(12) United States Patent
`Hirst
`
`USOO6294904B1
`(10) Patent No.:
`US 6,294,904 B1
`(45) Date of Patent:
`*Sep. 25, 2001
`
`(54) MULTIPLE FREQUENCY SWITCHING
`POWER SUPPLY AND METHODS TO
`OPERATE A SWITCHING POWER SUPPLY
`
`5,969,515 * 10/1999 Oglesbee .............................. 232/283
`6,127,816
`10/2000 Hirst ..................................... 323/284
`
`* cited by examiner
`
`(*) Notice:
`
`(21) Appl. No.: 09/656,669
`(22) Filed:
`Sep. 7, 2000
`
`(75) Inventor: B. Mark Hirst, Boise, ID (US)
`Primary Examiner Shawn Riley
`(73) Assignee: Hewlett-Packard Company, Palo Alto,
`CA (US)
`(57)
`ABSTRACT
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35 A multiple frequency Switching power Supply including a
`U.S.C. 154(b) by 0 days.
`pulse width modulation circuit having an output, having an
`input and having a clock Signal input. The power Supply also
`This patent is Subject to a terminal dis
`includes a Switching transistor having first and Second
`claimer.
`current-carrying electrodes and a control electrode. The first
`current-carrying electrode is coupled to a Voltage Source, the
`control electrode is coupled to the output of the pulse width
`modulation circuit and the Second current-carrying electrode
`is coupled to a power Supply output configured to provide a
`O
`O
`regulated output voltage. The power Supply additionally
`Related U.S. Application Data
`includes a voltage Sensing circuit coupled to the power
`9.
`sing
`p
`p
`(63) Continuation-in-part of application No. 09/368,401, filed on
`Supply output and having an output coupled to the pulse
`Aug. 4, 1999, now Pat. No. 6,127,816.
`width modulation circuit input and a Switch coupled to the
`7
`clock input of the pulse width modulation circuit. The Switch
`(51) Int. Cl.' ........................................................ G05F 1/40
`Supplies a first clock signal having a first frequency when the
`(52)
`... 323/283; 323/284
`(58) Field of Search ..................................... 3. 2. power Supply is in a normal mode of operation and Supplies
`/284,
`a Second clock signal having a Second frequency more than
`order of magnitude lower than the first frequency when the
`power Supply is in a Standby mode of operation.
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`6/1977 Herko et al. ......................... 323/266
`
`4,030,015
`
`14 Claims, 5 Drawing Sheets
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`CONTROLLER
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`Samsung/Dell, Exh. 1032, p. 1
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`U.S. Patent
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`Sep. 25, 2001
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`Sheet 1 of 5
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`US 6,294,904 B1
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`
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`DATA
`14 N
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`PRIMARY
`AOWER SUPPLY
`
`POWER
`MONITOR
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`Az z727
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`U.S. Patent
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`Sep. 25, 2001
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`Sheet 2 of 5
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`US 6,294,904 B1
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`Samsung/Dell, Exh. 1032, p. 3
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`Samsung/Dell, Exh. 1032, p. 3
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`U.S. Patent
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`Sep. 25, 2001
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`Sheet 3 of 5
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`US 6,294,904 B1
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`Samsung/Dell, Exh. 1032, p. 4
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`U.S. Patent
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`Sep. 25, 2001
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`Sheet 4 of 5
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`US 6,294,904 B1
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`Samsung/Dell, Exh. 1032, p. 5
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`U.S. Patent
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`Sep. 25, 2001
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`O)
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`US 6,294,904 B1
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`1
`MULTIPLE FREQUENCY SWITCHING
`POWER SUPPLY AND METHODS TO
`OPERATE A SWITCHING POWER SUPPLY
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`This application is a continuation in part of U.S. appli
`cation Ser. No. 09/368,401 filed Aug. 04, 1999 now U.S. Pat.
`No. 6,127,816.
`
`FIELD OF THE INVENTION
`The invention relates to multiple frequency Switching
`power Supplies and methods to operate a Switching power
`Supply.
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`ponents and (ii) Switching losses, caused by charge Storage
`and other effects in the Switching elements. Switching losses
`are proportional to Switching Speed.
`When the amount of electrical power being drawn from
`the Switching power Supply is reduced to very low levels, the
`conduction losses become very Small and the Switching
`losses are the dominant Source of Switching power Supply
`inefficiency. For example, Switching power Supplies that
`have efficiencies on the order of 95% under normal loading
`may have efficiencies of about 50% under standby
`conditions, due primarily to Switching losses.
`Moreover, increased concern over pollution caused by
`power generation and increasingly larger numbers of
`electrically-powered devices used in homes and industry
`combine to create new Standards and guidelines for power
`consumption budgets for electrical appliances. Also,
`increasing numbers of electrical appliances are maintained
`in a “ready-to-operate' State twenty-four hours a day.
`Further, the United States Government Environmental
`Protection Agency (EPA) has developed new guidelines for
`compliance with Energy Star power consumption limit
`guidelines for power budgets for Such appliances. AS a
`result, these appliances are being designed to incorporate
`power-saving "Standby modes whereby the appliance is
`both ready to be operated and is consuming as little elec
`tricity as possible whilst in the standby mode.
`Additionally, as the number and the diversity of
`electrically-powered appliances has increased, especially
`data communications, data Storage and data manipulation
`appliances, demand has grown for battery-powered electri
`cal appliances. Consumer trends for Such appliances place
`heavy emphasis on size and weight for the appliances. A
`particular emphasis on increased battery life, and hence on
`reduced consumption of electrical power, places a Substan
`tial premium on reduction of power consumption in Such
`appliances.
`What is needed is a new type of Switching power Supply
`capable of extremely low power consumption in a Standby
`mode while Still being capable of Supporting all required
`functions in a host appliance and which is also capable of
`Switching very rapidly to full power operation on demand.
`SUMMARY OF THE INVENTION
`In a first aspect, the invention provides a multiple fre
`quency Switching power Supply. The power Supply includes
`a pulse width modulation circuit having an output, having an
`input and having a clock Signal input. The power Supply also
`includes a Switching transistor having first and Second
`current-carrying electrodes and having a control electrode.
`The first current-carrying electrode is coupled to a Voltage
`Source, the control electrode is coupled to the output of the
`pulse width modulation circuit and the Second current
`carrying electrode is coupled to a power Supply output
`configured to provide a regulated output Voltage. The power
`Supply additionally includes a voltage Sensing circuit
`coupled to the power Supply output and having an output
`coupled to the pulse width modulation circuit input and also
`includes a Switch coupled to the clock input of the pulse
`width modulation circuit. The Switch supplies a first clock
`Signal having a first frequency to the pulse width modulation
`circuit when the power Supply is in a normal mode of
`operation and Supplies a Second clock signal having a
`Second frequency more than order of magnitude lower than
`the first frequency to the pulse width modulation circuit
`when the power Supply is in a Standby mode of operation.
`In another aspect, the invention provides a method to
`operate a Switching power Supply. The method includes
`
`BACKGROUND OF THE INVENTION
`Switching power Supplies are used in applications where
`power Supply efficiency is a concern. Several different types
`of Switching power Supplies are known and commonly used.
`A first type is known as a “buck' power Supply. Buck power
`Supplies are DC-to-DC converters providing a stable output
`Voltage from an input Voltage that is larger than the output
`Voltage. In other words, by bucking a portion of the elec
`tromotive force of the input Voltage, the buck Supply is able
`to provide a regulated, reduced output voltage. Ideally, the
`buck power Supply is able to perform Voltage Step-down and
`regulation functions with very little power loSS.
`A Second type of Switching power Supply is known as a
`"boosting power Supply. Boosting power Supplies incorpo
`rate a transformer or coil and one or more Switches coupled
`in Series with a primary winding of the transformer or in
`Series with the coil. Boosting power Supplies are able to
`Supply a DC output Voltage from a DC input voltage that is
`lower than the output voltage. Examples of Switching power
`Supplies using both principles of operation are described in
`U.S. Pat. No. 5,691,632, entitled “Switching Power Supply,”
`issued to Otake and hereby incorporated herein by reference.
`These examples use a Switching transistor as a Synchronous
`rectifier. Efficiency of Switching transistor operation is
`improved by reducing charge Storage effects.
`Other examples of Switching power Supplies are known.
`For example, U.S. Pat. No. 5,675,479, issued to Tani et al.
`and hereby incorporated herein by reference, discloses a
`Switching power Supply where efficiency is improved under
`light loading by lowering the Switching Speed of a Switching
`element. Under heavy load, output ripple is reduced by
`increasing Switching Speed.
`U.S. Pat. No. 5,390,101, issued to Brown and hereby
`incorporated herein by reference, discloses a Switching
`power Supply having a voltage controlled oscillator (VCO)
`to provide high efficiency operation throughout a wide range
`of input voltages and load conditions. VCO frequency is
`increased when output loading increases. U.S. Pat. No.
`4,683,529, issued to Bucher II and hereby incorporated
`herein by reference, discloses a Switching power Supply that
`uses pulse-width modulation together with Switching fre
`quency modulation to maintain high efficiency.
`All of these examples are concerned with maintaining
`efficiency over portions of a load curve where significant
`amounts of power are being drawn from the Switching
`power Supply. As a result, the range of frequencies over
`which they operate is relatively narrow. Additionally, power
`dissipation in Switching power Supplies (i.e., inefficiency) is
`composed of two principal components: (i) conduction
`losses, caused by parasitic resistance in power Supply com
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`determining when an output current from the power Supply
`falls below a threshold and Switching from one Switching
`frequency to another, discrete Switching frequency when the
`output current falls below the threshold.
`In another aspect, the invention provides a method to
`operate a Switching power Supply. The method includes
`determining when an output current from the power Supply
`is above a threshold and Supplying a first Switching Signal
`having a first frequency to a Switching transistor in the
`power Supply when the output current is above the threshold.
`The method also includes determining when the output
`current from the power Supply is below the threshold and
`Supplying a Second Switching Signal having a Second fre
`quency to the Switching transistor in the power Supply when
`the output current is below the threshold. The second
`frequency is less than one tenth of the first frequency.
`In another aspect, the invention includes provision of a
`control input on a multiple frequency Switching power
`Supply. Commands from a print engine controller Select a
`first Switching frequency for the power Supply when the
`printer is in a normal mode of operation and Select a Second
`Switching frequency for the power Supply when the printer
`is in a Standby mode of operation.
`DESCRIPTION OF THE DRAWINGS
`Preferred embodiments of the invention are described
`below with reference to the following accompanying draw
`ings.
`FIG. 1 is a simplified block diagram of a printer incor
`porating a multiple frequency Switching power Supply, in
`accordance with an embodiment of the present invention.
`FIG. 2 is a simplified Schematic diagram of a multiple
`frequency Switching power Supply, in accordance with an
`embodiment of the present invention.
`FIG. 3 is a simplified Schematic diagram of a multiple
`frequency Switching power Supply, in accordance with an
`embodiment of the present invention.
`FIG. 4 is a simplified Schematic diagram of a multiple
`frequency Switching power Supply, in accordance with an
`embodiment of the present invention.
`FIG. 5 is a simplified schematic diagram of a multiple
`frequency Switching power Supply, in accordance with an
`embodiment of the present invention.
`FIG. 6 is a simplified block diagram of a group of power
`supplies for the laser printer of FIG. 1.
`DETAILED DESCRIPTION OF THE
`INVENTION
`FIG. 1 is a simplified block diagram of a laser printer 10
`incorporating a multiple frequency Switching power Supply
`12, in accordance with an embodiment of the present inven
`tion. The laser printer 10 includes a data input port 14, a data
`memory 16, a controller 18 and a print engine 20. The laser
`printer 10 also includes a power consumption monitor 22, a
`primary AC-to-DC power Supply 24 and one or more
`efficient DC-to-DC Switching power supplies 12 coupled to
`the primary power Supply 24.
`Laser printers 10 may be formed to provide relatively
`high Speed print capability, as described, for example, in
`U.S. Pat. No. 5,195,176, entitled “Method and Apparatus to
`Enhance Laser Printer Speed and Functionality,” issued to
`Lung and hereby incorporated herein by reference. Color
`print capability is described in U.S. Pat. No. 5,018,805,
`entitled “Laser Printer,” issued to Kessler and hereby incor
`porated herein by reference. Improvements in or affecting
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`print engines in laser printers are described in U.S. Pat. No.
`5.245,442, entitled “Multi-Functional Laser Printer,’ issued
`to Yang and U.S. Pat. No. 5,079,569, entitled “Laser Printer
`With Paper Positioning and Tensioning Features,” issued to
`Bunch, Jr., which patents are hereby incorporated herein by
`reference.
`In laser printers 10, the data input port 14 and the data
`memory 16 are typically digital circuits that operate together
`under the direction of the controller 18 to accept input data
`at a relatively high Speed at the input port 14, Store the input
`data in the data memory 16 and then proceSS portions of the
`input data to provide printed output matter from the print
`engine 20.
`In laser printers 10, output data, in the form of printed
`material, is output from the print engine 20 at a much lower
`rate than the data input rate. When the laser printer 10 has
`input data Supplied to the data input port 14, the data
`memory 16 provides temporary Storage of the input data
`until the input data can all be processed.
`In order to reduce power consumption due to operation of
`the data input port 14, the data memory 16, the controller 18
`and the print engine 20, these components have been
`designed to operate with progressively lower power Supply
`Voltages over the last Several years. However, other kinds of
`functions, Such as an electromechanical drive for feeding
`paper through the print engine 20, require higher Voltages or
`currents in order to operate properly. As a result, the AC-to
`DC power Supply 24 is often used to provide a primary
`power Supply, with one or more efficient DC-to-DC Switch
`ing power Supplies 12 to provide the data-handling circuits
`14, 16 and 18 with electrical power.
`In normal operation, the power consumption monitor 22
`detects normal power consumption and provides control
`Signals to maintain normal power Supply functions. When,
`however, no input data are present at the data input port 14
`and the print engine 20 is not active, power consumption
`decreases. When the power consumption monitor 22 detects
`reduced power consumption, Signals from the power con
`Sumption monitor 22 place the multiple frequency Switching
`power Supply 12 in a Standby mode.
`In the Standby mode, the multiple frequency Switching
`power Supply 12 Switches from a first clock signal to a
`Second clock Signal having a lower frequency than the first
`clock signal to reduce Switching Speed and thus to reduce
`Switching losses in the multiple frequency Switching power
`Supply 12. In one embodiment, the Second clock Signal has
`a frequency that is one-tenth to one-one hundredth of the
`frequency of the first clock signal. In the Standby mode, the
`amount of electrical power being drawn from the power
`Supply in the Standby mode may be more than a factor often
`less than is drawn in the normal operating mode. For
`example, the power draw in the normal operating mode
`might be 200 Watts, with only 10 Watts being required in the
`Standby mode.
`There are significant advantages to discontinuously
`Switching to a Second clock signal having a frequency that
`is well outside of a range of frequencies for a first clock
`Signal. A first advantage is that a sharply reduced clock
`Signal frequency also provides a sharp increase in Switching
`power Supply efficiency because Switching losses are dra
`matically reduced. A Second advantage is that circuit com
`plexity is reduced compared to what would be required for
`a single oscillator to be able to provide Such diverse fre
`quencies.
`A third advantage is that the range of clock frequencies
`available through discontinuous Switching of clock frequen
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`cies is increased. This arises because the Small changes in
`loading that are used to provide incremental or continuous,
`rather than discontinuous, changes in clock signal frequency
`do not Support Such great modification of clock frequency
`from a VCO.
`VCOS operate over a limited range of frequencies having
`an upper frequency bound that is typically only two to three
`times as great as a lower frequency bound. In other words,
`a Switching power Supply that uses a VCO to vary Switching
`frequency within a range of loads typically cannot encom
`pass a wide enough frequency range to obtain the reduction
`in Switching frequency that is needed in order to provide
`greatly increased efficiency in the Standby mode.
`FIG. 2 is a simplified Schematic diagram of a multiple
`frequency (MF) Switching power Supply 30, in accordance
`15
`with an embodiment of the present invention. The MF
`Switching power supply 30 may be used for the multiple
`frequency switching power supply 12 of FIG. 1. While the
`MF Switching power supplies shown in FIGS. 2-5 are
`shown as being buck power Supplies, it will be appreciated
`that other kinds of Switching power Supplies may use the
`inventive concepts illustrated, including “boost”, “fly back”
`and "forward converter types of Switching power Supplies.
`The MF switching power supply 30 includes a power
`input port 32 to accept an input voltage V, a Switching
`transistor 34, a filtering circuit 36, a voltage divider 37, a
`power output port 38 to provide an output voltage V and
`a pulse width modulation (PWM) circuit 40 having an input
`42. The MF switching power supply 30 also includes a
`Switch 44 and multiple clock Signal Sources or oscillators 46
`and 48.
`The PWM circuit 40 input 42 may be switched by the
`Switch 44 to selectively couple one of several oscillators 46
`and 48 to the PWM circuit 40 in response to control signals
`from the power consumption monitor 22 of FIG. 1. When
`the power consumption monitor 22 detects reduced power
`consumption, the power consumption monitor 22 causes the
`Switch 44 to Switch from a first clock signal from a first
`oscillator 46 having a first clock frequency f. to a Second
`clock signal from a Second oscillator 48 having a Second
`clock frequency fo, reducing the clock frequency, and thus
`the Switching frequency of the Switching transistor 34, in the
`Standby mode. In one embodiment, the Switching frequency
`is reduced by one or more orders of magnitude in the
`Standby mode.
`When the power consumption monitor 22 detects
`increased power consumption, the power consumption
`monitor 22 causes the Switch 44 to Switch from the second
`oscillator 48 to the first oscillator 46, increasing the output
`current from the MFSwitching power supply 30 and restor
`ing the laser printer 10 of FIG. 1 to normal operation. In one
`embodiment, the first clock frequency f. is 100 kilohertz
`and the Second clock frequency f,
`is 1 kilohertz.
`It will be appreciated that the frequency f. of the first
`clock signal may be varied in a continuous fashion in order
`to operate the multiple frequency Switching power Supply 12
`at high efficiency over a range of loads in the normal mode.
`AVCO may be used for the first oscillator 46 to allow the
`clock frequency f. to increase as the current output from the
`multiple frequency Switching power Supply 12 is increased.
`For example, the first oscillator 46 may sense power
`Supply loading using the Voltage divider 37, as indicated by
`a dashed line in FIG. 2. Variation of Switching power supply
`operating frequency to maintain high efficiency over a range
`of loads is discussed in U.S. Pat. No. 5,691,632, issued to
`Otake and hereby incorporated herein by reference.
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`The MF switching power supply 30 operates by turning
`the Switching transistor 34 ON and OFF to supply electrical
`charge from the power input port 32 to the filtering circuit
`36. The resistive voltage divider 37 is formed from two
`resistors R and R2 and provides a node at a junction of the
`two resistors R and R for one input of the PWM circuit 40.
`A gate of the Switching transistor 34 is coupled to an output
`of the PWM circuit 40. A pulse width of the ON portion of
`the duty cycle of the Switching transistor 34 is varied in a
`conventional manner by the PWM circuit 40 in response to
`voltages sensed by the voltage divider 37 to regulate the
`amount of charge that is transferred per cycle from the
`power input port 32 to the filtering circuit 36 and thus to
`maintain the desired output voltage V at the output 38.
`In one embodiment, the limits over which the duty cycle
`may be varied in the PWM circuit 40 are different when the
`PWM circuit 40 is driven by the first oscillator 46 than when
`the PWM circuit 40 is driven by the second oscillator 48.
`This can provide hysteresis to reduce or avoid excessive
`Switching between the normal operating mode and the
`Standby mode.
`The filtering circuit 36 acts to Smooth out pulses from the
`Switching transistor 34. In one embodiment, the filtering
`circuit 36 includes a series-connected inductor L and a
`shunt-connected capacitor C. A diode D acts as a clamp to
`prevent transient changes in current through the inductor L.
`from driving the Side of the inductor L that is coupled to the
`Switching transistor 34 significantly below ground.
`In one embodiment, the MF switching power supply 30
`includes a ripple monitor 49. The amount of ripple that is
`present in the output Voltage Vo tends to increase as the
`amount of power being drawn from the MFSwitching power
`supply 30 increases. When the ripple monitor 49 determines
`that the ripple in the output voltage V is too high, the ripple
`monitor 49 provides a signal to the controller 18 of FIG. 1
`to increase the frequency of the MFSwitching power Supply
`30. In one embodiment, the frequency is increased by
`Switching to the first oscillator 46. In one embodiment, the
`frequency of the first oscillator 46 is increased when the
`ripple monitor 49 detects unacceptably high ripple, for
`example by changing a control voltage controlling a VCO
`employed for the first oscillator 46.
`FIG. 3 is a simplified Schematic diagram of a multiple
`frequency Switching power Supply 50, in accordance with an
`embodiment of the present invention. Many elements used
`in the MF switching power supply 50 are identical to
`elements used in the MFSwitching power Supply 30 of FIG.
`2. These elements are given the same reference numbers as
`are used in FIG. 2 and explanation of these elements will not
`be repeated. The dashed line coupling the first oscillator 46
`(f) to the voltage divider 37 of FIG. 2 has been eliminated
`from Subsequent Figures for clarity of illustration.
`The MF switching power supply 50 includes a modified
`filtering circuit 36". The modified filtering circuit 36
`includes three additional resistors, Rs, R and R. The
`resistor Rs is a low value resistor coupled between the
`inductor L and the capacitor C that develops a small
`voltage proportional to the current being drawn from the MF
`Switching power supply 50. A first voltage divider 52 is
`formed by resistors R and R2 coupled in Series between a
`first end of the resistor Rs and ground, and a Second Voltage
`divider 54 formed by resistors R and R coupled between
`a Second end of the resistor Rs and ground.
`A Voltage difference between Voltages developed in the
`first voltage divider 52 and the second voltage divider 54 is
`Sensed by a current Sensing circuit 56. The current Sensing
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`circuit 56, together with the current Sensing resistor Rs and
`the first and second voltage dividers 52 and 54, is one
`embodiment of the power consumption monitor 22 of FIG.
`1.
`In one embodiment, the current Sensing circuit 56
`includes a first operational amplifier 57, a resistor 58 and a
`second operational amplifier 59. The first operational ampli
`fier 57 has a non-inverting input coupled to a node joining
`the resistors R and R forming the first voltage divider 52.
`The first operational amplifier 57 has an inverting input
`coupled to a node joining the resistors R and R forming the
`second voltage divider 54. The resistor 58 sets again for the
`first operational amplifier 57.
`An output signal from the first operational amplifier 57 is
`coupled to an inverting input to the Second operational
`amplifier 59. A reference Voltage V is coupled to a
`non-inverting input to the Second operational amplifier 59.
`The reference Voltage V, together with the resistor 58,
`the resistor Rs and the first and second voltage dividers 52
`and 54, determine a current level at which the current
`Sensing circuit 56 Switches from a normal operating State to
`a Standby State and Vice versa. The output signal from the
`first operational amplifier 57 may optionally be filtered using
`an R-C filter 60 including a series resistor R and a shunt
`capacitor C, as shown in FIG. 3.
`An output of the second operational amplifier 59 is
`coupled to the Switch 44. When the current sensing circuit 56
`detects that the current being drawn from the output 38 has
`dropped below a threshold level, the output from the second
`operational amplifier 59 switches the PWM 40 from the first
`oscillator 46 providing the first clock signal (f) to the
`Second oscillator 48 providing the Second clock signal (f).
`This puts the MF Switching power supply 50 into the
`standby mode. When the current sensing circuit 56 detects
`that the current being drawn from the output 38 has
`increased above the threshold, the Second operational ampli
`fier 59 changes state, Switching the Switch 44 to provide the
`first clock signal (f). This puts the MF Switching power
`supply 50 back into the normal operating mode from the
`Standby mode.
`FIG. 4 is a simplified Schematic diagram of a multiple
`frequency Switching power Supply 70, in accordance with an
`embodiment of the present invention. The MF Switching
`power Supply 70 includes a duty cycle Sensing circuit 72
`having an input coupled to the gate of the Switching tran
`Sistor 34 and having an output coupled to the Switch 44.
`When the duty cycle sensing circuit 72 detects that the
`duty cycle of the Switching transistor 34 falls below a
`threshold, the duty cycle Sensing circuit 72 causes the Switch
`50
`44 to couple the Second oscillator 48 Supplying the Second
`clock signal (f) to the PWM 40. This sets the MF
`Switching power supply 70 to the standby mode of opera
`tion.
`When the duty cycle sensing circuit 72 detects need for
`increased power output from the MF Switching power
`supply 70, for example due to increased duty cycle of the
`Switching transistor 34, the duty cycle Sensing circuit 72
`Switches the MF switching power supply 70 from the
`Standby mode to the normal mode by coupling the first
`oscillator 46 to the PWM 40 to provide the first clock signal
`(f) to the PWM 40.
`FIG. 5 is a simplified schematic diagram of a multiple
`frequency (MF) Switching power Supply 74, in accordance
`with an embodiment of the present invention. In the embodi
`ment shown in FIG. 5, the MF switching power supply 74
`Switches from the normal mode of operation using the first
`
`8
`to the
`oscillator 46 having the first clock frequency f.
`Standby mode of operation using the Second clock signal
`from the second oscillator 48 having the second clock
`frequency f, in response to commands from the controller
`18 of FIG. 1. This permits the controller 18 to cause more
`than one MF Switching power Supply, Such as the power
`supply 74, to enter the standby mode when the laser printer
`10 of FIG. 1 enters the standby mode.
`FIG. 6 is a simplified block diagram of a group of power
`supplies 80 for the laser printer 10 of FIG. 1. Modern laser
`printers 10 include a number of power supplies 80, many or
`all of which may be Switching power Supplies. The power
`supplies 80 include a filter 82 that couples AC input power
`to an active power factor control power Supply 84. In one
`embodiment, the active power factor control power Supply
`84 converts 120 volts AC to 400 volts DC.
`An output from the active power factor control power
`Supply 84 is coupled to a capacitor 86 and to an input to a
`fly back converter 88. An output from the fly back converter
`88 is coupled to a primary winding of a transformer 90. A
`secondary winding of the transformer 90 is coupled through
`a diode 92 to a capacitor 94 to provide a +24 volt power
`Source. A +5 volt buck converter 96 is coupled to the +24
`volt power source to provide a +5 volt output, and a +3.3 volt
`buck converter 98 is coupled to the +5 volt output to provide
`a +3.3 volt output.
`The controller 18 is able to couple either a first oscillator
`100 having a first frequency for a second oscillator 102
`having a Second frequency f,
`to the active power factor
`power supply 84 via a Switch 104 in response to signals from
`the power monitor 22 of FIG. 1 or in response to other
`assessments of power need. The controller 18 also is able to
`couple either a first oscillator 106 having a first frequency
`for a Second oscillator 108 having a Second frequency f,
`to the fly back converter 88 via a Switch 110. The controller
`18 also is able to couple either a first oscillator 112 having
`a first frequency for a second oscillator 114 having a
`Second frequency f, to the +5 Volt buck converter 96 via a
`Switch 116. The controller 18 also is able to couple either a
`first oscillator 118 having a first frequency for a Second
`oscillator 120 having a Second frequency f, to the +3.3 volt
`buck converter 98 via a switch 122. The first frequency f.
`and the Second frequency f, need not be the same for each
`of the power supplies 84, 88, 96 and 98. The controller 18
`may switch all of the power Supplies 84, 88, 96 and 98
`together or may Switch them in a preferred order, or may Set
`them individually in response to power levels being drawn
`from each of them.
`In the embodiments of FIGS. 2 through 6, a small amount
`of hysteresis may be included to reduce or prevent unnec
`essary Switching between the two frequencies f and f.
`The hysteresis may be provided by Simply including a short
`delay, or by Setting thresholds for Switching from one State
`to be slightly different than thresholds for Switching back to
`the one State or may be implemented in other ways.
`It will be appreciated that the Switching transistor 34 of
`FIGS. 2 through 5 may be a MOS power transistor (as
`shown) or may be a bipolar power Switching transistor.
`Other types of Switching elements that may be pulse width
`modulated may also be employed.
`In compliance with the Statute, the invention has been
`described in language more or leSS Specific as to Structural
`and methodical features. It is to be understood, however, that
`the invention is not limited to the Specific features shown
`and described, Since the means herein disclosed compri