A low-cost, high-reliability battery charger for electric vehicles

A low-cost, high-reliability battery charger for electric vehicles

According to the characteristics of the lead-acid battery of the electric bicycle, when it is 36V/12AH, the voltage-limited constant-current charging method is adopted, and the maximum initial charging current should not exceed 3A. In other words, the maximum output of the charger reaches 43V/3A/129W, which is sufficient. During the charging process, the charging current will gradually decrease. In terms of the current switching power supply technology and the production level of switch tubes, the output power limit of the single-ended switching regulator has been increased to 180W, or even greater. The reliability of the single-ended self-excited switching regulator with an output power of less than 150W has reached an extremely high level. The application of MOS FET switch tubes has successfully solved the problem of secondary breakdown of switch tubes, making the reliability of switching power supplies even higher.
At present, the most widely used and earliest single-ended driver that can directly drive MOS FET switch tubes is MC3842. MC3842 has the function of load current control while stabilizing the output voltage, so it is often called a current-controlled switching power supply driver. This function undoubtedly has unique advantages when used in chargers. It can achieve constant voltage output with only a few peripheral components and can also control the charging current. In particular, the MC3842 can directly drive the MOS FET tube, which can greatly improve the reliability of the charger. Due to the wide application of MC3842, this article only introduces its characteristics.

MC3842 is a dual-row 8-pin single-ended output self-excited switching power supply driver integrated circuit. Its internal functions include: reference voltage regulator, error amplifier, pulse width comparator, latch, oscillator, pulse width modulator (PWM), pulse output driver stage, etc. There are many similar products of MC3842, among which the interchangeable ones are UC3842, IR3842N, SG3842, CM3842 (domestic), LM3842, etc. The internal block diagram of MC3842 is shown in Figure 1. Its characteristics are as follows:
single-ended PWM pulse output, output drive current is 200mA, and peak current can reach 1A.
The starting voltage is greater than 16V, and the starting current is only 1mA to enter the working state. After entering the working state, the working voltage is between 10 and 34V, and the load current is 15mA. Exceeding the normal working voltage, the switching power supply enters the undervoltage or overvoltage protection state, and the integrated circuit has no drive pulse output at this time. A
5V/50mA reference voltage source is built in, which is used as the sampling reference voltage after 2:1 voltage division.
The output drive pulse can drive both bipolar transistors and MOS field effect tubes. If driving a bipolar transistor, it is advisable to connect an RC cutoff acceleration circuit to the base of the switch tube, and limit the frequency of the oscillator to below 40kHz. If driving a MOS field effect tube, the oscillation frequency is set by an external RC circuit, and the operating frequency can reach up to 500kHz.
There are two pulse modulation (PWM) control terminals: overcurrent protection input (pin 3) and error amplifier input (pin 1). The error amplifier input terminal constitutes the main pulse width modulation (PWM) control system. The overcurrent detection input can control the pulses one by one, directly control the pulse width of each cycle, and make the output voltage regulation rate reach 0.01%/V. If the voltage of pin 3 is greater than 1V or the voltage of pin 1 is less than 1V, the pulse width modulation comparator outputs a high level to reset the latch, and it will not be reset until the next pulse arrives. If the level relationship between pins 1 and 3 is used to control the opening/closing of the latch in the external circuit, so that the latch only outputs a trigger pulse once per cycle, the anti-interference ability of the circuit will undoubtedly be enhanced, the switch tube will not be triggered by mistake, and the reliability will be improved.
The frequency of the internal oscillator is set by the external resistor and capacitor connected to pins 4 and 8. At the same time, the internal reference voltage is introduced into the external synchronization through pin 4. The external resistor and capacitor connected to pins 4 and 8 constitute a timing circuit, and the charging/discharging process of the capacitor constitutes an oscillation cycle. When the setting value of the resistor is greater than 5kΩ, the charging time of the capacitor is much greater than the discharging time, and its oscillation frequency can be approximated according to the formula: f = 1/Tc = 1/0.55RC = 1.8/RC.
The lead-acid battery charger with an output power of up to 120W composed of MC3842 is shown in Figure 2. In this charger, only the switching frequency part is hot ground, while the drive control system composed of MC3842 and the switching power supply output charging part are cold ground. The two grounding circuits are isolated by the input and output transformers. The transformer is not only simple in structure, but also easy to achieve the primary and secondary AC 2000V dielectric strength. The output voltage of this charger is set to 43V/1.8A. If necessary, the current can be adjusted to 3A for charging large-capacity lead-acid batteries (such as for charging batteries with a capacity of 30AH).
After the mains input is bridge rectified, a DC voltage of about 300V is formed, so the requirements for this rectifier and filter circuit are different from the usual ones. For battery chargers, it is not necessary to filter out the 100Hz pulsating current of the bridge rectifier. Strictly speaking, the 100Hz pulsating current is not only harmless to battery charging, but also beneficial. To a certain extent, it can play the role of pulse charging, so that the chemical reaction of the battery during the charging process has a chance to buffer and prevent the plate sulfation caused by continuous high-current charging. Although the initial charging current of 1.8A is greater than 1/10 of the rated capacity C of the battery, the intermittent large current also alleviates the temperature rise of the battery. Therefore, the C905 of the filter circuit uses a 47μF/400V electrolytic capacitor, which is not enough to filter out the ripple in the 120W load of the rectifier, but only reduces the output impedance of the rectifier power supply to reduce the loss of the switching circuit pulse in the power supply circuit. The reduction in the capacity of C905 reduces the output voltage of the rectifier to about 280V when it is fully loaded.
U903 is used as a single-ended output driver according to the typical application circuit of MC3842. The functions of its pins and the selection principles of peripheral components are as follows (see Figures 1 and 2).

Pin 1 is the output of the internal error amplifier. The error voltage is level-shifted by D1 and D2 inside the IC, and divided by R1 and R2 before being sent to the inverting input of the current control comparator to control the PWM latch. When pin 1 is at a low level, the latch is reset and the drive pulse output is turned off. It is not reset until the next oscillation cycle begins, and the pulse output is restored. The external circuit is connected to R913 (10kΩ) and C913 (0.1μF) to correct the frequency and phase characteristics of the amplifier.
Pin 2 is the inverting input of the internal error amplifier. When the charger is charging normally, the maximum output voltage is 43V. After the external circuit is divided by R934 (16kΩ), VR902 (470Ω), and R904 (1kΩ), a 2.5V sampling voltage is obtained, which is compared with the 2.5V reference voltage at the in-phase input of the error amplifier to detect the difference, and the output voltage is limited to 43V by controlling the output pulse duty cycle. When adjusting this voltage, the charger can be unloaded. Adjust VR902 to make the voltage of the positive and negative output terminals 43V.
Pin 3 is the charging current control terminal. Within the output voltage range set by pin 2, the charging current is controlled by R902. The action threshold of pin 3 is 1V. Within the voltage drop of R902 of 1V, the output voltage change is controlled by the internal comparator to achieve constant current charging. The constant current value is 1.8A, and R902 uses 0.56Ω/3W. When the charging voltage is limited to 43V, the charging current can be adjusted to a constant 1.75A~1.8A through the output voltage. When the battery is fully charged, the terminal voltage is ≥43V, the isolation diode D908 is cut off, there is no current in R902, the voltage of pin 3 is 0V, the constant current control is invalid, and the charging voltage is controlled by the sampling voltage of pin 2 not exceeding 43V. At this time, if it is fully charged, if the power is not cut off, a trickle charge of 43V voltage will be formed to keep the battery voltage at 43V. To prevent overcharging, the voltage upper limit of the 36V lead-acid battery should not make the battery cell voltage exceed 2.38V. Although this circuit is a battery sampling circuit, it actually limits the output voltage. If the output voltage exceeds the battery voltage by 0.6V, the battery voltage will also increase and be sent to the voltage sampling circuit to reduce it.
The 4th pin is connected to the external oscillator timing element, CT is 2200pF, RT is 27kΩ, and R911 is 10Ω. In this example, considering the difficulty in purchasing high-frequency magnetic cores, the frequency is set to about 30kHz. R911 is used for external synchronization and can be omitted in this circuit.
The 5th pin is the common ground terminal.
The 6th pin is the drive pulse output terminal. In order to achieve isolation from the mains, the switch tube is driven by T902. T902 can use a 5×5mm magnetic core, and the primary and secondary windings are each wound with 0.21mm enameled wire for 20 turns, and the windings are insulated with 2×0.05mm polyester film. R909 is 100Ω and R907 is 10kΩ. If there is no protection diode for the gate-source of Q901, a 10-15V voltage regulator can be added to the external circuit.
Pin 7 is the power supply terminal. In order to save the independent power supply circuit, the circuit is powered by the voltage of the battery terminal, and the power supply voltage is 18V. When the battery to be charged is connected, the minimum voltage is between 32.4V and 35V, and a stable voltage of 18V can be obtained by connecting an 18V voltage regulator. The filter capacitor C909 is 100μF.
Pin 8 is the 5V reference voltage output terminal, and at the same time, the voltage is divided into 2.5V by R3 and R4 inside the IC, which is used as the error detection reference voltage.
The pulse transformer T901 of the charger can use a commercially available core with a round core and a diameter of 12mm (a 1mm air gap is already provided at the core joint). The primary winding is wound with 82 turns of 0.64mm high-strength enameled wire, and the secondary winding is wound with 50 turns of 0.64mm high-strength enameled wire in parallel. Three layers of polyester film need to be placed between the primary and secondary.
The control drive system and secondary charging system of the charger are isolated from the mains, and the MC3842 is powered by the voltage of the battery to be charged, so there is no possibility of overvoltage or overcurrent. The T901 secondary has only a few components, and as long as they are qualified, the possibility of breakdown is almost zero, so its reliability is extremely high. The diode D911 in this part can choose common cathode or common anode, and the Schottky diode can be used in parallel. D908 can choose an ordinary diode with a rated current of 5A. It is sufficient to use a 220μF filter capacitor for the secondary rectifier circuit to have a certain ripple when the initial charging current is large, so as to play the role of pulse charging.
The charger circuit is extremely simple, but the reliability is high. The reason is that: MC3842 is a cycle-by-cycle control oscillator, which controls the voltage and current in each conduction cycle of the switch tube. Once the load is overcurrent, D911 will leak and break down; if the battery terminal is short-circuited, the voltage of the third foot will be higher than 1V, and the drive pulse will stop output immediately; if the sampling voltage of the second foot exceeds 2.5V due to the increase of the output voltage, the voltage of the first foot will be lower than 1V, and the drive pulse will also be turned off. For many years, MC3942 has been widely used in computer monitor switching power supply drivers. No matter what the situation is (its own damage or peripheral component failure), it will not cause the output voltage to increase, but there is no output or the output voltage is reduced. This feature makes the load circuit of the switching power supply extremely safe. In this charger, MC3842 and its external circuit are not related to the mains input part, and the battery voltage is used to power it after voltage reduction and voltage stabilization, so its failure rate is almost zero.
The only circuit related to the AC input in this charger is the switch circuit between the primary of T901 and the secondary of T902. There are two common reasons for the damage of the switch tube: First, when a bipolar switch tube is used, the temperature rises and causes thermal breakdown. This does not exist for the negative temperature coefficient characteristic of Q901. The resistance characteristic of the drain-source conduction of the field effect tube itself has the ability to balance its conduction current. In addition, since the reverse voltage of the switch tube is too high, when the switch tube is cut off, the peak of the reverse pulse is very easy to break through the switch tube. For this reason, the capacity of C905 is reduced in this circuit to appropriately reduce the rectifier voltage under the high current state of the switch tube conduction. Second, a ferrite core with a circular center column is used, and its leakage inductance is relatively smaller than that of a rectangular cross-section core, and the air gap is reserved in the center column, not on the side columns on both sides, which further reduces the leakage inductance. Under this condition, it is safer to use a switch tube with a higher VDS. In Figure 2, Q901 is 2SK1539, with a VDS of 900V, an IDS of 10A, and a power of 150W. Other MOS FET tubes with similar specifications can also be used as substitutes. If you are worried about the peak pulse breaking through the switch tube, you can connect the usual C, D, and R absorption circuits to the primary of T901. Since the initial charging current and the maximum charging voltage of the charger are designed to be at a relatively low value, and the trickle charging current is extremely small after full charge, it can basically be considered as timed charging. For example, a 12A lead-acid battery can be fully charged in 7 hours, and after full charge, whether the power is turned off or not has little effect on the battery and charger. During the trial, the power was connected to the power supply for charging at 8 pm, and the power was turned off at 7 am the next day. The shell temperature of the battery and charger did not exceed the room temperature when touched by hand.