DC voltage output is too high

by:Mainbon     2022-02-17
This kind of fault is often caused by the abnormality of voltage regulation sampling and voltage regulation control circuit. In the charger, the 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, 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 the voltage between the anode (Anode) and the cathode is 2.5V, and it is still 2.5∨ after waiting for a while, it is a good tube, otherwise it is a bad tube.
The DC voltage output is too low
According to maintenance experience, in addition to the voltage regulator control circuit causing the output voltage to be too low, there are the following reasons:
(1) The output voltage end is rectified and filtered. The failure of the capacitor can be judged by the substitution method.
(2) The performance of the switching power tube is degraded, which leads to the failure of the switching tube 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 but a large power, as an overcurrent detection resistor. The resistance of this resistor is generally 0.2 to 0.8Ω. If the resistance changes, welding, or poor contact will also cause the output voltage to be too low.
(4) The bad 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, 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 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 determine whether the resistance has changed value, deteriorated or opened welding, or has poor contact. If there is no problem, check again - whether the iron core of the high-frequency transformer is intact. In addition to this, it is also possible that the capacity of the output filter capacitor is reduced, or welding or virtual connection; the power output current limiting resistance changes in 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 is missing.

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