When charging a lithium battery, if the charging voltage is too high, there might be a risk of battery explosion; if the charging voltage is too low, the battery life may be affected. In addition, when charging discharging the lithium battery, other abnormal conditions, such as excessive current and short-circuit, may also occur. Sometimes, the severity of such abnormal conditions may reach a certain level and may pose dangers to personal safety. To prevent these anomalies, special protection circuits re used to protect the lithium battery during the charging/discharging processes.
FIG. 1 shows a conventional lithium battery protection circuit. As shown in FIG. 1, the protection circuit includes a control circuit 1 (integrated circuit or IC), high-voltage power transistors M1 and M2, resistors R1 and R2, and capacitor C1. The drain terminals of the power transistors M1 and M2 are connected together; the gate terminals of the power transistors M1 and M2 are connected to the control circuit 1; the source terminal of the power transistor M1 is connected to the ground; and the source terminal of the power transistor M2 is connected to one end of the resistor R2 and also to the negative electrode “B−” to external circuits. The other end of the resistor R2 is connected to control circuit 1.
Further, one end of the resistor R1 is connected to the positive electrode of the lithium battery, and the other end of the resistor R1 is connected to one end of the capacitor C1. The other end of the capacitor C1 is connected to the ground and also to the negative electrode of the lithium battery. Both resistor R1 and capacitor C1 are connected to the control circuit 1. Two ends of the lithium battery are respectively connected to the external circuit positive electrode “B+” and negative electrode. “B−” When a load or an external circuit is connected between the positive electrode “B+” and negative electrode “B−”, the lithium battery discharges and provides current to the load; when a charger is connected between the positive electrode B+″ and negative electrode. “B−” the lithium battery is charged by the charger.
The control circuit 1 includes a bias and reference circuit, a multi-channel switch, an over-discharge protection circuit, and an over-charge protection circuit. Both the over-discharge protection circuit and the over-charge protection circuit are connected to the multi-channel switch. The over-discharge protection circuit and the over-charge protection circuit are also connected to logic circuit 2 via the delay circuit. The logic circuit 2, on the one hand, is connected to the gate terminals of power transistors M1 and M2 external to the control circuit 1 and, on the other hand, is connected to system sleep circuit 5. The excessive-current protection circuit 3 and short-circuit protection circuit 4 are connected to resistor R2 external to control circuit 1, and also to the logic circuit 2 via the delay circuit.
During a lithium battery charging process by a charger, if the battery voltage is higher than an over-charge protection voltage (typically 4.2V˜4.3V), the logic circuit 2 turns off the power transistor M2, which further cuts off the charging circuit loop and stops charging tale lithium battery. After the power transistor M2 is turned off, because no load current flows through the charging circuit loop, the charger's output voltage becomes higher than usual. The external circuit negative electrode “B−” can have a negative high voltage (up to −20V), which requires logic circuit 2, excessive-current protection circuit 3, short-circuit protection 4, and power transistor M2 to be able to withstand the negative high voltage. Meeting such requirement is necessary to ensure that the protection circuit can be used in high-voltage charging applications, and also improves reliability of the protection circuit under different application conditions.
During a discharging process, if the voltage of the lithium battery drops below an over-discharge protection voltage (usually 2V˜2.5V), and the low-voltage condition lasts longer than a specified delay time the logic circuit 2 turns off the power transistor M1, which stops the discharging. This condition may also show that the lithium battery has been exhausted. In order to better protect the lithium battery, logic circuit 2 also starts system sleep circuit 5 to put the entire control circuit 1 into a sleep state. Thus, the power consumed by the control circuit 1 itself can be reduced. Further, during the discharging process, if there is excessive-current or short-circuit condition, the logic circuit 2 also turns off the power transistor M1 to stop the discharging to protect the lithium battery.
Although the protection circuit as shown in FIG. 1 can achieve the goal of protecting the lithium battery during the charging/discharging processes, only the control circuit 1 is an integrated circuit, and other components in the protection circuit are external components. Thus, the degree of integration is relatively low and the manufacturing cost is relatively high.
FIG. 2 shows another conventional lithium battery protection circuit. As shown in FIG. 2, compared with the protection circuit in FIG. 1, the previous external resistors R1 and R2 and the power transistor M1 and M2 are integrated into the control circuit 1. Level shift circuit 6 and substrate switching circuit 7 are added to combine the power transistors M1 and M2 into one power transistor. Thus, the chip area and cost are reduced. The level shift circuit 6 is connected to the logic circuit 2, the substrate switching circuit 7, and the gate terminal of power transistor M1. The substrate switching circuit 7 is connected to the substrate of the power transistor M1 and the level shift circuit 6.
Although the protection circuit as shown in FIG. 2 increases the degree of integration and lowers cost, the excessive-current protection circuit 3, short-circuit protection circuit 4, level shift circuit 6, and substrate switching circuit 7 generally use low-voltage MOS devices (a low-voltage MOS device may only be able withstand a relative low voltage between the gate-source and the source-drain). In general, no other additional protective measures are used. Therefore, when protecting an overcharging condition, the protection circuit may be unable to withstand the high negative voltage from the external circuit negative electrode “B−”. Thus, the protection circuit may have a low reliability and may be limited on its applications.
The disclosed methods and systems are directed to solve one or more problems set forth above and other problems.