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Induction coil burnout cause analysis and maintenance method

View: 73807/22/2019  

Contact has studied a lot of switching power supplies, up to the 48V/100A dedicated power supply for communication power room, as small as the mobile phone charger, and also handled many faults, but so far, have not encountered the burning of the inductor. This inductor is connected in series with the rectified and primary capacitor filtered output main loop, followed by a level of capacitor filtering to form a CLC π-type filter. Its burning is nothing more than the following reasons:


1 Inductance does not match the output power of the switching power supply. When the coil has a large DC resistance, the coil temperature continues to rise until it is burned out at full load or overload output. There are but many reasons for this.

2 The power supply is overloaded for a long time (more likely). This causes the inductor's coil resistance loss (DC) and core eddy current loss (AC) to increase, both of which become thermal energy, causing the inductor temperature to rise rapidly until burnout. The protection circuit only works when the switching power supply is overloaded by 50% (ie, the rated output power is 150%).

The rated output power of the power supply is actually the ultimate output power, which cannot be exceeded when used, and there must be a certain margin. This will enable continuous, safe and stable operation.

3 There is a problem with the quality of the inductor. If the quality of the inductor core is not good, when a large high-frequency AC component passes through the inductor, a large eddy current loss is generated in the core, so that the core coil temperature continues to rise until burned out.

4 A filter capacitor fails. This will cause all the pulsating AC components after rectification to be added to the inductor, causing the core eddy current loss to peak, and the temperature rises rapidly to cause the inductor to burn out. At this time, the output voltage is lowered, and the voltage is raised by the negative feedback, so that the output pulsating AC component is larger, the core eddy current temperature rises faster, resulting in a vicious cycle, causing the inductor to burn.

5 Inductor coil short circuit between turns. This is also one of the possible reasons, similar to the power transformer, if there is a short circuit between the turns of the coil, the transformer must burn.

Induction coil burnout cause analysis and maintenance method

The reason why the inductor is burnt out can be said to be very much. We can consider prevention from the following factors:

1. The design margin of the inductor coil is not enough; the manufacturer does not leave a certain room for cost saving. The design margin is originally a part of the design that takes into account various factors in the design process and deliberately designs more.

2, the quality of the enameled wire; manufacturers in order to reduce the cost of production, the use of temperature-resistant enameled wire below 130 ° C ~ 150 ° C.

3, the temperature rise of the inductor coil; generally speaking, the design requirements of the inductor coil are up to 60K, the heat resistance of the polyester enameled wire should be 155 °C, and some manufacturers reduce the number of inductor turns in order to reduce the cost. Improve the temperature rise of the inductor coil to 75K~90K, so that the inductor coiled wire is working under high temperature for a long time. Once it is overloaded for a long time, it may cause poor contact of the conductive parts, and the contact resistance will increase, which will greatly reduce the inductance. The insulation strength of the coil.

4. The reaction force between the suction force of the inductor coil; when the voltage is low, the suction will become difficult, the action time of the inductor coil is long, and the time for the inductor coil to withstand the strong current becomes longer, which makes the inductor coil heat more. The suction is more obvious and the suction is more difficult until it can not be sucked. The inductor operates at high temperatures, causing the resistance to increase and the current to become very large.

5. The working voltage range of the product design is not wide enough. Once the voltage is between 80% and 85%, the hot state may not be absorbed. When the voltage is higher than 120%, the inductor coil will easily overheat.

6. In the production process, the control is not strict or out of control; during production, the inner layer of the winding of the inductor coil is not sufficiently immersed in the paint, and the drying is not thorough. It is easy to cause poor soldering of the lead wire joint and incomplete insulation, resulting in inter-turn and inter-layer Short circuit and loss of insulation.

7. There is a shortage of the winding process of the inductor coil; when the winding machine is in production, the winding tension should not be too loose or too tight, otherwise the enameled wire will be elongated, resulting in partial insulation withstand voltage drop.

8. Before the input of the inductor coil, because the weather is humid, rainy, and the humidity is 80%, it is easy to cause moisture to intrude into the inside of the inductor coil, resulting in moisture in the insulation part.

9. In the process of storage and transportation of the induction coil, if the error is improper, the impurities such as moisture and grease will be mixed, and the insulation strength will be greatly reduced.

10. In the process of use, the insulation part of the inductor coil is damaged or mechanically damaged, causing the inductance coil to short-circuit or hit the ground. Then the inductor coil generates a large short-circuit current, causing the temperature to rise sharply and transfer the heat to Near the line, it will eventually burn the entire coil.

11. Human causes; when the user is unfamiliar with the use of non-inductive coils, the voltage regulation is often incorrect; the installation process is poor, the inspection of the inductor coil is not careful, causing the inductor to be mixed with other impurities, and the operation and maintenance are not in place. The relevant use technology has not been strictly implemented. During the period from installation to burnout, most of the inductor coils have not undergone routine routine maintenance and dirt treatment, resulting in poor heat dissipation conditions of the inductor coil and burning.

12, lightning strike; the general and other electronic components of the inductor coil were previously installed in the equipment, the organic rate line was struck by lightning, and the surge voltage on the inductor winding will be more than several times higher than the rated voltage, then the inductor coil is struck by lightning. Damage will be difficult to avoid.

The above causes the inductor to burn out, and can be used as long as it is simply repaired. The method is to rewind the coil, as long as the number of turns is not particularly large, the short circuit is at the end of the coil, and the rest of the inductor is intact, then the damaged part can be removed, leaving Continue to use, which has little effect on the performance of some of the inductors.

The cause of the filter inductor burnout

To understand the cause of the power filter damage, first, we must first understand the structure of the power filter. The power filter is mainly composed of a filter capacitor, a filter inductor and a resistor. It can be seen that the damage or failure of the power supply filter is nothing more than two reasons: saturation of the inductor, loss of capacity of the capacitor, and breakdown. So what causes the above situation? First, it is overvoltage; when the power supply voltage exceeds 20% of the rated voltage of the power supply filter, the inductance will be saturated; in addition, it may cause breakdown of the capacitor; Second, long-term use. As we all know, the capacitor is in the process of continuous charging and discharging, and the number of times of charging and discharging is limited.

The inductance L of the inductor is mainly determined by factors such as the number of turns of the coil, the structure, and the winding method. The more the number of turns of the inductor coil, the denser the wound coil is, the larger the inductance is. The core inside the coil is larger than the coreless one. The magnetic permeability of the core is larger, and the inductance is larger. The use of the inductor is different. The amount of inductance required is also different, for example. The resonant circuit applied to the short-wavelength band has a inductance of several micro-henries: the resonant circuit applied to the medium-wavelength band has an inductance of several thousand micro-henries; in power supply filtering. The inductance of the inductor is as high as 1 - 30H.

The relevant quality factor of the inductor, also known as the Q value or the figure of merit, is the main parameter that weighs the quality of the inductor. It refers to the ratio of the inductive reactance exhibited by the inductor to its equivalent loss resistance when operating at an AC voltage of a certain frequency. The higher the Q value of the inductor, the smaller the loss and the higher the efficiency.

Any current in the inductor in the circuit produces a magnetic field that acts on the circuit. According to Lenz's law, this flux tends to resist current changes by the induced voltage (back EMF). The quality factor of the inductor is related to the DC resistance of the coil wire, the dielectric loss of the coil bobbin, and the loss caused by the core and the shield. The basic role of the color ring inductance: filtering, oscillation, delay, notch, etc.

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