1. Introduction

Since lithium-ion batteries have high energy density, high cell voltage, no memory effect, low self-discharge rate and long service life compared to their main rival, NiMH batteries^[1], Li-ion batteries have already been applied as power sources for many 3C (computer, communication, and consumer electronics) products, especially in portable electronics devices^[2]. Additionally, many countries are taking steps to cut their nations' greenhouse gas emissions. In particular, auto emission standards are getting stricter and will force vehicles to become more fuel efficient. Because of these demands, the electric vehicle (EV) and the hybrid electric vehicle (HEV) have attracted more and more attention for fuel economy and eco-friendliness from automakers, governments, and customers^[3]. However, due to the fragility and sensitivity to voltage and current (for example an overcharge has a high risk of explosion and an overdischarge of the battery cell reduces the lifecycle of the battery cell^[4-7]})$, lithium-ion batteries are not considered for wide applications^[8]. So, the design and realization of a lithium-ion battery protection circuit with high reliability is a challenge and a difficult task for designers. Moreover, owing to the high demand for the length of time the battery is used, low consumption is another key factor for the protection circuit.

In order to realize the protection circuit, many methods have been proposed^[9-13]. The methods all have their advantages in protecting batteries from harm. However, most of these methods have been complex to understand and realize. To improve the reliability and lower the power consumption, this paper focuses on new and simple architectures for the protection circuit. The voltage bandgap reference (VBG) and $V_{\rm ref}$ produce a stable reference voltage with low temperature coefficient. The current detection circuits monitor the current of the battery to avoid short circuit (SC) and overcurrent (OC). The voltage of the battery is detected by the overvoltage detection (OVD) and undervoltage detection (UVD) circuits to make the battery work in the range of the normal voltage. The controller is the brain of the whole circuit; it can deal with all the information from other blocks and give appropriate orders to protect the battery from harm. OSC in the controller provides the clock for the circuit. Additionally, a sleep state control (SSC) circuit is developed to reduce the power consumption of the circuit.

2. The structure of the single lithium-ion battery protection circuit

A block diagram of the proposed single lithium-ion battery protection circuit is shown in Fig. 1. There are eight main blocks in the diagram: VBG, $V_{\rm ref}$, voltage detection (VD), OVD, UVD, overcurrent detection (OCD), short circuit protection (SCP) and controller. VBG and $V_{\rm ref}$ are bandgap reference circuits which could generate reference voltage to meet the circuit's application stability needs. VD monitors the voltage of the single lithium-ion battery and then sends the voltage to OVD and UVD in real time. $I_{\rm bat}$ is monitored and protected by OCD and SCP. According to the monitoring information above, the controller decides whether it is necessary to perform OV, UV, OC or SC protection schemes. If the lithium-ion battery is OV, DO is low so that the charging switch NMOS2 is turned off, then the circuit stops charging. Simultaneously, the circuit forbids discharging when the lithium-ion battery is UV, OC and SC. The power source for each component of the protection circuit is obtained from the battery itself.

3. Detailed description of the circuit

3.1 VBG and $\boldsymbol{V_\text{ref}}$ circuit

A schematic diagram of the proposed VBG is shown in Fig. 2. The VBG is a bandgap reference circuit relative to $V_{\rm DD}$. The negative temperature coefficient bipolar transistors npn1, npn2 and npn3 lower the temperature coefficient of the changeable power source circuit. The differential amplifier with active current mirror Mn4 constituted by Mn2, Mn3, Mp3 and Mp4 transforms differential input signals to a single-ended output signal $V_{\rm o1}$. Then $V_{\rm o1}$ is sent to some other blocks of the circuit through the common source stage amplifier made up of Mn5 and Mp6. The common source stage amplifier can enhance the efficiency and enlarge the output voltage swing. In order to solve the stability problem of the bandgap reference circuit, a Miller compensation circuit consisting of $R_{1}$ and Mp5 is developed. Mp1, Mp2 and Mn1 constitute a start-up circuit to guarantee that the amplifier works regularly when the amplifier is powered on.

Owing to the two approximately equal input voltages of the differential amplifier, Equation (1) can be written as

$\begin{equation} V_{\rm BE2} =V_{\rm BE3} +I_3 R_3, \end{equation}$

(1)

where $V_{\rm BE2}$ and $V_{\rm BE3}$ are the base-emitter voltages of npn1 and npn2 respectively. According to the formula $V_{\rm BE} =V_{\rm T} \ln \left( I/I_0 \right)$, Equation (1) can be refined as

$\begin{equation} V_{\rm T} \ln \left( I_2/I_3 \right)=I_3 R_3, \end{equation}$

(2)

where $V_{\rm T}$ is thermal voltage. Assuming $I_{2}/I_{3}=n$, Equation (2) can be rewritten as

$\begin{equation} I_3 =\frac{V_{\rm T} }{R_3 }\ln n. \end{equation}$

(3)

The output voltage $V_{\rm VBG}$ can be seen from Fig. 1 to be

$\begin{equation} V_{\rm VBG} =V_{\rm DD}-\left[{V_{\rm BE2} +I_3 R_4 +(I_2 +I_3 )R_5 } \right]. \end{equation}$

(4)

According to Eqs. (2) and (3), Equation (4) can be simplified as

$\begin{equation} V_{\rm VBG} =V_{\rm DD}-\left[{V_{\rm BE2} +\frac{(R_4 +R_5 +nR_5 )}{R_3 }V_{\rm T} \ln n} \right]. \end{equation}$

(5)

In Eq. (5), $V_{\rm VBG}$ has a negative temperature coefficient and $V_{\rm T}$ has a positive temperature coefficient, so appropriate values of $R_{3}$, $R_{4}$, $R_{5}$ and n can give a low temperature coefficient VBG. When $R_{3}$, $R_{4}$, $R_{5}$ and n are determined to be explicit values, $V_{\rm VBG}$ is a fixed value relative to $V_{\rm DD}$.

To convert $V_{\rm VBG}$ to $V_{\rm O}$, a $V_{\rm ref}$ circuit is developed, as shown in Fig. 3. At the output, a lowpass filter is used to purify $V_{\rm O}$, then $V_{\rm O}$ is preserved as the reference voltage of OCD.

3.2 Current detection circuit

As shown in Fig. 4, the current detection circuit includes two parts: a comparator and a Schmitt trigger. The comparator consists of a differential amplifier with active current mirrors and a common source single-stage amplifier. It can quickly compare the signal $V_{\rm M}$ generated by the charging or discharging current to the reference voltage $V_{\rm O}$, and then output a single-end signal. The common source single-stage amplifier amplifies the single-end signal and the Schmitt trigger reshapes the single-end signal, so the signal has greater ability to resist noise interference.

3.3 Voltage detection, overvoltage detection and undervoltage detection circuit

As shown in Fig. 5, the VD circuit adopts the structure of the resistive subdivision to set the size of the OV and UV. The parallel structure of the resistive subdivision enhances the stability of voltage detection and improves the accuracy of the voltage collected from the power source. OVD and UVD circuits are comparators consisting of a differential amplifier with active current mirrors and a common source single-stage amplifier. The comparators compare $V_{\rm O2}$ and $V_{\rm O3}$ to $V_{\rm VBG}$, then $V_{\rm OCH}$ and $V_{\rm ODH}$ are sent to the controller as the control signals of charging and discharging respectively.

3.4 Controller circuit

The controller circuit is the key part of the whole system. All the detected signals converge into this block, then this part outputs signals to control the whole circuit. There are seven main blocks in the diagram shown in Fig. 6: state detection (SD), SSC, OSC, divider, judgment of priorities (JOP), output control and prohibit charging and discharging (PCAD). SD receives and deals with the detected signal so that the work status of the battery can be obtained. JOP judges the priority of the signal from SD, then outputs the highest priority signal to the output control circuit. The output control circuit controls the charging signal CO and discharging signal DO to decide the battery's work state. The OSC and divider provide the clock signal for SD. In order to reduce power consumption, SSC makes OSC and the divider come into the idle state when the battery's voltage is under UV.

The SSC circuit is shown in Fig. 7. $R_{18}$, $R_{19}$, $R_{20}$, $R_{\rm f}$ and AMP constitute a subtracter, so $V_{\rm mc}$ can be written as

$\begin{equation} V_{\rm mc}=\frac{R_{18}+R_{\rm f}}{R_{18}} \frac{R_{19}}{R_{19}+R_{20}} V_{\rm DD}-\frac{R_{\rm f}}{R_{18}}V_{\rm M}. \end{equation}$

(6)

When $R_{18}$, $R_{19}$, $R_{20}$ and $R_{\rm f}$ have the same sizes, Equation (6) can be rewritten as

$\begin{equation} V_{\rm mc}=V_{\rm DD}-V_{\rm M}, \end{equation}$

(7)

then comparator COMP_2 compares $V_{\rm mc}$ and $V_{\rm SH}$ to judge the idle state. In order to enhance the driving capability of the idle state signal, the negative resistance inverter made up of Mp28, Mp29, Mn23 and Mn24 is developed. Finally, the enable signal EN is decided by the shorted load signal Short_De, UV signal $V_{\rm OD_De}$ and idle state signal $V_{\rm MC}$. As illustrated in Table 1, eight kinds of EN signal can be obtained by changing Short_De, $V_{\rm OD\_De}$ and $V_{\rm MC}$.

It can be seen from Table 1 that EN is only 0 if $V_{\rm MC}$ and $V_{\rm OD_De}$ are 0 and Short_De is 1. Therefore, the circuit comes into the idle state only when the load is not shorted, the voltage of the battery is under UV and $V_{\rm mc} < V_{\rm SH}$ simultaneously.

OSC provides the circuit clock. As shown in Fig. 8, the main structure of OSC is a ring oscillator. The clock cycle is decided by the charging and discharging times of the capacitors. The equation can be written as

$\begin{equation} T=\frac{UC}{I}, \end{equation}$

(8)

where U is the voltage difference on the capacity. C and I are the capacitance value and constant-current source respectively. So the clock cycle can be roughly written as

$\begin{equation} T=\frac{U_1 C_1 }{I_1 }+\frac{U_2 C_2 }{I_2 }+\frac{U_3 C_3 }{I_3 }+\frac{U_4 C_4 }{I_4}, \end{equation}$

(9)

because $C_{1}$-$C_{4}$ and $I_{1}$-$I_{4}$ are the same, so $U_{1}$-$U_{4}$ have the same value, which is equal to $V_{\rm DD}$ -$V_{\rm TH}$, where $V_{\rm TH}$ is the threshold voltage of Mp30-Mp33. Equation (9) can be simplified as

$\begin{equation} T=4\frac{\left( {V_{\rm DD}-V_{\rm TH} } \right)C_1 }{I_1 }. \end{equation}$

(10)

It can be seen from Eq. (10) that the clock cycle of the circuit could be set up by changing the size of the capacities and the constant-current power.

4. Simulation and measurement of the proposed battery protection circuit

The proposed battery protection circuit has been implemented in a 0.5 $\mu$m mixed-signal CMOS process with 3 V power supply. The total area of the chip is about 1.05 $\times$ 0.73 mm$^{2}$. Figure 9 shows a microphotograph of the battery protection circuit. It was simulated by Cadence Spectre and measured by an oscilloscope. Under a regular working environment, the total power of the proposed protection circuit is about 9 $\mu$W. Additionally, the power consumption is as low as 0.3 μW when the circuit comes into the idle state.

First, the simulation characteristics of VBG are shown in Fig. 10. It can be seen from Fig. 10(a) that $V_{\rm VBG}$ is proportional to $V_{\rm DD}$. The reference voltage relative to $V_{\rm DD}$ is 1.19 V, shown in Fig. 10(b). In order to get a stable reference voltage, the lowest work voltage of the circuit is 1.5 V. Figure 10(c) shows that the voltage fluctuation is only about 4 mV when the temperature changes from $-40$ to 120 ℃, so the temperature coefficient is about 21 ppm/℃.

Figure 11 shows the simulation result for $V_{\rm ref}$. The reference voltage relative to GND is 1.19 V and the lowest work voltage of the circuit is 1.8 V. So the proper operating voltage of the whole circuit must be above 1.8 V.

Second, the simulation characteristics of the current detection circuit are shown in Fig. 12. When $V_{\rm M}$ increases to 100 mV due to a large current or short circuit, the circuit stops discharging to protect the battery from harm. The protection is lifted as $V_{\rm M}$ decreases to 79 mV.

Third, the simulation outputs of the OVD and UVD circuits are shown in Fig. 13. It can be seen from Fig. 13(a) that the battery is prohibited from charging when the voltage is above 4.25 V. So the battery's highest voltage is 4.25 V. The battery recovers the ability to charge when the voltage is under 4.15 V. On the other hand, notice from Fig. 13(b) that the battery cannot be discharged to 0 V; it has a lowest voltage of \, 2.3 V. Therefore, the range of the battery's voltage is from 2.3 to 4.25 V. Figure 14 shows the simulation result for the OSC. The output frequency is 3.5 kHz.

Figure 15 shows the measured results for the battery protection circuit. Figure 15(a) shows the measured result for OV, and Figure 15(b) shows the measured result for UV. Both the measured results meet the reliability requirements. The disparity between them and the simulation results shown in Fig. 13 is mainly generated by the parasitical effects of the capacitors and the variation of the process and temperature. The protection criteria for triggering the protection schemes and the working conditions of the circuit are listed in Table 2.

A performance comparison between the proposed lithium-ion battery protection circuit and other related lithium-ion battery protection circuits is listed in Table 3.

5. Conclusion

A single lithium-ion battery protection circuit with high reliability and low power consumption is proposed in this paper. Some new circuit architectures are adopted to improve the reliability of the protection circuit. First, $V_{\rm BG}$ and $V_{\rm ref}$ adopt the structures of the differential amplifier and negative temperature coefficient bipolar transistors to get a stable and lower temperature coefficient reference voltage. Second, the charging and discharging current is monitored by a current detection circuit to protect the battery from SC and OC. At the same time, OVD and UVD circuits which have simple structure avoid the battery being damaged by OV and UV. Additionally, the controller handles all signals from other parts and makes the decision to protect the battery or not. Finally, the OSC is adjusted continuously and precisely by the charging capacitors and the constant-current source provides a clock signal for the controller, and the SSC circuit is developed to reduce the power consumption of the circuit. The proposed protection circuit is fabricated in a 0.5 μm mixed-signal CMOS process with 3 V power supply. The measurement results show that the reference voltage is 1.19 V, OV is 4.2 V and UV is 2.2 V. Under a regular working environment, the proposed protection circuit consumes about 3 μA under a 3 V power supply.