Battery charging circuit

Battery Charging Circuit

In order for the UPS to really play a protective role when the mains power is interrupted, on the one hand, the UPS is required to have a good switching function and switching time, and at the same time, the battery configured by the UPS must have a good state of charge, otherwise it will be difficult to achieve the expected results.

Therefore, after each discharge of the UPS battery and during daily use, care must be taken to ensure that the battery in the UPS has a good state of charge, which requires UPS designers and users to consider the charging of the battery. Commonly used charging circuits for batteries include constant voltage charging and constant current first and then constant voltage charging.

Constant voltage charging circuit

In order to simplify the circuit and reduce the cost, the backup UPS often uses a constant voltage charging circuit composed of a step-down transformer, a rectifier bridge module, an integrated voltage regulator chip, a resistor, a potentiometer, and a capacitor. The circuit is shown in Figure 6-14.

Constant voltage charging circuit

After the mains power passes through the transformer B, it becomes a 27V alternating current voltage, which is transformed into a pulsating direct current voltage through the rectifier module and then into a smooth direct current voltage of 33V through the filter capacitor C. This DC voltage is turned into a stable and adjustable DC voltage through the integrated voltage regulator chip. The output voltage V0 of the integrated chip is:

The output voltage V0 of the integrated chip

If the backup UPS adopts two sealed batteries of 12V and 6Ah, the final discharge voltage of the battery will be 21V, and the charged voltage will be 27.5V. If the charging voltage is selected too high, the charging current at the initial stage of charging will be too large, which will easily damage the battery; if the charging voltage is selected too low, the charging current will be too small in the later stage of the charging, which will cause insufficient charging. Therefore, the final charging voltage is selected to be 26~27V. At this time, the initial stage of charging The charging current should not exceed 0.2C, and the charging current in the later stage of charging is close to 0.05C.

Constant current first and then constant voltage charging circuit

The first constant current and then constant voltage charging is also called hierarchical charging. There are many forms of this kind of circuit, this section chooses one of the circuits to discuss. The block diagram of the circuit is shown in Figure 6-15.

Block diagram of hierarchical charging circuit

The rectifier in the picture is used as a shared DC power supply for the inverter and charger. The rectifier not only provides stable voltage and smooth current to the inverter, but also provides variable current and voltage to the battery through the charger. When the mains power supply is normal, the voltage VB at both ends of the battery is less than the voltage VA at the output end of the rectifier, and the isolation diode VD is cut off. When the mains power supply is interrupted, the output voltage VA of the rectifier is zero, the isolation diode VD is turned on, and the storage battery provides the working voltage for the inverter.

The hierarchical charging circuit is shown in Figure 6-16. It is mainly composed of main circuit, pulse width modulator, current modulation circuit, voltage modulation circuit and protection circuit.

Hierarchical charging circuit

Main circuit

In Figure 6-16, the main charging loop is composed of rectifier bridge VD1~VD6, isolation diodes VD7, VD8, switch tube VT1, current sampling resistor R1, current sensor CS, inductor L, freewheeling diodes VD9, VD10. The simplified circuit is shown in Figure 6-17 (a). In the figure, Vd represents the output voltage of the three-phase rectifier bridge, E represents the voltage across the battery, VD represents the freewheeling diode, and VT represents the switch tube. The signal applied to the gate of the switch tube is shown in Figure 6-17(b).

Charging main circuit and its waveform

The working process is as follows: during t0 ~ t1, the gate of the switch tube VT plus the pulse, VT saturation conduction, V T =0, the voltage of the current limiting inductor L is:

the voltage of the current limiting inductor

i increases linearly with time, and its waveform is shown in Figure 6-17(c). It charges the battery and converts electrical energy into chemical energy for storage; at the same time, it converts electrical energy into magnetic energy and stores it in a current-limiting inductor.

During t1 to t2, the gate of VT with no pulse and VT cut-off, the current through the current-limiting inductance showed a linear decline, generating an antipotential at both ends with a polarity of left “+” right “-“. Due to eL> E, the continuation diode VD conduction, Vd≈0, the magnetic energy in the current-limiting inductor is converted to electric energy to charge the battery.

After that, the above process will be repeated with T1 as the cycle, and the output waveform is shown in Figure 6-17 (d).

It can be seen from the above that the function of the main circuit is to convert a stable DC voltage Vd into a pulse voltage, and a smooth current into a changing current. The average current through the battery cannot exceed 1/10 of the rated capacity of the battery.

The average value of the output voltage V0 of the main circuit is:

The average value of the output voltage V0 of the main circuit

It can be seen from the above formula that changing the duty cycle δ can change the output voltage, which also changes the charging current.

Pulse width modulator

In Figure 6-16, the pulse width modulator is composed of comparator U1. Its non-inverting terminal is connected to the modulation signal , and the inverting terminal is connected to the angular wave signal. The simplified circuit is shown in Figure 6-18 (a).

Pulse width modulation waveform

Unipolar triangle wave, is a direct current signal, and its working process is: when >, the output of the comparator is high; when <, the output of the comparator is low. The waveform of the output signal V0 of the comparator is shown in Figure 6-18 (c). It can be seen from the figure: increase, the pulsebecomes wider; decrease, the pulse becomes narrower.

Triangular wave generator

In Figure 6-16, the triangle wave generator is composed of a square wave generator, a frequency divider, a follower and an integrator. Its function is to generate a triangular wave with a frequency of 20kHz. The working process is as follows.

Suppose comparator U5, the non-inverting terminal is VTH (+), and the output terminal is high potential. This potential is charged to C1 through R7, and VC1 gradually rises. When VC1>VTH+, the output terminal of the comparator changes from high potential to low potential. The terminal jumps from VTH(+) to VTH(-) so C1 discharges through R7, and VC1 gradually drops. When VC1<VTH(-), the circuit jumps again. This cycle is repeated, and a square wave is obtained at the output of the comparator. The waveform is shown in Figure 6-19 (8). The frequency of the square wave is:

Triangular wave forming circuit waveform diagram

The square wave is added to the input end of the frequency divider through the inverter U6. The frequency divider is composed of a D flip-flop U7. The D terminal of the D trigger is connected with the two frequency divider; the CP terminal is connected to the output terminal of the inverter U6; the output terminal Q is connected to the non-inverting terminal of the follower through R8 and C2. The function of U7 is to change a 40kHz square wave into a 20kHz square wave.

The function of R8, C2, and R9 in the figure is to isolate the DC component in the 20kHz square wave, so that the unipolar square wave becomes a bipolar square wave. U8 uses a single power supply, and a 7.5V power supply is used as a bias voltage to be applied to the non-inverting end of U8 through R9.

The follower is composed of operational amplifier U8, and its task is to separate the frequency divider from the integrator.

The integrator consists of R10, C3, and U9. constitute. When the 20kHz square wave is at the positive maximum value, it will charge C3 through R10 and increase according to the linear law, and the U9 output terminal potential will decrease according to the linear law; when the 20kHz square wave is at the minimum value, C3 discharges through R10, and VC3 according to the linear law Decrease, U9 output terminal potential rises according to a linear law. The waveform is shown in Figure 6-19 (c). It can be seen from the figure that the triangle wave is obtained from the output of U9.

Closed loop current regulation system

In the early stage of staged charging, constant current charging is used, so a closed-loop current regulation system is required. In Figure 6-16, the closed-loop current regulation system is composed of the main loop, current detection circuit, current error amplifier circuit, pulse width modulator and drive circuit.

•Current detection circuit.
The current detection circuit is composed of operational amplifiers U10 and U11, resistors R11~R17, potentiometer VR1 and capacitor C4, U10 and R11~R14 constitute a differential amplifier. i is the corresponding voltage after the charging current I0 passes through the current sensor CS, that is, i=KI0. U10 uses a single power supply, and a 7.5V power supply is used as a bias voltage to be applied to the non-inverting end of U10 through R13. Since R11=R12=R13=R14, the output voltage of the differential amplifier υ10 is:

The waveform is shown in Figure 6-19 (8). The frequency of the square wave

It can be seen from the above that the differential amplifier only plays a role of isolation. In the figure, U11, R15~R17, VR1, and C4 constitute an inverting amplifier. 7.5V is the bias voltage of U11; the voltage taken from VR1 is used to suppress the offset of U11; C4 can suppress the high frequency oscillation of U11. The output voltage of the inverting amplifier is:

The output voltage of the inverting amplifier

It can be seen from the above formula that the current detection signal 11 is proportional to the charging current.

•Current error amplifier circuit. The current error amplifier circuit is composed of U12, R18~R21, C5 and a given voltage -5.1V. 7.5V is the bias voltage of U15. Its output voltage 12 is:

Its output voltage

It can be seen from the above formula that υ12 decreases as υ11 increases
•Drive circuit.
In Figure 6-16, the drive circuit is composed of two transistors VT2 and VT3, and its working process is as follows.

The output terminal of the photocoupler U4 is “1”, the transistor VT2 is turned on, and VT3 is turned off. The 15V power supply charges the input capacitor Cinput of the switch tube VT1 through the transistor VT2. When VC input>Vth, the switch tube VT1 turns from cut-off to conduction and rises with VC input, and VT1 enters the saturation zone from the amplification zone. The output terminal of the photocoupler U4 is “0”, the product tube VT3 is turned on, VT2 is turned off, and the input capacitor C is discharged and reverse charged through R2, VT3, and voltage regulator tube VD20. When VC input<Vth, the switch tube VT1 turns from on to off.
The function of the drive circuit is to amplify the power of the pulse.

•Adjustment process.
Due to some reason, the charging current increases → the output voltage υi of the current sensor CS increases → the current detection signal υ11 increases → the current error amplification signal υ12 decreases accordingly → the pulse width modulator output pulse becomes narrow → the switch tube VT1 The on-time is shortened → the charging current is reduced. Therefore, the charging current is maintained unchanged, and the battery is charged with a constant current. In order to prevent the parasitic oscillation of the closed-loop current regulation system, PI correction links R20 and C5 are specially set in the current error amplifier circuit.

Closed loop voltage regulation system

In the later stage of hierarchical charging, constant voltage charging is used, so a closed-loop voltage regulation system is required. It shares the main loop, pulse width modulator and drive circuit with the closed-loop current regulation system: it differs from the closed-loop current regulation system in that it uses a voltage detection circuit and a voltage error amplifier circuit.

•Voltage detection circuit. In Figure 6-16, the voltage detection circuit is a two-stage inverting amplifier composed of U13~U14, R22~R28, and C6~C8. The voltage E at both ends of the battery is added to the ends of C6 and C7 through R22 and R23, and becomes C6 and C7. C6 is added to the inverting terminal of U13 through R24; υC7 is added to the inverting terminal of U14 through R25. The output voltage of U13 is:

The output voltage of U13

The relationship between the voltage across the capacitor and the voltage E across the battery is

The relationship between the voltage across the capacitor and the voltage E across the battery

Voltage detection signal 14 is proportional to the voltage E at both ends of the battery

In the formula, C is the constant of proportionality.

It can be seen from the above formula that the voltage detection signal υ14 is proportional to the voltage E at both ends of the battery.
•Voltage error amplifier circuit. In Figure 6-16, the voltage attenuation amplifier circuit is composed of U15, R29~R32, C9 and a given voltage of -5.1V. Its output voltage is:

U15 decreases as U14 increases

It can be seen from the above formula that υ15 decreases as υ14 increases.
•Adjustment process. For some reason, the output voltage of the main charging loop increases → the voltage across the battery increases → the detection voltage υ14 increases accordingly → the output signal υ15 of the voltage error amplifier decreases → the pulse width modulator output pulse becomes narrow → the main charging The loop output voltage υ0 decreases, so the charging circuit charges the battery with a constant voltage.

The role of isolation diodes

The current error amplification signal υ12 is added to the input end of the pulse width modulator through the isolation diode VD11, and the voltage error amplification signal is added to the input end of the pulse width modulator through the isolation diode VD12. At the initial stage of charging, the voltage across the battery is low, the voltage detection signal υ14 is also low, and the voltage error amplification signal v15 is relatively high. At this time, υ15 is greater than the current error amplification signal υ12. The isolation diode VD11 is turned on, and the isolation diode VD12 is turned off.

The current error amplified signal is added to the input end of the pulse modulator through the isolation diode VD11, and the pulse width modulator output pulse degree is controlled by the current signal. In the later stage of charging, the voltage across the battery is high, the voltage detection signal υ14 is also high, and the voltage error amplification signal15 is relatively low. At this time, υ15 is smaller than the current error amplification signal υ12. VD12 turns on and VD11 turns off. The voltage error amplification signal is added to the input end of the pulse width modulator through VD12. The pulse width modulator output pulse width is controlled by the voltage signal.

Overcurrent protection

The function of the overcurrent protection circuit is to prevent excessive charging current from damaging the battery during charging. In Figure 6-16, the overcurrent protection circuit is composed of a differential amplifier, a comparator and a control gate.

•Differential amplifier. The differential amplifier is composed of U16, R34~R35, C10~C11. When the charging current flows through the resistor R1 (0.5Ω), a corresponding voltage υi0 is generated at both ends of R1. This voltage is applied to both ends of C10 and C11 through R33~R34 to become v10 and υ11 、υ10 is added to the inverting terminal of U16 through R35, and 11 is added to the non-inverting terminal of U16 through R36. 7.5V is the bias voltage of U16. Since R35=R36=R37=R41, the output voltage of U16 is:

The overcurrent detection signal is proportional to the charging current

It can be seen that the overcurrent detection signal is proportional to the charging current.

•Comparator. The comparator is composed of U17, R38~R40, R46, and C12. After υ16 is added to the 15V power supply at the inverting terminal of the comparator through R39, and the 7.5V power supply is divided by R38 and R46, the comparison voltage VK is obtained. VK is added to the non-inverting end of comparator U17 through R40.

When the charging current is too large, V16>VK, the output terminal of the comparator is “0”, and the output terminal of the inverter U18 is “1”. It becomes a differential wave through C13 and R49 and added to the R terminal of the D flip-flop U19. The output terminal Q of the trigger is “0”, the NAND gate U2 is blocked, the pulse width modulator output pulse cannot be added to the input terminal of the drive circuit through U2, the switch tube T1 is cut off, and the charging stops.

When the charging current is normal, V16<VK, the output terminal of the comparator is “1”, the output terminal of the NOT gate U18 is “0”, and the D flip-flop is under the action of a clock pulse with a frequency of 20kHz, and its output terminal Q is “l”. The NAND gate U2 is opened, and the pulse width modulator output pulse is added to the gate of the switch tube VT1 through U1 and the drive circuit, VT1 works normally, and the charging circuit works normally.

Soft start

In Figure 6-16, the soft start circuit is composed of counter U22, NAND gate U21, NOT gates U20 and U23, resistors R42~R45 and R50~R52, potentiometer VR2, diodes VD14~VD19, and voltage regulator tube VD13.

In the figure, vi1 is a full-wave rectified voltage with a frequency of 100 Hz. vi1 passes through the resistor R43 and the voltage regulator tube VD13, and obtains the trapezoidal wave voltage at VD13, which is added to the CP terminal of U22 through the NAND gate U21. VD13 becomes a DC voltage through diode VD19, capacitor C14, and resistor R44, and this voltage is added to the R terminal of U22 through a non-gate U20. On the one hand, the output terminal Q10 of U22 is added to the input terminal of the NAND gate U21 through the NAND gate U23; on the other hand, it is connected to the D terminal of the D flip-flop through the diode VD14.

When the output Q10 of the counter U22 is “0”, the output of the NOT gate U23 is “1”. This potential is applied to the input terminal of the NAND gate U21. When the rising and falling edges of both ends of the voltage regulator tube VD13 arrive, the output terminal of the NAND gate U21 is “1”; when the voltage across the VD is its working voltage VZ, the output terminal of the NAND gate U21 is “0”, and its waveform is as follows As shown in Figure 6-20(c).

Triangular wave forming circuit waveform diagram.

The working process of the circuit is as follows:

At the moment of turning on, the voltage across capacitor C14 is “0”, and the output terminal of NOT gate U20 is “1”. The counter U22 is cleared, and its Q10 terminal is “0”. The diode VD14 is turned on, the D terminal of the D flip-flop is “0”, the output Q of the D flip-flop U19 is also “0” under the action of the clock pulse, and the NAND gate U2 is blocked.

Since Q10 of U2 is “0” and diodes VD15 and VD16 are turned on, point B in the circuit is “0”, and the voltage across capacitor C15 is suppressed to about 0V. V+ The voltage across the voltage regulator tube VD13 charges the capacitor C14 through the diode VD19, and VD14 gradually rises. When VD14 is greater than the turning voltage of the inverter U20, the output terminal of the inverter U20 changes from “1” to “0”, that is, the R terminal of the counter U22 is “0”. Under the action of the clock pulse, the counter U20 starts to count. After a delay of 29×10ms, its Q10 terminal changes from “0” to “1”.

Since the Q10 terminal is “1”, the diode VD14 is cut off, and the D terminal of the D flip-flop is “1”. Under the action of the clock pulse, the output terminal of the D flip-flop U19 changes from “0” to “1”, and the NAND gate U2 Was opened.

Since the Q10 terminal is “1”, the diode VD15 is off, and the 15V power supply charges C15 through R47, so the potential at point B in the circuit rises as VD15 rises. The pulse width output by the pulse width modulator U1 gradually widens with the extension of time, and the charging current gradually increases, thus preventing the excessive charging current from damaging the battery during startup.

Charge controller with smart chip

At present, semiconductor manufacturers in various countries in the world have launched many special integrated circuits for lead-acid battery chargers. The representative bq2031 is selected as a brief introduction below.

The charger composed of the single-chip COMS integrated circuit bq2031 can automatically switch the charging state according to the battery charging voltage and charging current, thereby completing the charging control of the lead-acid battery.

The internal block diagram of bq2031 is shown in Figure 6-21.

Block diagram of bq2031

It is composed of power-on reset circuit, temperature compensation reference voltage, maximum time timer, charge state controller, voltage/current regulator, oscillator and display control circuit.

The pin arrangement and function of bq2031

bq2031 adopts 16-pin DIP package or SOIC package, and the pin arrangement is shown in Figure 6-22.

bq2031 pin arrangement

The names and functions of each foot are shown in Table 6-5.

bq2031 pin function table

The main functions of bq2031

The connection of the constant voltage/constant current selection pin QSEL and the fast charge termination method selection pin TSEL is different. The bq2031 can use a variety of charging modes and fast charging termination methods. When adopting two-stage constant voltage and current-limiting charging mode, bq2031 keeps the battery voltage at a constant value that has nothing to do with the state of charge. The charging curve and various voltage thresholds of the two-stage constant-voltage current-limiting charging mode are shown in Figure 6-23.

Two-stage constant voltage and current limiting charging

When the power is applied to the VCC pin and the battery is connected, the charging process starts. The bq2031 is always detecting the temperature of the battery. When the TS pin voltage is between LTF (low temperature fault) and HTF (high temperature fault), the charger enters the pre-charging process. During this process, the charging current maintains a small constant value (Icond) and the battery voltage rises rapidly. When the voltage of the single battery rises to the lowest voltage Vmin allowed for fast charging, the charger switches to the fast charging state. In the fast charging state, the charging current is limited to the maximum value Imx. When the voltage of the single battery reaches 0.94 times the fast charge termination voltage Vblk, the capacity of the battery has reached more than 80% of the rated capacity, at this time, the charger enters the complementary charging state.

In the complementary charging state, the current-limiting charging continues until the single battery voltage vc is equal to the fast charging termination voltage Vblk. Then the battery voltage stabilizes at Vblk, and the charging current gradually decreases exponentially. When the charging current (iSNS) drops to the externally set complementary charging minimum current Imim, the complementary charging state ends. When using the constant voltage current limiting charging mode, the charging safety timer (MTO) can terminate the fast charging and supplement the charging state. After entering the complementary charging state, the charging timer (MTO) is reset. After the specified time is reached, the MTO terminates the complementary charging and the charger enters the maintenance charging state. In this state, the battery voltage is maintained at the float voltage (Vflt).

The advantage of the constant voltage charging mode is that it can automatically adjust the charging current of the battery according to the charging state of the battery, and all voltage values have temperature compensation. The constant voltage charging mode is suitable for cyclic charging and floating charging. In the fast charging state, the charger outputs a higher constant voltage, and then drops to the temperature compensated float voltage.

The charging curve of the two-stage constant current charging mode is shown in Figure 6-24.

Charging curve of two-stage constant current charging mode

In the constant current charging mode, after precharging, the charger charges the battery at a higher charging rate (charging current is Imax) until the battery voltage rises to close to the full charge voltage. When the voltage vcell of the single battery is equal to or greater than the fast charge termination voltage Vblk, the fast charge is terminated, and then the maintenance charge state is transferred. In this state, the charging current stabilizes at the trickle charging current Imin.

The charging curve of the constant current pulse charging mode is shown in Figure 6-25.

The charging curve of the constant current pulse charging mode

When this charging mode is adopted, the current-limiting fast charging state is the same as the above-mentioned constant-current charging mode, but the charging current is not a continuous current. During the charging process, when the voltage vcell of the single battery exceeds the fast charge termination voltage Vblk, or when the second-order increment of the single battery voltage ∆2V<0, the fast charge state is immediately terminated and the charger enters the maintenance state.

In this state, the charging current is zero (iSNS=0), so the battery voltage begins to drop. When the voltage of the single battery is lower than or equal to the set float voltage Vm1, the fast charging state restarts, and the voltage m of the single battery rises rapidly, when vs is equal to the second-order increment of Vhk or vcn ∆2V<0 When the charger is in the maintenance state again, the single battery voltage vcell slowly drops to the float voltage Vflt. Thereafter, repeat the above process. During the entire charging process, the width of the fast charging current skin impulse gradually decreases, and the duration of maintaining the charging state gradually increases. It should be noted that in the fast charging state, the timer (MTO) can terminate the fast charging state.

The main parameters of bq2031

•Limit parameters. The limit parameter values of bq2031 are shown in Table 6-6

Limit parameters of bq2031
•DC threshold voltage. When the ambient temperature TA is within the specified range and the power supply voltage VCC=5V±1%, the DC threshold voltages are shown in Table 6-7.

DC threshold voltage value

 

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