Switching power supply failure and inspection

Most peripherals now use switching power supplies for voltage conversion. Although the switching power supply has the characteristics of small size, high working efficiency, and good voltage regulation effect, but because the switching power supply is directly connected to the mains, changes in the mains voltage and surges may cause damage to the switching power supply. Switching power supply circuit is more complex, many fans are helpless to power damage, in fact, as long as we have a certain understanding of it, it is not difficult to repair it.

The principle of the switching power supply is roughly the same. Here, we take the example of the voltage-free output of the switching power supply of the HP 3748 printer as an example to explain the working principle and troubleshooting of the switching power supply.

Learn how it works

If you want to learn to troubleshoot the switching power supply, we have to understand how it works and what components are vulnerable. When the utility power is input from the input terminal, it first reaches the L-type or π-type filter circuit composed of capacitors and inductors for filtering, so as to eliminate the surge voltage and interference signals in the utility power, and improve the power quality. At the same time, a fuse is connected in series at the input end of the mains. When a short-circuit fault occurs in the power supply, the fuse is blown to prevent the fault from expanding. And, most of the switching power supply input terminals are now also varistors. This resistance when the voltage is normal, the resistance is infinite, does not affect the work of the circuit. Once the voltage is too high, the varistor will short-circuit, increase the current through the fuse, and fuse the fuse to avoid damage to other components due to high voltage.

The filtered alternating current is filtered by a diode bridge rectifier circuit and a high-voltage large-capacity capacitor to generate a high voltage DC voltage of 300 V. The voltage is then sent to an oscillation control circuit to generate an oscillating signal after resistance reduction and simple regulation. After the oscillation signal is amplified by the power oscillating tube, it is converted into a low-voltage AC voltage when it is matched with a high-frequency transformer. After a low-voltage AC voltage is once rectified and filtered, various low-voltage DC voltages available for the equipment can be generated. In addition, at the main voltage output end, a voltage sampling feedback circuit is also provided to feed the current voltage back to the oscillation control circuit. Once the main voltage generates a voltage drift due to a load change, the oscillation control circuit will change the oscillation pulse width to ensure the output voltage. Stability. At the same time, when the load is short-circuited, the sampling feedback signal will also notify the oscillation control circuit in time, stop the output of the voltage, and avoid damage to the power supply due to overload.

Fault detection method

After having a basic understanding of the switching power supply, let's look at the fault detection and maintenance methods.

First of all, you should observe whether the power fuse is damaged. If the fuse is damaged and cannot be replaced eagerly, you must first check the power supply for short circuit. Method: Use the multimeter's electric resistance to test the AC end of the power fuse (test point 1). The normal resistance should be more than tens of kilo-ohms. If the resistance is zero, it means that there is an AC short circuit in the power supply. In addition, we should also focus on checking whether the AC filter capacitor of the power supply is damaged. If there is a varistor, we should also check this resistance.

If the above test results are normal, we should then test the four rectifier diodes of the power supply (Test Point 2). Under normal conditions, the forward resistance of the diode is a few k (tested with a multimeter x 1k block), and the reverse resistance is close to infinity. If you find that the test result is abnormal, you need to replace it. After checking down, the next thing to do is to test the DC resistance of the power supply (test point 3). The normal resistance is also several k. If the resistance is zero, there is a DC short circuit. There are many causes of DC short-circuit, such as the short-circuit of the filter capacitor, the damage of the power oscillating tube, and the damage of the oscillation integrated block and peripheral circuit may cause short circuit. Here, it should be noted that before replacing the power oscillating tube, it must be determined that the oscillating integrated block and the peripheral circuit are normal, otherwise the power supply tube will be damaged again.

Excluding the above-mentioned problems of switching power supply, it shows that most of the faults exist in the oscillation control circuit, sampling feedback circuit or load. At this time, we should check whether the power supply circuit of the oscillating manifold is normal (test point 4). The normal voltage should be around 10V (special remind: Because the test voltage should be conducted under power, and the power board has high voltage Mains electricity, so pay special attention to personal safety, do not try to directly touch any part of the power supply). If there is no voltage or low voltage at this point, first check whether the buck resistor is damaged; secondly, check whether the oscillation integrated block and its external power supply circuit are normal. If no fault is found in the peripheral circuit, it is recommended to replace the oscillation manifold. Of course, sometimes the power supply cannot output the voltage normally, and it may be caused by the short circuit of the load that causes the power supply protection. For this power supply, we only need to remove the output line, check the output voltage is normal (test point five), you can determine the fault location. After the previous troubleshooting, the feedback circuit part should also be checked in the end. Generally speaking, this part of the fault is mainly concentrated on the optocoupler and its amplifier circuit, and special attention should be paid.

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