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

Photocoupler

Currently, the most widely used and original single-ended driver that can directly drive MOS FET switches is the MC3842. The MC3842 also has a load current control function while stabilizing the output voltage. It is often referred to as a current-controlled switching power supply driver. It is undoubtedly a unique advantage for the charger. It can be realized with only a few external components. The output is controlled while controlling the charging current. In particular, the MC3842 can directly drive the characteristics of the MOS FET tube, which can greatly improve the reliability of the charger. Due to the wide application of MC3842, please refer to: MC3842 pin, feature and function description for details.

A lead-acid battery charger with an output power of up to 120W consisting of MC3842 is shown in Figure 2. Only the switching frequency part of the charger is hot ground. The drive control system composed of MC3842 and the output charging part of the switching power supply 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 implement. The primary and secondary exchanges have an electric strength of 2000V. The charger output voltage is set to 43V/1.8A, and the current can be set to 3A if necessary, for charging a large capacity lead-acid battery (for charging a battery with a capacity of 30AH).

Charger circuit and principle introduction:

After the mains input is bridge rectified, a DC voltage of about 300V is formed, and thus the requirements of the rectifying and filtering circuit are different from those of the current. For the battery charger, the 100Hz pulsating current of the bridge rectification does not need to be filtered out. Strictly speaking, the 100Hz pulsating current is not only harmless to the battery charging, but is beneficial, and can play the role of pulse charging to a certain extent, so that charging During the process, the chemical reaction of the battery has a buffering opportunity to prevent plate vulcanization 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 high current also relieves the temperature rise of the battery. Therefore, the C905 of the filter circuit selects 47μF/400V electrolytic capacitor, which is not enough to filter the ripple in the load of the rectifier 120W, but only reduces the output impedance of the rectified power supply to reduce the switching circuit pulse in the power supply circuit. Loss. The capacity of the C905 is reduced, so that the output voltage of the rectifier is reduced to about 280V at full load. U903 uses the typical application circuit of MC3842 as a single-ended output driver. The principle of each pin action and peripheral component selection is as follows (see Figure 1, Figure 2).

Pin 1 is the output of the internal error amplifier. The error voltage is shifted by the D1 and D2 levels inside the IC. After R1 and R2 are divided, they are sent to the inverting input of the current control comparator to control the PWM latch. When pin 1 is low, the latch is reset, the drive pulse output is turned off, and it is not reset until the start of the next oscillation cycle, and the pulse output is restored. The external circuit is connected to R913 (10kΩ) and C913 (0.1μF) to correct the amplifier frequency and phase characteristics.
Pin 2 internal error amplifier inverting input. When the charger is charging normally, the maximum output voltage is 43V. The external circuit is divided by R934 (16kΩ), VR902 (470Ω), 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, and the difference is detected. The control limits the output voltage to 43V. When this voltage is adjusted, the charger can be left unloaded. Adjust the VR902 to make the positive and negative output voltages 43V.
Pin 3 is the charging current control terminal. In the output voltage range set by the second pin, the charging current is controlled by R902. The operation threshold of the third pin is 1V. Within the voltage drop of R902, the output voltage is controlled by the internal comparator to achieve constant current charging. The constant current value is 1.8A, and the R902 is 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. 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 the third pin is 0V, the constant current control is invalid, and the charging voltage is controlled by the second pin to not exceed 43V. At this time, if it is fully charged, in the case of no power failure, a trickle charge of 43V voltage will be formed, and the battery voltage will be maintained at 43V. In order 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 external oscillator timing component, CT is 2200pF, RT is 27kΩ, R911 is 10Ω. In this example, it is considered that the high frequency magnetic core is difficult to purchase, and the frequency is set to about 30 kHz. R911 is used for external synchronization and can be used in this circuit.
The fifth foot is a common ground.
Pin 6 is the drive pulse output. In order to achieve isolation from the mains, the switch tube is driven by T902. T902 can use 5×5mm magnetic core, the primary and secondary windings are wound by 200.2 with 0.21mm enameled 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 can be incorporated in the external circuit.
Pin 7 is the power supply terminal. In order to save the independent power supply circuit, the circuit is stepped down by the battery terminal voltage, 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 the 18V voltage regulator. The filter capacitor C909 is 100 μF.
Pin 8 is the 5V reference voltage output terminal, and is 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 be used with a commercially available core with a circular core and a diameter of 12 mm (the air gap of the core post is already provided with a 1 mm air gap). The primary winding is wound with a 0.64mm high-strength enamelled wire and the secondary winding is wound with a 0.64mm high-strength enameled wire and wound around 50 turns. 3 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, without the possibility of overvoltage and overcurrent, while the T901 secondary has only a few components, as long as The choice is qualified and the possibility of breakdown is almost zero, so its reliability is extremely high. This part of the diode D911 can be selected as a common cathode or a 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, so that the initial charging current has a certain ripple when it is large, and it acts as a pulse charge.

The charger circuit is extremely simple, but the reliability is high. The reason is: MC3842 is a cycle-by-cycle control oscillator, and voltage and current are controlled in each conduction period of the switch tube. Once the load is over-current, D911 leaks* Wear; if the battery terminal is short-circuited, the voltage of the third pin must be higher than 1V, the drive pulse will stop output immediately; 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, drive The pulse will also be turned off. For many years, the MC3942 has been widely used in computer display switching power supply drivers. Under no circumstances (its own damage or peripheral component failure) will not cause the output voltage to rise, but no output or output voltage is reduced. This feature makes the switching power supply The load circuit is extremely safe. In the charger, the MC3842 and its external circuits are independent of the mains input section. In addition, the battery voltage is stepped down and stabilized to supply power, so that the failure rate is almost zero.

The only circuit in the charger that is related to the mains input is the switching circuit between the T901 primary and the T902 secondary. The causes of common switching tube damage are nothing but two: First, when the bipolar switching tube is used, the temperature rises. Thermal breakdown. 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. In addition, 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 is to use a ferrite core with a central column as a round shape. The leakage inductance is relatively smaller than that of the rectangular cross-section core, and the 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 access the normal 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 basically regarded as timing 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 the trial, the power supply was charged at 8:00 pm, and the power was turned off at 7:00 the next morning. The temperature of the outer casing of the battery and the charger did not exceed the room temperature.