U.S. patent application number 14/624524 was filed with the patent office on 2018-05-31 for adjustable cable voltage compensation for battery chargers.
The applicant listed for this patent is Shanghai SIM-BCD Semiconductor Manufacturing Co., Ltd.. Invention is credited to QINGHUA SU, HU WANG, JINGJING ZHAO.
Application Number | 20180152036 14/624524 |
Document ID | / |
Family ID | 50572026 |
Filed Date | 2018-05-31 |
United States Patent
Application |
20180152036 |
Kind Code |
A9 |
ZHAO; JINGJING ; et
al. |
May 31, 2018 |
ADJUSTABLE CABLE VOLTAGE COMPENSATION FOR BATTERY CHARGERS
Abstract
A battery charger controller is configured to add a compensation
current in the feedback control loop such that the output voltage
varies with the output current to compensate charging cable voltage
drop. In some embodiments, the output voltage is also proportional
to a compensation resistor. Therefore, cable voltage drop
compensation can be adjusted using a resistor that is external to
the controller IC. The external resistor may be one of the feedback
resistors connected at a voltage feedback pin. In another
embodiment, the adjustable resistor is the resistor between the
feedback resistors and the voltage feedback pin. In still another
embodiment, the adjustable resistor is the resistor in parallel
with a compensation capacitor. In embodiments of the invention,
adjusting the resistance of the external compensation resistor can
change the voltage drop compensation and allow the power supply to
meet requirements of different charging cable applications.
Inventors: |
ZHAO; JINGJING; (Shanghai,
CN) ; SU; QINGHUA; (Shanghai, CN) ; WANG;
HU; (Shanghai, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Shanghai SIM-BCD Semiconductor Manufacturing Co., Ltd. |
Shanghai |
|
CN |
|
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20160241068 A1 |
August 18, 2016 |
|
|
Family ID: |
50572026 |
Appl. No.: |
14/624524 |
Filed: |
February 17, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02M 3/33507 20130101;
H02J 2207/20 20200101; H02J 7/0072 20130101; H02J 7/02 20130101;
H02J 7/022 20130101; H02J 7/00 20130101 |
International
Class: |
H02J 7/00 20060101
H02J007/00; H02M 3/335 20060101 H02M003/335 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2014 |
CN |
201410035354.X |
Claims
1. A switch mode power supply (SMPS), comprising: a transformer
having a primary winding for coupling to an external input voltage,
a secondary winding providing an output voltage of the power supply
to a load, and an auxiliary winding; a voltage divider coupled to
the auxiliary winding for providing a feedback signal
representative of the output voltage of the power supply, the
voltage divider having first and second feedback resistors
connected at a feedback node; a power switch for coupling to the
primary winding of the power supply; and a controller coupled to
the power switch, the controller including: a first terminal for
coupling to the feedback node for receiving the feedback signal; a
current source coupled to the first terminal, the current source
configured for providing a compensation current that is
proportional to an output current of the power supply; and a
control signal generation circuit configured for generating a
control signal for controlling the power switch based at least on
comparing a voltage at the first terminal with a reference voltage
such that the output voltage of the power supply increases with the
output current; wherein the controller is configured such that a
portion of the output voltage increases with the resistance of a
compensation resistor.
2. The switch mode power supply of claim 1, the compensation
resistor is characterized by a resistance such that the output
voltage of the power supply is compensated for a voltage drop on a
charging cable when the switch mode power supply is charging a
battery through the charging cable.
3. The switch mode power supply of claim 2, wherein the first
feedback resistor is the compensation resistor.
4. The switch mode power supply of claim 2, wherein the
compensation resistor is coupled between the feedback node and the
first terminal of the controller.
5. The switch mode power supply of claim 2, wherein the current
source comprises a voltage-controlled current source configured to
generate the compensation current based on a voltage determined by
charging a compensation capacitor during an on time of a secondary
current.
6. The switch mode power supply of claim 5, wherein the
compensation resistor is coupled in parallel with the compensation
capacitor.
7. The switch mode power supply of claim 5, wherein the on time of
the secondary current is determined through a low-pass filter.
8. The switch mode power supply of claim 5, wherein the on time of
the secondary current is determined using a digital circuit.
9. The switch mode power supply of claim 1, wherein control signal
generation circuit comprises a pulsed frequency modulation (PFM)
control circuit.
10. A controller integrated circuit (IC) for a switch mode power
supply (SMPS), comprising: a first terminal for receiving a
feedback signal representative of an output voltage of the power
supply; a current source coupled to the first terminal of the
controller, the current source configured for providing a
compensation current that is proportional to an output current of
the power supply; a control signal generation circuit configured
for generating a control signal based at least on comparing a
voltage at the first terminal with a reference voltage; and a
second terminal for providing the control signal to the power
switch to regulate the output voltage of the power supply such that
a portion of the output voltage is proportional to a product of the
output current and the resistance of a compensation resistor that
is external to the controller IC.
11. The controller of claim 10, where in the current source is
coupled between the first terminal and a ground of the
controller.
12. The controller of claim 10, wherein the compensation resistor
is coupled to the first terminal of the controller.
13. The controller of claim 10, wherein the current source
comprises a voltage-controlled current source configured to
generate the compensation current based on a voltage determined by
charging a compensation capacitor during an on time of a secondary
current.
14. The controller of claim 13, wherein the controller further
comprises a third terminal (CPC) for coupling to the capacitor.
15. The controller of claim 14, wherein the compensation resistor
is coupled to the third terminal of the controller.
16. A method for compensating for voltage drop on a charging cable
for charging a battery, the method comprising: providing a battery
charger for connecting to a battery through the charging cable, the
charging cable being characterized by a cable resistance, the
battery charger including a controller integrated circuit (IC) and
an external compensation resistor, the controller IC having: a
first terminal for receiving a feedback signal representative of an
output voltage of the power supply; a current source coupled to the
first terminal of the controller, the current source configured for
providing a compensation current that is proportional to an output
current of the power supply; a control signal generation circuit
configured for generating a control signal based at least on
comparing a voltage at the first terminal with a reference voltage;
and a second terminal for providing the control signal to the power
switch to regulate the output voltage of the power supply such that
a portion of the output voltage is proportional to a product of the
output current and the resistance of a compensation resistor that
is external to the controller IC. wherein the method further
comprises determining the resistance of the external compensation
resistor based on information regarding the charging cable such
that the battery receives a constant charging voltage.
17. The method of claim 16, wherein the battery charger further
comprises: a transformer having a primary winding, a secondary
winding, and an auxiliary winding, the primary winding for coupling
to an external input voltage, the secondary winding for providing
an output voltage to a load; a voltage divider coupled to the
auxiliary winding for providing a feedback signal representative of
the output voltage of the power supply, the voltage divider having
first and second feedback resistors connected at a feedback node;
and a power switch for coupling to the primary winding of the power
supply.
18. The method of claim 17, wherein the feedback node is coupled to
the first terminal of the controller, and the first feedback
resistor is the compensation resistor.
19. The method of claim 17, wherein the feedback node is coupled to
the first terminal of the controller through the compensation
resistor.
20. The method of claim 16, wherein the controller IC further
comprises a compensation terminal for coupling to an external
compensation capacitor and the compensation resistor.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority to Chinese Patent
Application No. 201410035354.X, filed Jan. 24, 2014, commonly owned
and incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to the field of
switch mode power supply (SMPS). More particularly, embodiments of
the present invention relate to SMPS used as battery chargers that
can be adapted to compensate for voltage drops from different
charging cables.
[0003] Switching power supply products have been widely used
because of their small size, light weight, and high power
conversion efficiency. For example, they are used in industrial
automation and control, military equipment, scientific equipment,
LED lighting, industrial equipment, communications equipment,
electrical equipment, instrumentation, medical equipment,
semiconductor cooling and heating, air purifiers, electronic
refrigerator, LCD display, audio-visual products, security,
computer chassis, digital products, equipment, and other
fields.
[0004] A switch mode power supply (SMPS) usually includes a
transformer that has a primary winding coupled to an input voltage
and a secondary winding for providing an output. In charger
applications of a switch mode power supply, the output can achieve
CV (Constant Voltage)/CC (Constant Current) characteristic by
different methods. For example, in secondary side regulation (SSR),
a sensing signal at the secondary side is provided as a feedback
signal to a controller. In primary side regulation (PSR), the
sensing signal from an auxiliary winding at the primary side is
provided to a controller. In either case, the output is maintained
at the output terminal of the power supply by the controller, which
can be based on pulse width modulation (PWM) or pulse frequency
modulation (PFM).
[0005] Switch mode power supplies used as battery chargers are
configured to charge batteries of electronic devices such as
portable computers, cell phones, and digital cameras. The battery
is usually connected to the battery charger through a charging
cable. The voltage drop on the cable varies with the load. For
example, the charging cable voltage drop is much larger at heavy
loads than that at light loads, causing the voltage received at the
device to vary. Therefore, the voltage drop on the charging cable
needs to be compensated.
[0006] In a conventional compensation method, the cable voltage
compensation is achieved by adding a voltage, which is proportional
to the load current, to a CV (constant voltage) reference voltage
to get a constant voltage at the output cable terminal. Some
adjustable compensation methods require an extra pin to be added to
the controller integrated circuit (IC).
BRIEF SUMMARY OF THE INVENTION
[0007] The inventors observed that conventional methods of
compensating for charging cable voltage drop have many drawbacks.
For example, the conventional compensation method that adds a
compensation voltage to a reference voltage is not desirable,
because it requires additional electronic elements and is not
cost-effective. The additional compensation circuitry may require
several revisions of the controller IC to calibrate to the charging
cable. Further, the compensation is fixed by the circuit and cannot
provide different compensation for different charging cables. In
embodiments of the invention, the compensation is adjusted by
adjusting the value of a resistor external to the controller IC. As
a result, costly rework of the controller IC can be avoided, and
the same controller IC can be calibrated to work with different
charging cables. Some conventional methods provide adjustable
compensation, but require an extra pin to be added to the
controller IC, increasing design complexity and system cost. In
embodiments of the invention, no extra pin is needed, because the
adjustment resistor is connected to an existing pin of a standard
package for controller IC. Alternatively, the resistor may be part
of a standard power supply circuitry, eliminating the need for the
extra adjustment resistor, further reducing system complexity and
cost.
[0008] Embodiments of the present invention provide circuits and
methods for compensating for the voltage drop of charging cable
over the entire load range. Further, the cable compensation is
adjustable and adaptable to meet application specifications such as
different output currents, different output voltages, and different
charging cable resistances. Examples are described using a closed
loop constant voltage (CV) regulation in primary side regulated
(PSR) system. However, it is understood that the invention can be
applied to other controllers as well.
[0009] According to an embodiment of the invention, a battery
charger controller introduces a current source in the feedback
control loop such that the output voltage varies with the output
current to compensate for charging cable voltage drop. In some
embodiments, a portion of the output voltage is also proportional
to the resistance of a compensation resistor that is external to
the controller IC. Therefore, cable voltage drop compensation can
be easily adapted using the resistor. In an embodiment, the
adjustable resistor outside the controlling IC may be one of the
feedback resistors connected at a voltage feedback pin, and no
additional component is required. In another embodiment, the
adjustable resistor is a resistor between the feedback resistors
and the voltage feedback pin. In still another embodiment, the
adjustable resistor is a resistor in parallel with a compensation
capacitor. In embodiments of the invention, adjusting the
resistance of the compensation resistor can change the voltage drop
compensation and allow the power supply to meet requirements of
different charging cable applications.
[0010] According to another embodiment of the invention, a switch
mode power supply (SMPS) includes a transformer having a primary
winding for coupling to an external input voltage, a secondary
winding providing an output voltage of the power supply to a load,
and an auxiliary winding. A voltage divider is coupled to the
auxiliary winding for providing a feedback signal representative of
the output voltage of the power supply, the voltage divider having
first and second feedback resistors connected at a feedback node.
The SMPS also includes a power switch for coupling to the primary
winding of the power supply and a controller coupled to the power
switch. The controller has a first terminal for coupling to the
feedback node for receiving the feedback signal and a current
source coupled to the first terminal. The current source is
configured for providing a compensation current that is
proportional to an output current of the power supply. The
controller also includes a control signal generation circuit
configured for generating a control signal for controlling the
power switch based at least on comparing a voltage at the first
terminal with a reference voltage such that the output voltage of
the power supply increases with the output current. Further, the
controller is configured such that a portion of the output voltage
increases with the resistance of a compensation resistor.
[0011] According to another embodiment of the invention, a method
for compensating for voltage drop on a charging cable for charging
a battery includes providing a battery charger for connecting to a
battery through the charging cable, which is characterized by a
cable resistance. The battery charger includes a controller
integrated circuit (IC) and an external compensation resistor. The
controller IC has a first terminal for receiving a feedback signal
representative of an output voltage of the power supply, a current
source coupled to the first terminal of the controller, the current
source configured for providing a compensation current that is
proportional to an output current of the power supply, and a
control signal generation circuit configured for generating a
control signal based at least on comparing a voltage at the first
terminal with a reference voltage. The controller IC also includes
a second terminal for providing the control signal to the power
switch to regulate the output voltage of the power supply such that
a portion of the output voltage is proportional to a product of the
output current and the resistance of a compensation resistor that
is external to the controller IC. The method also includes
determining the resistance of the external compensation resistor
based on information regarding the charging cable such that the
battery receives a constant charging voltage.
[0012] A further understanding of the nature and advantages of the
present invention may be realized by reference to the remaining
portions of the specification and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a simplified schematic diagram of an SMPS (a
switch mode power supply) used as a battery charging system
according to an embodiment of the present invention;
[0014] FIG. 2 is a schematic diagram of a controller for a switch
mode power supply according to an embodiment of the present
invention;
[0015] FIG. 3 is a schematic diagram illustrating a portion of a
controller associated with charging cable compensation according to
an embodiment of the invention;
[0016] FIG. 4 is a waveform diagram illustrating selected timing
parameters in the circuit of FIG. 3;
[0017] FIG. 5 is a simplified schematic diagram illustrating a
voltage-controlled current source (VCCS) according to an embodiment
of the present invention;
[0018] FIG. 6 is a waveform diagram illustrating timing signals in
a battery charger with charging cable compensation according to an
embodiment of the present invention;
[0019] FIG. 7 is a simplified schematic diagram of a battery
charging system including charging cable compensation according to
another embodiment of the present invention;
[0020] FIG. 8 is a schematic diagram illustrating a portion of the
controller associated with charging cable compensation in battery
charging system in FIG. 7 according to an embodiment of the
invention;
[0021] FIG. 9 is a simplified schematic diagram of a battery
charging system including charging cable compensation according to
yet another embodiment of the present invention; and
[0022] FIG. 10 is a schematic diagram illustrating a portion of the
controller associated with charging cable compensation in battery
charging system in FIG. 9 according to an embodiment of the
invention;
DETAILED DESCRIPTION OF THE INVENTION
[0023] Embodiments of the present invention provide compensation
methods to compensate the IR (current-resistance) voltage drop of
charging cable to achieve accurate voltage control at the output
cable terminal. The compensation can be adjusted for different
charging cables by varying the resistance of a resistor external to
the controller integrated circuit.
[0024] FIG. 1 is a simplified schematic diagram of a battery
charging system according to an embodiment of the present
invention. The battery charging system includes a primary-side
regulated power supply 200, a battery 122, and charging cables 131
and 132, which connect battery 122 to the power supply. As shown in
FIG. 1, the output voltage at the output terminals of the power
supply is Vo, and the voltage at the terminals of the battery 122
is Vo_cable.
[0025] Switched mode power supply (SMPS) 100 includes a transformer
102, which includes a primary winding 141 for coupling to an input
voltage Vin and a secondary winding 142 for providing the output
voltage Vo through a rectifying diode 120 and a capacitor 119.
Transformer 102 also has an auxiliary winding 143 for providing a
feedback signal Vfb representative of the output of the power
supply. In FIG. 1, Vs denotes the voltage at the secondary winding,
and Va denotes the voltage at the auxiliary winding. Ns is the coil
turns in the secondary winding, and Na is the coil turns of the
auxiliary winding.
[0026] Power supply 100 also includes a power switch 101 coupled to
primary winding 141 and controller 200 for receiving a control
signal to turn on and off power switch 101 to control the primary
current through primary winding 141 in order to regulate output
voltage Vo. In FIG. 1, power switch 101 is shown as a bipolar power
transistor. However, in other embodiments, a power MOSFET or
another type of power switch can also be used. In the embodiment of
FIG. 1, controller 200 is a single integrated circuit (IC) having
several terminals, for example, VCC, GND, FB, CS, DRIVE, and CPC.
Controller 200 receives its operating power at the VCC terminal
from capacitor 106. During startup, capacitor 106 is charged by a
current provided by input voltage Vin through resistors 107 and
108. In normal operation, capacitor 106 is charged by a current
provided by auxiliary winding 143 through resistor 104 and diode
105. Terminal FB receives feedback signal Vfb from auxiliary
winding 123 through a voltage divider 130 formed by resistors 117
and 118, with resistances Rfb1 and Rfb2, respectively. Terminal CS
receives a current sense voltage signal representative of the
primary current through a current sense resistor 116 with a
resistance Rcs. Controller 200 is configured to provide a control
signal at the DRIVE terminal to control power switch 101, based on
information provided at the FB and CS terminals. The CPS terminal
is coupled to a compensation capacitor Ccpc with a voltage
Vcps.
[0027] In the battery charging system, the resistance of the
charging cables causes a voltage drop between the output voltage of
the power supply Vo and the voltage presented by the cable
Vo_cable, which is the input voltage to the battery. If the
controller is configured to maintain a constant Vo, then Vo_cable
would be equal to Vo minus the voltage drop on the charging cables.
Let the output current of the power supply be Io, which represents
the load current of the power supply. Then the voltage drop is
proportional to Io times the resistance of the charging cable. In
order to compensate for the voltage drop of charging cable over the
load current range, Vo needs to be controlled such that Vo
increases with the load current Io in order to maintain Vo_cable at
a constant. Embodiments of the present invention provide circuits
and methods for compensating for charging cable voltage drop that
can be adjusted for different cables and load conditions. In some
embodiments, the adjustment is made by selecting a resistance of a
resistor external to the controller integrated circuit. Further, no
additional dedicated pin connection needs to be added to the
controller IC for connection to the adjustment resistor.
[0028] FIG. 2 is a schematic diagram of a controller 200 for a
switch mode power supply according to an embodiment of the present
invention. As shown in FIG. 2, controller 200 includes circuits for
performing pulse frequency modulation (PFM) control functions.
However, it is understood that a pulse width modulation (PWM)
control can also be used. In FIG. 2, controller 200 includes a
terminal FB for receiving a feedback signal Vfb. Controller 200
also has a control signal generation circuit configured for
generating a control signal at the DRIVE terminal for controlling
an on time and off time of the power switch. In FIG. 2, a
comparison of feedback signal voltage signal Vfb with a reference
voltage is performed by error amplifier EA. Feedback signal Vfb and
current sense signal Vcs from the CS terminal are used by the
controller to turn on and off the power switch to control the
current flow in the primary winding and to regulate the power
supply output voltage Vo in the constant voltage control mode. In
response to the primary current flow, a secondary current is
induced in the secondary winding. In the description below, the
on-time of the secondary current is designated as Tons, and the
off-time of the secondary current is designated as Toffs. In CC
(constant current) regulation, the CC loop control function keeps a
fixed proportion between on-time Tons and off-time Toffs of
secondary side by discharging or charging a capacitor in the
controller as shown in FIG. 2. Controller 200 also includes
references voltages V1-V5 that are design parameters selected
according to specific applications.
[0029] As shown in FIG. 2, controller 200 also includes a current
source Icmp coupled between the FB terminal and a ground GND. The
current source is configured for providing a compensation current
Icmp that is proportional to an output current Io of the power
supply. In embodiments of the invention, compensation current Icmp
is added to the feedback terminal to modify the control signal such
that the output voltage of the power supply Vo increases with the
output current Io. More details are described with reference to
FIG. 3.
[0030] FIG. 3 is a schematic diagram illustrating a portion of
controller 200 associated with charging cable compensation
according to an embodiment of the invention. As shown in FIG. 3,
auxiliary winding voltage Va is detected by the FB pin of the
controller IC as a voltage signal Vfb through a feedback voltage
divider block including resistances Rfb1 and Rfb2. Vfb and a
reference voltage Vref are coupled to an error amplifier EA, which
calculates an error voltage Vea that represents the difference
between Vfb and Vref.
[0031] As shown in FIG. 3, in order to provide compensation for the
charging cable voltage drop, a compensation current Icmp is
inserted in controller 200 between the FB terminal and ground GND.
In embodiments of the invention, Icmp is derived by a voltage
controlled current source (VCCS) from a voltage Vcpc that is
proportional to the output current Io. As a result, Icmp is
proportional to Io, as described below.
[0032] In a switch mode power supply, the output current Io is the
mean value of the current through the secondary side diode Vd. If
the peak value of the secondary diode current is ipks, the time of
ON time of the secondary diode is Tons, and the switching period is
Tsw, then,
Io = 1 2 ipks Tons Tsw ##EQU00001##
As shown, output current Io is proportional to the duty cycle of
secondary on-time Tons/Tsw.
[0033] According to embodiments of the invention, the duty cycle of
secondary on-time can be determined from the transient voltage at
the FB pin of the controlling IC. The duty cycle can be determined,
for example, through a low-pass filter in an analog method, or by
calculating the time ratio of secondary on-time and switching time
period in a digital method. FIG. 4 is a waveform diagram
illustrating selected timing parameters in the power supply. As
shown in FIG. 4, timing information about Tons and Tsw is contained
in the voltage signal Vfb. A Tons detector is configured to
determine Tons by the positive voltage of Vfb filtered as V_Tons.
Here, the high voltage of V_Tons is at VDD which is a constant
voltage generated from VCC inside the controlling IC. V_Tons is
then filtered by a low-pass filter comprising Rlp and Ccpc as shown
in FIG. 3. The capacitor voltage Vcpc can be shown to be
proportional to the output load current Io.
Vcpc = VDD Tons Tsw = 2 VDD ipks Io ##EQU00002##
[0034] As shown in FIG. 3, a voltage-controlled current source
(VCCS) is connected to Vcpc to generate compensation current Icmp,
which is proportional to Vcpc and is also proportional to the
output load current Io.
Icmp = kcpc Vcpc = 2 kcpc VDD ipks Io = ki Io ##EQU00003##
where kcps and ki are constants. As shown in FIG. 3, current Icmp
flows from the FB pin inside the controller IC and causes Vfb to be
lowered by an amount proportional to Io, the output load current.
As explained below, the controller is now configured to regulate
the power supply to maintain an output voltage Vo that compensates
for the voltage drop across the charging cable.
[0035] As shown in FIG. 3, error voltage Vea is derived from error
amplifier EA when Vfb deviates from Vref. As depicted in FIG. 2,
Vea is used to produce a drive signal DRIVE that turns on the power
switch to change the output voltage Vo, which is reflected in
auxiliary winding voltage Va to allow the controller to maintain
Vfb at Vref. As described below, in embodiments of the invention,
when output current Io changed from open load (Io_min) to full load
(Io_max), the additional compensation voltage at Va will be
increased linearly.
[0036] At open load, when Io=0, the relationship between Va and
Vref is,
Va = ( 1 + Rfb 1 Rfb 2 ) Vref ##EQU00004##
Further, the relationship between Va and Vo is determined by the
coil turn ratios,
Va = Na Ns Vs = Na Ns ( Vo + Vd ) ##EQU00005##
where Na and Ns are the coil turns of auxiliary winding and
secondary winding, respectively, Vs is voltage at the secondary
winding, and Vd is the voltage across the diode on the secondary
side.
[0037] From the above two equations, the relationship between
output voltage Vo and output current Io is,
Vo = Va Ns Na - Vd = [ ( 1 + Rfb 1 Rfb 2 ) Vref ] Ns Na - Vd
##EQU00006##
[0038] Similarly, at heavy load, when Io is not zero, the
relationship between Va and Vref includes the compensation current
Icmp,
Va = ( 1 + Rfb 1 Rfb 2 ) Vref + Icmp Rfb 1 ##EQU00007## where ,
Icmp == ki Io ##EQU00007.2##
Then the relationship between output voltage Vo and output current
Io is,
Vo = Va Ns Na - Vd = [ ( 1 + Rfb 1 Rfb 2 ) Vref + Icmp Rfb 1 ] Ns
Na - Vd = [ ( 1 + Rfb 1 Rfb 2 ) Vref + ki Io Rfb 1 ] Ns Na - Vd
##EQU00008##
[0039] The above equation can be rewritten as follows.
Vo = K 1 + K 2 Io ##EQU00009## where : K 2 = ki Ns Na Rfb 1
##EQU00009.2##
It can be seen that the controller is configured to cause the
output voltage Vo of the switch mode power supply to include a
component "K2Io" which is proportional to a product of the output
current Io and a coefficient K2, whose value depends linearly on
the resistance of resistor Rfb1. Thus, the output voltage of the
charger Vo is configured to increase linearly with the output load
current Io, with a proportionality constant that varies with the
resistance of resistor Rfb1, which is external to the controller
IC.
[0040] Assuming the resistance of the charging cable is Rcable,
then,
Vo=Vo_cable+RcableIo
By selecting a resistance for resistor Rfb1 to compensate for the
charging cable voltage drop, and the output voltage at the charging
cable Vo_cable can be maintained at a constant. In this embodiment,
Rfb1 is the cable compensation resistor Rcmp, which can be adjusted
to meet different application specifications with different
charging cable resistances in different kinds of cables and
different cable lengths.
[0041] FIG. 5 is a simplified schematic diagram illustrating a
voltage-controlled current source (VCCS) according to an embodiment
of the present invention. As shown VCCS 500 includes an amplifier
510 coupled to an MOS transistor 520 and a resistor 530. VCCS 500
is configured such that output current Icpc is proportional to
input voltage Vcps. In one embodiment, Icpc is coupled to the FB
terminal through a current mirror. Of course, the VCCS and current
mirror can also be implemented using other known circuit
techniques.
[0042] FIG. 6 is a waveform diagram illustrating timing signals in
a battery charger with charging cable compensation according to an
embodiment of the present invention. As shown, the output load
current Io increases from light load to heavy load between time t1
to time t2. As described above, voltage Vcps increases with the
load, so does the compensation current Icmp which is derived from
Vcps. As a result the output voltage Vo increases with output load
to compensate for the charging cable resistance.
[0043] FIG. 7 is a simplified schematic diagram of a battery
charging system including charging cable compensation according to
another embodiment of the present invention. Battery charging
system of FIG. 7 is similar to the battery charging system
described above in connection with FIG. 1. The battery charging
system includes a primary-side regulated power supply 700 as the
battery charger, a battery 122, and charging cables 131 and 132,
which connect battery 122 to the power supply. Many of the similar
components are not described here. Similar to battery charger 100,
battery charger 700 also includes adjustable charging cable
compensation. However, battery charger 700 has a different charging
cable compensation method. As shown in FIG. 7, battery charger 700
includes a resistor Rfbh coupled between the FB terminal of the
controller and the common node between feedback resistors Rfb1 and
Rfb2. The compensation method is described below with reference to
FIG. 8.
[0044] FIG. 8 is a schematic diagram illustrating a portion of
controller 200 associated with charging cable compensation in
battery charger 700 in FIG. 7 according to an embodiment of the
invention. In this embodiment, a resistor Rfbh is added between the
FB pin and the node between Rfb1 and Rfb2. As shown in FIG. 8,
auxiliary winding voltage Va is detected by the FB pin of the
controller IC as a voltage signal Vfb through a feedback voltage
divider block including resistances Rfb1 and Rfb2. Vfb and a
reference voltage Vref are coupled to an error amplifier EA, which
calculates an error voltage Vea that represents the difference
between Vfb and Vref.
[0045] As shown in FIG. 8, in order to provide compensation for
charging cable voltage drop, a compensation current Icmp is
inserted in controller 200 between the FB terminal and ground GND.
In embodiments of the invention, Icmp is proportional to Io. As
described below, Icmp is derived by a voltage-controlled current
source (VCCS) from a voltage Vcpc that is proportional to the
output current Io as described below.
[0046] The relationship between Va and Vref at open load is,
Va = ( 1 + Rfb 1 Rfb 2 ) Vref ##EQU00010##
Further, the relationship between Va and Vo is determined by the
coil turn ratios,
Va = Na Ns Vs = Na Ns ( Vo + Vd ) ##EQU00011##
where Na and Ns are the coil turns of auxiliary winding and
secondary winding, respectively, and Vd is the voltage across the
rectifying diode on the secondary side.
[0047] From the above two equations, the relationship between
output voltage Vo and output current Io is,
Vo = Va Ns Na - Vd = [ ( 1 + Rfb 1 Rfb 2 ) Vref ] Ns Na - Vd
##EQU00012##
[0048] At heavy load, when Io is not zero, and if the current
through the resistor Rfb1 is much more than the current Icmp inside
the IC,
Va = ( 1 + Rfb 1 Rfb 2 ) ( Vref + Icmp Rfbh ) ##EQU00013##
where Icmp=kiIo. Then the relationship between output voltage Vo
and output current Io is,
Vo = Va Ns Na - Vd = [ ( 1 + Rfb 1 Rfb 2 ) ( Vref + Icmp Rfbh ) ]
Ns Na - Vd = [ ( 1 + Rfb 1 Rfb 2 ) ( Vref + ki Io Rfbh ) ] Ns Na -
Vd ##EQU00014##
[0049] It can be seen that the controller is configured to cause
the output voltage Vo of the switch mode power supply to include a
component proportional to a product of the output current Io and a
coefficient, whose value depends linearly on the resistance of
resistor Rfbh, which is external to the controller IC. In this
example, Rfbh is the cable compensation resistor Rcmp.
Rcmp=Rfbh
By varying Rcmp (Rfbh), the cable compensation voltage can be
changed to meet different application specification with different
charging cable resistance in different kinds of cables and
different cable lengths.
[0050] FIG. 9 is a simplified schematic diagram of a battery
charging system including charging cable compensation according to
another embodiment of the present invention. Battery charging
system of FIG. 9 is similar to the battery charging system
described above in connection with FIG. 1. The battery charging
system includes a primary-side regulated power supply 900 as the
battery charger, a battery 122, and charging cables 131 and 132,
which connect battery 122 to the power supply. Many of the similar
components are not described here. Similar to battery charger 100,
battery charger 900 also includes adjustable charging cable
compensation. However, battery charger 900 has a different charging
cable compensation method. As shown in FIG. 9, battery charging
system 900 includes a resistor Rcpc coupled in parallel with
compensation capacitor Ccpc. The compensation method is described
below with reference to FIG. 10.
[0051] FIG. 10 is a schematic diagram illustrating a portion of
controller 200 associated with charging cable compensation in
battery charger 900 in FIG. 9 according to an embodiment of the
invention. In this embodiment, a resistor Rcpc is added to the CPC
pin and in parallel with capacitor Ccpc. As shown in FIG. 10,
auxiliary winding voltage Va is detected by the FB pin of the
controller IC as a voltage signal Vfb through a feedback voltage
divider block including resistances Rfb1 and Rfb2. Vfb and a
reference voltage Vref are coupled to an error amplifier EA, which
calculates an error voltage Vea that represents the difference
between Vfb and Vref.
[0052] As shown in FIG. 10, in order to provide compensation for
charging cable voltage drop, a compensation current Icmp is
inserted in controller 200 between the FB terminal and ground GND.
In embodiments of the invention, Icmp is proportional to Io. As
described below, Icmp is derived by a voltage controlled current
source (VCCS) from a voltage Vcpc that is proportional to the
output current Io. Further, Icmp can be adjusted by varying
compensation resistor Rcpc coupled in parallel with compensation
capacitor Ccpc, as described below.
[0053] At open load, or when Io=0, the relationship between Va and
Vref is,
Va = ( 1 + Rfb 1 Rfb 2 ) Vref ##EQU00015##
Further, the relationship between Va and Vo is determined by the
coil turn ratios,
Va = Na Ns Vs = Na Ns ( Vo + Vd ) ##EQU00016##
where Na and Ns are the coil turns of auxiliary winding and
secondary winding, respectively, and Vd is the voltage across diode
119 on the secondary side.
[0054] From the above two equations, the relationship between
output voltage Vo and output current Io is,
Vo = Va Ns Na - Vd = [ ( 1 + Rfb 1 Rfb 2 ) Vref ] Ns Na - Vd
##EQU00017##
[0055] Similarly, at heavy load, when Io is not zero, the
relationship between Va and Vref includes the compensation current
Icmp,
Va = ( 1 + Rfb 1 Rfb 2 ) ( Vref + Icmp Rfb 1 ) ##EQU00018## where ,
Vcpc = VDD Tons Tsw Rcpc Rcpc + Rlp = 2 VDD ipks Rcpc Rcpc + Rlp Io
##EQU00018.2## Icmp = kcpc Vcpc = 2 kcpc VDD ipks Rcpc Rcpc + Rlp
Io = ki Rcpc Rcpc + Rlp Io ##EQU00018.3##
Then the relationship between the output voltage and the output
current is shown as follows,
Vo = Va Ns Na - Vd = [ ( 1 + Rfb 1 Rfb 2 ) Vref + Icmp Rfb 1 ] Ns
Na - Vd = [ ( 1 + Rfb 1 Rfb 2 ) Vref + ki Io Rcpc Rcpc + Rlp Rfb 1
] Ns Na - Vd ##EQU00019##
[0056] It can be seen that the controller is configured to cause
the output voltage Vo of the switch mode power supply to include a
component proportional to a product of the output current Io and a
coefficient whose value depends linearly on the resistance of
resistor Rcpc. Thus, the output voltage of the charger Vo is
configured to increase linearly with the output load current Io,
with a proportionality constant that varies with the resistance of
a resistor Rcpc, which is external to the controller IC.
[0057] By selecting a proper resistance for resistor Rcpc to
compensate for the charging cable voltage drop, and the output
voltage at the charging cable Vo_cable can be maintained at a
constant. In this embodiment, Rcpc is the cable compensation
resistor Rcmp, which can be adjusted to meet different application
specifications with different charging cable resistances in
different kinds of cable and different cable lengths.
[0058] Thus, according to embodiments of the invention, a method
for compensating for voltage drop on a charging cable in a battery
charger includes providing a battery charger for connecting to a
battery through a charging cable, which is characterized by a cable
resistance. The battery charger includes a controller integrated
circuit (IC) and an external compensation resistor. The battery
charger is configured such that the output voltage increases
linearly with the resistance of the compensation resistor. The
method also includes determining the resistance of the external
compensation resistor based on information regarding the charging
cable such that the output voltage increases with the output
current to compensate for charging cable voltage drop. In some
embodiments, the resistance of the external compensation resistor
can be selected experimentally for different charging cables by
measuring the voltage at the output terminals of the charging
cables when the output current is varied from light load to heavy
load. Alternatively, the resistance of the external compensation
resistor can be selected using simulation techniques.
[0059] In some embodiments, the controller IC includes a first
terminal for receiving the feedback signal representative of an
output voltage of the battery charger and a current source coupled
between the first terminal and a ground of the controller. The
current source is configured for providing a compensation current
that is proportional to an output current of the battery charger.
The controller also includes a control signal generation circuit
configured for generating a control signal based at least on
comparing a voltage at the first terminal with a reference voltage
for regulating the output voltage such that the output voltage
increases with the output current. The controller is configured to
cause the output voltage Vo of the switch mode power supply to
include a component proportional to a product of the output current
Io and a coefficient whose value depends linearly on the resistance
of the compensation resistor, which is outside of the controller
IC.
[0060] In some embodiments, the battery charger further includes a
transformer having a primary winding, a secondary winding, and an
auxiliary winding. The primary winding is for coupling to an
external input voltage, and the secondary winding is for providing
an output voltage to the load. The battery charger also includes a
voltage divider coupled to the auxiliary winding for providing a
feedback signal representative of the output voltage of the power
supply. the voltage divider has first and second feedback resistors
connected at a feedback node. The battery charger also has a power
switch for coupling to the primary winding of the power supply. In
a specific embodiment, the feedback node is coupled to the first
terminal of the controller, and the first feedback resistor is the
compensation resistor. In another embodiment, the feedback node is
coupled to the first terminal of the controller through the
compensation resistor. In yet another embodiment, the controller IC
also has a compensation terminal for coupling to an external
compensation capacitor and the compensation resistor.
[0061] While the above is a description of specific embodiments of
the invention, the above description should not be taken as
limiting the scope of the invention. Therefore, it is appreciated
that the examples and embodiments described herein are for
illustrative purposes only and that various modifications or
changes in light thereof will be suggested to persons skilled in
the art and are to be included within the spirit and purview of
this application.
* * * * *