`Baranowski et al.
`
`US00570347OA
`Patent Number:
`(11
`45 Date of Patent:
`
`5,703,470
`Dec. 30, 1997
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`54 BATTERY CHARGER WITH POWER
`DSSPATION CONTROL
`
`75 Inventors: Robert Baranowski, Crystal Lake;
`Matthew Whiting Taylor, Gurnee, both
`of 1.
`
`73) Assignee: Motorola, Inc., Schaumburg, Ill.
`
`(21) Appl. No.: 657,699
`22 Filled:
`May 29, 1996
`(51) Int. Cl'................... H01M 1046
`52 U.S. C. ............................... 320/49; 320/51; 320/54
`58)
`Field of Search .................................... 32015, 12, 13,
`320/21, 27, 30, 35, 36,39, 40, 49, 51,
`54
`
`56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`323/1
`3,474,324 10/1969 Arnold .....
`323/1
`3496,450 2/1970 Thiele ...
`4,395,639 7/1983 Bring ...................................... 32O/3
`4472,672 9/1984 Pacholok ...
`320/32
`4,644,247 2/1987 Burmenko ..........
`... 32O/39
`5,525,893 6/1996 Alberkrack et al. .................... 320/40
`
`OTHER PUBLICATIONS
`"Advance Information, Power Field Effect Transistor,
`N-Channel Enhancement-Mode Silicon Gate TMOS with
`Current Sensing Capability," Motorola Semiconductor
`Technical Data, pp. 3-693, No date.
`Primary Examiner Edward Tso
`Attorney, Agent, or Firm-Sylvia Chen
`57
`ABSTRACT
`Abattery charger uses an unregulated DC transformer (100)
`as a power source for charging a battery pack (101) through
`a pass device (104). A charger controller (150) instanta
`neously computes a desired charging current value for
`maximizing the charging efficiency of the battery charger
`based on the present voltage of the battery pack (101),
`ambient temperature data received from a thermistor (121),
`and charging rates and other charging parameters received
`from a data-storage device (123) in the battery pack. The
`charger controller sends the calculated desired charging
`current information to a power controller (103). The power
`controller (103) monitors the instantaneous power dissipa
`tion of the pass device (104) and scales the desired charging
`current value to prevent excessive power dissipation in the
`pass device (104). By allowing the charging voltage to vary
`and dynamically adjusting the charging current, various
`types of batteries can be recharged without the use of an
`expensive tracking regulator.
`20 Claims, 3 Drawing Sheets
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`UNREGULATED
`TRANSFORNER
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`PASS
`DEVICE
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`POWER
`SENSE
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`CURRENT
`CONTROL
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`DESIRED
`CURRENT
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`POWER
`CONTROLLER
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`CHARGER
`CONTROLLER
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`BATTERY
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`TEMPERATURE
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`DATA
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`BATTERY CHARGER WITH POWER
`DSSPATION CONTROL
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`FIELD OF THE INVENTION
`This invention relates generally to battery chargers, and
`more particularly to fast charging of batteries using a pass
`device.
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`software, or a combination of hardware and software. The
`controller adjusts the current control signal, based on the
`present pass device power dissipation, to ensure that the
`power dissipation of the pass device does not exceed cal
`culated long-term and short-term power maximums. Also,
`polling a thermistor in the battery packallows the controller
`to more accurately determine the power maximums of the
`pass device, because the maximum power that can be
`dissipated by the device varies with the ambient temperature
`around the device.
`This battery charger dynamically adjusts for varying
`charging voltages, which allows replacement of the expen
`sive external tracking regulator found in traditional battery
`charging topologies by a simple, unregulated DC trans
`former. Additionally, the voltage slump of a low-cost,
`unregulated DC transformer at high current amounts reduces
`the power that needs to be dissipated in the pass device, and
`the feedback loop monitors and exploits this characteristic.
`With software flexibility and proper selection of a low-cost,
`unregulated DC transformer, many types of batteries can be
`efficiently charged to capacity, including nickel-cadmium
`(NiCad), nickel-metal-hydride (NiMH), and lithium-ion
`(Lilon) batteries.
`FIG. 1 shows a block diagram of a battery charger
`according to a preferred embodiment. In this approach, an
`inexpensive, unregulated DC transformer 100 such as a wall
`transformer provides an unregulated voltage to pass device
`104 for charging battery pack 101. Battery pack101 may be
`a battery pack for a portable radiotelephone and include a
`data storage device 123, which can be an electronic pro
`grammable read-only memory, such as an EPROM or
`EEPROM, retaining information such as charge rates and
`other charging parameters. Charger controller 150 can
`receive information from the data storage device through
`data input 124 to aid in the efficient charging of the battery.
`Charger controller 150 can also determine the ambient
`temperature before charging a battery by polling a ther
`mistor 121, which is built into most battery packs, using
`temperature sense input 122. Because maximum power
`dissipation of the pass device varies with temperature, the
`ambient temperature data can be used to scale the calculated
`maximum allowable power dissipation of the pass device.
`This scaling can improve charging times beyond any worst
`case times based upon a worst-case dissipation scenario of
`the pass device 104.
`Charger controller 150 also receives the instantaneous
`battery voltage through battery sense input 109. Thus, with
`information from data input 124, temperature sense input
`122, and battery sense input 109, charger controller 150 can
`compute a desired charging current value based on stored
`charging rates and other charging parameters, the ambient
`temperature, and the present battery voltage. This computed
`desired charging current value is transmitted to power con
`troller 103 through desired current output 107.
`Power controller 103 produces a current control signal
`based on the desired charging current information from
`charger controller 150 and the instantaneous power dissipa
`tion of the pass device 104 as sensed through power sense
`input 110. A power FET with current sensing capability
`could easily be used to determine the instantaneous power
`dissipation of the pass device using the current output from
`the FET and calculating the voltage drop across the pass
`device 104. Power controller 103 scales the desired charging
`current value from charger controller 150 based on the
`information from power sense input 110 to create a current
`control signal. The current control signal is sent from current
`control output 111 of power controller 103 to the pass device
`104 to keep the power dissipated by pass device 104 within
`acceptable power ratings.
`
`BACKGROUND OF THE INVENTON
`Battery chargers generally use a regulator which rectifies
`an alternating current (AC) to produce a direct current (DC)
`for charging a battery. In one type of charger, called a series
`pass charger, a linear switch pass device such as a metal
`oxide-semiconductor field-effect transistor (MOSFET) is
`connected between a regulator and the battery. When a
`battery is charging, the power dissipated by the pass device
`is equal to the difference between the input and output
`voltages of the pass device multiplied by the maximum
`charging current. When a battery is deeply discharged, the
`20
`battery voltage, which is the voltage at the output of the pass
`device, is much less than the regulator voltage, which is the
`voltage at the input of the pass device. During this condition,
`the power dissipated by the pass device could exceed
`maximum power ratings of typical device packages found in
`portable electronic devices. During this period of high
`power dissipation by the pass device, excess heat is gener
`ated and the overall efficiency of the charger is very poor.
`Conventional fast chargers for portable devices that are
`space and heat critical use an external tracking regulator to
`limit power dissipation in the charger's pass device. The
`tracking regulator provides a voltage that is a constant
`positive offset from the voltage of the battery being charged,
`thus holding the difference between the input and output
`voltages of the pass device relatively constant. A micropro
`cessor senses the battery voltage and creates an analog
`control voltage proportional to a desired charging current.
`The charging current is controlled by a hardware feedback
`loop that senses a voltage drop across a sense resistor, scales
`it, and compares it to the control voltage. When the charging
`Software calls for a change in the charging current based on
`a change in the battery voltage, the microprocessor changes
`the control voltage accordingly.
`By keeping the voltage drop across the pass device
`relatively constant and only varying the charging current, the
`charger can easily control the power dissipation of the pass
`device. The tracking regulator that keeps the voltage drop
`constant, however, is application specific and expensive.
`Thus, there is a need for a battery charger that both limits
`power dissipation and eliminates the need for an expensive
`tracking regulator.
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`BRIEF DESCRIPTION OF THE DRAWNGS
`FIG. 1 shows a block diagram of a battery charger
`according to a preferred embodiment.
`FIG. 2 shows a battery charger according to a first
`preferred embodiment,
`FIG. 3 shows a battery charger according to a second
`preferred embodiment.
`DETALED DESCRIPTION OF THE
`PREFERRED EMBODMENTS
`The battery charger with power dissipation control
`includes a feedback loop that senses a present battery
`voltage, monitors the instantaneous power dissipation of a
`pass device, and creates a current control signal for charging
`a battery pack of a device such as a portable radiotelephone.
`A controller in the feedback loop may include hardware,
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`Note that when the power controller 103 directs a reduc
`tion in charging current to limit pass device power
`dissipation, the battery charging time might increase. The
`amount of time increase is based upon the design of the
`transformer. If the transformer has a large voltage drop when
`sourcing large currents, a typical occurrence in unregulated
`DC transformers, the drop across the pass device will be
`less, which allows a higher charging current. If, however, the
`transformer voltage drop is very large, a large charging
`current cannot be used when a battery is near full charge.
`An advantage to this approach is that the charger controls
`the power dissipation of the pass device by allowing the
`charging voltage to fluctuate and controlling only the charg
`ing current. A first feedback loop through charger controller
`150 determines the desired current based on the present
`battery voltage, the ambient temperature, and information
`from a battery packdata-storage device. This desired current
`information is sent to a second feedback loop which includes
`power controller 103. Power controller 103 scales down the
`desired current value if the desired current value would
`cause excessive power dissipation at the pass device to
`produce a current control signal. The scaling is based on the
`instantaneous power dissipation information from power
`sense input 110. The resulting current control signal from
`current control output 111 allows the largest current through
`pass device 104 that does not exceed short-term and long
`25
`term power maximums.
`Thus, the voltage at the output of the unregulated DC
`transformer 100 can vary, and the power controller 103 will
`dynamically adjust the charging current to avoid excessive
`power dissipation through the pass device 104. The charger
`controller 150 in combination with the power controller 103
`30
`calculate the absolute maximum and short-term maximum
`power dissipations of the pass device and ensure that these
`maximums are never exceeded, regardless of the condition
`of the unregulated DC transformer.
`FIG. 2 shows a battery charger according to a first
`preferred embodiment. The charger controller 150 and
`power controller 103 shown in FIG. 1 are implemented
`using a microprocessor 250 which senses the battery voltage
`through battery sense input 209 and the charging voltage
`from the transformer through supply sense input 210. Pref
`erably sense inputs 209. 210 are connected to an analog-to
`digital converter in microprocessor 250. Note that software
`functionality in any of these embodiments, including
`analog-to-digital conversion, may be implemented using
`hardware and vice versa.
`Using information from the sense inputs, which represent
`the voltages on both sides of the pass device 204, the
`microprocessor 250 computes the voltage drop across the
`pass device. The microprocessor can then insure that the
`power dissipation of the pass device does not exceed the
`calculated maximum allowable by regulating a control sig
`nal from current control output 211, which directs the
`charging current control signal through operational ampli
`fiers 202, 203. Preferably, the control signal at desired
`current output 211 is a pulse-width-modulated (PWM)
`waveform. As an alternative, microprocessor 250 can use a
`digital-to-analog converter to produce the control signal at
`desired current output 211.
`The maximum power dissipation allowed through pass
`device 204 is calculated from ambient temperature informa
`tion through temperature sense input 222 connected to a
`thermistor 121 preferably in the battery pack 101 and
`chargingrates and other charging parameters from data input
`224 connected to data storage device 123 in the battery pack
`101. Other information, such as the pass device or trans
`former characteristics and ratings can also be used to deter
`mine the calculated maximum power dissipation through the
`pass device.
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`The voltage from the unregulated DC transformer 100 is
`fed through sense resistor 205 and diode 206 to charge the
`cells in battery pack 101. During charging, the voltage
`across sense resistor 205 is held constant by a hardware
`feedback loop including operational amplifiers 202,203 and
`controlled by the signal from current control output 211.
`Operational amplifier 203 controls analog switch pass
`device 204 to set the output current of operational amplifier
`202 as directed by the signal from desired current output 211
`of the microprocessor, Operational amplifier 203 has a
`lowpass filter 215 at its output for stability.
`FIG. 3 shows a battery charger according to a second
`preferred embodiment. In this approach, both the desired
`current feedback loop and the dynamic adjustment of the
`current control signal is brought into the microprocessor
`software. A low-cost, unregulated DC transformer 100 is
`again shown as the power source for charging battery pack
`101 through diode 306. An analog-to-digital conversion
`device in microprocessor 350 monitors the voltage on both
`sides of sense resistor 305 as received through supply sense
`input 310 and voltage sense input 312. The microprocessor
`software uses this voltage information and the known resis
`tance value of the sense resistor to calculate the current
`through the sense resistor, which is also the current through
`pass device 304. In this embodiment, sense resistor 305 is
`one-half of an ohm. Other resistor values may be substituted
`with a minor change in the microprocessor software.
`Control for analog switch pass device 304 comes from a
`pulse-width-modulated (PWM) waveformat current control
`output 311 of the microprocessor 350. This waveform is
`filtered by a lowpass filter 315 and amplified by transistor
`device 320 before going to the gate of the pass device 304.
`As an alternative, the PWM signal at current control output
`311 may be substituted with an analog signal from digital
`to-analog (D/A) conversion software in the microprocessor
`350.
`The desired current value is based on the ambient tem
`perature from temperature sense input 322, the present
`battery voltage as received through battery sense input 309,
`and data from data input 324. This desired current value is
`adjusted based on the instantaneous power dissipation of the
`pass device 304 to create a current control signal. The
`instantaneous power dissipation of the pass device 304 is
`equal to the instantaneous current through the sense resistor
`305 multiplied by the instantaneous voltage across the pass
`device as calculated from voltage sense input 312 and
`battery sense input 309. Thus, the current control signal
`varies in reaction to any change in charging voltage from the
`unregulated DC transformer 100 or desired current as cal
`culated by the microprocessor 350.
`Many unregulated transformers begin to show output
`voltage ripple when the current exceeds half of the rated
`current of the transformer. This voltage ripple will com
`monly be at the full-wave-rectified 120/100 Hz, which is
`sensed by supply sense input 310 and can be filtered out by
`the microprocessor software. Once the software determines
`the frequency and magnitude of the ripple, an error function
`in the microprocessor 350 could be used to create a correc
`tion signal, and a summer implemented in software could
`minimize the ripple. Note that with a hardware feedback
`loop such as that shown in FIG. 2, operational amplifiers
`filter out any voltage ripple.
`Thus, the battery charger with power dissipation control
`provides a low cost alternative to traditional fast battery
`charging methods. While specific components and functions
`of the battery charger with power dissipation control are
`described above, fewer or additional functions could be
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`employed by one skilled in the art within the true spirit and
`scope of the present invention. The invention should be
`limited only by the appended claims.
`We claim:
`1. A battery charger comprising:
`a pass device configured for connection to an unregulated
`transformer;
`a charger controller configured for connection to a battery
`for calculating a desired battery charging current value
`based on a battery voltage;
`a power controller connected to the charger controller and
`connected to the pass device for converting the desired
`battery charging current value from the charger con
`troller to a current control signal based on an instan
`taneous power dissipation of the pass device.
`2. A battery charger according to claim 1 wherein the
`charger controller comprises:
`a battery sense input configured for connection to a
`battery for sensing a present battery voltage.
`3. A battery charger according to claim 1 wherein the
`power controller comprises:
`a power sense input connected to the pass device for
`sensing instantaneous power dissipation of the pass
`device.
`4. A battery charger according to claim 1 wherein the
`power controller comprises:
`a current control signal output connected to a gate of the
`pass device for controlling current across the pass
`device.
`5. A battery charger according to claim 1 wherein the
`charger controller comprises:
`a software feedback loop.
`6. A battery charger according to claim 1 wherein the
`power controller comprises:
`a software feedback loop.
`7. A battery charger according to claim 1 wherein the
`charger controller comprises:
`a temperature sense input configured for connection to a
`thermistor.
`8. A battery charger according to claim 1 wherein the
`charger controller comprises:
`a data input configured for connection to a data storage
`device.
`9. A battery charger comprising:
`a pass device having an input configured for connection to
`an unregulated transformer and an output configured
`for connection to a battery; and
`a controller comprising:
`a first voltage sense input connected to the input of the
`pass device;
`a second voltage sense input configured for connection
`to the battery; and
`a current control output connected to a gate of the pass
`device, wherein the current control output has a
`decreased output current when a voltage difference
`between the first voltage sense input and the second
`voltage sense input exceeds a predetermined thresh
`old.
`10. A battery charger according to claim 9 further com
`prising:
`a sense resistor connected between the output of the pass
`device and the battery;
`a circuit connected to the current control output having an
`operational amplifier for applying a current control
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`signal based on an instantaneous power dissipation of
`the pass device to a gate of the pass device.
`11. A battery charger according to claim 9 wherein the
`controller further comprises:
`a temperature sense input for connection to a thermistor.
`12. A battery charger according to claim 9 wherein the
`controller further comprises:
`a data input for connection to a data-storage device.
`13. A battery charger comprising:
`a pass device having an input configured for connection to
`an unregulated transformer; and
`a controller comprising:
`a first voltage sense input connected to the input of the
`pass device;
`a second voltage sense input configured for connection
`to a battery;
`a third voltage sense input configured for connection to
`the unregulated transformer; and
`a current control signal output connected to a gate of
`the pass device.
`14. A battery charger according to claim 13 further
`comprising:
`a sense resistor connected between the first voltage sense
`input and the third voltage sense input.
`15. A battery charger according to claim 13 further
`comprising:
`a transistor connected between the current control output
`and the pass device.
`16. A battery charger according to claim 13 further
`comprising:
`a diode connected between the pass device and the second
`voltage sense input.
`17. A battery charger according to claim 13 wherein the
`controller further comprises:
`a temperature sense input for connection to a thermistor.
`18. A battery charger according to claim 13 wherein the
`controller further comprises:
`a data input for connection to a data-storage device.
`19. A method for charging a battery comprising the steps
`of:
`connecting an unregulated transformer to a pass device;
`connecting the pass device to a battery;
`sensing a battery voltage of the battery;
`computing a desired charging current value for the bat
`tery;
`sensing an instantaneous power dissipation of the pass
`device;
`producing a current control signal based on the desired
`charging current value and the instantaneous power
`dissipation of the pass device;
`transmitting the current control signal to the pass device.
`20. A method for charging a battery comprising the steps
`of:
`connecting an unregulated transformer to a pass device;
`connecting the pass device to the battery;
`sensing a battery voltage of a battery;
`sensing an output voltage of the unregulated transformer;
`producing a current control signal;
`transmitting the current control signal to the pass device.
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