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Charger Principle of Electric Vehicle Based on KA3842

Warm hints: The word in this article is about 1360 and reading time is about 12 minutes.  
Guidance: According to the circuit structure, one of the common electric vehicle chargers is a single-switch power supply driven by KA3842 field effect transistor, which cooperates with LM358 dual op-amp to realize three-stage charging mode. 

Principle Diagram of Charger for KA3842 Electric Vehicle

Principle Diagram of Charger for KA3842 Electric Vehicle
Principle Diagram of Charger for KA3842 Electric Vehicle

220V AC is suppressed by T0 bidirectional filter, D1 rectifier is pulsating DC, and then C11 filter is used to form stable 300V DC. U1 is a KA3842 pulse width modulation integrated circuit. Its five legs are negative poles, seven legs are positive poles, six legs are pulse output direct drive field effect transistor Q1 (K1358) and three legs are maximum current limit. Adjusting the resistance of R25 (2.5 ohms) can adjust the maximum current of charger. Two feet are voltage feedback, which can adjust the output voltage of the charger. Four pins external oscillating resistance R1 and oscillating capacitor C1. T1 is a high frequency pulse transformer with three functions. The first is to turn high-voltage pulse into low-voltage pulse. The second is to isolate high voltage in order to prevent electric shock. The third is to provide working power for KA3842. D4 is a high frequency rectifier (16A60V) C10 is a low voltage filter capacitor, D5 is a 12V regulator diode, U3 (TL431) is a precise reference voltage source, and cooperates with U2 (optical coupler 4N35) to automatically adjust the voltage of charger. The voltage of the charger can be fine-tuned by adjusting the W2 (fine-tuning resistance). D10 is the power indicator. D6 is a charging indicator. R27 is a current sampling resistor (0.1 ohm, 5w). Changing the resistance of W1 can adjust the high constant voltage of the charger.

At the beginning of power-on, there is a voltage of about 300 V on C11. This voltage is loaded all the way through T1 to Q1. The second route passes through R5, C8, C3 to reach the 7th foot of U1. Force U1 to start. U1’s 6 feet output square wave pulse, Q1 works, current through R25 to the ground. At the same time, T1 coil generates induction voltage, which provides reliable power supply to U1 through D3 and R12. The voltage of T1 output coil is stabilized by D4 and C10 rectifier filter. This voltage charges the battery all the way through D7 (D7 prevents the battery current from backfilling into the charger). The second route, through R14, D5 and C9, provides 12V working power supply for LM358 (dual operational amplifier, one foot as power source, eight foot as power source) and its peripheral circuits. D9 provides the reference voltage for LM358, which reaches the second and fifth legs of LM358 through R26 and R4 partial voltage. When charging normally, the upper end of R27 has a voltage of about 0.15-0.18V. This voltage is added to the third leg of LM358 through R17, and a high voltage is sent out from one leg. This voltage passes through R18, forcing Q2 to turn on, D6 (red light) to turn on, the second way injects 6 feet of LM358, 7 feet output low voltage, forcing Q3 to turn off, D10 (green light) to turn off, and the charger enters the constant current charging stage. When the battery voltage rises to about 44.2V, the charger enters the constant voltage charging stage, the output voltage is maintained at about 44.2V, and the current decreases gradually. When the charging current is reduced to 250 mA& mdash; 400 mA, the voltage of the upper end of R27 decreases, the three-leg voltage of LM358 is lower than 2-leg, one-leg output voltage is low, and Q2 is off.

KA3842 Component parameters

KA3842 Component parameters
KA3842 Component parameters
  • The foot① is the output end of the error amplifier. The external resistor and capacitor elements are used to improve the gain and frequency characteristics of the error amplifier.
  • The foot② is the input of the feedback voltage, which is compared with the reference voltage of 2.5V at the same phase of the error amplifier to generate the error voltage and control the pulse width.
  • The foot③ is the input of current detection. When the detection voltage exceeds 1V, the pulse width is reduced so that the power supply is in intermittent working state.
  • The foot④ is the timing end.The operating frequency of the internal oscillator is determined by the external resistance-capacitance time constant, f = 1.8/(RT& times; CT);
  • The foot⑤ is the public end;
  • The foot⑥ is the push-pull output terminal and the interior is the totem pole type. The driving capacity of the rising and falling time is only 50ns: amp; plusmn; 1A;
  • Foot⑦ is the DC power supply terminal, with under-voltage and over-voltage locking function, and the power consumption of the chip is 15 MW.
  • Foot⑧ is 5V reference voltage output terminal with 50mA load capacity.

Common Faults of Electric Vehicle Charger Based on KA3842

  1. High Voltage Fault
  2. Low Voltage Fault
  3. High-voltage and low-voltage faults.

The main phenomena of high voltage faults are that the indicator lamp is not on. They are characterized by fuse fuse breakdown, rectifier diode D1 breakdown, capacitor C11 bulging or bursting. Q1 breakdown, R25 open circuit. U1 7 feet short circuit to ground. R5 open circuit, U1 no start voltage. The above components can be repaired by replacing them. If the 7 legs of U1 have voltage above 11 V and the 8 legs have voltage of 5 V, it shows that U1 is basically normal. Emphasis should be placed on detecting whether the pins of Q1 and T1 are virtual welded. If Q1 breaks down continuously and Q1 does not burn, it is usually D2 and C4 fails. If Q1 breaks down and burns, there is leakage or short circuit in the low-voltage part. Excessive or abnormal output pulse waveform of KA3842’s 6 feet leads to Q1 overheating and burning. Other phenomena of high voltage faults are flashing indicator lamp, low and unstable output voltage. Generally, the lead of T1 has virtual welding, or D3, R12 open circuit, KA3842 and its peripheral circuit have no working power supply. Another rare high-voltage fault is that the output voltage is higher than 120V, usually U2 failure, R13 open circuit or U3 breakdown makes U1 2-pin voltage lower, 6-pin send out ultra-wide pulse. At this time, the power can not be powered on for a long time, otherwise the low-voltage circuit will be burned seriously. Most of the low-voltage faults are the connection between charger and positive and negative electrodes of batteries, which results in R27 burning down and LM358 breakdown. The phenomenon is that the red light is always on, the green light is not on, the output voltage is low, or the output voltage is close to 0V. The above components can be repaired by replacing. In addition, the output voltage of W2 drifts due to jitter. If the output voltage is too high, the battery will be overcharged, seriously dehydrated and scalded, which will eventually lead to thermal runaway and charge and explosion of the battery. If the output voltage is low, the battery will be undercharged. When both high and low voltage circuits are faulty, all the diodes, triodes, optical couplers 4N35, field effect transistors, electrolytic capacitors, integrated circuits, R25, R5, R12, R27, especially D4 (16A60V, fast recovery diodes), C10 (63V, 470UF) should be detected before switching on. Avoid blind power-on to further expand the scope of the fault. Some of the output terminals of chargers have special functions such as anti-reverse connection and short circuit protection. In fact, the output terminal plus a relay, in the case of reverse connection, short circuit relay does not work, charger no voltage output. Another part of the charger also has the function of anti-reverse connection and anti-short circuit. Its principle is different from the previous one. The starting voltage of its low-voltage circuit is provided by the charged battery and connected with a diode (anti-reverse connection). When the power supply starts normally, the charger will provide low-voltage power supply.

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