BACKGROUND
Field
[0001]The present disclosure relates generally to battery chargers, and more specifically to battery chargers with a constant current control loop.
Background
[0002]Conventional battery chargers use a constant current—constant voltage (CC-CV) control loop charging profile, as shown in FIG. 1, with the structure of FIG. 2. Note that a smaller battery pack has disadvantages, including unexpected increasing charge time, and a sensitive transition from CC to CV.
[0003]FIG. 1, 100, shows when the voltage of a battery pack, VBAT115, is below a threshold voltage VCVto enter CV from CC, the charger regulates charging current ICHG120. During CC charging, equivalent capacitance CBATin battery cell 125 is charged, and ICHGincreases VBATat a constant rate. For VBATbelow the threshold voltage VCCto enter CC from pre-charging, ICHGwill be at a 0.1 C charging rate. For VBATabove VCC(and below VCV), ICHGwill be at a 1 C charging rate. The value C varies according to the battery capacity, and is defined by the equation C [A]=(battery capacity) [Ah]/1 [h]. Pre-charging, a part of CC charging, is when the battery requires a smaller charge current, and VBAT<VCCin 110.
[0004]After VBATreaches VCV, VBATis regulated at VCV. During CV charging, ICHGdecays exponentially due to the equivalent series resistance RESRin battery cell 125, and the equivalent capacitance CBAT. After ICHGis reduced to its target, the charging cycle is terminated.
[0005]FIG. 2, 200, shows a CC-CV charger. The charger has CC and CV control circuits, and each circuit functions apart from each other. The CC control circuit controls the gate voltage VGof the power device gate (MMAN), to regulate ICHG, by monitoring the mirrored current of ICHG, using the Charging Current Detector, passing through the sense device (MSENSE). The circuit is enabled when VBATsensed by the voltage detector is below VCV. The Voltage Detector measures VBAT.
[0006]When VBATgets close to VCV, the CV control circuit is enabled, to take control over VG, and the CC control circuit is disabled. During CV charging, VGis controlled to source appropriate ICHGto the battery, to reduce the gap between VCVand VBAT. As a result, VBATis regulated. Regardless of CC and CV charging, VG(and ICHGindirectly) works as the controlled input to the feedback system.
[0007]FIGS. 3A (3 ohm) and 3B (6 ohm) are graphical representations showing how a larger RESRincreases the CV region, and the resulting charge time, as VBATvoltage (315, 335) is shifted by RESR×ICHG(320, 340). Note, that for the case of increasing charge time, RESR330 of a smaller battery is larger than RESR310 of a larger battery, and that an aged battery will increase RESRas well. VCELL(325, 345) represents the voltage of the battery cell across RESR.
[0008]For the case of transition from CC to CV, a smaller battery requires a smaller current than a larger battery requires. This results in increased difficulty to enter CV from CC. The difficulty of the circuit in FIG. 2 comes from VGbeing controlled for both CC and CV charging, and from an architecture where the CC and the CV circuits work independently, which is apparent during the transition from CC to CV.
[0009]FIGS. 4A, 4B, and 4C are a circuit diagram and voltage/current charts showing a discontinuity in the battery charge current at the transition from CC to CV charging in 420, due to the use of independent CC 425 and CV 43 control circuits in 410. The CV control circuit may source overcurrent 455 (ICHG>1 C) as shown in 440, where ICHGexceeds CC charge current 1 C, at the beginning of CV charging. During the period when ICHG>1 C, the battery is overcharged (i.e. VCELLexceeds VCV), and may be damaged.
[0010]FIG. 5, 500, shows another charging profile that uses the CC-CV structure of FIG. 2. Here, there the transition state called PRE-CV relaxes the transition from CC to CV of FIG. 4B, since the transition point is moved to a smaller charge current point than the 1 C value. PRE-CV is a part of CC, where the charger digitally changes the CC setting to be smaller before entering CV, and reduces the risk for over-charging (>1 C) at the transition point between CC and CV. The voltage of the battery pack is VBAT515, the threshold voltage to enter CV from CC is VCV, and the charging current ICHGis 520.
[0011]FIGS. 7A and 7 B are current/voltage diagrams during CC step down, the battery charger digitally changes the CC setting to be smaller than the charge termination current, and introduces another disadvantage. CC step down 715 needs ΔT to be the same charge amount 730 as normal CV settling 725, for an area-A, in 710. Accordingly, it still needs the CV state to secure the increasing charge time of the CV region, especially when the system requires smaller termination current 720 (<0.1 C), in 740.
[0012]In FIGS. 7A and 7B, the charged amount for CV settling is:
[0013]∫ICHG(t)dt=-τ·VBATRESR[e-tτ]T0T1(1)
And the charged amount for PRE-CV settling is:
[0014]∫ICHG(T1)dt=[VBATRESRe-T1τ·t]T0T2(2)
where (1)=(2) (3)
and T0=k0τ, T1=k1τ, τ=RESR·CBAT. (4)
[0015]Substituting (4) into (3)
ΔT=τ{ek1-k0−(1+k1−k0)} (5)
where a larger k value means deep charging.
For example, at Time 1:
CBAT=6 kF, RESR=0.1 ohm, and PRE-CV charging is from 1.00 A to 0.90 A:
ΔT=τ{ek1-k0−(1+k1−k0)}=1.73 [sec]
For example, at Time 2:
CBAT=6 kF, RESR=0.1 ohm, and PRE-CV charging is from 0.10 A to 0.05 A:
ΔT=τ{ek1-k0−(1+k1−k0)}=92.06 [sec]
SUMMARY
[0016]Accordingly, it is an object of one or more embodiments of the present disclosure to provide a battery charger with a constant current control loop and no constant voltage control loop, for use in linear and switching chargers.
[0017]It is a further object of one or more embodiments of the disclosure to include digital controls and a comparator in a battery charger, for decreasing charging current towards termination.
[0018]Still further, it is an object of one or more embodiments of the disclosure to eliminate a constant voltage loop and amplifier, without increasing charging time, in a battery charger.
[0019]Still further, it is an object of one or more embodiments of the disclosure to simplify porting a battery charger design to other process technology nodes, and reducing size of the charger circuit.
[0020]Other objects will appear hereinafter.
[0021]The above and other objects of the present disclosure may be accomplished in the following manner. A battery charger is disclosed, comprising a constant current control loop, configured to provide a charge current to a battery, a digital voltage limiter, configured to sense a battery voltage, and dynamic controls, configured to calculate a dynamic control signal for controlling the charge current.
[0022]The above and other objects of the present disclosure may be further accomplished with a method for constant current control in a battery charger. The steps include providing a battery charger with a constant current (CC) control loop. The steps also include sensing a battery voltage and digitizing the battery voltage. The steps also include calculating a dynamic control signal for controlling a charge current.
[0023]In various embodiments the function may be used for short circuit detection, pre-charge voltage detection, or re-charge voltage detection.
[0024]In various embodiments a fixed control voltage is configured with a programmable digitized gain.
[0025]In various embodiments a Buck switching converter may be configured with a dynamic control voltage from the digital voltage limiter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026]The present disclosure will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which like reference numerals designate similar or corresponding elements, regions and portions and in which:
[0027]FIG. 1 shows when the voltage of a battery pack is below a threshold voltage to enter constant voltage (CV) from constant current (CC), the charger regulates the charging current.
[0028]FIG. 2 shows a CC-CV battery charger, where the charger has CC and CV control circuits, functioning apart from each other.
[0029]FIGS. 3A (3 ohm) and 3B (6 ohm) are graphical representations showing a larger equivalent series resistance of a battery increases the CV region, and the resulting charge time, as the voltage of the battery is shifted.
[0030]FIGS. 4A, 4B, and 4C are a circuit diagram and voltage/current charts showing a discontinuity in the battery charge current at the transition from CC to CV charging, due to the use of independent CC and CV control circuits.
[0031]FIG. 5 illustrates another charging profile that uses the CC-CV structure of FIG. 2.
[0032]FIG. 6 includes voltage/current curves demonstrating the present disclosure without a constant voltage (CV) control loop, and a battery charging profile.
[0033]FIGS. 7A and 7B are current/voltage diagrams showing that during CC step down, the battery charger digitally changes the CC setting to be smaller than the charge termination current, and introduces another disadvantage.
[0034]FIGS. 8A, 8B, and 8C are circuit diagrams and a voltage/current chart showing the proposed Digital Voltage Limiter (DVL) of the disclosure replacing the constant voltage (CV) control loop of the prior art.
[0035]FIGS. 9A and 9B illustrate an Accelerated Settle Down (ASD) technique to achieve the constant current control loop for the charger.
[0036]FIGS. 10A and 10B include voltage curves showing how the digitalized step down of the charging current in both FIGS. 6 and 9 takes into account over-charging.
[0037]FIG. 11 illustrates where the switching charger may require both a Buck converter and analog to digital control (ADC), in the prior art.
[0038]FIG. 12 shows where the proposed architecture of the disclosure only requires a Buck converter, and no ADC, in a switching charger.
[0039]FIG. 13 includes the circuitry of the proposed Digital Voltage Limiter (DVL), in a battery charger of the present disclosure.
[0040]FIGS. 14A and 14B include voltage/circuit curves demonstrating how the charge time of the Accelerated Settle Down is calculated.
[0041]FIG. 15 illustrates a test bench circuit, which includes both the CC-CV control logic of the prior art, and the CC-DVL control logic of the present disclosure.
[0042]FIGS. 16A and 16B show waveform results for CC-CV control logic compared with CC-CC step down logic, and CC-CV control logic compared with CC-Accelerated Settle Down (ASD) logic.
[0043]FIGS. 17A and 17B illustrates simulation results for the architecture of the prior art compared to that of the present disclosure, when the equivalent series resistance varies.
[0044]FIG. 18 shows how the circuit area used for the small size solution provided by the present disclosure compares to the area used for the circuit layout for a similar battery charger of the prior art.
[0045]FIG. 19 is a flow chart of a method for constant current control, in a battery charger.
DETAILED DESCRIPTION
[0046]The present disclosure proposes a new battery charger architecture, providing a constant current (CC) control loop, for use in linear chargers and switching battery chargers. Advantages include the use of digital controls and a comparator, for decreasing charging current towards termination. The technique of the disclosure eliminates a constant voltage loop and amplifier, without increasing charging time.
[0047]The CC control loop of the present disclosure achieves a faster, more consistent charging time than conventional CC-CV chargers. Removal of the constant voltage (CV) control loop reduces circuit area and simplifies design time. In addition, there is less sensitivity to variation of external parameters (RESR, CBAT, etc), and disadvantages of the conventional CC-CV charging structure are addressed.
[0048]FIG. 6, 600, includes voltage/current curves demonstrating how the present disclosure without a constant voltage (CV) control loop, and a battery charging profile. Now (PRE-CV+CV) are replaced by a constant current (CC) control loop step down, where the charger digitally changes the CC setting to be smaller than the charge termination current. The voltage of the battery pack is VBAT615, the transition for the voltage settling is CC step down, and the charging current ICHGis 620. The state of the battery moves from pre-charging, to a CC phase, then to CC-CC step down, and to stop charging, as the charge current rises to 1 C and then below its target value 0.1 C.
[0049]The present disclosure uses two techniques to achieve the constant current control loop of the battery charger, a Digital Voltage Limiter, illustrated in FIGS. 8A, 8B, and 8C, and an Accelerated Settle Down method, illustrated in FIG. 9A and 9B.
[0050]FIG. 8B shows the Digital Voltage Limiter (DVL) 820 of the disclosure replacing the constant voltage (CV) control loop 810, as shown in FIG. 8A, of the prior art. FIG. 8C includes voltage and current curves for the disclosed battery charger. DCCis the dynamic digital control for the constant current (CC) in 830, from which the value of ICHG860 depends. As a result of the DVL charging, ICHGis dynamically changed, which differs from the CV logic of the prior art. DCVis a digital signal used for controlling a feedback gain setting, and is a fixed value. DCVdetects a peak voltage, e.g. 4.2V, and when the peak voltage is detected by the DVL, changes DCCto change the charging current. If the system changes DCVin the CC region, it has the effect of VBAT840 acting as a voltage monitor. For the specific DCV, a High or Low signal is output from the comparator. By sweeping DCV, the voltage VBATis evident. After sweeping DCVfrom the lower voltage, DCVreaches 4.2 V. For the selling of DCV=4.2 V, the sequence described above is performed. The comparator used is a standard comparator.
[0051]The digital control circuit calculates DCCaccording to VBATin the DVL region. DCVis a digital control signal for a particular feedback gain, and {VBAT}×{feedback gain (DCV)}={the reference voltage} may be input to the DVL comparator. The sensed VBATis digitized by the DVL, and also input to the comparator. The comparator in FIG. 8B gives 1-bit output data, and for a specific value of DCV, there is a 1-bit comparator output. By seeing a DCVvalue that changes the comparator output from Low to High, the digitized version of VBATis achieved. The comparator output DVLOUTis fed to the dynamic controls, for gate control of the power and mirror devices in FIG. 13. This then determines the required DCCfrom DCV, to trace the CC-DVL charging profile.
[0052]Similar to a Zener diode voltage clamp, the Digital Voltage Limiter is clamped to a set VBATstate. The DVL doesn't require loop stability, or a special topology for its comparator. Any topology is permitted, as long as the input offset is small, which is managed by trimming, or an auto zero function, for example.
[0053]FIGS. 9A and 9B illustrate an Accelerated Settle Down (ASD) technique to achieve the constant current control loop for the charger. ASD 910 is used during DVL calculation of DCCcharging in 950, and has a faster charging time associated with current ICHG960 than a constant voltage control loop of prior art. By increasing and decreasing the charging current periodically as shown in FIGS. 9A and 9B, the average charge current is larger than the current controlled by the scheme shown in FIG. 7A, leading to a shorter charging time. The variable alpha controls the average charging current, and a higher alpha is always preferred. A smaller delta T is needed for a shorter charging time. Therefore, by having delta T′<delta T, the present disclosure can achieve a smaller charging time. Note that VCELLis the voltage across the battery cell in FIG. 13.
[0054]FIGS. 10A and 10B include voltage curves showing how the digitalized step down of the charging current ICHG(620, 960) in FIGS. 6 and 9B takes into account over-charging. For example, the calculation of ICHG1030, in 1020, is for VCELL1025>4.20V+0.5% in 1010. The system continues charging if the digital control is too slow to stop charging, for example when RESR≈0.
[0055]TOVER=4.221V-4.20V500uVs=42[sec](6)
[0056]TOVERis over charge time, allowed before charging is stopped, as to not damage the battery. Equation (6) is TOVERof the prior art.
[0057]For example, if there are 10 steps controlled by a 1 ms clock, the time is still smaller than TOVER.
10 steps×1 msec=10 [msec]«TOVER=42 [sec] (7)
[0058]In equation (7), 10 msec is arbitrarily chosen. For the present disclosure, any number of steps and clock frequency can be chosen, as long as (steps)*(1/(clock freq.))<42 [sec]. What's important is there is much more controllability for the number of steps and clock frequency chosen, than in the prior art, leading to a simpler design.
[0059]The Digital Voltage Limiter and the Accelerated Settle Down techniques used in the constant current control of the battery charger of the disclosure resolve the disadvantages of the CC step down and CV control loop of the prior art. Additional advantages include an alternative to comparators for VBATthreshold monitoring. For example, a charger IC at the system level needs to monitor VBATfor functions with threshold voltages. In the prior art, one comparator is required for each threshold voltage. In the present disclosure, the digital voltage limiter logic replaces the comparators needed for the above functions. These functions involve short circuit detection, pre-charge voltage detection, and re-charge voltage detection.
[0060]The battery charger of the disclosure, with only a constant current (CC) control loop, and the Digital Voltage Limiter (DVL) and Accelerated Settle Down (ASD) logic, has an improved system-level robustness to equivalent series resistance (ESR) variations. This is beneficial when the ESR of a large or small capacity battery or an aged battery tends to be larger. In the conventional architecture, the stability of the constant voltage (CV) control loop feedback relies on the ESR value, and much design effort needs to be taken to make the design insensitive to the variation of ESR. Since the proposed architecture does not have voltage feedback control, there are less stability problems caused by ESR variation. The system can be robust over the change in ESR due to battery aging and can extend battery life.
[0061]Background calibration of the CC control loop with DVL and ASD can be used to mitigate error in charge current and the error caused by ESR variation. The use of the disclosed function makes the current regulation more accurate and precise to prevent over-current that may cause damage to the battery.
[0062]The present disclosure offers versatile VBATmonitoring by the DVL logic, and can be used elsewhere as a way of battery status monitoring such as a fuel gauge. In addition, the charging profile can be customized with programmable digital gain control, and a charge profile configured based on the system status such as VBATor battery temperature. For example, if soft start is required, charge current can be increased gradually with time. The same function may be realized in conventional architecture, but flexibility is very limited due to the nature of the analog circuit. In the proposed architecture the change required is only in digital gain control where less effort required.
[0063]FIG. 11, 1100, illustrates where the battery charger may require both a Buck switch converter and analog to digital control (ADC) logic to monitor VBAT, in the prior art.
[0064]FIG. 12, 1200, shows where the proposed architecture of the disclosure only requires a Buck switching converter, and no ADC, in the battery charger. Here, DCVdynamically controls the VBATvoltage in the DVL logic, and the comparators needed are replaced by the Digital Voltage Limiter logic for all functions required. This results in easier design with minimal increase in area.
[0065]FIG. 13, 1300, includes the circuitry of the proposed Digital Voltage Limiter (DVL), in a battery charger of the present disclosure. For constant current (CC) charging, the sense device loop consists of a charge current controller 1310 for always controlling ICHG. In the proposed architecture, ICHGis controlled by one control loop, unlike the conventional architecture of FIG. 2, where two control loops exist, i.e. both CC and CV controls. As a result, the possibility of ICHGdiscontinuity between CC and CV charging is removed. And during charging, CC setting DCCis updated according to the battery voltage VBATsensed and digitized by the DVL logic in 1320, of the present disclosure.
[0066]During CC charging, ICHGis constant, for example at a 0.1 C charging rate for pre-charging, or a 1 C charging rate for fast-charging:
ICHG(t)=KDCC(n) (8)
[0067]where K is the constant value to calculate the charging current from the digital code, and DCC(n) is the on register map value.
[0068]During DVL charging, once the DVL logic detects the limit of VBAT, the system counts DVLOUT(=1′b1) to arrive to NDET. (∫DVLOUT=NDET), where 1′b1 and 1′b0 are logic-1 and logic-0 binary numbers, respectively, and NDETis the total number of “1”s appearing on DVLOUT, in a specific period of time.
[0069]The charging current of (8) becomes (9) in the next step:
ICHG(t1)=KDCC(n−1) (9)
If DVLOUTkeeps 1′b1, the charging current can go to (10) in the next step:
ICHG(t2)=KDCC(n−2) (10)
[0070]If DVLOUTchanges 1′b0, then it goes back to (9) and resets ∫DVLOUT. So, the logic needs to wait for ∫DVLOUT=NDETagain to be (10). When the charging current ICHGis reduced below the termination current rate (e.g. 0.1 C), charging is terminated.
[0071]FIGS. 14A and 14B include voltage/circuit curves demonstrating how the charge time of the Accelerated Settle Down is calculated by ICHG′ 1460 in 1450. WLOis fixed to 1, and NDETchanges WHI. Note that NDET=1 means α=0.5 in FIGS. 14A and 14B, where the ASD duty cycle is α=WHII (WHI+WLO), where a higher alpha is always preferred. The Accelerated Settle Down logic uses the pulse control via DCCto generate ICHG′ and:
ICHG(n−1)<ICHG′<ICHG(n) (11)
[0072]This means, the system is waiting for RESR×ICHG(n−1)=VBATwith charging ICHG′[>ICHG(n−1)], so, that it can have the recovery time (Tβ−T1). The recovery time can then secure the charge time increasing, and finally it can be faster than CV settling time for a deeper charging requirement.
[0073]According to (4), prior art CV charging is given by:
ΔTCV=T1−T0=τ(k1−k0) (12)
[0074]The charge time of the Accelerated Settle Down of the disclosure is calculated by
[0076]ICHG′=α·{ICHG(n)-ICHG(n-1)}+ICHG(n-1)(0<α<1)(13)ΔTAC=T1-T0=CBATICHG′dV=τ·ek1-k0-1α·ek1-k0+1-α(14)ΔTACΔTCV=1(k1-k0).ek1-k0-1α·ek1-k0+1-α(k1-k0>0)(15)
[0077]The initial state of the settle down is not so different (1420 in 1410):
[0078]ΔTACΔTCV≈1α·(k1-k0)+1≈1(0<k1-k0≪1)(16)
[0079]The deeper state of the settle down will be about 33% reduced (1430 in 1410), comparing delta Tac for the disclosure to delta Tcv of the prior art CV approach:
[0080]ΔTACΔTCV≈0.67(k1-k0=1,α=0.9)(17)
For example:
CBAT=210F, RESR=3 ohm, CV charging is from 10 mA to 5 mA:
[0081]ΔTCV=T1-T0=τ(k1-k0)=436.7[sec]=0.5:ΔTAC=T1-T0=cBATICHG′dV=420.[sec]<ΔTCV=0.9:ΔTAC=T1-T0=cBATICHG′dV=331.6[sec]<ΔTCV
[0082]FIG. 15, 1500, illustrates a test bench circuit, which includes both the CC-CV control logic 1520 of the prior art, and the CC-DVL control logic 1510 of the present disclosure. The test bench circuits of FIG. 15 are used to generate the waveform results of FIGS. 16A and 16B, and the simulation results of FIGS. 17A and 17B, for the voltage of the battery pack VBAT, the voltage of the battery cell WELLacross RESR, and the charging current ICHG.
[0083]FIGS. 16A and 16B show waveform results for CC-CV control logic compared with CC-CC step down logic both in the prior art, in 1610, and CC-CV control logic compared with CC-Accelerated Settle Down (ASD) logic of the disclosure, in 1650. Signals 1615, 1625, and 1635 represent CC-CV signals VBAT, VCELL, and ICHGrespectively, and signals 1620, 1630, and 1640 represent CC-CC step down signals VBAT, VCELL, and ICHG, respectively, in 1610. Signals 1655, 1665, and 1675 represent CC-CV signals VBAT, VCELL, and ICHGrespectively, and signals 1660, 1670, and 1680 represent CC-ASD signals VBAT, VCELL, and ICHG, respectively, in 1650.
[0084]FIGS. 17A and 17B illustrate simulation results for the architecture of the prior art CC-CV logic, compared to that of the present disclosure CC-DVL logic, when the equivalent series resistance varies from 3 ohms in 1710, to 6 ohms in 1750. When the ESR is 3 ohms, the charge time for the disclosed constant current-digital voltage limiter (CC-DVL) logic is 283 seconds less than the constant current-constant voltage (CC-CV) logic. When the ESR is 6 ohms, the charge time for the disclosed CC-DVL logic is 564 seconds less than the CC-CV logic.
[0085]FIG. 18, 1800, shows how the circuit area used for the small size solution provided by the present disclosure compares to the area used for the circuit layout for a similar battery charger of the prior art. The disclosed battery charger in 1810 achieves a 38% smaller solution than the prior art battery charger in 1850. In this example, where the two projects are comparable with about the same target specification for the battery charger, the disclosed battery charger uses an area of 1.0 mm{circumflex over ( )}2, whereas the prior art charger uses 1.61 mm{circumflex over ( )}2.
[0086]FIG. 19, 1900, is a flow chart of a method for constant current control, in a battery charger. The steps include 1910, providing a battery charger with a constant current (CC) control loop. The steps also include 1920, sensing a battery voltage and digitizing the battery voltage. The steps also include 1930, calculating a dynamic control signal for controlling a charge current.
[0087]The main advantage of one or more embodiments of the present disclosure include replacing comparator logic with digital voltage limiter logic, resulting in easier design with minimal increase in area. In addition, the disclosed battery charger has an improved system-level robustness to equivalent series resistance variations. The use of the disclosed function makes the current regulation more accurate and precise to prevent over-current that may cause damage to the battery. Also, the charging profile can be customized with programmable digital gain control and a charge profile configured based on the system status, such as battery voltage or temperature.
[0088]While particular embodiments of the present disclosure have been illustrated and described, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.
