Battery car lead-acid battery charger circuit - Power Circuit - Circuit Diagram

At present, one of the most commonly used and original single-ended drivers capable of directly driving MOS FET switches is the MC3842. This driver not only stabilizes the output voltage but also features a load current control function. It is often referred to as a current-controlled switching power supply driver, offering a unique advantage for chargers. With just a few external components, it can control the output while managing the charging current. Specifically, the MC3842's capability to directly drive MOS FETs significantly enhances the reliability of the charger. Given its widespread application, more detailed information about the MC3842, including its pin configuration, features, and functions, can be found elsewhere.

A lead-acid battery charger utilizing MC3842 and delivering up to 120W of output power is illustrated in Figure 2. The switching frequency portion of the charger is connected to the hot ground, whereas the drive control system made up of MC3842 and the output charging part of the switching power supply is connected to the cold ground. These two grounding circuits are isolated via the input and output transformers. Not only is the transformer structurally simple, but it is also straightforward to implement. With a primary-to-secondary insulation strength of 2000V, the charger’s output voltage is set to 43V/1.8A. If needed, the current can be adjusted to 3A for charging a large-capacity lead-acid battery (suitable for batteries with a capacity of 30AH).

Charger Circuit and Principle Introduction:

After the AC input is bridge rectified, a DC voltage of approximately 300V is generated. Thus, the requirements for the rectifying and filtering circuit differ from those of the current. For a battery charger, the 100Hz pulsating current from the bridge rectification does not need to be filtered out. In fact, strictly speaking, this pulsating current is not only harmless but also beneficial to battery charging, playing a role in pulse charging to some extent. This allows the chemical reaction of the battery to have a buffering opportunity, preventing plate sulfation caused by continuous high current charging. Although the initial charging current of 1.8A is greater than 1/10 of the battery's rated capacity C, the intermittent high current helps relieve the temperature rise of the battery. Therefore, the C905 filter circuit uses a 47μF/400V electrolytic capacitor, which is not enough to filter the ripple in the 120W rectifier load but reduces the output impedance of the rectified power supply to minimize pulse losses in the power supply circuit. By reducing the capacity of C905, the output voltage of the rectifier drops to about 280V under full load. U903 uses the typical application circuit of MC3842 as a single-ended output driver. The principles of each pin's function and the selection of peripheral components are as follows (refer to Figure 1 and Figure 2).

Pin 1 is the output of the internal error amplifier. Inside the IC, the error voltage is shifted by the D1 and D2 levels. After division by R1 and R2, it is sent to the inverting input of the current control comparator to control the PWM latch. When pin 1 is low, the latch resets, turning off the drive pulse output, which is not reset until the start of the next oscillation cycle, restoring the pulse output. The external circuit is connected to R913 (10kΩ) and C913 (0.1μF) to correct the amplifier's frequency and phase characteristics.

Pin 2 is the inverting input of the internal error amplifier. When the charger is charging normally, the maximum output voltage is 43V. The external circuit divides this by R934 (16kΩ), VR902 (470Ω), and R904 (1kΩ) to obtain a sampling voltage of 2.5V, which is compared with the 2.5V reference voltage of the non-inverting input of the error amplifier, detecting any difference. The control ensures that the output voltage remains at 43V. When adjusting this voltage, the charger can be left unloaded. Adjust VR902 to ensure the positive and negative output voltages reach 43V.

Pin 3 is the charging current control terminal. Within the output voltage range set by the second pin, the charging current is controlled by R902. The operating threshold of the third pin is 1V. Within the voltage drop across R902, the output voltage is controlled by the internal comparator to achieve constant current charging. The constant current value is 1.8A, with R902 being 0.56Ω/3W. When the charging voltage is limited to 43V, the charging current can be adjusted to be constant from 1.75A to 1.8A by the output voltage. When the battery is fully charged, the terminal voltage is ≥43V, isolating diode D908 is cut off, there is no current in R902, the voltage of the third pin is 0V, the constant current control becomes invalid, and the charging voltage is controlled by the second pin to not exceed 43V. At this point, if it is fully charged and there is no power failure, a trickle charge of 43V voltage forms, maintaining the battery voltage at 43V. To prevent overcharging, this voltage upper limit of the 36V lead-acid battery should not cause the cell voltage to exceed 2.38V. Although the circuit samples the battery, it actually limits the output voltage. If the output voltage exceeds the battery voltage by 0.6V, the battery voltage also rises and is sent to the voltage sampling circuit to reduce it.

Pin 4 is the external oscillator timing component, with CT being 2200pF, RT being 27kΩ, and R911 being 10Ω. In this example, since the high-frequency magnetic core is difficult to purchase, the frequency is set to about 30kHz. R911 is used for external synchronization and can be utilized in this circuit.

Pin 5 is the common ground.

Pin 6 is the drive pulse output. To achieve isolation from the mains, the switch tube is driven by T902. T902 can use a 5×5mm magnetic core, with the primary and secondary windings wound by 200 turns using 0.21mm enamel-coated wire, and the windings are insulated with 2×0.05mm polyester film. R909 is 100Ω and R907 is 10kΩ. If the Q901 internal gate-source has no protection diode, a 10~15V Zener diode can be incorporated in the external circuit.

Pin 7 is the power supply terminal. To save the independent power supply circuit, the circuit is stepped down by the battery terminal voltage, with the power supply voltage being 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 the 18V voltage regulator. The filter capacitor C909 is 100μF.

Pin 8 is the 5V reference voltage output terminal, divided into 2.5V by R3 and R4 inside the IC as the error detection reference voltage.

The pulse transformer T901 of the charger can use a commercially available core with a circular core and a diameter of 12mm (the air gap of the core post is already provided with a 1mm air gap). The primary winding is wound with 0.64mm high-strength enamel wire, and the secondary winding is wound with 0.64mm high-strength enamel wire and wound around 50 turns. Three layers of polyester film are required between the primary and secondary.

The charger's control drive system and secondary charging system are isolated from the mains, and the MC3842 is powered by the battery voltage to be charged, eliminating the possibility of overvoltage and overcurrent. While the T901 secondary has only a few components, as long as the choices are qualified, the possibility of breakdown is almost zero, making its reliability extremely high. This part of the diode D911 can be selected as a common cathode or common anode, and the Schottky diode is applied in parallel. The D908 is available with a common diode rated at 5A. It is sufficient to select 220μF for the secondary rectifier circuit filter capacitor, allowing the initial charging current to have a certain ripple when it is large, acting as a pulse charge.

The charger circuit is extremely simple yet highly reliable. The reason is that the MC3842 is a cycle-by-cycle control oscillator, controlling voltage and current during each conduction period of the switch tube. Once the load is overcurrent, D911 leaks or wears; if the battery terminal is short-circuited, the voltage of the third pin must be higher than 1V, immediately stopping the drive pulse output; if the output voltage of the second pin rises above 2.5V due to the output voltage, the voltage of the first pin is lower than 1V, also turning off the drive pulse. For many years, the MC3942 has been widely used in computer display switching power supply drivers. Under no circumstances (whether its own damage or peripheral component failure) will it cause the output voltage to rise, but rather result in no output or reduced output voltage. This feature makes the switching power supply load circuit extremely safe. In the charger, the MC3842 and its external circuits are independent of the mains input section. Additionally, the battery voltage is stepped down and stabilized to supply power, resulting in an almost zero failure rate.

The only circuit in the charger related to the mains input is the switching circuit between the T901 primary and the T902 secondary. The common causes of switching tube damage are two: First, when a bipolar switching tube is used, thermal breakdown occurs due to rising temperature. This point does not exist for the negative temperature coefficient characteristic of Q901. The resistance characteristic of the drain-source conduction of the field-effect transistor itself has the ability to balance its on-current. Furthermore, since the back pressure of the switching tube is too high, when the switching tube is turned off, the spike of the reverse pulse is extremely easy to break through the switching tube. For this reason, in the circuit, by reducing the capacity of C905, the rectified voltage is appropriately lowered in a large current state in which the switching transistor is turned on. The second reason is using a ferrite core with a round-shaped central column. The leakage inductance is relatively smaller than that of a rectangular cross-section core, and an air gap is reserved in the center column instead of the side columns on both sides, further reducing the leakage inductance. It is safer to use a switch with a higher VDS under these conditions. In Figure 2, Q901 is 2SK1539, which has a VDS of 900V, an IDS of 10A, and a power of 150W. It can also be replaced with other MOS FET tubes of similar specifications. If you are concerned that the spike will break through the switch, you can incorporate the standard C, D, R absorption loop at the primary of the T901. Since the initial charging current and the maximum charging voltage of the charger are designed to be at a low value, and the trickle charging current is extremely small after being fully charged, it can be essentially regarded as timed charging. For example, a lead-acid battery at 12A can be fully charged in 7 hours, and after full charge, whether the power is cut off has little effect on the battery and the charger. During testing, the power supply was charged at 8:00 PM and the power was turned off at 7:00 AM the next morning. The temperature of the battery and charger casing did not exceed room temperature.