Buffer battery power supply system

[0002] The present invention is a continuation-in-part application under 37 C.F.R 1.53(b) of application Ser. No. 09/272,738 filed Mar. 8, 1999, and assigned to the same assignee.

[0003] 1. Field of the Invention

[0004] The present invention relates to a battery power supply system, and, more particularly, to a buffered battery charger circuit capable of controlling the power supplied to an active system and to a rechargeable battery, and including circuitry for self-adjusting allocation of total power supply ensuring that an active system gets priority of power while maintaining a charge current for the rechargeable battery. Particular utility of the present invention is in a power supply system for portable electronic units; although other utilities are contemplated herein.

[0005] 2. Description of Related Art

[0006] FIG. 1 is a simplified block schematic of a typical prior art power supply topology 20 for a portable electronic system 24. The active system 24 gets power, conditioned by the system DC/DC converter 22, either from the battery 18 or from the external input power adapter 10. The input power adapter 10 gets the power from an external primary power source, such as an AC outlet or a DC source, and provides the power directly to both the system DC/DC converter 22, through the separating diode 12, and to the battery charger 14. The battery 18 is connected and provides power to the system DC/DC converter 22 through the separating diode 16, as long as the primary power source is not available. When the primary power source is available, the battery is isolated from the power input of the system DC/DC converter 22 by the reversed polarized (reversed biased) diode 16. In addition, the battery 18 is charged when power is supplied by the primary power source, through the charger 14. This topology in FIG. 1 has the disadvantage of big and fast voltage transients at the node 25, which is the input of the system DC/DC converter 22.

[0007] FIG. 2 shows a simplified block diagram of a buffer battery power supply 20′ topology. The battery pack 18 is permanently connected to the input of the system DC/DC converter 22 and provides the requested power. The external input power adapter 10 powers the battery charger 14 when an external primary power source is available. The external input power adapter 10 is intended to adapt the parameters of the primary source to the charger input requirements. The battery charger 14 powers in parallel both the system DC/DC converter 22 and the battery 18 to charge it or to maintain the voltage of the fully charged battery at the optimal level. This “buffer battery topology” limits the voltage variations at the system DC/DC converter input (node 25) to normal battery pack voltage variations and does not allow fast voltage transients at this input. Furthermore, when the power requested by the system 24 temporarily exceeds the capability of the input power adapter 10, both the input power adapter 10 and the battery 18 will deliver in parallel the power to the system 24 through the converter 22. Disadvantageously, however, the circuit 20′ shown in FIG. 2 provides no mechanism by which the power supplied by the battery charger can be reduced or increase based on preset limits or demand from the battery, the system, or both.

[0008] Similarly, U.S. Pat. No. 5,698,964 issued to Kates et al. Provides a battery charging circuit topology. This circuit monitors the current from an AC adapter (i.e., I_in) and adaptively utilizes all available current to charge the batteries. The system DC/DC converter is powered directly by the AC adapter after its connection; the battery is disconnected from the system. Thus the voltage at the input of the system DC/DC converter abides a heavy transient, from the low voltage of a discharged battery to the AC adapter voltage, every time higher than the maximum charged battery voltage. Furthermore, as the AC adapter output voltage could vary, no real control is provided for the power delivered by the AC adapter to both the system (e.g. portable electronic device) and the battery. A similar topology is provided in U.S. Pat. No. 5,723,970 issued to Bell, which suffers similar and/or additional drawbacks mentioned above.

[0009] The approach in the prior art to provide battery charge circuitry and a path to an active system is typically accomplished using separate paths between a power source and a rechargeable battery, and a power source an a load. In the case of the present invention, the source, battery and load (system) are all in parallel thus, the conventional charging/discharging approaches would be inadequate, since the voltage conditions on the battery must be accounted for when providing power to the system.

[0010] Thus, there exists a need to provide a buffered battery power supply system that can control both the total output power and the power delivered to the battery. Moreover, there exists a need to provide a system that will significantly reduce the voltage transients that may appear at the electronic device, the battery, or both. Also, there exists a need to provide a buffer topology (where the battery and system are in parallel with a source) that permits charging of the battery when the battery is deeply discharged, and that permits a variety of choices for the source voltage in addition to conventional PWM-type source voltages.

[0011] Accordingly, the present invention solves the aforementioned drawbacks by providing a buffer battery power supply system that includes feedback control of both the total output current delivered by the battery charger circuit and the voltage delivered to the battery. Feedback control is provided based on the total output power (total output current x total output voltage) delivered by the battery charger circuit. To permit charging of a deeply discharged battery while also supplying power to a system (or a DC/DC converter), the present invention also includes a battery switch circuit that selectively decouples the battery from a load (system) when the battery is in a deeply discharged state, yet still provides a path for a trickle charge (low current) to charge the battery sufficiently to begin regular charging.

[0012] In one embodiment of the present invention, a power supply system is provided that includes a charger circuit for generating a duty cycle for delivering power to an active system and a battery. A first feedback loop is provided to sense the total output current generated by the charger circuit and a second feedback loop is provided to sense the current delivered to said battery by the charger circuit. The first and second feedback loops including error circuits for generating an error signal to the charger circuit. The charger circuit adjust the duty cycle so thereby controlling the total output current delivered to the active system and the battery based on the value of the error signal. Also, a battery switch circuit is provided that decouples said battery from said active system when said battery voltage is less than the minimum voltage required to power said active system, and couples said battery to said charger circuit to receive a charging current.

[0013] In another embodiment of the present invention, a power supply system is provided that includes an input power source, and a charger circuit for generating a duty cycle for controlling the input power source to deliver controlled power to an active system and a battery. A first feedback loop is provided to sense the total output current generated by the charger circuit, the first feedback loop generating a first error signal based on the total output current and a preset threshold total output current signal. A second feedback loop is provided to sense the current delivered to the battery by the charger circuit, the second feedback loop generating a second error signal based on the current delivered to the battery and a preset threshold battery current signal. A third feedback loop is provided for sensing the total output power generated by the charger circuit, the third feedback loop generating a third error signal based on the total output power and a preset threshold total output power signal. Using the first, second or third error signals, the charger circuit adjusts the duty cycle for controlling the total output current and power delivered to the active system and the battery. This embodiment also includes a battery switch circuit is provided that decouples said battery from said active system when said battery voltage is less than the minimum voltage required to power said active system, and couples said battery to said charger circuit to receive a charging current.

[0014] In method form, the present invention provided a method of regulating the current delivered by a charger circuit to an active system and a battery. The method includes the steps of sensing a first error signal based on the total output current of the charger circuit and a preset threshold total output current signal. The method also includes sensing a second error signal based on the current delivered to the battery by the charger circuit and a preset threshold battery current signal. One of the first or second error signals is provided to the charger circuit as feedback signals. The charger circuit adjusts the current delivered based on the first or second feedback error signals. The method also includes the steps of sensing the voltage of said battery and decoupling said battery from a load if said battery voltage is less than a required minimum voltage for said load; and coupling said battery to said charger circuit if said battery voltage is less than a required minimum voltage for said load.

[0015] In still other embodiments, the present invention provides a power supply system, comprising a power source; a battery in parallel with said power source; a load in parallel with said battery and said power source; and a battery switch circuit placed between said battery and said power source, said battery switch circuit adapted to decouple said battery from said active system and said load when said battery voltage is less than the minimum voltage required to power said active system, and simultaneously couple said battery to said charger circuit to receive a charging current.

[0016] In preferred form, the battery switch circuitry comprises a controllable switch coupled between said battery said charger circuit and said active system, an impedance in parallel with said controllable switch, and a comparator to control the activation state of said controllable switch, wherein said comparator comparing said minimum voltage required to power said active system and said battery voltage and generating a control signal to control the conduction state of said controllable switch, and wherein said impedance providing a path between said battery and said charger circuit if said switch is nonconducting. In most preferred form, the impedance is chosen to have an impedance value larger than said active system.

[0017] It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to preferred embodiments and methods of use, the present invention is not intended to be limited to these preferred embodiments and methods of use. Rather, the present invention is of broad scope and is intended to be limited as only set forth in the accompanying claims.

[0018] Other features and advantages of the present invention will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, wherein like numerals depict like parts, and wherein:

[0019] FIG. 1 is a block diagram of a power supply circuit topology of the prior art;

[0020] FIG. 2 is block diagram of another power supply circuit topology of the prior art;

[0021] FIG. 3 is a block diagram of one preferred embodiment of the power supply system of the present invention;

[0022] FIG. 4 is a detailed circuit diagram of the embodiment of FIG. 3;

[0023] FIG. 5 is a detailed circuit diagram of another embodiment of the power supply system of the present invention;

[0024] FIG. 6 is a detailed circuit diagram of one example of a current-voltage multiplying circuit provided in the embodiment of FIG. 5;

[0025] FIG. 7 is a detailed circuit diagram of another example of a current-voltage multiplying circuit provided in the embodiment of FIG. 5;

[0026] FIG. 8 is another circuit diagram of a conventional buffer battery power supply system;

[0027] FIG. 9 is a circuit diagram of another power supply toplogy according to the present invention and includes a battery switch circuit;

[0028] FIG. 10 is circuit diagram of another power supply topology according to the present invention and includes the battery switch circuit of FIG. 9; and

[0029] FIGS. 11 and 12 are circuit diagrams of the topologies of FIGS. 4 and 5, respectively, and further include the battery switch circuit of FIG. 9.

[0030] FIG. 3 shows the block diagram of one preferred embodiment of the battery power system 30 according to the present invention. The battery pack 18 is, as discussed above, permanently connected to the system DC/DC converter through the sense resistor 34. Preferably, resistor 34 is a very small value resistor is intended to sense the current to and from the battery 18, with negligible voltage drop. The battery charger 32 is connected to the system DC/DC converter input (node 25) through the separating diode 12 and the sense resistor 26. When the input power adapter 10 has a primary power source available and it is connected to the system 30, the battery charger 32 provides the requested power to the active system 24 through the DC/DC converter 22, and simultaneously charges the battery 18. The battery charging current, sensed by the sense resistor 34, and the voltage on the node 25 are regulated by the battery charger 32, which makes use of feedback connections from the current sense resistor 34 and from the node 25. Furthermore, the total battery charger output current is sensed by sense resistor 26 and is limited to a safe value by using the feedback connections from the sense resistor 26 to the battery charger 32. The battery charger 32 reacts by reducing the charging current. The feedback connections via sense resistors 26 and 34 are described in more detail below.

[0031] Referring to the FIG. 4, a detailed circuit diagram of the battery charger circuit 32 according to one embodiment of the present invention is depicted. The switching MOS transistors 40 and 42, the Schottky diode 46, the inductor 44, the capacitor 48 and the pulse width modulator 38 together form a controlled buck converter. In this embodiment, the duty cycle of the buck converter pulses is controlled by the pulse width modulator 38 through at least three feed back loops: I) a voltage loop built around the error amplifier 60, II) a battery charging current loop using the battery charging current sense amplifier 64 and the error amplifier 58, and III) a total output current limiting loop embedding the output current sense amplifier 62 and the comparator 56. The diodes 50, 52 and 54 assure the largest negative value (i.e., the diode having the largest reverse bias) goes through to the PWM 38. This is a measure of the largest error, thus providing control of that output parameter which had reached the limit value.

[0032] When the input power adapter 10 provides power to the charger input, the pulse width modulator 38 starts to generate pulses which drive the gates of power MOS transistors 40 and 42. As a result a voltage appears on the output of the charger. The duty cycle of the pulses depends on the feed back voltage received by the PWM 38 from the feed back loops. As long as no one of the set limits is exceeded, the duty cycle increases, which in turn raises the output voltage of the buck converter. The set limits are preferably defined as preset inputs to the comparators, as discussed below. When the output voltage of the buck converter exceeds the battery voltage, the circuit 80 switches ON and the output current reaches the node 25. This current is shared between the system DC/DC converter 22 and the battery 18. The current flowing to the battery generates a voltage drop across the sense resistor 34. This voltage is amplified by the sense amplifier 64 and is compared with the programmed value IDAC by the error amplifier 58. When the charging current exceeds the programmed value IDAC, the output of the error amplifier 58 goes negative, flows through diode 52, and reduces the duty cycle of the PWM to keep the charging current at the programmed value. Likewise, error amplifier 60 compares the battery voltage with the programmed value VDAC and, when the charging voltage exceeds the programmed value VDAC, the duty cycle of the buck converter is decreased. Similarly, the total output current of the buck converter generates a voltage drop on the sense resistor 26. This voltage drop is amplified by the sense amplifier 62 and compared with the preset value Iout_max by the error amplifier 56. When the total output current exceeds the preset value, the output of the error amplifier 56 goes negative, the signal flows through diode 50, and the duty cycle is reduced as to keep the total output current at the preset limit. This decrease produces a drop of the battery charging current. As it is known, because of the battery's low internal resistance, the battery charging current decreases very quickly with the reduction of the voltage. Conversely, the current sunk by the system DC/DC converter 22 is only marginally affected by this voltage variation. Thus, the total current I_TOTat node 25 is kept constant while the battery charging current decreases. Thus, the current allocated to the system DC/DC converter 22 is increased. Up to a preset limit, the whole output current of the buck converter is allocated to the system 24. Furthermore, if the system requests more power, the voltage will drop even more and the battery will join the buck converter in providing the power. This features allows to use smaller and cheaper input power adapters.

[0033] Signals VDAC and IDAC are programmed signals that represent the maximum current and voltage that can be delivered safely to the battery 18, i.e., the threshold values permitted by the battery for safe operation. In some instances, battery 18 supplies signals VDAC and IDAC (i.e., if battery 18 is a so-called “smart battery” that supplies signals indicative of its maximum allowable power), which could be in digital form. Thus, D/A converters (not shown) are provided to convert VDAC and IDAC into analog signals for comparison at error amplifiers 60 and 58, respectively, as described above. Alternatively, VDAC and IDAC can be generated by other programmable circuitry (not shown), as is known in the art. In addition, reference signal Iout_max is another preset threshold value that represents the maximum allowable current that PWM is permitted to deliver to prevent overcurrent from being delivered by the charger circuit 32. Iout_max can be generated by a voltage divider circuit (not shown) or other current generating circuits known in the art.

[0034] It should be noted that, in this embodiment, diode 12 is preferably replaced with circuit 80. Both diode 12 (FIG. 3) and circuit 80 prevent reverse current from reaching the PWM coming from the battery 18. However, circuit 80 has the additional advantage over a diode in that a negligible forward voltage is needed to turn ON circuit 80. Consequently, circuit 80 has a very small voltage drop compared to a diode, and thus, circuit 80 creates a negligible loss in the system. Circuit 80 carries out the task of cutting out the reverse current from the battery to the charger. Circuit 80 includes a MOS transistor 70 embedding the body diode 72. The MOS transistor 70 is driven by the comparator 66. The comparator 66 is designed to have a definitely positive offset, as provided by bias source 68. As long as the voltage on the MOS drain is negative with the respect to its source, the output of the comparator 66 goes high and the MOS transistor 70 turns OFF. When the MOS drain voltage exceeds the offset, the output of the comparator 66 goes low and the MOS transistor 70 turns ON. As the result, the circuit 80 behaves like a diode with very low forward voltage drop.

[0035] The power supply system shown in FIG. 4 limits the total output current of the buck converter. Since the buck converter output voltage depends on the battery voltage, in the case of a fully discharged battery, this output current limiting method forces the buck converter to deliver a lower power than its rating. Accordingly, another solution is to control and to limit the buck converter output power, in addition to control of the parameters shown in FIG. 4. FIG. 5 shows a system 32′ similar to the one in FIG. 4, but with an additional power limiting loop. The voltage drop across the sense resistor 26, proportional to the total output current I_out, is applied both to the output current sense amplifier 62 and to the multiplier 82. Through the second connection to the multiplier senses also the output voltage V_out. By multiplying the total output current value by the output voltage value, the multiplier 82 provides at its output a voltage PWR_OUT proportional to the output power. As for the other loops, the PWR_OUT voltage is compared by the comparator 84 with the set limit. The amplified error drives the Pulse Width Modulator 38, through the diode 86. The function of this diode is similar with the one of the other diodes 20, 52, 54, as described above in reference to FIG. 4.

[0036] An illustrative circuit 82 for multiplying the buck converter output current and voltage values is shown in FIG. 6. The voltage drop across the sense resistor 26 is applied to the transconductance amplifier 88. This provides a current, K×I_out, which is proportional with the voltage drop, therefore with the total output current. The MOS transistor 90 chops this current by the same duty cycle as the buck converter. To that end, the duty signal is supplied to the control line of transistor 90. The resulting current is integrated by the integrating circuit built around the operational amplifier 92 using the integrative RC group 94. The integrator 92 output voltage will be proportional with the total output power of the buck converter.

[0037] Another illustrative circuit 82′ for multiplying the output current and the output voltage values is shown in the FIG. 7. This circuit is based on a well-known property of an amplifying differential stage. The output voltage of such a stage is roughly proportional to the product of the common source current, I=k×V_out, and the differential input voltage. The differential amplifier stage shown in the FIG. 7 includes common-source transistors 98 and 100 each tied to reference voltage V_CC. The differential input is connected to the total current sense resistor 26. Therefore, the output voltage, provided by amplifier 96, will be proportional to the buck converter output power.

[0038] Thus, it is apparent that there has been provided a buffer battery power supply circuit that satisfies the objectives set forth herein. Those skilled in the art will recognize that the present invention is subject to modification and/or alterations, all of which are deemed within the scope of the present invention, as defined in the appending claims.

[0039] For example, although the preferred embodiments shown in FIG. 4 and 5 specifically mention the use of a controlled buck converter circuit, those skilled in the art will recognize that the buck converter circuit can be replaced with other controllable power supplies known in the art, including, for example, boost, buck-boost, and other similar circuit topologies. Such topologies may also be derived from frequency width modulation (FWM) circuitry and or other switching topologies.

[0040] Other modifications are possible. For example, the diodes 50, 52, 54, and 56 can equally be replaced with other reverse bias switches known in the art, including for example, biased transistor circuits.

[0041] The topologies disclosed in FIGS. 3-7 provide control over charging and discharging conditions at node 25. In the case where Lithium Ion and NiMH batteries are employed, several conditions may exist which would require further control over the battery charging and discharging.

[0042] FIG. 8 depicts a conventional power supply topology 200 which controllably delivers current to both the load 24 and the battery 18, via switch 204. In this topology, switch 204 is controlled via comparator 206, which compares node voltage 25 to a battery maximum threshold voltage, Vbatt_max. Once the battery is greater than (or equal to) the threshold voltage, the switch stops conducting and power to the load is supplied entirely by the battery via a discharge current. In this topology, the source (supply) is chosen as a conventional current limited power source 202, which is capable of clamping current supplied to the system or battery (or both) regardless of the draw required by these components. Note that in this topology, a PWM power source is not required since the output of the source is dependant only on the draw requirements of the battery and system (not using feedback). Also, since the conduction state of the switch 204 is dependent on the voltage condition of the battery 206, the internal resistance of the battery will cause the voltage to drop once the switch closes, thereby decreasing the voltage at node 25 below the minimum threshold and opening the switch again. This oscillation will continue whenever a power supply is present. As with most system components, including cell phones, the system requires a minimum voltage before the system is activated. Thus, if the battery cannot supply this voltage, source 200 is required. However, a drawback to this topology depicted in FIG. 8 is if the battery is in a deeply discharged state (i.e., unable to supply a minimum system voltage Vsys_min), then the external power source (if connected) will only supply power to the battery for charging, not to the system for use.

[0043] FIG. 9 is a circuit topology according to the present invention which solves the shortcomings of the topology disclosed in FIG. 8. For FIG. 9, it is assumed that the power source 202 is similar to the current limited source of FIG. 8. In this example, a battery switch circuit 210 is provided between node 25 and the battery 18 that operates to decouple the battery from the system 24 when the battery is in a deeply discharged state, while simultaneously permitting a trickle charge current to reach the battery from the source, thus slowly charging the battery, while permitting the source 202 to power the system. As is understood in the art, a deeply discharged LiIon or NiMH battery cannot take a large current fast charge once the battery is in a deeply discharged state (discharged more than 95% of fully charged), rather a trickle charge, or slow current charge must be supplied until the battery cells reach a state capable of receiving a larger charging current. Deeply discharged, as used herein, generally means that either the battery requires a low current (trickle) charge, or that the battery has insufficient voltage to meet the minimum system requirements (i.e., Vbatt<Vsys_min). Battery switch circuit 210 includes a switch 212, an impedance 214 in parallel with switch 212, and comparator 216 for controlling the conduction state of switch 212. In operation, comparator compares the voltage of the battery 18 to a minimum system voltage, Vsys_min. If the voltage of the battery is less than the value of Vsys_min, switch 212 remains open (nonconducting). However, to permit charging of the battery even during these conditions, impedance 214 permits a small current to reach the battery for charging purposes. Thus, since it is intended to charge the battery while simultaneously supplying power to the system, it is desirable that the impedance value is chosen to be sufficiently large so that a negligible current is diverted away from the system. It should also be noted that although the figures depict a resistor as the impedance 214, it should be recognized that the resistor can include more complex impedance devices and/or switches without departing from the scope of the present invention. It is important to note that since the battery and system are arranged in parallel with the source, a dynamic allocation of current supplied to the system and the battery is achieved. Also, the system always gets priority of available current, and any remaining current is supplied to the battery for charging. If the system requires more current than is available, the battery will begin to supply current (either across impedance or through the closed switch) to supply the difference in current required by the system (assuming that the battery is sufficiently charged to begin conducting). FIG. 10 is yet another example of a dynamic source current allocation circuit topology according to the present invention. The circuit of FIG. 10 is similar to the circuit of FIG. 9, except that instead of a current limited source, a current and voltage limited source 220 is provided. Source 220, as is understood in the art, has a rectangular I,V characteristics such that the source will deliver (if required) only a maximum current and any further current draw causes the voltage to decrease. In other words, source 220 is capable of delivering a maximum current I max at a maximum voltage Vmax, until the load (system or battery) requires more current, in which case Vmax decrease thereby preventing any further delivery of current. To prevent overvoltage and overcurrent conditions on the battery, Vmax and Imax are chosen in accordance to the maximum battery voltage and current, Ibatt_max and Vbatt_max.

[0044] FIGS. 11 and 12 are similar to FIGS. 4 and 5, respectively (described in detail above), but further includes the battery switch circuit 200 in accordance with the present invention. In these examples, the source power supply is provided through a PWM circuit, using feedback in accordance with the principles set out above.

[0045] Thus, the battery switch circuit of the present invention can be provided with conventional current limited power supplies, or more complex supplies as shown in FIG. 10, 11 and 12.