Design of current expansion circuit for lithium-ion battery charger

　　The capacity of lithium-ion 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 chargers matched with them is 450 to 1000mAh. Since the current is not large, linear chargers are 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 relatively large 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 charger with a charging current of 500-1000mA is used for charging, the charging time is too long. If a battery with a charging current of 3000mAh and 5400mA is charged at a charging rate of 0.5C, 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 CN3056 charger has been introduced in the December 2006 issue of this magazine and the 2007 Power Supplement ("Linear Lithium Secondary Battery Charger Chip CN3056"). Here is just a brief 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), and the circuit is 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; the battery is not installed or the contact is poor, and 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

　　Charger 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 size 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 current 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. In Figure 5, the red line is the voltage characteristic of the charging battery, the black line is the charging current characteristic, the solid line is the expansion characteristic, and the dotted line is the non-expansion 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.

　In Figure 2, two voltage dividers (detected voltage VBAT) are formed by R5, R6 and R7, R8, and the detected voltages are input into the window comparator composed of comparator P1 and comparator P2 respectively. R3 and R4 are pull-up resistors of P1 and P2 respectively, and D2 and D3 are isolation diodes. The charging battery voltage VBAT and the outputs of P1 and P2 and the working state of P-MOSFET are shown in Table 1.

　　Table 1 Charging battery voltage and P-MOSFET working state

　　As can be seen from Figure 2: the -VGS voltage of the P-MOSFET is provided by R2 and RP to D1, so the P-MOSFET should be always on after power-on. Now it is required that the P-MOSFET should be turned off when the battery voltage (VBAT) is less than 3.0V and greater than 4.15V. The control circuit should add a high level to the gate G of the P-MOSFET when VBAT<3.0V and VBAT>4.15V, so that -VGS=0.7V, which is less than the conduction threshold voltage -VGS(th), then the P-MOSFET is turned off (turned off). The window comparator composed of P1, P2 comparators and other components now realizes this control requirement: no matter whether P1 or P2 outputs a high level, VIN is added to the gate of the P-MOSFET through R4 or R3 and D3 or D2, forcing the gate voltage to be VIN=0.7V, then -VDS=0.7V and turned off, meeting the control requirements (see Figure 6). In the figure, D1, D2, and D3 are isolation diodes, which are essential for correct control.

　　Figure 6 Window comparator circuit

　　Power consumption and heat dissipation of P-MOSFET

　　1 Calculation of power consumption of P-MOSFET with expanded current tube

　　The power consumption PD of P-MOSFET during current expansion is related to the output voltage VIN, the battery voltage VBAT, the forward voltage drop VF of the Schottky diode and the current expansion current ID. The calculation formula is as follows:

　　PD=VIN-(VBAT+VF)×ID (1)

　　The maximum power consumption is at VIN(max) and VBAT(min), that is, when the current expansion starts (VBAT=3V), then the above formula can be written as:

　　PDmax=VIN(max)-(3V+VF)×ID (2)

　　If VIN(max) = 5.2V, when ID = 1A, VF = 0.4V, then PDmax = 1.8W. The maximum allowable power dissipation of the selected P-MOSFET should be greater than the calculated maximum power dissipation.

　　2 Heat dissipation of P-MOSFET

　　SMD power MOSFET uses the copper layer of the printed circuit board to dissipate heat, that is, a certain heat dissipation area should be reserved when designing the printed circuit board. For example, when the MTD2955E using the DPAK package is calculated to have PDmax=1.75W, a heat dissipation area of ​​11mm2 is required; if PDmax=3W, a heat dissipation area of ​​26mm2 is required. If a double-sided copper clad board is used (some metallized holes are made on the upper and lower layers to connect each other and use air circulation), its area can be reduced. If the heat dissipation is not good, the temperature of the power MOSFET rises, and the output of ID will rise with the increase in temperature. Therefore, sufficient heat dissipation should be taken seriously, and it is best to determine the appropriate heat dissipation area through experiments to make ID stable.

　　It should also be pointed out that P-MOSFETs in different packages have different heat dissipation areas at the same maximum power consumption. For example, when using Si99XXDY series P-MOSFETs in SO-8 package, the package size is small and there is no metal heat dissipation pad on the back, so its heat dissipation area is much larger than that of DPAK package. The specific heat dissipation area is determined by experiments.

　　Two types of power MOSFET

　　Two P-MOSFETs are introduced here: Si9933DY and MTD2955E.

　　1 Main parameters of Si9933DY and MTD2955E

　　The main parameters of Si9933DY and MTD2955E are shown in Table 2.

　　2 Pinout

　　The pinout of Si9933DY is shown in Figure 7, and the pinout of MTD2955E is shown in Figure 8.

　　Figure 7 Si9933DY pinout

　　Figure 8 MTD2955E pinout

　　3 Output Characteristics

　　When using Si9933DY, two MOSFETs can be connected in parallel to double the power and reduce PDS(ON) by half. When using Si9933DY, the current can be expanded to 1A. When using MTD2955E, the current can be expanded to 2A or more.

　　Figure 9 Si9933DY output curve

　　Figure 10 MTD2955E output curve

　　Conclusion

　　The above simple current expansion circuit can increase the charging current to 2-3A. However, since the current expansion tube works in a linear state, the tube consumption is large and the efficiency is 60%-70%. If a larger charging current is required, it can still be used, which can achieve higher efficiency.