(12) United States Patent
`Jain et al.
`
`USOO634.4986B1
`(10) Patent No.:
`US 6,344,986 B1
`(45) Date of Patent:
`Feb. 5, 2002
`
`(54) TOPOLOGY AND CONTROL METHOD FOR
`POWER FACTOR CORRECTION
`
`(75) Inventors: Praveen K. Jain; Yan-Fei Liu, both of
`Kanata (CA)
`(73) Assignee: Astec International Limited (HK)
`(*) Notice:
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 09/595,425
`(22) Filed:
`Jun. 15, 2000
`(51) Int. Cl." .......................... H02M 5/42; H02M 3/335
`(52) U.S. Cl. ....................................... 363/89; 363/21.12
`(58) Field of Search ......................... 363/16, 20, 21.01,
`363/21.12, 21.14, 21.18, 84, 89, 95, 97,
`131
`
`
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`5.434,767 A * 7/1995 Bataeseh et al. .............. 363/16
`5,515,257 A
`5/1996 Ishii ............................ 363/21
`5,600,549 A 2/1997 Cross
`5,734,562 A 3/1998 Red
`6,031,748 A 2/2000 Hong .......................... 363/89
`OTHER PUBLICATIONS
`Hewlett Packard 6th Annual Power Systems Symposium,
`Feb. 26–28, 1996, Palo Alto, California, “Novel Circuits for
`
`Implementing Power Factor Correction in Off-Line Power
`Converters', 10 pages.
`* cited by examiner
`Primary Examiner Matthew Nguyen
`(74) Attorney, Agent, or Firm-Coudert Brothers LLP
`(57)
`ABSTRACT
`In a power factor corrected AC-to-DC power Supply System,
`a DC-to-DC power converter is coupled to the output of an
`AC-to-DC power converter in order to produce a regulated
`DC output Signal from a rectified AC input Signal. The
`AC-to-DC power converter and the DC-to-DC power con
`verter each includes a Switch for controlling the operation of
`their respective power converter. The AC-to-DC converter
`includes an inductor. The System provides power factor
`correction for minimizing harmonic distortion by including
`a controller that receives the regulated DC output Voltage as
`a feedback signal, and in response, produces a Series of drive
`pulses having predetermined constant duty cycle. These
`pulses are simultaneously fed to each Switch, to operate the
`respective converters alternately between ON and OFF
`states. When the AC-to-DC converter is driven by a fixed
`duty cycle of the Series of pulses, power factor correction is
`improved Since the current flowing through the inductor is
`substantially proportional to the waveform of the rectified
`AC input Signal. By preselecting the value of the inductor,
`the AC-to-DC converter is operable in a discontinuous mode
`when the instantaneous rectified AC input signal is low and
`in a continuous mode when the instantaneous rectified AC
`input Signal is high.
`
`22 Claims, 2 Drawing Sheets
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`Current Sense
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`Feedback
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`MyPAQ, Exhibit 2024
`IPR2022-00311
`Page 1 of 8
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`U.S. Patent
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`Feb. 5, 2002
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`Sheet 1 of 2
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`US 6,344,986 B1
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`asues qualuno !
`f7Z\!
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`89
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`OEZ
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`IPR2022-00311
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`U.S. Patent
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`Feb. 5, 2002
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`Sheet 2 of 2
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`US 6,344,986 B1
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`Time 15.400mS
`FIG.2
`
`Harmonic
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`SE"ero 931703580.1320,08000550 0500038
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`input Current
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`Current
`(ampS)
`EC-1 OOO-3-2
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`NO
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`0.686 0.383 0.2020. 101|0.071 0.060
`
`FIG.3
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`New, Circuit based Non PFC Circuit Two Stage PFC
`On This invention
`Circuit
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`POWer
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`d
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`-39.6dbm(max)
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`Eficiency 77.5%
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`-48dbm(max)
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`78.5%
`FIG.4
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`-31.8dbm(max)
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`72.7%
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`MyPAQ, Exhibit 2024
`IPR2022-00311
`Page 3 of 8
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`1
`TOPOLOGY AND CONTROL METHOD FOR
`POWER FACTOR CORRECTION
`
`US 6,344,986 B1
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`2
`power converters. Specifically, the IEC-1000-3-2 standard
`requires that the harmonic components of the input current
`be below a certain level. Accordingly, it is desirable to
`provide power factor correction for AC-to-DC power con
`verters in order to achieve a low input current harmonic
`content in the AC Supply, and, equivalently, a higher power
`factor.
`One known embodiment for providing power factor cor
`rection involves using an AC-to-DC converter followed by
`a DC-to-DC converter where the former is a boost converter
`and the latter is a flyback converter. In order to provide
`power factor correction with this topology, the drive signal
`of the boost converter must be controlled to force the current
`flowing through the inductor in the boost converter to follow
`the Sinusoidal waveform of the rectified input AC signal.
`However, conventional control techniques used to achieve
`this result suffer from a number of drawbacks. The predomi
`nant drawback is that a complex control circuit is needed, as
`well as bulky and heavy filter components to filter ripple
`current and to meet the EMI specification of the power
`Supply. The result is a high cost control circuitry and the
`need to use more printed circuit board (PCB) space, which
`in turn contributes to higher fabrication costs. The overall
`size of the AC-DC converter is also increased.
`A conventional boost converter generally comprises an
`inductor which is coupled between a Source of AC power
`(e.g., a rectifier producing an unregulated AC voltage) and
`a Switch. The Switch is preferably a MOSFET transistor,
`which is in turn coupled in parallel with a Series combination
`of a rectifier and an output filter capacitor acroSS which the
`load is connected. The output capacitor is Selected to be
`large in order to ensure that the load receives a Substantially
`constant DC voltage. This constant DC signal appears across
`the load and is greater than the peak Sinusoidal value of the
`input AC Voltage.
`One well known control method for providing power
`factor correction in a boost converter is to Set the duration of
`the ON state (e.g., a time T out of a period T) of the FET
`Switch to a constant value. The constant duration of T is
`predetermined by certain operating conditions, Such as the
`input voltage, output voltage and output current of the boost
`converter. Energy is Stored in the boost converter's inductor
`during this time T. Additionally, certain circuit parameters,
`Such as the value of the boost inductor, affects the duration
`of T. When the FETswitch is switched to its OFF state for
`a certain time period (e.g., T), the polarity across the
`inductor reverses So that the energy that was Stored in the
`inductor during T is transferred via the diode to the output
`capacitor. A constant DC voltage, V., appears across the
`capacitor and has the following relationship with respect to
`the rectified input AC voltage:
`
`V.
`WTri = - -
`DC - 1 - T. T.
`
`(1)
`
`While this control method ensures that the boost converter
`operates at the boundary of continuous and discontinuous
`modes of operation, it Suffers from three drawbackS. First,
`because the duration of T is fixed, the ripple current
`passing through the inductor is large. Accordingly, a bulky
`and heavy Electromagnetic Interference (EMI) filter is
`required to filter out this ripple current in order to meet the
`EMI specification of the power supply. Second, the Switch
`ing frequency of the transistor in the boost converter must be
`varied in order to regulate the constant DC voltage V.
`Such variable Switching frequency control is usually unde
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates to Switching power Supplies,
`and more particularly, to an AC-to-DC power factor cor
`recting power converter comprising a boost power converter
`coupled to a DC-to-DC power converter where both con
`verters are driven by the same Signal derived from the output
`Voltage of the power converter.
`2. Description of the Prior Art
`Many electronic devices, Such as computers and many
`household appliances, require one or more regulated DC
`15
`Voltages. The power for Such electronic devices is ordinarily
`Supplied by power converters that convert an AC line
`Voltage into the regulated DC Voltages required by the
`devices.
`Electrical power converters commonly include a rectifier
`circuit which converts the AC line Voltage to an unregulated
`DC voltage, also known as a rectified line Voltage, and a
`DC-to-DC converter for converting this unregulated DC
`Voltage into one or more regulated DC output Voltages. If a
`Simple rectifier circuit is used, Such power converters com
`25
`monly draw high currents near the peak of the AC voltage
`cycle, and Substantially Zero current around the Zero
`crossing points of the Voltage cycle. Thus, the input current
`drawn by the converter has a highly non-sinusoidal wave
`form with correspondingly high harmonic content.
`AS is known in the electrical power art, current harmonics
`above the fundamental frequency of the Voltage do not
`contribute to the power drawn from a typical AC voltage
`Source, with the result that the actual or true power drawn by
`the power Supply is lower than the apparent power drawn.
`The distinction between apparent power and true power is
`important because power Supplies are rated according to the
`apparent power drawn rather than the true power drawn. AS
`a basis of comparison, the true power and apparent power
`drawn by a device are divided to form a ratio called the
`“power factor.” Power factors less than about 80 percent can
`pose barriers to the performance or improvement of many
`types of electronic devices that operate on direct current,
`including Such devices as personal computers,
`minicomputers, and appliances using microprocessors. For
`example, the high current peaks associated with lower
`power factors can cause circuit breakers on the AC line to
`trip, which limits System design in terms of the functional
`load it places on the AC line. Additionally, the harmonics
`asSociated with the high, non-sinusoidal current peaks often
`result in power-line distortion, noise, and electromagnetic
`interference (EMI). In general, improving the power factor
`of the device reduces the harmonic content and electromag
`netic noise.
`To address these problems, many power Supplies include
`power factor correction circuitry that is designed to raise
`power factors and eliminate harmonic distortion. Such cir
`cuits are often referred to as power factor correction circuits
`(abbreviated “PFC). Power factor correction circuits gen
`erally rectify the AC line Voltage and produce an unregu
`lated DC voltage (referred to herein as the “PFC voltage')
`in a manner that has a relatively high power factor within a
`given range of AC line Voltages. A Switching power con
`verter then converts the PFC voltage into the required
`regulated Voltages.
`The International Electrotechnical Commission (IEC) has
`Set Standards Specifying certain requirements for AC-to-DC
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`US 6,344,986 B1
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`3
`Sirable for applications involving telecommunications
`because of possible interference. Third, because the control
`chip and accompanying current Sensing circuit are necessary
`in order to provide power factor correction, the inclusion of
`these components occupies a significant portion of the PCB
`area and results in increased size and cost of the power
`Supply.
`Another well known control method for providing power
`factor conversion in a boost converter and for producing a
`constant DC voltage that is substantially free from distortion
`is conventionally known as the average current mode control
`method. With this technique, the boost converter operates in
`a continuous conduction mode. In particular, to obtain a high
`power factor, the average value of the current passing
`through the inductor in the boost converter is Sensed and
`forced to follow in phase the rectified sinusoidal waveform
`of the input AC voltage V. In this control method, a
`multiplier is needed to generate the reference current for the
`boost inductor current. The average value of the boost
`inductor current must be Sensed to achieve average current
`control.
`A key drawback of the average current control method is
`the high cost of the control circuit.
`What is needed is improved power factor correction in a
`power converter whereby the boost and DC-DC converters
`are driven with a control method that overcomes the above
`described drawbacks.
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`Because only one feedback control loop is required in the
`present invention to operate the Switches within the power
`converters of the power Supply System, the circuitry is
`Simplified. Moreover, the response of the power Supply
`System output Voltage is fast because the feedback loop can
`be selected to have a wide bandwidth.
`Additionally, because a regulated output Voltage may be
`obtained for a universal input Voltage range (e.g., 86V is to
`265Vs), an auto range circuit is not required.
`These and other objects of the present invention will
`become apparent to those skilled in the art from the follow
`ing detailed description of the invention and preferred
`embodiments, the accompanying drawings, and the
`appended claims.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a circuit diagram of a preferred embodiment of
`a power Supply System according to the present invention
`that includes power factor correction.
`FIG. 2 is a graphical representation of the input current
`waveform as a function of time for the DC-to-DC converter
`used in the present invention depicted in FIG. 1.
`FIG. 3 is a table listing measured harmonic components
`of the input current of the present invention as compared to
`the IEC standard values.
`FIG. 4 is a table comparing the power factor and effi
`ciency obtained by the System according to the present
`invention as compared to a converter that does not have
`power factor correction and a prior art two Stage converter
`having power factor correction.
`
`DESCRIPTION OF A PREFERRED
`EMBODIMENT
`The present invention recognizes that prior art AC-to-DC
`power Supplies having power factor correction require cur
`rent Sensing and Signal comparison circuitry in order for the
`current in the boost converter to be Substantially propor
`tional to the input AC Voltage. A separate control circuit is
`required for a second DC-DC converter such as a flyback
`converter, that provides DC regulation for the output volt
`age. This results in circuit complexity. By providing a means
`to obtain power factor correction, yet without Such addi
`tional circuitry, the AC-to-DC power Supply System accord
`ing to the present invention provides power conversion with
`power factor correction that is accomplished more cost
`effectively and efficiently. This is accomplished by provid
`ing a feedback control to Simultaneously drive the power
`Switches of the boost and flyback converters. In addition, the
`boost converters inductor is Selected to provide inductor
`current that is discontinuous when the instantaneous recti
`fied AC input Voltage is low and continuous when the
`instantaneous rectified AC input voltage is high.
`The power Supply System of the present invention pro
`duces a regulated DC output voltage. FIG. 1 depicts a
`preferred embodiment of an AC-to-DC power supply system
`10. An AC power source 12 is coupled to a rectifier 14 in
`power Supply System 10. Power Source 12 comprises a
`sinusoidal voltage waveform. Rectifier 14 as shown is
`typically a diode bridge circuit; however, one of ordinary
`skill in the art will appreciate that other embodiments for
`providing rectification of Sinusoidal waveforms originating
`from an AC power source may be substituted. Both the
`power Source 12 and rectifier 14 are preferably connected to
`a common ground 16. The rectifier 14 provides a rectified
`AC input Voltage measured acroSS from node 18 to ground
`
`SUMMARY OF THE INVENTION
`The present invention is directed to a power Supply
`System having power factor correction. The System includes
`an input rectifier for generating a rectified input AC voltage
`from an AC power Source for two Stages of power conver
`Sion. The first Stage is a boost converter. It is coupled to the
`input rectifier and converts the rectified input AC voltage
`into a substantially constant first DC voltage. The boost
`converter includes a first Switch. The Second Stage converter
`is a DC-to-DC converter and is coupled to the output of said
`boost converter. The second converter converts the first DC
`40
`Voltage to a Second Voltage, the output Voltage of the power
`Supply System. The Second converter regulates the output
`Voltage to a desired level. The Second converter includes a
`Second Switch. The power Supply System also includes a
`controller for providing feedback control as a function of the
`output voltage, Said controller generating a drive signal for
`Said first and Second Switches So as to cause Said Switches to
`be switched on and off simultaneously.
`Accordingly, it is an object of the present invention to
`provide a power Supply System for providing power factor
`correction in a cost efficient manner. Cost efficiency of
`power factor conversion is achieved because the present
`invention does not require current Sensing circuitry to Sense
`the boost converters inductor current, nor the corresponding
`additional control circuitry for the boost converter as
`required in the prior art. PCB Space is also reduced.
`It is another object of the present invention to reduce EMI
`by fixing the Switching frequency of the controller's drive
`Signal at a constant value. By maintaining a constant
`frequency, the low frequency ripple component appearing at
`the output Voltage is Substantially reduced. This is essential
`for power Supplies used with applications involving tele
`communications where the level of C-message noise must
`be very low. Additionally, the current ripple of the boost
`inductor is Small with the present invention. This is advan
`tageous because leSS EMI filtering is required to meet the
`above-mentioned IEC standard.
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`16. The rectified AC input voltage typically falls within an
`expected input Voltage range (e.g., 86 Vrats to 256 Vrats,
`where RMS refers to the root-mean-square of the input AC
`voltage) as is well-known in the art.
`Power supply system 10 has a first stage which includes
`a boost converter 20 coupled to rectifier 14, and also a
`Second Stage which includes a DC-to-DC power converter
`22 coupled to the output of converter 20. Power converter 20
`is a conventional boost converter and DC-DC converter 22
`is preferably a conventional flyback power converter.
`The boost converter 20 comprises an inductor 26, a Switch
`28, a rectifier diode 30, and a capacitor 32. Switch 28 is
`preferably a MOSFET power transistor having a control
`node 34 (e.g., a gate) and two conduction terminals 36 and
`38 (e.g., a Source and a drain, respectively). One end of
`inductor 26 is coupled to rectifier 14 at node 18. The other
`end of inductor 26 is coupled to one end of diode 30 and to
`one conduction terminal 36 of Switch 28, as indicated at
`node 40. Diode 30 and capacitor 32 form a series combi
`nation which is connected in parallel acroSS the two con
`duction terminals 36 and 38. As already described, a feature
`of a boost converter is to provide power factor correction
`when the current flowing through the inductor 26 has
`substantially the same waveform as the rectified AC input
`Voltage appearing at node 18.
`DC-to-DC converter 22 functions to correct the voltage
`received from the boost converter 20 to a lower output
`regulated DC voltage, the output Voltage of the power
`supply system 10. The preferred embodiment for converter
`22 is a conventional flyback converter, as illustrated in FIG.
`1. Flyback converter 22 comprises a transformer 42 having
`a primary winding 44 and a Secondary winding 46, a Switch
`48, rectifier diode 50, and an output filter capacitor 52 across
`which a load 54 is connected. Although various switches
`may be used, Switch 48 is preferably a MOSFET power
`transistor similar to Switch 28. Switch 48 has a control node
`56 as well as two conduction terminals 58 and 60. Primary
`winding 44 includes a first terminal connected to node 24
`and a Second terminal connected to conduction terminal 58
`of Switch 48. Coupled across the secondary winding 46 is
`the series combination of diode 50 and the parallel combi
`nation of capacitor 52 and load 54.
`AS is conventionally known, the operation of a flyback
`power converter is analogous to an energy Storage converter.
`More specifically, the flyback power converter functions to
`cyclically Store energy in the power transformer and to
`transfer Such Stored energy to a capacitor, to produce the
`desired output Voltage. The level of the output voltage is
`regulated and controlled by varying the on time of Switch 48
`(and thus its duty cycle), thereby controlling the amount of
`energy that is Stored in the transformer and transferred to the
`capacitor and load per cycle. Switch 48 is connected in
`series with the primary winding 44 of the transformer. The
`converter 22 operates by Switching the transistor alternately
`to ON and OFF states. When the Switch 48 is in its ON state,
`current flows through primary winding 44 So that magnetic
`energy is stored in transformer 42. When the Switch 48 is in
`its OFF State, current flowing through the primary winding
`44 is cut off, Such that the Voltage appearing acroSS trans
`former 42 is reversed. This reversal causes forward biasing
`of diode 50 so that current flows out of the secondary
`winding 46 into capacitor 52 and out to load 54. This
`operation permits the Stored energy in transformer 42 to be
`transferred out of the Secondary winding 46 and Stored in
`capacitor 52, So as to produce the desired constant output
`voltage to load 54. The duration of time in which this energy
`is released from the Secondary winding to the load is known
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`as the flyback period or cycle. AS is well known, when the
`transistor is in the ON state, current does not flow through
`Secondary winding 46.
`In the present invention, flyback converter 22 receives the
`first DC voltage V, from boost converter 20 across node 24
`and ground 16. Flyback converter 22 converts V, to output
`Voltage V, acroSS node 62 and output ground 63. The output
`Voltage V is a regulated DC voltage of lower value than
`V, and within the desired Voltage range for the particular
`Specification of the power Supply.
`Regulation of the output DC voltage is accomplished in a
`conventional way. Preferably, feedback of the output voltage
`V from node 62 is provided on signal line 64, which is
`coupled to an input terminal 66 of a controller 68. An output
`terminal 70 of controller 68 is coupled to signal line 72,
`which in turn is coupled to control node 56 of FET switch
`48. The preferred embodiment of controller 68 is a pulse
`width modulation (PWM) control circuit, which produces a
`Series of constant frequency drive pulses whose pulse widths
`are varied depending on whether the output DC voltage is
`above or below its specified value. The rates of the on-time
`of Switch 48 to the period of each Switch cycle is the duty
`cycle of the Switch.
`During operation, power Supply System 10 produces a
`rectified input AC signal from power Source 12. At the first
`Stage of power conversion, the rectified AC Signal is con
`verted to a first DC voltage via boost converter 20. The
`output of boost converter 20 is a DC voltage with small
`ripple. This first DC voltage is then fed as an input to the
`Second Stage of power conversion, comprising flyback con
`verter 22. The operation of flyback converter 22 has been
`described above. Because the first DC voltage is
`unregulated, the flyback converter is used to convert and to
`regulate the first DC voltage to the desired level. The power
`factor is independent of either Voltage or current control
`techniques.
`As seen in FIG. 1, the output of pulse width modulation
`control circuit 68 is a Series of fixed frequency drive pulses
`that are coupled at the same time to the gates 34 and 56 of
`the MOSFET transistor Switches 28 and 48, respectively, in
`boost converter 20 and flyback converter 22. As described
`above, the duty cycle of Switch 48 is varied to control the
`output DC voltage level. A key advantage of the present
`invention is that it is unnecessary for the PWM control
`circuit 68 to know the required duty cycle for the series of
`pulses that are provided to boost Switch 28 in order for the
`boost inductor current to approximately follow the wave
`shape of the input AC Signal and thus provide power factor
`connection. It has been determined that, by causing the duty
`cycle of Switch 28 to match the duty cycle of Switch 48, there
`is no need to Separately calculate the duty cycle for the boost
`Switch 28. This results because power factor correction is
`inherent to the topology of the present invention. Although
`a boost and flyback topology has been shown in FIG. 1, it
`will be appreciated that various other AC-to-DC converter
`topologies (e.g., boost and forward) may be similarly used
`without the need to Separately calculate the duty cycle for
`the boost Switch 28.
`It is noted that the value of the boost inductor 26 is
`preSelected So that when the instantaneous value of the
`rectified input AC voltage is low, the inductor operates in a
`discontinuous mode, and when the instantaneous value of
`the rectified input AC voltage is high, the inductor operates
`in a continuous mode. In a preferred embodiment, the boost
`converter operates in the discontinuous mode within 0 to 45
`degrees around the Zero crossings of the input AC Signal
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`waveform. This effectively shapes the inductor current
`waveform So as to eliminate the harmonic components that
`introduce distortion. Moreover, because the frequency of
`Switching of Switch 28 is fixed, EMI is reduced as well as the
`low frequency ripple component at the System output.
`Experiments by the inventors indicate that typical values for
`a boost inductor 26 vary between 300 uH and 1000 uH,
`depending upon the power level used. In an exemplary
`example, a boost inductor 26 value of 500 uH would be used
`for an output voltage power of 150 W, and with a universal
`input Voltage range of 90Vs to 265Vs.
`It is also desirable that the feedback loop comprising
`signal lines 64, 72 and controller 68 have a wide bandwidth,
`that is with a frequency much higher than the frequency of
`the AC input signal at node 18. The present inventors have
`noticed that if the bandwidth is at about one tenth to one fifth
`of the Switching frequency for Switches 28 and 48, then low
`frequency ripple at the output voltage may be Substantially
`reduced. Moreover, when the feedback loop operates with a
`high bandwidth, the System response of the output Voltage is
`increased. By capturing and returning the output Voltage to
`drive both power converters 20 and 22, the present invention
`is cost-effective because it eliminates the need for Separate
`control circuits for each converter.
`Furthermore, if current mode control is used in DC-to-DC
`converter 22, the current through the Switch 48 in the flyback
`converter may also be Sensed through the dotted line rep
`resenting a current sense line 74. With this embodiment, a
`sensor 76 would be placed at conduction terminal 60.
`Operated in manner described above, the present inven
`tion permits improved power factor correction, as compared
`with the prior art, because the inductor current in the boost
`converter will be Substantially proportional to the Sinusoidal
`waveform of the input AC voltage when the boost converter
`is driven by a fixed duty cycle. This provides improved
`power factor correction. Moreover, because the gate drive
`Signal for the boost converter is the same as that for the
`flyback converter, the output voltage of the power Supply
`System may be regulated without the additional circuits and
`components of the prior art. Consequently, the present
`invention provides power factor correction at a lower cost
`than with conventional techniques. AS compared with con
`ventional AC-to-DC power Supplies lacking power factor
`correction, the topology and control method according to the
`present invention shapes the input current of the boost
`converter in a manner that conforms to the regulatory
`requirement for the reduction of harmonic current compo
`nents. Additionally, the present invention provides a well
`regulated DC output Voltage from the input AC voltage
`without using auto-range circuitry.
`Referring to FIG. 2, an input current waveform for the
`inductor in the AC-to-DC converter is shown to be Substan
`tially sinusoidal and thus proportional to the input AC
`Voltage. It is noted that only at or near the Zero Voltage axis
`80, does some amount of distortion occur. However, this
`amount of distortion is negligible and will not create a level
`of harmonic distortion that the IEC-1000-3-2 standard
`intends to preclude. Rather, with the present invention, an
`output voltage V of 5 volts (at 5 amperes) and of 19 volts
`(at 3 amperes) is easily obtained. The present invention
`provides measured harmonic components of the input cur
`rent using the power system 10 that are well below the IEC
`Standard values for Such current harmonic components, as
`illustrated in FIG. 3. Accordingly, the present invention is
`well-Suited for AC-to-DC power Supplies having a power
`level of up to 150 Watts.
`As seen in the table provided in FIG. 4, the AC-to-DC
`power Supply System of the present invention performs with
`
`45
`
`50
`
`55
`
`60
`
`65
`
`8
`power factor of 91.9% having an EMI of -39.6 dbm, and
`an efficiency of 77.5%. This result is considerably better
`than conventional converters lacking power factor correc
`tion which have a power factor of 70%, EMI of -48 dbml,
`and efficiency of 78.5%. By contrast, conventional two
`Stage converters with power factor correction have a power
`factor of 99%, but with a tradeoff being an EMI of -31.8
`dbm,
`and 72.7% efficiency.
`It will be appreciated that embodiments other than as
`described above may be utilized and structural or logical
`changes may be implemented without departing from the
`Scope of the present invention. Consequently, the detailed
`description is not to be construed in a limiting Sense, and the
`Scope of the present invention is defined by the appended
`claims and their equivalents.
`What is claimed is:
`1. An AC-to-DC power Supply System having power
`factor correction, comprising:
`an input rectifier for generating a rectified input Voltage
`for Said System from a Source of AC power;
`a boost converter coupled to Said rectifier for converting
`Said input Voltage to a first DC voltage, Said boost
`converter having a first Switch;
`a DC-DC converter coupled to the output of said boost
`converter for converting Said first DC voltage to a
`second DC output voltage, said DC-DC converter
`having a Second Switch; and
`a controller for providing feedback control as a function
`of Said output voltage, Said controller generating a
`Single drive signal that is coupled to both Said first and
`Said Second Switches So as to cause Said Switches to be
`Switched on and off simultaneously.
`2. The system of claim 1, wherein said rectifier is a diode
`bridge.
`3. The system of claim 1, wherein said DC-to-DC con
`verter is a flyback converter.
`4. The System of claim 1, wherein Said controller is a pulse
`width modulation circuit
`having an input for receiving Said output voltage, and
`having an output for providing a Series of drive pulses
`with a predetermined fixed duty cycle for selectively
`Switching both said first Switch and the second switch
`to alternating ON and OFF states at the same time.
`5. The system of claim 1, wherein said boost converter
`further includes an inductor coupled between Said rectifier
`and Said first Switch, and
`a first Series combination of a diode and capacitor con
`nected in parallel with Said first Switch, Said inductor
`having a Selected value to provide current flow through
`the inductor that is discontinuous when the instanta
`neous rectified AC input Voltage is low and continuous
`when the instantaneous rectified AC input Voltage is
`high.
`6. The system of claim 5, wherein said first Switch
`includes a control node coupled to the output, Said drive
`Signal being applied to the control node So that Said first
`Switch is operable to permit current flow through Said
`inductor.
`7. The system of cla