Repair methods for common faults of electric vehicle chargers

Electric vehicles have entered our lives with their advantages of convenient travel, low carbon and environmental protection, but the high failure rate of its chargers is a headache. For this reason, based on my years of maintenance experience, I have summarized the maintenance methods of common faults of electric vehicle chargers for your reference.

Common fault repair

Since the input circuit of the electric vehicle charger works under high voltage and high current, the failure rate is the highest, such as high voltage and high current rectifier transistors, filter capacitors, switch power tubes, etc.; the second most vulnerable parts are the rectifier diodes, protection diodes, filter capacitors, current limiting resistors, etc. of the output rectifier part; and the feedback part and protection circuit part of the pulse width modulation controller.

1. The fuse is blown

Generally speaking, a blown fuse indicates that there is a short circuit or overcurrent fault in the charger's internal circuit. This is because the charger has been working under high voltage and high current for a long time, and the internal components have a high failure rate. In addition, fluctuations and surges in the grid voltage will cause the current in the charger to increase instantly, causing 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 electrolyte overflows, and smell it for any strange smell. Then measure the resistance value of the power input terminal. If it is less than 20OkΩ, it means that there is a local short circuit at the back end. Then measure the positive and negative resistance values ​​of the four 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 switch power tube is broken down and damaged, and whether UC3842 and surrounding components are broken down and burned out. It should be noted that because the measurement is on the circuit, it may cause errors in the measurement results or misjudgment. Therefore, the components can be welded down for measurement when necessary. If there is still no such situation, measure whether the input power line and the output power line are internally short-circuited. Generally speaking, in the case of a fuse blown failure, the rectifier diode, power filter capacitor, switch power tube, and UC3842 are vulnerable parts, and the probability of damage can reach more than 95%. It is easy to troubleshoot if you focus on checking these components.

2. No DC voltage output or unstable voltage output

If the fuse is intact, under load idling conditions, the main causes of this type of failure are: overvoltage, overcurrent protection circuit open circuit, short circuit phenomenon; oscillation circuit does not work; power supply load is too heavy, the rectifier diode in the high-frequency rectifier filter circuit is broken down; filter capacitor leakage, etc.

Repair method: First, use a multimeter to measure whether the 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 can be confirmed that the control circuit of the power supply is faulty. Finally, use a multimeter to statically measure whether the rectifier diode and low-voltage filter capacitor in the high-frequency filter circuit are damaged. If the above components are damaged, replace them with new ones, and the general fault can be eliminated. But please note: the output line is broken or the welding is open or the welding is virtual, which can also cause this fault. Pay attention to this situation during maintenance.

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

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

Repair method: First, you should determine whether the charger's variable control chip UC3842 is in working state or damaged. The specific judgment method is: power on and measure the voltage of UC3842's 7-pin to ground. If the voltage of 7-pin is normal and there is +5∨ voltage on 8-pin, and there are different voltages on 1, 2, 4, and 6-pin, it means that the circuit has started to oscillate and UC3842 is basically normal. If the voltage of 7-pin is low and there is no voltage on the other pins, it means that UC3842 is damaged. The most common damage is the breakdown of 7-pin to ground, the breakdown of 6 and 7-pin to ground, and the breakdown of 1 and 7-pin to ground. If none of these pins are broken down and the charger still cannot start normally, it also means that UC3842 is damaged and should be replaced directly. If it is determined that the chip is not broken, then check whether the current limiting resistor of the switch gate is open, poorly welded or changed, and whether the switch power tube itself has poor performance. In addition, the disconnection of the power output line or poor contact can also cause this fault, so you should also pay attention to it during maintenance.

4. DC voltage output is too high

This type of fault is often caused by abnormalities in the voltage stabilization sampling and voltage stabilization control circuits. In the charger, the DC output, sampling resistor, error sampling amplifier, optocoupler, power supply control chip, etc. together form a closed control loop. Any problem in any part will cause the voltage to increase.

Repair method: Since the charger has an overvoltage protection circuit, the overvoltage protection circuit will be activated first if the output voltage is too high. Therefore, when encountering this kind of fault, we can disconnect the overvoltage protection circuit to make it inoperative, and then measure the main voltage of the power supply at the moment of startup. If the measured value is more than 1V higher than the normal value, it means that the reason for the high output voltage is indeed in the control loop. At this time, we should focus on checking whether the sampling resistor has changed value 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 be damaged. We can use the following method to judge the precision voltage stabilizing amplifier: connect the reference end (Ref) of TL431 to its cathode (Cathode), connect a 1OkΩ resistor in series, and connect a 5∨ voltage. If the voltage between the anode (Anode) and the cathode is 2.5V, and it is still 2.5∨ after 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 stabilization control circuit causing the output voltage to be too low, there are also the following reasons:

(1) The output voltage end rectifier transistor and filter capacitor are faulty, which can be judged by substitution method.

(2) The performance of the switching power tube is reduced, causing the switching tube to fail to conduct normally, increasing the internal resistance of the power supply and reducing its load capacity.

(3) The source of the switching power tube is usually connected to a resistor with a small resistance but high power as an overcurrent protection detection resistor. The resistance of this resistor is generally between 0.2 and 0.8Ω. If the resistor changes value or is open-welded or has poor contact, it will also cause the output voltage to be too low.

(4) A defective high-frequency pulse transformer will not only cause the output voltage to drop, but also cause insufficient excitation of the switching power tube, thus damaging the switching tube.

(5) The high-voltage DC filter capacitor is defective, resulting in poor load capacity of the power supply.

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

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

Maintenance method: First, use a multimeter to check whether the 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 problems, measure the current limiting resistor of the electrode of the switch power tube and the overcurrent protection test resistor of the source to see if the value has changed, deteriorated, or the welding is broken, or the contact is poor. If there are no problems, check whether the iron core of the high-frequency transformer is intact. In addition, it is possible that the output filter capacitor capacity has decreased, or the welding is broken, or the connection is virtual; the power supply output current limiting resistor has changed or the connection is virtual; the power supply output line is virtual, etc.

None of these factors should be overlooked and all should be carefully checked to ensure that there are no mistakes.

6. The cooling fan does not rotate

The main reason for this failure is that the transistor (usually 8550 or 8050) that controls the fan is damaged, or the fan itself is damaged or the fan blades are stuck by debris. However, some chargers use intelligent heat dissipation. For chargers that use this method to dissipate heat, the probability of the thermistor being damaged is very high.

Repair method: First use a multimeter to measure whether the transistor that controls the fan is damaged. If the tube is not damaged, it is possible that the fan itself is damaged. You can unplug the fan from the circuit board and connect a 12V DC power (pay attention to the positive and negative poles) to see if it rotates and whether there is any foreign object stuck. If the fan rotates when the wires of the fan are swung, it means that the wires are broken or the connectors are in poor contact. If it still does not rotate, the fan must be broken. For chargers that use intelligent heat dissipation, in addition to the above inspections, you should also check whether the thermistor has poor contact or is damaged, or has open welding. But it should be noted that this thermistor has a negative temperature coefficient, so you should pay attention when replacing it.