ON Semiconductor's low-cost CCR charging solution for rechargeable batteries

Charging single-cell Li-ion batteries commonly found in portable devices such as mobile phones, digital cameras (DSCs), and music players has always been a challenging problem, as specific application requirements must be met while ensuring safe and trouble-free charging operations. This article will discuss how ON Semiconductor's constant current regulator (CCR) can be used in a low-cost charging circuit for rechargeable batteries, providing a simple controller for charging termination.

Battery Type and Charging Technology Selection

The three most common rechargeable batteries are Nickel Metal Hydride (NiMH), Nickel Cadmium (NiCad), and Lithium Ion (Li-ion). Battery charging rates are denoted by the letter "C". "C" defines the battery capacity in 1.0 hour. For example, a battery rated at 800 mAh (milliamp-hour) can be charged at 0.5C, so it takes more than 2 hours to fully charge the battery at 400 mA charging current. Figure 1 is a basic block diagram of a charging circuit.

Figure 1: Basic block diagram of charging circuit

1) NiMH and NiCd batteries

NiMH batteries are rated at 1.2 V/cell and should be charged at up to 1.5-1.6 V/cell. There are several different techniques that can be used to decide when to interrupt charging, including: peak voltage detection, negative delta voltage, delta temperature (dT/dt), temperature thresholds, and timers. For high-end chargers, these techniques may all be combined in one charger.

The CCR charger is a peak voltage detection circuit that terminates charging at a predetermined peak value, providing a suitable solution for charging the above batteries. Its predetermined peak voltage is 1.5 V/cell, which can charge the battery to about 97% of its capacity. NiCd batteries can be charged using this circuit. Their performance is very similar to NiMH batteries, so this method is suitable.

2) Lithium-ion battery

For lithium-ion batteries, a common charging method is to charge the battery to 4.2 V/cell through trickle charging at 0.5C to 1C. During the charging process, the temperature rise of the lithium-ion battery should be kept below 5°C. A higher temperature rise indicates that spontaneous combustion may occur. The battery temperature rise is the largest during the trickle charge portion of the charging cycle and is most likely to spontaneously ignite. Due to this problem, high-end charging can use intelligent ICs (such as ON Semiconductor's NCP1835B) to monitor and control the charging process of lithium-ion batteries.

Constant current regulator (CCR) charging circuit design

The CCR controller discussed in this article does not use trickle charging, thus eliminating the problem of possible spontaneous combustion and keeping the battery in a safe operating area helps to increase the battery life. However, without trickle charging, the battery will only be charged to about 85%.

1) Set the reference voltage

The reference voltage can be set using the TL431, a three-terminal programmable shunt regulator. It provides a constant 2.5 V output at its reference pin. When two external resistors are connected as shown in Figure 2, the reference voltage can be selected from 2.5 V to 36 V. For our purposes, we set R2 to 1.0 kΩ and adjust Rref to the reference voltage we want to match. The formula used to find the R2/Rref ratio is:

The resistor connected to the cathode of the TL431 is used to limit the current and separate the reference voltage from the input voltage.

Figure 2: Reference voltage setting

2) Hysteresis loop comparator

The LM311 is a single comparator that compares a reference voltage to the battery voltage. The battery voltage is connected to the inverting input. Hysteresis is provided by the feedback resistor (Rh) between the output and the non-inverting input. R3 is a 1.0 kΩ resistor that is used to simplify the ratio of R3/Rh. The bandwidth of the hysteresis loop can be changed by adjusting Rh. Increasing Rh reduces the bandwidth and vice versa. It is recommended that the bandwidth of the hysteresis be greater than 200 mV because the battery voltage will drop slightly when charging is terminated. The inverting input formula for high voltage and low voltage is:

A 1.0 kΩ resistor (R4) is connected to the output of the comparator as a pull-up resistor.

Figure 3: Hysteresis setting

Figure 4: Schematic diagram of charging circuit

3) Current switch

The two bipolar junction transistors (BJTs) (Q3 and Q6) in the circuit act as switches to control the charging current. The base of Q6 is controlled by the output of the comparator through a 5.6 kΩ resistor (R6). The collector of Q6 is connected to the base of Q3 through a 1.0 kΩ resistor (R5). When the output of the comparator goes low, Q6 is turned off, causing Q3 to turn off and terminate the charging current.

4) Steady flow

The battery charging current is controlled using a CCR. The current can be adjusted using an adjustable CCR and/or parallel CCRs. This demo board is designed specifically for two parallel CCRs (Q4 and Q5) (you can connect more than two CCRs in parallel to be able to achieve any current you want). For the experiments discussed in this article, the CCR (NSI45090JDT4G) is adjustable from 90 mA to 160 mA. The three currents used for data analysis are 90, 180, and 300 mA.

5) Indicator LED

To indicate that the battery is charging, a CCR, Q7 and an LED are used in combination. The CCR provides a constant current to the LED. The LED will also be "on" when no battery is connected to the charger. When the LED is "off", it indicates that the battery is fully charged.

6) Set different test currents

Table 1 shows the variable component values ​​that determine the charge current and the charge termination voltage. Two NSI45090JDT4G CCRs were used to give a current output of 90 mA with Radj = 10 while testing at 180 mA.

7) Test results

The CCR charging circuit was tested by charging Li-Ion and NiMH batteries at 90 mA, 180 mA, and 300 mA. Table 2 shows the critical voltages monitored while the battery was being charged. Table 3 shows the same critical voltages after the circuit terminated the battery charge. During the test, the battery temperature began to rise rapidly (see Table 4), and the battery voltage reached the reference voltage before the test was terminated.

Table 4 contains the temperature data for the batteries. In all cases, the ambient temperature was approximately 25°C. For Li-Ion batteries, it can be concluded that the higher the charge current, the higher the battery temperature rise. The same is true for NiMH batteries when charging at 0.1C. It is important to remember when to choose what charge rate to use.

Table 1: Resistor values ​​used for testing

Table 2: Voltage during charging

Table 3: The voltage is terminated just after charging is completed

Table 4: Battery temperature

Charging current, power consumption and battery voltage

1) Charging current over time

Using a constant current regulator the charging current can be kept constant until charging is terminated, as shown in Figure 5.

Figure 5: Charging current over time

2) Power consumption of BJT and diode

Nowadays, people are very concerned about the power consumption of circuits. Reducing the input voltage is one way to improve the circuit performance. This is one of the reasons for using low VCE(sat) transistors. As shown in Table 1, the VCE of the transistor is very low. Figure 6 also depicts the power consumed by the PNP transistor over time. As one would expect, the dissipated power (PD) increases when the charging current increases. However, at a charging current of about 300 mA, the transistor consumes less than 15 mW.

In addition to using a low VCE(sat) BJT, a low forward voltage drop (VF) Schottky diode in a DSN2 package can be used to reduce power dissipation. This diode is used for reverse current protection. The ON Semiconductor NSR10F40NXT5G was chosen because it has the lowest VF on the market. The power dissipation in the diode was measured to be approximately 95 mW at the highest charge current. Figure 7 shows the power dissipation of the DSN2 low VF Schottky barrier diode when the battery is being charged.

The input voltage can be reduced to the lowest possible level by using low VCE(sat) BJTs and low VF Schottky diodes.

Figure 6: PNP transistor power dissipation over time

Figure 7: Diode power dissipation over time

3) Power consumption of CCR

Power dissipation is a very important parameter when using a CCR. It is the device that drops all the voltages to ensure constant current battery charging. As the device starts to heat up, the current starts to drop. To minimize the temperature rise of the CCR, copper foil is placed in most of the empty space on the board. The cathode of the CCR is then connected to the copper foil in this area to act as a heat sink. When using multiple CCRs in parallel, it is important to remember that the power dissipation of each CCR is simply the value of the CCR's individual current multiplied by the voltage, not the total charging current value. Figure 8 shows the power dissipated by the CCR over time. When using multiple CCRs to obtain higher charging currents, only one CCR data is shown.

Figure 8: CCR power dissipation over time

4) Battery voltage over time

Figure 9 depicts the battery voltages for all six test cases. For the Li-Ion battery voltages, one would expect to see the voltage begin to flatten out when it reaches 4.2 V. In a more advanced circuit, this would be suitable for trickle charging. However, as mentioned above the circuit is designed to stop charging at a predetermined voltage, in this case 4.15 V.

Figure 9: Battery voltage over time

in conclusion

In summary, a constant current regulator, or CCR, can provide a constant current for battery charging. In addition, when the controller discussed above is implemented with a CCR, it is possible to charge different battery chemistries with different currents using the same circuit. This way, specific application requirements can be met while ensuring safe and trouble-free charging operations.