US007110270B2
`
`(12) United States Patent
`Balakrishnan et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 7,110,270 B2
`Sep. 19, 2006
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`METHOD AND APPARATUS FOR
`MAINTAINING A CONSTANT LOAD
`CURRENT WITH LINE VOLTAGE IN A
`SWITCH MODE POWER SUPPLY
`
`Inventors: Balu Balakrishnan, Saratoga, CA (US);
`Alex B. Djenguerian, Saratoga, CA
`(US); Kent Wong, Fremont, CA (US);
`David Michael Hugh Matthews,
`Sunnyvale, CA (US)
`Assignee: Power Integrations, Inc., San Jose, CA
`(Us)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 47 days.
`Appl. N0.: 10/892,300
`
`Notice:
`
`Filed:
`
`Jul. 15, 2004
`
`Prior Publication Data
`
`US 2004/0251886 A1
`
`Dec. 16, 2004
`
`Related US. Application Data
`
`Continuation of application No. 10/253,307, ?led on
`Sep. 23, 2002, now Pat. No. 6,781,357.
`Provisional application No. 60/325,642, ?led on Sep.
`27, 2001.
`
`Int. Cl.
`(2006.01)
`H02M 3/335
`US. Cl. ...................................... .. 363/21.7; 363/49
`
`Field of Classi?cation Search ............. .. 363/21.1,
`363/21.12, 97, 21.4, 56, 49; 323/288, 222,
`323/284, 282
`See application ?le for complete search history.
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,371,824 A *
`
`2/1983 Gritter ...................... .. 318/722
`
`(Continued)
`FOREIGN PATENT DOCUMENTS
`
`EP
`
`0744818 A1
`
`11/1996
`
`OTHER PUBLICATIONS
`
`In?neon Technologies Preliminary Datasheet, “ICE2AS01; Olf
`Line SMPS Current Mode Controller,” Datasheet, Version 2.1, Feb.
`2001, In?neon Technologies AG, Miinchen, Germany.
`(Continued)
`Primary ExamineriRajnikant B. Patel
`(74) Attorney, Agent, or F irmiBlakely SokoloiT Taylor &
`Zafman LLP
`
`(57)
`
`ABSTRACT
`
`A power supply including a regulation circuit that maintains
`an approximately constant load current With line voltage. In
`one embodiment, a regulation circuit includes a semicon
`ductor switch and current sense circuitry to sense the current
`in the semiconductor switch. The current sense circuitry has
`a current limit threshold. The regulation circuit current limit
`threshold is varied from a ?rst level to a second level during
`the time when the semiconductor switch is on. One embodi
`ment of the regulation circuit is used in a power supply
`having an output characteristic having an approximately
`constant output voltage below an output current threshold
`and an approximately constant output current below an
`output voltage threshold.
`
`25 Claims, 5 Drawing Sheets
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`
`ON SEMICONDUCTOR EXHIBIT 1002
`Page 1 of 14
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`

`
`US 7,110,270 B2
`Page 2
`
`US. PATENT DOCUMENTS
`
`6,781,357 B1
`
`8/2004 Balakrishnan et al.
`
`7/1983 O’Sullivan et al.
`4,392,103 A
`6/1987 Hill ....................... .. 363/21.17
`4,674,020 A
`5,268,631 A * 1 2/ 1993 Gorman et a1, ___________ __ 323/246
`5,285,366 A *
`2/1994 Zaretsky __________________ __ 363/561
`5,479,090 A 12/1995 Schultz
`5,680,034 A 10/1997 Redl
`6,154,377 A * 11/2000 Balakrishnan et al.
`6,166,521 A 12/2000 Mercer et al.
`6,665,197 B1
`12/2003 Gong et al.
`
`363/2101
`
`OTHER PUBLICATIONS
`
`National Semiconductor: Linear and Switching Voltage Regulator
`Fundamentals, ’Online: XP002316042 Retrieved from the Internet:
`URL: http://Web.archive.org/Web/20010602192453/http://WWW.na
`tional.com/appinfo/poWer?les/f4.pdf>, no date.
`European Search Repon, EP 02256760, Feb, 2, 2005,
`
`* cited by examiner
`
`ON SEMICONDUCTOR EXHIBIT 1002
`Page 2 of 14
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`ON SEMICONDUCTOR EXHIBIT 1002
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`U.S. Patent
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`Sep. 19, 2006
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`ON SEMICONDUCTOR EXHIBIT 1002
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`Sep. 19, 2006
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`ON SEMICONDUCTOR EXHIBIT 1002
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`Sep. 19, 2006
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`ON SEMICONDUCTOR EXHIBIT 1002
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`ON SEMICONDUCTOR EXHIBIT 1002
`Page 7 of 14
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`US 7,110,270 B2
`
`1
`METHOD AND APPARATUS FOR
`MAINTAINING A CONSTANT LOAD
`CURRENT WITH LINE VOLTAGE IN A
`SWITCH MODE POWER SUPPLY
`
`REFERENCE TO PRIOR APPLICATIONS
`
`This application is a continuation of US. application Ser.
`No. 10/253,307, ?led Sep. 23, 2002 now US. Pat. No.
`6,781,357, Which claims the bene?t of and priority to US.
`provisional application Ser. No. 60/325,642, ?led Sep. 27,
`2001, entitled “Method And Apparatus For Maintaining A
`Constant Load Current With Line Voltage In A SWitch Mode
`PoWer Supply.”
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`This invention relates generally to poWer supplies and,
`more speci?cally, the present invention relates to a sWitched
`mode poWer supply.
`2. Background Information
`All electronic devices use poWer to operate. A form of
`poWer supply that is highly ef?cient and at the same time
`provides acceptable output regulation to supply poWer to
`electronic devices or other loads is the sWitched-mode
`poWer supply. In many electronic device applications, espe
`cially the loW poWer off-line adapter/ charger market, during
`the normal operating load range of the poWer supply an
`approximately constant output voltage is required beloW an
`output current threshold. The current output is generally
`regulated below an output voltage in this region of approxi
`mately constant output voltage, hereafter referred to as the
`output voltage threshold.
`In knoWn sWitched mode poWer supplies Without second
`ary current sensing circuitry, minimiZing the variation of the
`output current at the output voltage threshold is performed
`With complex control schemes. Typically, these schemes
`include the measurement of input voltage, output diode
`conduction time and peak primary current limit. Some or all
`of this measured information is then used to control the
`regulator in order to reduce the variation of the output
`current at the output voltage threshold.
`
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`SUMMARY OF THE INVENTION
`
`45
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`A poWer supply that maintains an approximately constant
`load current With line voltage beloW the output voltage
`threshold is disclosed. In one embodiment, a regulation
`circuit includes a semiconductor sWitch and current sense
`circuitry to sense the current in the semiconductor sWitch.
`The current sense circuitry has a current limit threshold. The
`regulation circuit current limit threshold is varied from a ?rst
`level to a second level during the time When the semicon
`ductor sWitch is on. In one embodiment, the regulation
`circuit is used in a poWer supply having an output charac
`teristic having an approximately constant output voltage
`beloW an output current threshold and an approximately
`constant output current beloW an output voltage threshold.
`In another embodiment, a poWer supply is described, Which
`includes a poWer supply input and a poWer supply output
`and that maintains an approximately constant load current
`With line voltage beloW the output voltage threshold. In one
`embodiment, the poWer supply has an output characteristic
`having an approximately constant output voltage beloW an
`output current threshold and an approximately constant
`output current beloW an output voltage threshold. A regula
`
`50
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`55
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`60
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`65
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`2
`tion circuit is coupled betWeen the poWer supply input and
`the poWer supply output. The regulation circuit includes a
`semiconductor sWitch and current sense circuitry to sense
`the current in the semiconductor sWitch. The current sense
`circuitry has a current limit threshold. The regulation circuit
`current limit threshold is varied from a ?rst level to a second
`level during the time When the semiconductor sWitch is on.
`In another aspect, the current limit threshold being reached
`coincides With the poWer supply output characteristic tran
`sitioning from providing an approximately constant output
`voltage to supplying an approximately constant output cur
`rent. In yet another aspect, the semiconductor sWitch is a
`MOSFET. Additional features and bene?ts of the present
`invention Will become apparent from the detailed descrip
`tion and ?gures set forth beloW.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The present invention detailed illustrated by Way of
`example and not limitation in the accompanying ?gures.
`FIG. 1 is a schematic of one embodiment of a sWitched
`mode poWer supply regulator in accordance With the teach
`ings of the present invention.
`FIG. 2 is a diagram illustrating one embodiment of
`saWtooth, duty cycle and intrinsic current limit Waveforms
`in accordance With the teachings of the present invention.
`FIG. 3 shoWs one embodiment of a poWer supply that has
`an approximately constant voltage and constant current
`characteristic in accordance With the teachings of the present
`invention.
`FIG. 4 shoWs one embodiment of a poWer supply that has
`an approximately constant voltage and constant current
`characteristic in accordance With the teachings of the present
`invention.
`FIG. 5 is a diagram illustrating the typical relationship
`betWeen the output current and output voltage of one
`embodiment of a poWer supply in accordance With the
`teachings of the present invention.
`
`DETAILED DESCRIPTION
`
`Embodiments of methods and apparatuses for maintain
`ing a poWer supply output current substantially constant
`independent of input voltage at the point Where the poWer
`supply output characteristic transitions from providing an
`approximately constant output voltage to supplying an
`approximately constant output current are disclosed. In the
`folloWing description, numerous speci?c details are set forth
`in order to provide a thorough understanding of the present
`invention. It Will be apparent, hoWever, to one having
`ordinary skill in the art that the speci?c detail need not be
`employed to practice the present invention. In other
`instances, Well-knoWn materials or methods have not been
`described in detail in order to avoid obscuring the present
`invention.
`Reference throughout this speci?cation to “one embodi
`ment” or “an embodiment” means that a particular feature,
`structure or characteristic described in connection With the
`embodiment is included in at least one embodiment of the
`present invention. Thus, the appearances of the phrases “in
`one embodiment” or “in an embodiment” in various places
`throughout this speci?cation are not necessarily all referring
`to the same embodiment. Furthermore, the particular fea
`tures, structures or characteristics may be combined in any
`suitable manner in one or more embodiments.
`In one embodiment here, a sWitched mode poWer supply
`is described in Which the output current beloW the output
`
`ON SEMICONDUCTOR EXHIBIT 1002
`Page 8 of 14
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`

`
`US 7,110,270 B2
`
`3
`voltage threshold, is regulated to be approximately constant.
`This provides an approximate constant voltage/constant
`current output characteristic. The output current level at the
`output voltage threshold in knoWn poWer supplies sensed at
`the output of the poWer supply to provide feedback to a
`regulator circuit coupled to the primary Winding of the
`poWer supply. If hoWever, the approximate constant current
`functionality is achieved Without feedback from the second
`ary Winding side of the poWer supply, the output current at
`the output voltage threshold is a function of a peak current
`limit of the primary regulator.
`Embodiments of the present invention reduce the varia
`tion of the output current at the output voltage threshold by
`reducing the peak current limit variation With changing input
`voltage. In general, the intrinsic peak current limit is set by
`internal circuitry in the regulator to be constant. In one
`embodiment, once the drain current reaches a current limit
`threshold, the sWitching cycle should, in theory, terminate
`immediately. HoWever, a ?xed delay is inherent from the
`time the threshold is reached until the poWer metal oxide
`semiconductor ?eld effect transistor (MOSFET) is ?nally
`disabled. During this delay, the drain current continues to
`ramp up at a rate equal to the direct current (DC) input
`voltage divided by the primary inductance of the transformer
`(drain current ramp rate). Therefore, the actual current limit
`is the sum of the intrinsic current limit threshold and a
`ramp-rate dependent component (the overshoot), Which is
`the drain current ramp rate multiplied by the ?xed delay.
`Thus, at higher DC input voltages, the actual current limit
`ramps to a higher level above the intrinsic current limit level
`than at loW DC input voltages. This can result in variations
`in the output current delivered to the load at the output
`voltage threshold over a range of input line voltages.
`The actual current limit is the sum of the intrinsic current
`limit and the ramp-rate dependent component (the over
`shoot). The goal is to maintain a constant actual current limit
`over DC input voltage variations. Since the ramp-rate com
`ponent (the overshoot) increases With respect to the DC
`input voltage, the only Way to maintain a relatively constant
`current limit Would be to reduce the intrinsic current limit
`threshold When the DC input voltage rises.
`In discontinuous poWer supply designs, the point in time
`during the sWitching cycle in Which the current limit is
`reached is dependent on the DC input voltage. In fact, the
`time it takes from the beginning of the cycle to the point
`Where current limit is inversely proportional to the DC input
`voltage. Thus, the time elapsed from the beginning of the
`cycle can be used to gauge the DC input voltage.
`Therefore, in order to create an intrinsic current limit
`Which decreases relative to the DC input voltage, the time
`elapsed can be used. It is simply necessary to increase the
`intrinsic current limit as a function of the time elapsed
`during the cycle. A ?rst approximation for increasing the
`intrinsic current limit With time can be obtained by using the
`Equation 1 beloW:
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`ILIA/HNTRINSICIKr+K2>1< lelapsed’
`
`(Equation 1)
`
`Where is I L ,M_ ,NTR INSIC the intrinsic current limit, K1 and K2
`are constants and telapsed is the time elapsed.
`In one embodiment, the time elapsed can be detected by
`the internal oscillator output Waveform. In one embodiment,
`this Waveform is a triangular one. It starts at its minimum at
`the beginning of the cycle. It gradually ramps until it reaches
`the point of maximum duty cycle.
`In one embodiment, he ramp is substantially linear With
`time. In another embodiment, the ramp can also be nonlinear
`
`60
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`4
`depending on the requirements of the poWer supply in Which
`the regulator is used. The intrinsic current limit threshold is
`basically proportional to the voltage seen at the input of the
`current limit comparator. This bias voltage is the product of
`the resistor value and the current delivered to this resistor.
`One Way to increase the intrinsic current limit linearly as a
`function of the elapsed time Would then be to derive a
`linearly increasing (With elapsed time) current source and
`deliver this current to the resistor. This linearly increasing
`(With elapsed time) current source can thus be derived from
`the oscillator.
`FIG. 1 shoWs a schematic of one embodiment of a
`sWitched mode poWer supply in accordance With the teach
`ings of the present invention. All of the circuitry shoWn in
`this schematic is used to control the sWitching of the poWer
`MOSFET 2. The timing of the sWitching is controlled by
`oscillator 5. Oscillator 5 generates three signals: Clock 10,
`DMAX (Maximum duty cycle) 15, and SaWtooth 20. The
`rising edge of Clock signal 10 determines the beginning of
`the sWitching cycle. As shoWn in the illustrated embodiment,
`When Clock signal 10 is high, output latch 90 is set, Which
`results in a control signal output from output latch 90 to
`enable poWer MOSFET 2 to begin conducting. The maxi
`mum conducting time is determined by DMAX 15 signal
`being high. When DMAX 15 signal goes loW, latch 90 is
`reset, thus causing the control signal output from latch 90 to
`disable poWer MOSFET 2 from conducting.
`The intrinsic current limit is, to the ?rst order proportional
`to the voltage on node 22. As stated earlier, the goal of the
`invention is to generate an intrinsic current limit propor
`tional to the time elapsed in the sWitching cycle. The saW
`tooth Waveform 20 can be used to perform this task. As the
`base voltage of NPN transistor 30 rises, the emitter voltage
`also rises at the same rate. Thus, the current through resistor
`25 is linearly increasing With time elapsed during the
`sWitching cycle. After mirroring this current through current
`mirror 40, the linearly increasing (With elapsed time) current
`source 27 is derived. The current limit threshold 22 is thus
`proportional to the product of the combination of linearly
`increasing current source 27 and constant current source 50
`With the resistor 17. The voltage on node 37 is proportional
`to the poWer MOSFET drain voltage because of the voltage
`divider netWork formed by resistors 55 and 60. The drain
`current is proportional to the drain voltage. As the drain
`current 7 ramps up during the sWitching cycle, the voltage
`on node 37 rises proportionately. After the voltage on node
`37 exceeds the voltage on current limit threshold node 22,
`comparator 70 disables the poWer MOSFET by ultimately
`resetting latch 90.
`PWM Comparator 32 modulates the duty cycle based on
`the feedback signal coming from the output of the poWer
`supply. The higher the feedback voltage, the higher the duty
`cycle Will be.
`FIG. 2 shoWs an embodiment of three Waveforms: saW
`tooth 20, duty cycle max 15, and intrinsic current limit 22.
`The saWtooth Waveform 20 and the duty cycle max Wave
`form 15 are generated by the oscillator 5. The duty cycle
`max 15 signal determines the maximum duration of a poWer
`MOSFET sWitching cycle, When it is high. The saWtooth
`Waveform 20 starts increasing at the loW point When the duty
`cycle max Waveform 15 goes high. This signals the begin
`ning of the poWer MOSFET sWitching cycle. The high point
`of the saWtooth 20 is reached at the end of the cycle, at the
`same time the duty cycle max signal 15 goes loW. The
`intrinsic current limit 22 signal starts at the loW point at the
`beginning of the cycle and then linearly increases With
`elapsed time throughout the cycle. At a time elapsed of Zero,
`
`ON SEMICONDUCTOR EXHIBIT 1002
`Page 9 of 14
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`

`
`US 7,110,270 B2
`
`5
`the intrinsic current limit is at K1. As time elapsed increases,
`the current limit increases by a factor of K2*teZaPSed. As can
`be seen in FIG. 2 therefore, the intrinsic current limit
`(ILIM-INTRINSIC) is the Sum of K1 and K2*telapsed'
`FIG. 3 shoWs one embodiment of a poWer supply that has
`an approximately constant voltage and constant current
`characteristic in accordance With the teachings of the present
`invention. An energy transfer element 220 is coupled
`betWeen DC output 200 and HV DC input 255. In one
`embodiment, energy transfer element is a transformer
`including an input Winding 225 and an output Winding 215.
`Regulation circuit 250 is coupled betWeen HV DC input 255
`and energy transfer element 220 to regulate DC output 200.
`In the illustrated embodiment, feedback information respon
`sive to DC output 200 is provided to the regulator 250 at its
`control pin. The current at the control pin is proportional to
`the voltage across resistor 235, Which in turn is related to the
`output voltage at DC output 200.
`In operation, the regulator circuit reduces the duty cycle
`of the poWer MOSFET When the voltage across resistor 235
`increases above a threshold. In this section, the output is in
`approximately constant voltage mode. The regulator circuit
`reduces the current limit of the poWer MOSFET When the
`voltage across resistor 235 decreases beloW a threshold. The
`current limit is reduced as a function of the voltage across
`resistor 235 to keep the output load current constant. Thus,
`the load current is proportional to the current limit of the
`poWer MOSFET in regulator 250. By keeping the current
`limit invariant to line voltage, the output load current Would
`remain constant at all line voltages.
`FIG. 4 shoWs one embodiment of a poWer supply that has
`an approximately constant voltage and constant current
`characteristic in accordance With the teachings of the present
`invention. The feedback information is provided to the
`regulator 350 at its control pin. The current at the control pin
`is proportional to the voltage across resistor 335, Which in
`turn is related to the output voltage. The regulator circuit
`reduces the duty cycle of the poWer MOSFET When the
`voltage across resistor 335 increases above a threshold. In
`this section, the output is in approximately constant voltage
`mode. The regulator circuit reduces the current limit of the
`poWer MOSFET When the voltage across resistor 335
`decreases beloW a threshold. The current limit is reduced as
`a function of the voltage across resistor 335 to keep the
`output load current approximately constant. Thus, the load
`current is proportional to the current limit of the poWer
`MOSFET in regulator 350. By keeping the current limit
`substantially constant With line voltage, the output load
`current Would remain substantially constant at all line volt
`ages.
`FIG. 5 is a diagram illustrating the typical relationship
`betWeen the output current and output voltage of one
`embodiment of a poWer supply in accordance With the
`teachings of the present invention. As can be seen in curve
`400, the poWer supply utiliZing the invention exhibits an
`approximately constant output current and constant output
`voltage characteristic. That is, as output current increases,
`the output voltage remains approximately constant until the
`output current reaches an output current threshold. As the
`output current approaches the output current threshold, the
`output voltage decreases as the output current remains
`approximately constant over the drop in output voltage until
`a loWer output voltage threshold is reached When the output
`current can reduce further as shoWn by the range of char
`acteristics. It is appreciated that the constant output voltage
`
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`and constant output current characteristics of the present
`invention are suitable for battery charger applications or the
`like.
`In the foregoing detailed description, the method and
`apparatus of the present invention has been described With
`reference to speci?c exemplary embodiments thereof. It
`Will, hoWever, be evident that various modi?cations and
`changes may be made thereto Without departing from the
`broader spirit and scope of the present invention. The
`present speci?cation and ?gures are accordingly to be
`regarded as illustrative rather than restrictive.
`What is claimed is:
`1. A poWer supply regulator, comprising:
`a comparator having ?rst and second inputs and an output,
`the ?rst input of the comparator adapted to sense a
`current adapted to How betWeen ?rst and second ter
`minals of a sWitch, the second input of the comparator
`coupled to receive a variable current limit threshold
`signal; and
`a control circuit adapted to generate a control signal in
`response the output of the comparator, the control
`signal adapted to be coupled to a control terminal of the
`sWitch to control sWitching of the sWitch to provide a
`poWer supply to have an output characteristic having an
`approximately constant output current beloW an output
`threshold voltage, the variable current limit threshold
`signal adapted to vary betWeen a ?rst level and a second
`level during a time When the sWitch is adapted to be on
`in response to the control signal.
`2. The poWer supply regulator of claim 1 Wherein the
`current limit threshold signal adapted to vary betWeen the
`?rst level and the second level during a time When the sWitch
`is adapted to be off in response to the control signal.
`3. The poWer supply regulator of claim 1 further com
`prising an oscillator adapted to generate a saWtooth Wave
`form, Wherein variable current limit threshold signal is
`generated in response to the saWtooth Waveform.
`4. The poWer supply regulator of claim 3 Wherein the
`control circuit includes a latch adapted to provide the control
`signal, Wherein the latch includes a reset input coupled to the
`output of the comparator.
`5. The poWer supply regulator of claim 4 Wherein the latch
`further includes a set input coupled to an output of the
`oscillator.
`6. A poWer supply regulator, comprising:
`a comparator having ?rst and second inputs and an output,
`the ?rst input of the comparator to sense a voltage
`developed by a sWitch, during an on time of the sWitch,
`the second input of the comparator coupled to receive
`a current limit threshold signal to increase during the on
`time of the sWitch; and
`a control circuit to generate a control signal in response to
`the output of the comparator, the control signal to be
`coupled to a control terminal of the sWitch to control
`sWitching of the sWitch to provide a poWer supply to
`have an output characteristic having an approximately
`constant output current beloW an output threshold volt
`age, the variable current limit threshold signal to vary
`betWeen a ?rst level and a second level during a time
`When the sWitch is to be on in response to the control
`signal.
`7. The poWer supply regulator of claim 6 further com
`prising an oscillator to generate a saWtooth Waveform,
`Wherein variable current limit threshold signal is generated
`in response to the saWtooth Waveform.
`
`ON SEMICONDUCTOR EXHIBIT 1002
`Page 10 of 14
`
`

`
`US 7,110,270 B2
`
`7
`8. The power supply regulator of claim 7 wherein the
`control circuit includes a latch to provide the control signal,
`Wherein the latch includes a reset input coupled to the output
`of the comparator.
`9. The poWer supply regulator of claim 8 Wherein the latch
`further includes a set input coupled to an output of the
`oscillator.
`10. A poWer supply regulation circuit, the circuit com
`prising:
`a sWitch including a ?rst terminal, a second terminal and
`a control terminal, said sWitch being operable to couple
`or decouple the ?rst terminal and the second terminal in
`response to a control signal received at the control
`terminal;
`voltage sense circuitry to sense a voltage developed
`across the sWitch during an on time of the sWitch,
`representative of a current in the sWitch, the voltage
`sense circuitry having a variable current limit threshold
`to increase betWeen a ?rst level and a second level
`during a time When the sWitch is on, the control signal
`responsive to the variable current limit threshold to
`provide a poWer supply With an output characteristic
`having an approximately constant output voltage beloW
`an output current threshold and an approximately con
`stant output current beloW an output voltage threshold.
`11. The poWer supply regulation circuit of claim 10
`further comprising an oscillator to provide a saWtooth Wave
`form, the variable current limit threshold is to be varied in
`response to the saWtooth Waveform provided by the oscil
`lator.
`12. The poWer supply regulator circuit of claim 11 further
`comprising a comparator coupled to the voltage sense cir
`cuitry to compare the current in the sWitch With the variable
`current limit threshold.
`13. The poWer supply regulator circuit of claim 12 further
`comprising a latch to provide the control signal, the latch
`coupled to be reset in response to the comparator.
`14. The poWer supply regulator of claim 10 Wherein the
`sWitch comprises a metal oxide ?eld effect transistor (MOS
`FET).
`15. A poWer supply coupled to receive an input voltage,
`comprising:
`an energy transfer element coupled betWeen a poWer
`supply input and a poWer supply output; and
`a regulation circuit coupled betWeen the poWer supply
`input and the energy transfer element, the regulation
`circuit including:
`a sWitch coupled betWeen the poWer supply input and the
`energy transfer element to control delivery of energy to
`the poWer supply output;
`current sense circuitry to sense a voltage developed across
`the sWitch during an on time of the sWitch, represen
`tative of a current in the sWitch, the current sense
`circuitry having a variable current limit threshold to
`increase betWeen ?rst and second levels during a time
`When the sWitching device is on.
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`8
`16. The poWer supply of claim 15 Wherein the poWer
`supply is to transition from providing an approximately
`constant output voltage to providing an approximately con
`stant output current When an output current threshold is
`reached.
`17. The poWer supply of claim 15 Wherein the regulation
`circuit further comprises an oscillator to provide a saWtooth
`Waveform, the variable current limit threshold is to be varied
`in response to the saWtooth Waveform provided by the
`oscillator.
`18. The poWer supply of claim 17 Wherein the regulation
`circuit further comprises a comparator coupled to the current
`sense circuitry to compare the current in the sWitch With the
`variable current limit threshold.
`19. The poWer supply of claim 18 Wherein the regulation
`circuit further comprises a latch to provide a control signal
`coupled to be received by the sWitching device, the latch
`coupled to be reset in response to the comparator.
`20. The poWer supply of claim 15 Wherein the sWitching
`device comprises a metal oxide ?eld effect transistor (MOS
`FET).
`21. A method for regulating a poWer supply, comprising:
`enabling or disabling a How of energy from a poWer
`supply input to a poWer supply output With a sWitching
`device in response to a control signal;
`sensing a current through the sWitching device by sensing
`a voltage developed across the sWitching device during
`an on time of the sWitching device;
`disabling the sWitching device in response to the current
`through the sWitching device reaching a variable cur
`rent limit threshold; and
`increasing the current limit threshold betWeen ?rst and
`second levels during a time When the sWitching device
`is on.
`22. The method of claim 21 varying the variable current
`limit threshold such that the poWer supply output is regu
`lated to provide an approximately constant output voltage
`beloW an output current threshold and an approximately
`constant output current beloW an output voltage threshold.
`23. The method of claim 21 Wherein increasing the
`current limit threshold betWeen ?rst and second levels
`comprises:
`generating a saWtooth Waveform; and
`deriving the variable current limit threshold from the
`saWtooth Waveform.
`24. The method of claim 21 Wherein disabling the sWitch
`ing device comprises resetting a latch from Which the
`control signal is provided in response to the current through
`the sWitching device reaching the variable current limit
`threshold.
`25. The method of claim 24 further comprising receiving
`a feedback signal responsive to an output level of the poWer
`supply output and resetting the latch in response to the
`feedback signal.
`
`ON SEMICONDUCTOR EXHIBIT 1002
`Page 11 of 14
`
`

`
`US007110270C1
`(12) EX PARTE REEXAMINATION CERTIFICATE (8365th)
`United States Patent
`(10) Number:
`US 7,110,270 C1
`Balakrishnan et a].
`(45) Certi?cate Issued:
`Jun. 28, 2011
`
`(54) METHOD AND APPARATUS FOR
`MAINTAINING A CONSTANT LOAD
`CURRENT WITH LINE VOLTAGE IN A
`SWITCH MODE POWER SUPPLY
`
`
`(75) Inventors: Balu Balakrishnan, Saratoga, CA (US); . .
`
`Alex B. DJenguerIan, Saratoga, CA
`(Us); KeIItWOIIg, Fremont, CA (Us);
`David Michael Hugh Matthews,
`Sunnyvale, CA (US)
`
`(73) Assignee: Power Integrations, Inc., San Jose, CA
`(Us)
`
`Reexamination Request:
`No. 90/009,393, Jan. 20, 2009
`
`Reexamination Certi?cate for:
`Patent No.:
`7,110,270
`Issued:
`Sep. 19, 2006
`Appl. No.:
`10/892,300
`Filed:
`Jul. 15, 2004
`
`(63)
`(60)
`
`(51)
`
`(52)
`(58)
`
`(56)
`
`Related US. Application Data
`
`Continuation of application No. 10/253,307, ?led on Sep.
`23, 2002, now Pat. No. 6,781,357.
`Provisional application No. 60/325,642, ?led on Sep. 27,
`2001.
`
`Int. Cl.
`H02M 3/335
`H02M 3/28
`
`(2006.01)
`(2006.01)
`
`US. C