Application case: Universal charger solution based on SE9020

SE9020C/D is a single-cell lithium battery charging control IC that can automatically identify the battery polarity. This IC integrates the functions required for a universal charger solution, such as complete battery polarity identification, automatic charging control, and charging protection. It can charge lithium batteries without almost any external components, and the charging current reaches 250mA (TYP). SE9020C/D uses a small and simple SOT23-6L and COB package , which is suitable for portable devices such as universal chargers.

SE9020C/D has an internal integrated polarity detection circuit , which can quickly identify the polarity of the inserted lithium battery and automatically charge the lithium battery with the correct polarity. SE9020C/D charges the lithium battery at a constant voltage and the charging current is fixed at design time and limited to 250mA (TYP). This can effectively ensure the charging quality and reduce the risk of battery overheating during charging. While charging, the chip automatically detects the battery voltage . When the battery is fully charged, the chip automatically stops charging to avoid chip overcharging and effectively protect the battery.

SE9020C/D supports multiple charging status display modes, including red/green LED flashing mode and colorful light display mode. The letter C in SE9020C/D means color, which refers to the colorful light mode. The letter D means Double, which refers to the flashing mode. In the colorful light display mode, LED1 is used to display whether the power supply and battery are connected and the connection status, and the colorful light is used to indicate the charging status. When charging, the colorful light flashes, and when charging is completed, the colorful light goes out. In the flashing light display mode, LED1 is used to display whether the power supply and battery are connected and the connection status, and LED2 is used to display the charging status. When charging, LED2 flashes, and when charging is completed, LED2 goes out.

SE9020C/D also has protection functions such as automatic restart due to battery undervoltage, lithium battery short circuit protection, over-temperature protection, over-current protection, etc. to prevent dangerous situations such as battery explosion.

Compared with the traditional universal charger for lithium battery charging, SE9020C/D adopts a safer and more efficient charging mode:

Traditional lithium battery charging method

1. Charging mode

Constant current and voltage limited charging (CC-mode)

Constant current charging uses a constant current source to charge lithium batteries. The working principle of constant current charging is that during the charging process, a current detection unit is used to collect the TEST2 signal to monitor the current of the entire charging circuit, and a control signal CONTROL2 is fed back to control and adjust the current source so that the current source outputs a constant current to charge the lithium battery. In addition, a battery voltage detection unit is used to detect the TEST1 signal to monitor the voltage of the lithium battery. If the voltage of the lithium battery reaches 4.2V, a charging cut-off signal is output through CONTROL1 to turn off the current source. The entire charging process is completed.

Constant voltage non-current limiting charging (CV-mode)

Constant voltage charging is a method of charging the battery by detecting the battery voltage and adjusting the output current of the current source. During the charging process, the voltage detection unit samples the signal of TEST1 and outputs the CONTROL1 signal to control the current source. When the battery voltage increases, the charging current gradually decreases. Achieve a balanced state of energy conservation. In addition, the current detection unit monitors TEST2. When the current decreases to the preset value, the CONTROL2 signal is output to shut down the entire charging circuit. The entire charging process is completed.

Pros and Cons:

The advantage of constant current charging mode is that it maintains a constant current charge. During the entire charging process, the charging current can be controlled to maintain a preset value, maintaining a stable charging time. The disadvantage is that when the battery voltage reaches the preset voltage value (judgment of the full state), the charging current is immediately turned off. At this time, the battery is not really full. After charging is completed, the battery voltage will drop rapidly by about 100mV, and it will also affect the battery discharge time.

The advantage of the constant voltage charging mode is that it can detect the battery voltage in real time to adjust the charging current. When the battery voltage reaches the preset voltage, the charging current gradually decreases. This constant voltage charging process ensures that the battery is truly fully charged. After charging is completed, the battery voltage remains basically unchanged. At the same time, it has a longer discharge time. The disadvantage is that the charging current decreases quickly as the battery voltage rises, affecting the charging time.

(II) Implementation method

The traditional universal charger uses separation devices to form the entire system. The principle is shown in Figure 2. The traditional separation device solution uses a simple NPN on-voltage as a controller to shut down charging. Two NPN tubes and two PNP tubes are used as controllers to switch the battery polarity. The influence caused by the voltage drop of the NPN tube and the PNP tube itself cannot be avoided. At the same time, the saturation voltage is different when the same charger charges different batteries, which makes the charging voltage highly discrete and difficult to control within 1% accuracy. In addition, in the application of traditional charging solutions, due to the insufficient internal gain of the circuit, there is often an intermediate state in the switching between the charging process light and the full display light, which is easy to mislead the user during use. At the same time, a large number of separation devices reduce the processing efficiency of the board-level circuit. There is no perfect protection circuit, which makes the battery charging process have safety hazards.

SE9020 Charging Application Analysis

By analyzing the traditional charging mode, we found that current limiting-constant voltage charging is a more scientific universal charger charging method. Constant voltage charging ensures that the battery is truly full, and current limiting charging ensures that there is no risk of overheating when charging lithium batteries of various specifications. Through comparative tests under the same test conditions, the battery charged by the current limiting-constant voltage charging mode has a longer discharge time. In addition, the use of highly integrated ICs to replace traditional discrete device solutions is a future development trend both from a technical level and from a production and processing perspective.

How charging works

SE9020 adds a unit to limit the maximum charging current on the basis of traditional constant voltage charging . It controls the current of the charging circuit and slows down the speed of the charging current reduction, so that the battery voltage can maintain a large charging current when it is close to the full value. At the same time, the constant voltage process allows the battery to be truly fully charged. Considering factors such as charging and discharging time, the charging efficiency of SE9020 is better than the traditional constant current charging mode. Figure 3 and Figure 4 are comparisons of the SE9020 charging process and the traditional constant current mode. SE9020 has a built-in unit for automatically identifying the polarity of the battery. By detecting the voltage direction of the battery, it automatically switches the charging circuit, so that the battery can be charged normally regardless of whether it is connected forward or reverse.

Figure 3 SE9020 charging principle

SE9020 has a built-in accurate reference circuit to provide an accurate reference for the entire charging process. At the same time, the voltage drop of the switch controlling the polarity is reduced to zero as the charging current gradually decreases. It has a built-in full judgment unit to ensure that the charging saturation voltage has a high accuracy of 1% without the risk of overcharging. SE9020 also has a RE-CHARGE function. When the IC detects that the battery voltage is lower than 4.05V, the circuit will restart the charging process. When the power supply voltage is as low as 5V, SE9020 can still have a large charging current, which has greater compatibility with USB charging solutions. It has an accurate and fast indicator light logic control unit, which enables the indicator light to switch accurately and quickly when switching between various states during the charging process.

Figure 5 Comparison of the discharge curve of the battery after charging with SE9020 and the discharge curve of the battery after charging with the traditional constant current mode

Complete charging protection function

SE9020C/D also has various protection functions that the separation device solution does not have: battery short circuit protection. When the battery voltage is lower than 2.5V, the circuit determines that the battery is short-circuited and shuts off the entire charging circuit, avoiding the danger caused by the large current generated by the battery short circuit. SE9020 also has a temperature protection circuit. When the temperature is too high, the entire charging circuit is shut down, effectively preventing dangerous situations caused by battery overheating.

Rich charging indicator light function

SE9020 has two built-in power indicator light modes, L1/L2 flashing light mode and L1/colorful light mode. The indicator light logic under various charging states is shown in Table 1.

Area optimization and cost saving

Compared with the traditional solution of discrete devices, SE9020 has a high degree of integration and does not require any peripheral components except the LED indicator. SE9020 application Circuit As shown in Figure 6 and Figure 7.

Through the above comparison, we can find that SE9020 has the following advantages over traditional charger application solutions:

1. Fully charged , accurate voltage , automatic identification of positive and negative battery levels, can be combined with USB interface, in line with national standards, bringing more added value to customers, 100% product charging consistency.

2. Built-in high-precision reference voltage unit, so that the full charge voltage can reach a high accuracy of 1%. The traditional constant current charging solution uses the forward voltage of the external LED as the reference voltage. (The accuracy of the LED forward voltage is usually 15%, so it is necessary to manually select the LED with 1% accuracy from the LED with 15% accuracy. This greatly reduces the processing efficiency).

3. Optimized charging mode makes charging more efficient.

4. Complete charging protection function to improve product safety.

5. Rich logic indicator functions and various packaging forms provide customers with more choices.

6. High integration reduces the number of workers on the assembly line, greatly improves productivity, increases the yield of finished products, improves procurement and production management, and improves production efficiency.

7. The product can be made smaller and more refined, and the PCB board can be reduced by 30%.

8. The finished product test is simple and the repair rate is greatly reduced

9. Higher grade, easier to win orders.