Maintenance methods for common faults of electric vehicle chargers

1. The fuse tube is blown

In general, a blown fuse indicates that there is a short circuit or overcurrent fault in the internal circuit of the charger. This is due to the high failure rate of internal devices when the charger works for a long time in the state of high voltage and high current. In addition, the fluctuation of grid voltage and surge will cause the current in the charger to increase instantaneously and cause the fuse to blow.

Maintenance method: first carefully check the components on the circuit board to see if the surface of these components is burnt or the electrolyte overflows. Then measure the resistance value of the input end of the power supply. If it is less than 20OkΩ, it means that there is a partial short circuit at the back end. Then measure the positive and negative resistance values of the 4 rectifier diodes and the resistance values of the two current limiting resistors to see if there is a short circuit or burnout. Finally, measure whether the power filter capacitor can be charged and discharged normally, whether the switching power tube is broken down and damaged, whether the UC3842 and surrounding components are broken down, burned out, etc. It should be noted that, because it is measured on the road, it may make the measurement results wrong or cause misjudgment, so if necessary, the components can be soldered down for measurement. If the above is still not the case, measure whether the input power line and output power line are internally short-circuited. In general, in the fuse failure, rectifier diodes, power supply filter capacitors, switching power tubes, and UC3842 are vulnerable parts, and the probability of damage can reach more than 95%. It is easy to troubleshoot these components by focusing on inspection.

2. No DC voltage output or unstable voltage output

If the fuse is in good condition, under the idle condition of load. The main reasons for this kind of failure are: overvoltage, overcurrent protection circuit appears open circuit, short circuit; vibration circuit does not work; power supply is overloaded, high frequency rectifier filter circuit Medium rectifier diode is broken down: filter capacitor leakage, etc.

Maintenance method: First, use a multimeter to measure whether the various components of the high-frequency pulse transformer are damaged: after eliminating the breakdown of the high-frequency rectifier diode and the short-circuit of the load, measure the DC voltage of each output terminal, if the output is still zero at this time. , it is certain that the control circuit of the power supply is faulty. Finally, use a multimeter to statically measure whether the rectifier diodes and low-voltage filter capacitors in the high-frequency filter circuit are damaged. If the above components are damaged, replace them with new ones. The general fault can be eliminated . But pay attention: the output line is broken or open welding, virtual welding can also cause this kind of failure, you should pay attention to this situation during maintenance.

3. No DC voltage output, but the fuse wire is intact

This phenomenon indicates that the charger is not working, or has entered the protection state after working.

Maintenance method: First of all, it should be judged whether the change control chip UC3842 of the charger is in the state of the king or has been damaged. The specific judgment method is: power on and measure the voltage of pin 7 of UC3842 to ground. If the voltage of pin 7 is normal and the voltage of pin 8 has +5∨ voltage, and pins 1, 2, 4, and 6 will also have different voltages, it means that the circuit has been activated. Vibration, UC3842 is basically normal. If the voltage of pin 7 is low and the other pins have no voltage, it means that the UC3842 is damaged. The most common damages are pin-to-ground breakdown, pins 6 and 7, and pin-to-ground breakdowns 1 and 7. If these feet are not broken down and the charger still cannot start normally, it also means that the UC3842 is damaged and should be replaced directly. If it is judged that the chip is not damaged, then check whether the current-limiting resistance of the gate of the switch is open, soldered or changed value, and whether the performance of the switch power tube itself is poor. In addition, the power output line disconnection or poor contact can also cause this kind of failure, so it should also be paid attention to during maintenance.

4. DC voltage output is too high

This kind of fault is often caused by the abnormality of voltage regulation sampling and voltage regulation control circuit. In the charger, DC output, sampling resistor, error sampling amplifier, optocoupler, power control chip, etc. together form a closed control loop , any problem will cause the voltage to rise.

Maintenance method: Since the charger has an overvoltage protection circuit, if the output voltage is too high, the overvoltage protection circuit will act first. Therefore, when encountering this kind of fault, we can disconnect the overvoltage protection circuit, so that the voltage protection circuit does not work, and then measure the main voltage of the power supply at the moment of power-on. If the measured value is more than 1V higher than the normal value, it means that the cause of the high output voltage is indeed in the control loop. At this time, it is important to check whether the sampling resistance is changed or damaged, and whether the precision reference voltage source (TL431) or the optocoupler (PC817) has poor performance, deterioration or damage. Among them, the precision reference voltage source (TL431) is very easy to damage. We can use the following method to judge the precision voltage regulator amplifier: connect the reference terminal (Ref) of TL431 to its cathode (Cathode), connect a 1OkΩ resistor in series, and connect 5 ∨ Voltage. If there is 2.5V between the anode (Anode) and the cathode, and it is still 2.5∨ after waiting for a while, it is a good tube, otherwise it is a bad tube.

5. DC voltage output is too low

According to maintenance experience, in addition to the voltage regulator control circuit will cause the output voltage to be too low, there are the following reasons:

(1) The failure of the three-pole rectifier and filter capacitor at the output voltage end can be judged by the substitution method.

(2) The performance of the switching power tube is degraded, which causes the switching tube to fail to conduct normally, which increases the internal resistance of the power supply and reduces the load capacity.

(3) The source of the switching power tube is usually connected to a resistor with a small resistance value but a large power, which is used as an overcurrent detection resistor. The resistance value of this resistor is generally 0.2~0.8Ω. If the resistance changes, welding, or poor contact will also cause the output voltage to be too low.

(4) The defective high-frequency pulse transformer will not only cause the output strider voltage to drop, but also cause insufficient excitation of the switching power tube, which will cause repeated damage to the switching tube.

(5) The high-voltage DC filter capacitor is not good, resulting in poor load capacity of the power supply.

(6) The power output line is in poor contact and has a certain contact resistance, which causes the output voltage to be too low.

(7) The grid voltage is too low. Although the charger can still output the rated charging voltage at low voltage, when the grid voltage is lower than the minimum voltage limit of the charger, the output voltage will also be too low.

Maintenance method: First, use a multimeter to check whether the lower high-voltage DC filter capacitor has deteriorated, whether the capacity has decreased, and whether it can be charged and discharged normally. If there are no above problems, measure the current limiting resistance of the electrode of the switching power tube and the overcurrent protection of the source to measure whether the resistance has changed value, deteriorated or opened welding, and has poor contact. If there is no problem, check again - whether the iron core of the high-frequency transformer is intact. In addition, it is possible that the capacity of the output filter capacitor is reduced, or welding, virtual connection; the power output current limiting resistor changes value or virtual connection; the power output line is virtual connection, etc.

Don't let go of these troubled elements, you should check them carefully to make sure nothing goes wrong.

6. The cooling fan does not rotate

This kind of failure is mainly caused by damage to the triode (usually 8550 or 8050) that controls the fan, or the fan itself is damaged or the fan blade is stuck by debris. However, some chargers use intelligent heat dissipation. For chargers that use this method to dissipate heat, the probability of damage to the thermistor is very high.

Maintenance method: First, use a multimeter to measure whether the triode that controls the fan is damaged. If the tube is not damaged, it may be that the fan itself is damaged. You can unplug the fan from the circuit board and connect a 12V DC power supply. (Pay attention to the positive and negative poles), to see if it rotates, and also to see if there is any foreign matter stuck. If the fan is turned by swinging the wire of the fan, it means that there is a broken wire inside the wire or the connector is in poor contact. If it still does not rotate, the fan must be broken. For chargers with intelligent heat dissipation, in addition to the above inspections, it is also necessary to check whether the thermistor is in poor contact or damaged, or if it is open for welding. However, it should be noted that this thermistor has a negative temperature coefficient and should be paid attention to when replacing it.