Patent Publication Number: US-2011068735-A1

Title: Systems and Methods of Accurate Control of Battery Pre-charge Current

Description:
TECHNICAL FIELD 
     The present disclosure is generally related to electronics and, more particularly, is related to battery chargers. 
     BACKGROUND 
     Rechargeable batteries are an important power source for today&#39;s products, especially for portable appliances such as notebook computers, mobile phones, and digital cameras. The importance of rechargeable batteries is increasing as the usefulness/functionality of portable electronic equipment is increasing. The reasons are several: an ongoing integration of functions (such as a mobile phone with a digital camera), the higher computing speed in notebook computers, and the convenience of large color displays. As a consequence of this high level of power consumption in portable devices, the use of rechargeable batteries has become more cost effective than using a standard battery. Even more important are the environmental benefits of rechargeable batteries. Using rechargeable batteries tremendously reduces the amount of hazardous materials dumped into our environment, the consumption of materials, and the energy required to produce the equivalent in nonrechargeable batteries. 
     Charging rechargeable batteries is an important facet in maximizing battery use and lifespan. Because fast charging a Li-ion cell can accelerate the degradation of the cell at lower voltages, a much lower charging current, as low as at 1/20 full charge rate, is used to pre-charge the battery. There are many methods of pre-charging with tradeoffs. There are heretofore unaddressed needs with previous pre-charging methods and systems. 
     SUMMARY 
     Example embodiments of the present disclosure provide systems and methods of accurate control of battery pre-charge current. Briefly described, in architecture, one example embodiment of the system, among others, can be implemented as follows: a charge control device comprising a monitor device; a charging driver; a discharging driver; and a semiconductor device configured in parallel with at least one of the charging driver and the discharging driver, the monitor device configured to monitor a voltage drop across a resistor in series with the semiconductor device and to control the current through at least one of the charging driver and the discharging driver. 
     Embodiments of the present disclosure can also be viewed as providing methods of accurate control of battery pre-charge current. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following: monitoring pre-charge current through a resistor in series with a semiconductor device, the semiconductor device in parallel with at least one of a charging driver and a discharging driver; and controlling the current of at least one of the charging driver and the discharging driver based on the pre-charge current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of an example embodiment of a charger for a battery pack. 
         FIG. 2  is a circuit diagram of an example embodiment of the battery charger circuit of  FIG. 1  with an external pre-charge FET. 
         FIG. 3  is a circuit diagram of an example embodiment of the battery charger circuit of  FIG. 1  with an approximate mirror FET. 
         FIG. 4  is a circuit diagram of an example embodiment of the battery charger circuit of  FIG. 1  with an open circuit gate voltage drive. 
         FIG. 5  is a circuit diagram of an example embodiment of the battery charger circuit of  FIG. 1  with PWM control. 
         FIG. 6A  is a circuit diagram of an example embodiment of systems and methods of accurate control of battery pre-charge current using external n-channel components. 
         FIG. 6B  is a circuit diagram of an example embodiment of systems and methods of accurate control of battery pre-charge current using internal n-channel components. 
         FIG. 7  is a circuit diagram of an example embodiment of systems and methods of accurate control of battery pre-charge current using external p-channel components. 
         FIG. 8  is a circuit diagram of an example embodiment of systems and methods of accurate control of battery pre-charge current using external n-channel components, monitoring low-level discharge current. 
         FIG. 9  is a flow diagram of an example embodiment of methods of accurate control of battery pre-charge current. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be described more fully hereinafter with reference to the accompanying drawings in which like numerals represent like elements throughout the several figures, and in which example embodiments are shown. Embodiments of the claims may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. The examples set forth herein are non-limiting examples and are merely examples among other possible examples. 
     At low temperatures, or when dealing with a heavily discharged battery, some cell chemistries require that the battery is charged with a reduced level of current until said battery reaches an acceptable charge or temperature. Typically this is achieved by either pulsing an in-line field effect transistor (FET) in the battery pack on/off periodically, having a separate high-impedance charge path through a discrete FET device, or by using an intelligent current-limited charger. 
       FIG. 1  provides a simplified schematic of a typical lithium-ion battery pack such as those used in most laptop computers and other portable electronic equipment. Although lithium-ion is used in an example embodiment, the systems and methods disclosed herein are applicable to any rechargeable battery chemistry. During pre-charge, it may be preferable for the monitoring electronics to limit the current supplied to the battery to avoid damaging the chemistry. 
       FIG. 1  provides an example embodiment of one implementation for pre-charge. Circuit  100  of  FIG. 1  provides an intelligent current-limited charger. Battery monitoring integrated circuit (IC)  105  monitors the current supplied by battery pack  130  by sensing the voltage drop across resistor  140 . Battery monitoring IC  105  controls charge current by controlling field effect transistor (FET)  110 . Battery monitoring IC  105  controls discharge current by controlling FET  115 . In this approach, charge FET  110  and discharge FET  115  are turned ON. The selected charger determines the approximate state of charge and limits the current supplied to circuit  100 . This may be an expensive solution in that much of the electronics required for the intelligent charger already exists in the battery pack causing undesirable duplication of circuitry. There may also be a safety concern if a user connects an incompatible charger. 
       FIG. 2  provides an example embodiment of another implementation for pre-charge. Circuit  200  includes battery monitoring integrated circuit (IC)  205 , which monitors the current supplied by battery pack  230  by sensing the voltage drop across resistor  240 . Battery monitoring IC  205  controls charge current by controlling field effect transistor  210 . Battery monitoring IC  205  controls discharge current by controlling field effect transistor  215 . Circuit  200  of  FIG. 2  adds external pre-charge FET  220  and series resistor  225  to circuit  100  of  FIG. 1 . When these components are used, FET  210  is OFF and the pre-charge path is enabled via FET  220 . Resistor  225  limits the current into the battery. Circuit  200  of  FIG. 2  is a common approach, but requires additional components and the current is not well controlled. The current control largely depends on the battery voltage and the applied charger voltage. Both resistor and MOS device may be integrated into the monitoring integrated circuit (IC), but thermal management becomes difficult as considerable power will then be dissipated by the semiconductor die. If the MOS device is integrated and resistor is not, an extra pin requirement, PCHG, is added to the IC, but thermal management is still an issue, which leads to a large on-chip FET requirement with suitably low on-state resistance. 
       FIG. 3  provides an example embodiment of another implementation for pre-charge control. Circuit  300  of  FIG. 3  introduces current driven internal FET  350 . FET  350 , internal to IC  305 , is used to amplify current I REF  into external FET  310  in an approximate mirror configuration. The difficulty with this approach is that without specifying the exact external FET to use for FET  310 , the current control is poor. Even if the external FET to use for FET  310  is specified, the control current may be poor due to manufacturing tolerances in both IC  305  and external FET  310 , including variations over changes in temperature. There will also be inaccuracy at the R SAFETY1  input of IC  305  due to R SAFETY1   * I REF . Hence, buffer  360  is preferred to avoid this inaccuracy. 
       FIG. 4  provides an example embodiment of another implementation for pre-charge. Circuit  400  of  FIG. 4  introduces open-loop gate control with open circuit gate voltage drive. Circuit  400  of  FIG. 4  works much like circuit  300  of  FIG. 3 , except that a fixed voltage is programmed on the gate of external FET  410 . As in  FIG. 3 , the variation with different external FETs and temperature may be large. 
       FIG. 5  provides an example embodiment of another implementation for pre-charge. Circuit  500  of  FIG. 5  introduces a pulse-mode control or pulse width modulation technique. In the pulse-mode technique, sense resistor  540  is used to sense the pre-charge current. However, R sense    540  may be very small to prevent excessive power loss during high discharge conditions. As a non-limiting example, 5 mohms may be used as sense resistor  540 . The pre-charge current will, then, also be small, such as a non-limiting example of 50 mA. This leads to a voltage drop on sense resistor  540  of only 250 uV. However, controlling this sense voltage to a +/−10% accuracy is not trivial due to offsets within such a control loop. 
     To amplify this voltage, FET  510  and FET  520  can be turned on at considerably higher current for a non-continuous period of time using variable frequency oscillator  570 . Oscillator  570  could also be fixed frequency with variable duty cycle in example embodiments. As a non-limiting example, turning on FET  510  and FET  520  with a duty cycle of 1:10 would result in a peak charge current of 500 mA and a 2.5 mV signal on R sense    540 . This is undesirable because pulsing in this manner creates electromagnetic interference, may confuse a connected charger, is only an approximation of the low current desired for the battery chemistry, and is still difficult to control as sense resistor values are trending downwards to reduce energy losses during operation and to reduce cost and size. 
     Example embodiments of the systems and methods of accurate control of battery pre-charge current disclosed herein offer improvements over previous solutions.  FIG. 6A  provides an example embodiment of circuit  600  using external n-channel charge FET  610  and external n-channel discharge FET  615  for accurate control of battery pre-charge current. Low V forward  diode  690  and series resistor  685  form a parallel path around discharge FET  615 . Diode  690  is shown in  FIG. 6A  as a Schottky diode (V forward  of approximately 0.2V); however, many alternatives embodiments exist (for example, a low threshold metal oxide semiconductor device). It is preferable for the component to have a voltage drop significantly less than that of parasitic diode  680  in FET  615  when carrying current in one direction and substantially higher voltage drop in the other direction. During a pre-charge condition, discharge FET  615  is turned off, and amplifier  675  monitors the voltage across resistor  685 . An example embodiment of circuit  600  uses a servo amplifier, which generally includes an integrated feedback loop to actively control the output of amplifier  675  at a desired level. However, other suitable components may be used. Amplifier  675  controls the gate of charge FET  610  such that a desired current is flowing from the charger to the battery. 
     In an example embodiment, resistor  685  may be integrated into IC  605  for matching purposes. In another example embodiment, as shown in circuit  601  of  FIG. 6B , diode  690  may also be integrated into IC  605  to eliminate the external components for this example embodiment of a system for accurate control of battery pre-charge current, without thermal problems since the voltage drop across the resistor/diode combination is less than 0.7V. Excess power in circuit  601  of  FIG. 6B  may be dissipated across external FET  610 . The difference between forward bias voltage of Schottky diode  690  and parallel parasitic diode  680  enables a sizeable voltage (for example, several hundred mV) across resistor  685  before parasitic diode  680  reaches forward bias. This allows circuitry in the control loop to have small area, short design time, and low cost. 
     An alternative embodiment is provided in  FIG. 7 .  FIG. 7  provides circuit  700  using external p-channel charge FET  710  and p-channel discharge FET  715 . 
     An additional alternative embodiment of systems and methods of accurate control of battery pre-charge current is provided in  FIG. 8 . In circuit  800  in  FIG. 8 , a method of accurate control of battery pre-charge current is applied to the discharge path. Rather than control a servo loop in response to sensed current, circuit  800  allows monitoring of very low discharge currents, such as may occur if a load device I load  is in a steady state or a shutdown state, drawing greater than zero current, but substantially less than ‘normal operation’ currents. In this alternative embodiment, charge FET  810  is turned off, enabling accurate measurement of small current flow across a substantially larger resistor  817 , thereby maintaining accurate state of charge even during load system shutdown or standby modes. In a non-limiting example, resistor  840  may be 5 mohms and the standby load current may be 1 mA with a resulting voltage of 5 uV. If resistor  817  is 300 ohms, then the same 1 mA current discharge produces a 300 mV drop across resistor  817 , which is considerably less challenging to accurately monitor. 
       FIG. 9  provides flow diagram  900  of an example embodiment of a method for accurate control of battery pre-charge current. In block  910 , the pre-charge current through a resistor in series with a semiconductor device is monitored, the semiconductor device in parallel with at least one of a charging driver and a discharging driver. In block  920 , the current of at least one of the charging driver and the discharging driver is controlled based on the pre-charge current.