Patent Publication Number: US-2007097572-A1

Title: Protective circuit

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a protective circuit for a rechargeable battery. In particular, the present invention relates to a low-power, low “rush current” protective circuit suitable for use with a lithium ion battery or lithium ion polymer battery.  
      2. Discussion of the Related Art  
      Lithium ion batteries and lithium polymer batteries are widely use in portable electronic devices because of their high energy density per unit weight or per unit volume. However, if not properly used, they can be hazardous. In some instances, inadvertent large discharge currents or large charging currents have been known to cause fire or even explosion. Therefore, as a safety measure, each lithium ion battery or lithium polymer battery is always provided a protective circuit that limits the current drawn from the battery in the event of an unusual or abnormal operating condition occur. Because the protective circuit is always operating, regardless of whether or not a load is connected across the battery, a practical protective circuit cannot be allowed to draw more than a few microamperes of current.  
      A protective circuit for a battery typically has the following states: (a). the on state, in which the switch is closed to allow normal discharging or charging currents to flow, and the total on-resistance R total  is no more than a few tens of milli-ohms; (b) a charging-only state, in which current flows in the charging direction is allowed and current flows in the discharging direction is blocked; and (c) A discharging-only state, in which current flows in the discharging direction is allowed but current flows in the charging direction is blocked.  
       FIG. 1  illustrates schematically protective circuit  100  used in a rechargeable battery in the prior art. At this time, protective circuit  100  of  FIG. 1  is the most widely used protective circuit in portable electronic devices. As shown in  FIG. 1 , protective circuit  100  includes control circuit  101  and metal-oxide-semiconductor field effect transistors (MOSFETs)  102  and  103 . As shown in  FIG. 1 , MOSFETs  102  and  103  are shown connected in a common-drain configuration in series with battery  105 , with their gate terminals being controlled by control circuit  101 . Battery  105  is typically a lithium ion battery or a lithium polymer battery. Parasitic diodes  104   a  and  104   b  are also expressly shown in  FIG. 1  for illustrative purpose. During a discharging operation, a load (e.g., load  107 ) is connected across terminals  106   a  and  106   b . During a charging operation, a charger (e.g., charger  108 ) is connected across terminals  106   a  and  106   b . Two MOSFETs are used to allow current to be completely switched off, either during charging and discharging.  
      In a typical implementation, protective circuit  100  is mounted on a printed circuit board, with MOSFETs  102  and  103  provided in a single package, and control circuit  101  provided in a separate integrated circuit. Because the bulk (or substrate) terminals of MOSFET  102  and  103  are common with their respective source terminals, parasitic diodes  104   a  and  104   b  of MOSFETs  102  and  103  each allow current flow from its respective source terminal to its drain terminal, even when the voltage applied to its gate terminal is below its threshold voltage.  
      In protective circuit  100 , MOSFETs  102  and  103  are sized to achieve a low total resistance R total , which is equal to the sum of the MOSFETs&#39; individual on-resistance (R ds(on) ). Hence, each R ds(on)  equals ½ R total . As the on-resistance of a MOSFET is inversely proportional to the device area on the semiconductor substrate (“die area”), the total die area for MOSFETs  102  and  103  is roughly four times the size of an alternative “single-MOSFET” protective circuit, which is discussed next.  
       FIG. 2  shows protective circuit  200 , which includes control circuit  74 , MOSFET  78  and switches  82  and  86 . Protective circuit  200  is disclosed in U.S. Pat. No. 6,670,790 to Stellberger, entitled “Power Switch for Battery Protection”, which was filed on Dec. 14, 2001 and issued on Dec. 30, 2003. As shown in  FIG. 2 , MOSFET  76  is connected in series with battery  105 , with its gate terminal being controlled by control circuit  74 . Battery  70  is typically a lithium ion battery or a lithium polymer battery. Parasitic diodes  97  and  98  are also expressly shown in  FIG. 2  for illustrative purpose. During a discharging operation, a load (e.g., load  88 ) is connected across terminals  99  and  94 . During a charging operation, a charger (e.g., charger  90 ) is connected across terminals  99  and  94 .  
      The bulk terminal of MOSFET  78  in protective circuit  200  is difficult to control because of the bulk-to-source junction and the bulk-to-drain junction. If either of these junctions become forward biased, the parasitic lateral and vertical bipolar transistors may become conducting and the resulting current may be detrimental. To prevent these junctions to become forward biased, switches  82  and  86  are controlled such that the bulk terminal of MOSFET  78  is connected to the source terminal or the drain terminal of MOSFET  78 , whichever has the lower potential.  
      One disadvantage of protective circuit  200  is its requirement that control circuit  74  monitors the current direction during both charging and discharging operations, so as to determine which one of the source and drain terminals of MOSFET  78  has the lower potential. This determination may be difficult sometimes. Because the on-resistance R ds(on)  of MOSFET  78  is in the range of tens of milliohms, and the current drawn from the battery is typically between a few microamps (μA) to about 1 ampere (A), the voltage drop across the source and drain terminals of MOSFET  78  is less than a few tens of millivolts (mV). Consequently, protective circuit  200  must include a high-precision comparator. Such a comparator requires precious die area and draws a significant operating current.  
      Another disadvantage of protective circuit  200  occurs when battery  70  switches from discharging to charging, or vice versa. During the switch over, switches  82  and  86  must operate in a coordinated fashion to switch the bulk terminal of MOSFET  78  from its source terminal to its drain terminal, or vice versa. This condition is illustrated by  FIG. 3 . In  FIG. 3 , representing a discharging operation, switch  82  is closed and switch  86  is open. Switch  82  shorts the bulk terminal of MOSFET  78  to terminal  96  to avoid forward biasing parasitic diode  97 . If battery  70  is over-discharged through load  88 , the voltage on gate terminal of MOSFET  78  falls below the threshold voltage, and may even be close to the ground voltage. As a result, MOSFET  78  is non-conducting. If charger  90  is engaged at this point to recharge to depleted battery, switch  82  must open and switch  86  must close within a very short period of time to connect the bulk terminal of MOSFET  78  to terminal  94  to prevent forward biasing parasitic diode  98 . However, because protective circuit  200  typically draws only a few μA of current, protective circuit  200  cannot respond very quickly—perhaps requiring a few hundred micro-seconds (μs)—to this sudden change from a discharging operation to a charging operation. In the meantime, a large current flows in switch  82  and parasitic diode  98 . This large current may result in circuit latch-up and other undesirable or detrimental effects. This large current may also be hazardous, from the viewpoint of safety.  
      Thus, a safe, low power protective circuit which requires a small die area is desired.  
     SUMMARY  
      According to one embodiment of the present invention, a protective circuit for a battery includes: (1) an MOS transistor having a first drain/source terminal coupled to one terminal of the battery; (2) a switch selectable to couple the bulk terminal of the MOS transistor to (a) the first drain/source terminal, (b) a second drain/source terminal, or (c) float; and (3) a control circuit which provides control signals for the gate terminal of the MOS transistor and the switch. By allowing the bulk terminal to float during normal operation (i.e., charging or discharging operation), a precision, low-power comparator used in the prior art is eliminated, thereby allowing the protective circuit to have a small foot-print.  
      In one embodiment, the protective circuit further includes a resistor. The switch connects the bulk terminal of the MOS transistor to the first drain/source terminal through this first resistor, thereby limiting any “rush” current which occurs when the battery circuit switches from discharging to charging, or vice versa, over a very short time period. Consequently, safe operation of the battery circuit is achieved.  
      In accordance with another embodiment of the present invention, a protective circuit for a battery includes: (1) an MOS transistor having a first drain/source terminal coupled to one terminal of the battery; (2) a switch selectable to couple the bulk terminal of the MOS transistor to (a) the first drain/source terminal, or (b) a second drain/source terminal; and (3) a control circuit which provides control signals for the gate terminal of the MOS transistor and the switch. In this embodiment, while a low-power comparator is required in the control circuit, the protective circuit may further include a resistor, which operates to limit any “rush” current that occurs when the battery circuit switches from discharging to charging, or vice versa, over a very short time period. Consequently, safe operation of the battery circuit is also achieved. 
    
    
      The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.  
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates schematically protective circuit  100  used in a rechargeable battery in the prior art.  
       FIG. 2  illustrates schematically protective circuit  200  used in a rechargeable battery in the prior art.  
       FIG. 3  illustrates the operation of protective circuit  200  of  FIG. 2  during a switch over from discharging to charging.  
       FIG. 4  shows protective circuit  400 , in accordance with one embodiment of the present invention.  
       FIG. 5  shows protective circuit  500 , in accordance with a second embodiment of the present invention. 
    
    
      To facilitate cross-referencing and to simplify the detailed description, like elements in the figures are provided like reference numerals.  
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 4  shows protective circuit  400 , in accordance with one embodiment of the present invention. As shown in  FIG. 4 , protective circuit  400  includes control circuit  401 , MOSFET  402 , resistors  404   a  and  404   b , and switch  405 . MOSFET  402  includes parasitic diodes  403   a  and  403   b , corresponding respectively to the junctions at its source and drain terminals. Under control of control circuit  401 , switch  405  can selectively float the bulk terminal of MOSFET  402 , or to connect the bulk terminal of MOSFET  402  to its source terminal or its drain terminal, through resistors  404   a  and  404   b , respectively. During normal operation (i.e., either in a discharging operation or a charging operation), the bulk terminal of MOSFET  402  is allowed to float. Unlike protective circuit  200  of  FIG. 2 , by floating the bulk terminal of MOSFET  402 , protective circuit  400  is not required to detect whether the operation is charging or discharging (hence, there is no need to determine which one of the drain terminal and the source terminal is at a lower potential). Accordingly, control circuit  401  does not require a precision comparator.  
      During a change from a discharging operation to a charging operation, or vice versa, switch  405  is switched to connect the bulk terminal of MOSFET  402  to the drain terminal or the source terminal of MOSFET  402  thorough resistor  404   a  or resistor  404   b , so as to limit the current in parasitic diode  403   b  or parasitic diode  404   a , respectively. The appropriate resistance values should limit the current in each of the parasitic diodes to no more than a few milliamps (mA). Such a current normally does not cause catastrophic destruction in the integrated circuit. In one embodiment, resistors  404   a  and  404   b  are integrated into switch  405  by properly sizing the MOS switches.  
       FIG. 5  shows protective circuit  500 , in accordance with a second embodiment of the present invention. As shown in  FIG. 5 , protective circuit  500  includes control circuit  501 , MOSFET  402 , resistors  404   a  and  404   b , and switches  502   a  and  502   b . As discussed above, MOSFET  402  includes parasitic diodes  403   a  and  403   b , corresponding respectively to the junctions at its source and drain terminals. Under control of control circuit  501 , switches  502   a  and  502   b  selectively connect the bulk terminal of MOSFET  402  to its source terminal or its drain terminal, through resistors  404   a  and  404   b , respectively. Like protective circuit  200  of  FIG. 2 , without a float setting, control circuit  501  includes a comparator circuit to detect which one of the source terminal and the drain terminal of MOSFET  402  has a lower potential, so as to allow switches  502   a  and  502   b  to connect the bulk terminal of MOSFET  402  to either its source terminal or its drain terminal. In this embodiment, the resistors  404   a  and  404   b  are sized, as discussed above, according to the expected rush currents in parasitic diodes  403   b  and  403   a , respectively.  
      The detailed description above is provided to illustrate the specific embodiments of the present invention and is not intended to be limiting. Numerous modifications and variations within the scope of the present invention are possible. The present invention is set forth in the claims below.