According to the characteristics of lead-acid battery electric bicycle, when it is 36V/12AH, the use of pressure limiting constant current charging mode, the maximum initial charging current should not exceed 3A. That is, the maximum charger output to 43V/3A/129W, has to meet. During charging, the charge current will be gradually reduced. Switching power supply technology and the current production level switch, the single-ended switching regulator output power limit has been increased to 180W, or even greater. 150W output power below its excited single-ended switching regulators, its reliability has reached a very high level. MOS FET switch applications, successfully solved the problem of second breakdown switch, so that a higher level of reliability switching power supply.
Currently, the most widely used, but also can directly drive the first switch MOS FET single-ended drive is MC3842. MC3842 in a stable output voltage at the same time, also has a load current control, often referred to as current control type switching power supply drive, and no doubt for the charger has the unique advantage of this feature, only very few external components to achieve constant pressure output, while controlling the charge current. Especially the MC3842 tube can be directly driven MOS FET characteristics, can greatly improve the reliability of the charger. Wide application of the MC3842, this article only describes the features.

MC3842 8-pin dual-row single-ended output is Switching Power driver IC, and its interior features include: reference voltage regulator, error amplifier, pulse width comparator, latch, oscillator, pulse-width modulator ( PWM), pulse output driver stage and so on. MC3842 more similar products, which are interchangeable UC3842, IR3842N, SG3842, CM3842 (domestic), LM3842 and so on. MC3842 internal block diagram in Figure 1. Its characteristics are as follows:
Single-ended PWM pulse output, the output drive current is 200mA, the peak current up to 1A.
Start voltage greater than 16V, starting current is only 1mA to enter the working state. Into the working state, the working voltage between 10 ~ 34V, load current of 15mA. Beyond the normal operating voltage, switching power supply into voltage or over-voltage protection owed to the state, this time without driving pulse output circuits.
Features 5V/50mA voltage reference, the 2:1 sample as a reference voltage divider.
Driving pulse output can drive bipolar transistors can also be driven MOS FET. If the driver transistor, should be the base of the switch closing speed access RC circuit, while the 40kHz oscillator frequency limit below. If the driver MOS FET, the oscillation frequency set by the external RC circuit, the operating frequency up to 500kHz.
Equipped with overcurrent protection input (Pin 3) and the error amplifier input (pin 1) of two pulse modulation (PWM) control side. Constitute the main input error amplifier pulse-width modulation (PWM) control system, over-current detection input pulse can be individually controlled, direct control of the pulse width of each cycle, the output voltage regulation rate of 0.01% / V. If the voltage is greater than 3 feet or 1 foot 1V voltage is less than 1V, pulse width modulation comparator output high to reset latch, until the arrival of a pulse when the next re-set. If you use the first level of 1,3 feet between the outer latch circuit control on / off, so that the output latch only once per cycle, trigger pulse, no doubt enhanced anti-jamming circuit, the switch will not be misled trigger, reliability will be improved.
Internal oscillator frequency from the first 4,8 external resistors and capacitors to set foot. Meanwhile, the internal reference voltage through the introduction of 4 feet outside the synchronization. 4,8 feet first external resistors, capacitors constitute timing, the capacitor charge / discharge process constitutes an oscillating cycle. When the set value is greater than 5kΩ resistor, the capacitor charging time is much larger than the discharge time, the oscillation frequency can be approximated according to the formula derived: f = 1/Tc = 1/0.55RC = 1.8/RC.
Formed by the MC3842 up to 120W output power of lead-acid battery charger shown in Figure 2. The charger is only part of the switching frequency for the hot places, MC3842 composed of drive control systems and switching power supply output charge parts are cold, the two grounding circuit consists of input and output isolation transformer, the transformer is not only simple but also very easy to implement first-class anti-2000V AC dielectric strength. The charger output voltage is set to 43V/1.8A, if necessary, the current can be adjusted as 3A, used for larger capacity lead-acid battery charging (such as is used to recharge the battery capacity 30AH).
The bridge rectifier AC input, the form of about 300V DC voltage, and thus the requirements of this rectifier and filter circuit is often different. The battery charger, the bridge rectifier of the 100Hz ripple current is no need to filter out clean, strictly speaking, the pulse of 100Hz on the battery charging current is not only harmless, but beneficial, to a certain extent, the effect of pulse charging can play, so that charging the process of chemical reaction buffer battery chance to prevent the formation of a continuous high current charging plate curing phenomenon. Although the 1.8A charge current is greater than the initial rated capacity of C Battery 1 / 10, the intermittent high-current battery temperature also can be alleviated. Therefore, the filter circuit used 47μF/400V the electrolytic capacitor C905, and its role in not enough to 120W load in the rectifier ripple filter clean, but only reduce the output impedance of the power rectifier to reduce the pulse power supply circuit switching circuit in the loss. Reduce the capacity of C905, making the converter output voltage at full load is reduced to 280V or so.
U903 by MC3842 typical application circuit as a single-ended output drivers, and its role in the pin and external component selection principles are as follows (see Figure 1, Figure 2).

1 pin for the internal error amplifier output. IC internal voltage error by D1, D2 level shifter, R1, R2 divider, the comparator into the reverse current control input, control, PWM latch. When a pin is low, the latch is reset, turn off the drive pulse output, until the next oscillation cycle starts before re-set, recovery pulse output. External circuit access R913 (10kΩ), C913 (0.1μF), for the correction of frequency and phase characteristics of the amplifier.
Pin 2 internal error amplifier inverting input. Charger when the normal charge, the maximum output voltage is 43V. Foreign route R934 (16kΩ), VR902 (470Ω), R904 (1kΩ) partial pressure, got the sampling voltage 2.5V, and the error amplifier inverting input of the 2.5V reference voltage comparison, the difference detected by the output pulse duty cycle control the output voltage is limited to 43V. Adjusting the voltage, the charger can load. Adjust VR902, make positive and negative output voltage is 43V.
3 feet for the charge current control terminal. 2 pin set in the output voltage range, through the R902 to control the charge current, 3 feet of the action threshold is 1V, 1V or less pressure drop in the R902, through an internal comparator output voltage changes, to achieve constant current charging . Constant current is 1.8A, R902 choose 0.56Ω/3W. In the charging voltage is limited to 43V, the output voltage can be adjusted by a constant charge current is 1.75A ~ 1.8A. Battery is fully charged, the terminal voltage ≥ 43V, blocking diode D908 deadline, R902 is no current, 3 pin voltage is 0V, constant current control is invalid sample from the pin 2 voltage control charging voltage does not exceed 43V. At this point, if fully charged, the case without power, will form a 43V voltage trickle charging, the battery voltage at 43V. In order to prevent overcharge, 36V lead-acid batteries of this voltage limit should not be the battery cell voltage exceeds 2.38V. Although the sampling of the circuit the battery, actually limits the output voltage, such as the output voltage exceeds the battery voltage is 0.6V, the battery voltage also will be increased to make it into the lower voltage sampling circuit.
4 pin external oscillator timing components, CT to 2200pF, RT is 27kΩ, R911 is 10Ω. The case, taking into account the purchase of high-frequency core problems, the frequency is set to 30kHz or so. R911 for external synchronization, the circuit can not. For a total of 5 feet to side. 6 feet for the drive pulse output. In order to achieve electrical isolation with the city, driven by the T902 switch. T902 can be 5 × 5mm core, winding the first class of 20 turns with 0.21mm enameled wire, winding between the 2 × 0.05mm polyester film with insulation. R909 is 100Ω, R907 is 10kΩ. If Q901 unprotected internal gate-source diode can be incorporated into an external circuit 10 ~ 15V regulator. 7 feet for the supply side. In order to save the independent power supply circuit, the circuit from the battery terminal voltage step-down power supply, power supply voltage is 18V. When the pending charge the battery access, the minimum voltage between 32.4V ~ 35V, 18V regulator can get access to a stable 18V voltage. Filter capacitor C909 is 100μF. 8 pin to 5V reference voltage output, while the IC internal by R3, R4 partial pressure of 2.5V, as the error detection reference voltage.
Charger commercially available pulse transformer T901 core pillar diameter