Design of current expansion circuit for lithium-ion battery charger

The capacity of lithium-ion batteries or lithium polymer batteries used in small portable electronic products is relatively small, mostly in the range of 400 to 1000mAh, and the maximum charging current of the corresponding charger is 450 to 1000mAh. Since the current is not large, a linear charger is generally used.

The new linear lithium-ion battery charger has complete functions, good performance, simple circuit, small printed board area, low price, and the entire charger can be included in the product. If the USB port is used for charging, it is very convenient to use.

In recent years, some portable electronic products with slightly larger power consumption (such as portable DVD, mining lamp, camera, portable measuring instrument, small power tools, etc.) often use lithium-ion batteries with a capacity of 1500mAh to 5400mAh. If a 500-1000mA charging current charger is used for charging, the charging time is too long. If a 3000mAh and 5400mA battery is charged at a 0.5C charging rate, the capacity requirement of the rechargeable battery is 1500mA and 2700mA.

Someone proposed: Can we add a current expansion circuit to the 1A linear charger circuit to expand the charging current to 2-2.5A, and solve the charging problem of 3000-5400mAh lithium-ion batteries? If the charger with current expansion has good performance, simple circuit and low cost, it is a good idea. I designed a current expansion circuit according to this idea. This circuit is based on the 1A linear charger model CN3056, and is composed of a current expansion circuit and a control circuit.

CN3056 Introduction

The charger composed of CN3056 charges in constant current and constant voltage mode. If the charging battery voltage is <3V, there is a small current pre-charging mode; the charging current can be set, and the maximum charging current is 1A; the precision density is 4.2V±1%, and it has thermal regulation, undervoltage lockout and battery temperature detection, over-temperature protection, and charging status and temperature difference indication functions; 10-pin small size DFN package (3mm×3mm).

If the charging rate is between 0.5 and 1C and the battery temperature is between 0 and 45℃ (room temperature charging), the battery temperature detection circuit and the battery over-temperature indication circuit can be omitted in the CN3056 charger circuit (the pins TEMP and FAULT are grounded), as shown in Figure 1. VIN is the power input terminal, CE is the enable terminal (high level is valid); RISET is the charging current ICH setting resistor, RISET (Ω) = 1800 (V) / ICH (A); CHRG is the charging status signal output terminal: this terminal is high level when charging, and the LED is on; this terminal is high impedance when charging is completed, and the LED is off; if the battery is not installed or the contact is poor, the LED flashes. VIN is generally 4.5~5V, and 10μF and 6.8μF are input and output capacitors to ensure stable operation of the charger.

Figure 1 Charging circuit composed of CN3056

Electrical current expansion circuit

The charger current expansion circuit is composed of adding a current expansion circuit to the original charger circuit. The current expansion circuit consists of two parts: the current expansion part and the control part. Based on the CN3056 charger, the circuit with the current expansion part and the control part is shown in Figure 2. Now we introduce their working principles respectively.

Figure 2 Charger circuit

1. Current expansion circuit

The circuit of the current expansion part is shown in Figure 3. It consists of a P-channel power MOSFET (VT), a voltage divider composed of R and RP, and a Schottky diode D4. The voltage divider is used to adjust the -VGS of the P-MOSFET to obtain the required current expansion current ID. The output characteristics of the P-MOSFET (taking Si9933DY as an example) are shown in Figure 4. When -VGS=2.1V and VDS>0.5V, its output characteristics are almost a horizontal straight line; at different VDS, ID is a constant current. It can also be seen from Figure 4 that when -VGS increases, ID also increases accordingly.

Figure 3 Bracket circuit

Figure 4 P-MOSFET output characteristics

2. Control circuit

The purpose of the control circuit is to maintain the original three-stage charging mode, without current expansion in the pre-charging stage and the constant voltage charging stage, and current expansion only in the constant current stage, as shown in Figure 5.

Figure 5 Current performance of the bracket circuit

The original charger charges with a current of 1A. If the expansion current is 1A, the charging current is 2A during the constant current charging stage. The red line in Figure 5 is the voltage characteristic of the charging battery, the black line is the charging current characteristic, the solid line is the expansion current characteristic, and the dotted line is the non-expansion current characteristic. It can be seen from Figure 5 that the charging time t5 with expansion current is shorter than the time without expansion current (the time coordinates in Figure 5 are not drawn to scale); and it can also be seen that the expansion current is only performed during the constant current charging stage.

To ensure that the current expansion starts at the battery voltage of 3.0V and ends at the battery voltage of 4.15V, the control circuit sets a window comparator to control the P-MOSFET to turn on when the battery voltage (VBAT) is between 3.0 and 4.15V. Outside this window voltage, the P-MOSFET is turned off.