract:
AC/DC cascaded power converters having high DC conversion ratio and improved AC line harmonics provide low input harmonic currents, high power factor and efficient operation for low voltage DC outputs when coupled directly to a source of unfiltered rectified AC voltage. The power converter incorporates an intermediate storage element that provides most or all of the energy storage capacitance within the power converter and a blocking device that enables continuous energy transfer from AC line to output to achieve unity power factor and regulated output while maintaining low AC input current ripple.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to U.S. provisional application 60/404,795 filed Aug. 17, 2002 and from which it claims benefits under 35 U.S.C. §119(e). 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to power supplies, and more specifically to multi-stage AC/DC power supplies and methods for directly converting rectified alternating current (AC) input to produce a low voltage direct current (DC) output with low input current harmonic distortion. 
     2. Background of the Invention 
     Switching power converters are typically used in a wide variety of applications. Off-line AC/DC converter applications (for operation from an AC power line source) are a common example. Power factor correction is often incorporated in such power converters and typically takes one of two forms: a separate power factor correction stage at the input of the power converter or a power converter designed with power factor correction as part of the overall control function of the power converter. 
     Recently, the development of high-efficiency semiconductor light sources has generated a demand for low power high-efficiency off-line converters. Use of high-efficiency semiconductor light sources in traffic control, industrial and household applications is on the rise. Compatible and efficient power converters for supplying power from an AC line to a low voltage output at a relatively low power level are therefore a necessity. It is further necessary in some semiconductor light source applications to provide power factor correction (PFC) and a low level of AC input harmonic current, even in low power applications. In some applications, since a multitude of power supplies for the semiconductor light sources operate in parallel off of an AC line power source, power factor correction and reduction of AC line harmonic currents are a necessity to provide high efficiency and a low level of line interference. 
     Two-stage converters combining a power-factor correction (PFC) stage with a down converter stage are typically used when high efficiency and low input current harmonics are required. However, typical two-stage converters are costly for low power applications. Single-stage power factor corrected power supplies have been proposed, but for low voltage DC outputs they are generally restricted to transformer-coupled applications due to the step-up characteristic of a boost input stage that provides the PFC functionality. An attempt to operate these power converters as direct-coupled converters (as opposed to transformer coupled converters) would require the second stage down converter to operate at a very low duty cycle in order to produce a low output voltage, leading to inefficient operation. 
     Quadratic power converters for providing low voltage outputs from a wide range of input voltage were proposed by D. Maksimovic and S. Cuk in the article “Switching Converter with Wide DC Conversion Range”, May 1989 proceedings of the HFPC and also in “Switching Converters with Wide DC Conversion Range” published in the Institute of Electrical and Electronic Engineer&#39;s (IEEE) Transactions on Power Electronics on January 1991. The topologies use a single switch to control cascaded buck and buck-boost stages. However, PFC functionality cannot be provided using these topologies. In order to provide good power factor performance, the input buck-boost stage must operate in the discontinuous conduction mode (DCM) with a nearly fixed duty ratio. Operation in continuous conduction mode defeats the PFC operation, as the input current is no longer a function of the duty ratio. Also, a low frequency filter (energy storage device) must be incorporated in the power supply topology to provide regulated DC output throughout the AC line cycle. 
     Therefore, it would be desirable to provide low output voltage cascaded converters that may be directly coupled to an AC line without a need for a step-down transformer while maintaining power factor correction and a low level of input current harmonics. 
     SUMMARY OF THE INVENTION 
     The above objective of providing low output voltage cascaded converters that may be direct-coupled to the AC line, while maintaining power factor correction and a low level of input current harmonics is achieved in a method and apparatus. The apparatus includes a rectifier for rectifying an AC voltage input to produce a rectified DC output, a first converter stage for correcting input power factor and converting the rectified power output to an intermediate DC voltage lower than a maximum level of the rectified DC output, and a second converter stage for converting the intermediate DC voltage to an output DC voltage. The first converter stage includes an input circuit comprising a first inductor series coupled with a first diode, a switch for coupling in series with a first diode. The series connected circuit is connected to said rectifier so that the first converter section is prevented from conducting energy into the rectifier when the voltage at the rectifier output falls below a threshold. 
     The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiments of the invention, as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram depicting a prior art power supply circuit. 
     FIG. 2 is a schematic diagram depicting a first converter stage of a power supply circuit in accordance with an embodiment of the present invention. 
     FIG. 3 is a generalized schematic diagram depicting a power supply in accordance with an embodiment of the present invention. 
     FIG. 4 is a detailed schematic diagram depicting a power supply in accordance with an embodiment of the present invention. 
     FIG. 5 is a detailed schematic diagram depicting a power supply in accordance with another embodiment of the present invention. 
     FIG. 6 is a detailed schematic diagram depicting a power supply in accordance with yet another embodiment of the present invention. 
     FIG. 7 is a detailed schematic diagram depicting a power supply in accordance with yet another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a prior art power supply is depicted. A bridge BR 1  full-wave rectifies an AC line input to produce an input power source. Filter capacitor C_IN filters the voltage at the output of bridge BR 1  to produce a DC input voltage having substantially eliminated AC line components. The input DC voltage across capacitor C_IN is applied to a first converter (buck-boost) stage that includes an inductor L 1 , a capacitor C 1 , a diode D 1  and a switch (transistor) Q 1 . When switch Q 1  is activated by a pulse width modulator PWM, inductor L 1  charges as current is passed from the input DC across capacitor C_IN and energy is stored in inductor L 1 . When switch Q 1  is deactivated by pulse width modulator PWM, diode D 3  conducts, transferring the stored energy to capacitor C 1 . By the action of the first converter stage, an intermediate DC voltage is produced across capacitor C 1 . 
     A second (buck) converter stage is coupled to the first converter stage and includes a diode D 1 , a diode D 2 , an inductor L 2  and a capacitor C_OUT. The second converter stage is a DC/DC converter also controlled by switch Q 1 . When switch Q 1  is activated by pulse width modulator PWM, the voltage across capacitor C 1  conducts through diode D 1 , drawing current through inductor L 2 , therefore, transferring energy to capacitor C_OUT and storing energy in inductor L 2 . When switch Q 1  is deactivated by pulse width modulator PWM, diode D 1  is reverse biased and diode D 2  conducts, transferring the stored energy in inductor L 1  to capacitor C_OUT. By the action of the second converter stage, an output DC voltage (negative with respect to the common node) is produced across capacitor C_OUT and thus delivered to the load (represented by a resistor R_LOAD). 
     For the circuit of FIG. 1, filter capacitor C_IN must hold up the voltage at the input of the buck-boost converter stage (i.e., reduce variations of the rectified AC input voltage) so that inductor L 1  is always be supplied with sufficient energy to maintain the energy transfer needed to the second converter stage for maintaining the output voltage at C_OUT. There will be no input AC current whenever the rectified AC input voltage is lower than the voltage across the filter capacitor C_IN since the diodes of the bridge BR 1  are reverse biased. Therefore, power factor correction cannot be achieved in the circuit of FIG. 1 when capacitor C_IN is utilized to filter the rectified AC input voltage. Capacitor C 1  also cannot be used to store energy to achieve power factor correction. In order to achieve average input current of the buck-boost converter stage that is proportional to the rectified AC input voltage, inductor L 1  must operate in discontinuous conduction mode (DCM), such that the current through inductor L 1  drops to substantially zero before switch S 1  turns on again. However, DCM operation of inductor L 1  is not possible when the rectified AC input voltage falls below a level defined by the voltage level across capacitor C 1  and output voltage and current conditions of the second converter stage. Under the above conditions, the current in inductor L 1  reverses direction rather than remaining at zero once the energy stored in inductor L 1  has been released. As a result, the buck-boost converter stage cannot transfer energy from the input stage and power factor correction is not to be achieved. The resulting input current is similar to that of an AC bridge rectifier with a peak charging filter capacitor connected to the output of the AC bridge. Therefore, the only energy storage device available without degrading power factor performance is output capacitor C_OUT. However, using capacitor C_OUT to deliver energy to the output during dropouts of the rectified AC input voltage will require a very large capacitor. Low-voltage aluminum electrolytic capacitors are typically employed for this purpose. Such capacitors contribute significantly to the failure rate of power supplies. The large electrolytic capacitors may further add significant cost and undesirably increase the size of the power converter. 
     While the topology of the circuit of FIG. 1 provides an efficient and low-cost methodology for converting AC input voltages to a DC output as mentioned above, the circuit of FIG. 1 introduces substantial distortion in the AC input current. Therefore, the power supply of FIG. 1 is not useful in applications where high quality AC line rectification is required along with good regulation of the DC output voltage or current. 
     Referring now to FIG. 2, a schematic diagram of a first buck-boost converter stage  400  of a power supply circuit in accordance with an embodiment of the present invention is shown. An inductor  118  is connected in series with a blocking diode  117  to an input node  104  that receives a positive DC input voltage with respect to a common node  103 . The voltage on input node  104  may be substantially DC or a pulsating rectified AC voltage. Inductor  118  is charged by activating switch (transistor)  119 , pulling current from inductor  118  by alternately connecting switching node  106  to common node  103 . Blocking diode  117  prevents reverse flow of current to input node  104  when the voltage at input node  104  falls below the voltage at switching node, permitting only unidirectional current flow through inductor  118 . 
     When switch  119  is activated, inductor  118  is energized from a zero current to a peak current proportional to the product of the on state period and the instantaneous voltage present at node  104 . Simultaneously, capacitor  107  delivers stored energy to output node  110 . When switch  119  is deactivated, current flows through a flyback diode  116  and blocking diode  117 , causing the energy stored in inductor  118  to transfer to capacitor  107 . As the current through inductor  118  decreases, diode  116  becomes reverse-biased. Blocking diode  117  prevents reverse flow of current when the voltage at node  106  exceeds the instantaneous voltage at node  104 , as a reverse current would otherwise occur once the inductor  118  current has reached zero. Diode  117  enables first power converter stage  400  to maintain a DC voltage at capacitor  107 , while enforcing discontinuous conduction mode in inductor  118 . Assuming that the duty ratio of switch  119  is kept constant, average input current into node  104  will be proportional to the instantaneous voltage at node  104  and good power factor performance will be achieved. Capacitor  107  is selected to be sufficiently large in order to maintain a substantially DC voltage at capacitor  107 . 
     First converter stage  400  can operate as a step-down DC voltage converter producing an output voltage across the energy storage capacitor  107  that is lower than the typical hold-up capacitor voltage of prior art single-stage PFC power supplies (higher than 375V typically). The step-down feature of first converter stage  400  permits construction of a single-stage PFC low output voltage AC/DC power supply without the need for a step-down power transformer, while operating at reasonably high duty ratio of switch  119 , and with improved overall efficiency. 
     Accordingly, embodiments of the present invention include first converter stage  400  and variants thereof to provide direct off-line operation with improved power factor and low harmonic distortion of input AC line current in a low output voltage AC/DC converter. 
     Referring now to FIG. 3, a generalized schematic diagram of a power supply in accordance with an embodiment of the present invention is depicted. An AC/DC rectifier  501  is coupled to a first converter stage  400  in accordance with the schematic of FIG.  2 . Rectifier  501  is typically a full-wave bridge rectifier and may include a capacitor for filtering high-frequency switching component of the input current of first converter stage  400 . 
     A DC/DC converter  502  provides a second converter stage for converting the intermediate DC voltage produced by first converter stage  400  to a low voltage DC output. A load, represented by resistor R_LOAD, is connected to the output of second converter stage  502 . The power converter circuit of FIG. 3 represents a generic model of a variety of multi-stage converter circuits that can be built using buck-boost stage  400  of FIG.  2 . An exemplary set of power converters that may be implemented are illustrated in the following description and associated figures, but the present invention is not limited to the particular embodiments depicted herein. 
     Referring now to FIG. 4, a two-stage power converter including the buck-boost stage  400  of FIG. 2 and a buck output stage is depicted. The buck output stage is coupled to first converter stage  400  by a diode  108  and includes a diode  112 , an inductor  111  and a capacitor  114 . The second converter stage is controlled along with buck-boost stage  400  by switch  119 . When switch  119  is activated by pulse width modulator PWM, the voltage across capacitor  107  appears as a negative bias to diode  108 , drawing current through inductor  111 . Therefore, energy from capacitor  107  is delivered to capacitor  114  and partially stored in inductor  111 . When switch  119  is deactivated by pulse width modulator PWM, diode  108  is reverse biased and diode  112  conducts, transferring the stored energy in inductor  111  to capacitor  114 . By the action of the second converter stage, an output DC voltage (negative with respect to the common node) is produced across capacitor  114  and thus delivered to the load (represented by a resistor  115 ). The power converter of FIG. 4 may be further adapted to provide galvanic isolation by incorporating a transformer in the second DC/DC converter stage. 
     The power converters of FIGS. 5-7 are similar to that of FIG. 4, thus only differences appearing therein will be described below. The output stages of the power converters of FIGS. 5-7 differ in form from the power converter of FIG. 4, but all of the output stages are controlled by a switch  119  within first converter stage  400 . 
     Referring now to FIG. 5, a two-stage power converter including the buck-boost stage  400  of FIG. 2 and a second buck-boost output stage is depicted. The buck-boost output stage is coupled to first converter stage  400  by a diode  208  and includes a diode  212 , an inductor  211  and a capacitor  214 . When the switch (Q 1 ) is activated by pulse width modulator PWM, diode  208  is forward biased and inductor  211  charges as current is passed from the intermediate DC voltage across capacitor  107  and energy is stored in inductor  211 . When switch (Q 1 ) is deactivated by pulse width modulator PWM, diode  208  is reverse biased and diode  212  conducts, transferring the stored energy to capacitor  214 . By the action of the second converter stage, an output DC voltage (positive with respect to the common node) is produced across capacitor  214  and thus delivered to the load (represented by a resistor  215 ). The power converter of FIG. 5 may be further adapted to provide galvanic isolation by incorporating a transformer in the second DC/DC converter stage. 
     Referring next to FIG. 6, a two-stage power converter including the buck-boost stage  400  of FIG. 2 with addition of a secondary winding  301  to inductor  118  coupled to output capacitor  314  by a diode  302 . A second buck output stage is depicted in FIG. 6 in the way of example, rather than in the limiting sense, as any of the second stage converter embodiments of this description or other topologies can be used. Operation of the circuit of FIG. 6 is identical to operation of the circuit of FIG. 4, except that when voltage at capacitor  107  exceeds voltage at capacitor  314  multiplied by a turn ratio between windings  118  and  301 . One possible reason for voltage increase at capacitor  107  is associated with imbalance of energy transfer between the first converter stage operating in DCM and second converter stage operating in CCM. At light load (high R_LOAD), this imbalance can create high voltage stress at capacitor  107 . When voltage at capacitor  107  exceeds voltage at capacitor  314  multiplied by a turn ratio between windings  118  and  301 , as the switch (Q 1 ) is deactivated by pulse width modulator PWM, diode  302  becomes forward biased yielding an alternative path for the energy stored in inductor  118  to be transferred directly to output capacitor  314 . Therefore, voltage at capacitor  107  will be limited to the voltage at capacitor  314  multiplied by the turn ratio between windings  118  and  301 . 
     Referring finally to FIG. 7, a two-stage power converter including the buck-boost stage  400  of FIG. 2 and a second buck output stage configured as a current source for driving a semiconductor light source  601  is depicted. The buck output stage is coupled to first converter stage  400  by a diode  508  and includes a diode  512 , an inductor  511  and a capacitor  602 . Operation of the circuit of FIG. 7 is identical to operation of the circuit of FIG. 4, but filter capacitor  602  may be optional or may be a small non-electrolytic capacitor, since constant average current through semiconductor light source  601  will generally be the only control required. The above is especially true if continuous conduction mode with respect to inductor  511  is maintained. The capacitance value of capacitor  107  is selected to set the amount of input AC ripple transferred to the output current of the power converter (i.e., to semiconductor light source  601 ) and to limit the peak-to-average current ratio to a predetermined level. Capacitance  107  may also be a small non-electrolytic capacitor, yielding a more compact, low cost and reliable power converter. Using a non-electrolytic capacitor for capacitor  107  dramatically increases the service live of the power converter of FIG.  7  and permits operation at elevated ambient temperatures. The power converter of FIG. 7 can also be operated from a phase-controlled power source such as are commonly used for dimming light sources (due to the use of intermediate storage capacitor  107 ). The above characteristics make the power supply of FIG. 7 ideal for driving semiconductor light sources in applications where one or more semiconductor light sources are used in what were previously incandescent lighting applications, such as traffic control and lighting applications. 
     The present invention presents a new class of AC/DC power converters that achieve unity power factor and low ripple of output current or voltage by means of an intermediate capacitor as the first storage means within the power converter and a blocking means for preventing discharge of the storage means and permitting input energy to be delivered to the output continuously. The power converters of the present invention have improved input AC line current harmonic distortion and power factor correction when operated directly off of an input AC line voltage. The class of converters presented herein are further ideal for driving semiconductor light sources in applications replacing traditional uses for incandescent or other light sources. In latter applications, the power converters disclosed herein have improved reliability and extended operating temperature range due to lack of electrolytic capacitors altogether. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.