Discussion on Foil-type Inductive Capacitors under High Temperature and High Voltage

1. Working temperature of capacitor in energy-saving lamp:

Because the lamp tube of the energy-saving lamp is integrated with the electronic ballast, when the energy-saving lamp is normally lit, the temperature of the lamp tube is above 100℃. Because the lamp tube is close to the sealed electronic ballast plastic shell, affected by heat transfer, part of the heat is dissipated into the space, and the other part is conducted to the electronic ballast. In addition, due to comprehensive factors such as the heating of components inside the plastic shell, the temperature of the space inside the plastic shell can reach about 105℃. That is to say, the capacitor must work at an ambient temperature of about 105℃.

2. Rated voltage of capacitor in energy-saving lamp:

In the circuit of electronic energy-saving lamps, the highest rated voltage of capacitors is generally above 1000V, and the withstand voltage requirement is relatively high. For example, in an oscillation circuit, due to the high oscillation frequency, the generated reverse peak voltage (or instantaneous overvoltage) can easily cause the capacitor to break down. Source: http://tede.cn

1. Analysis of the failure mechanism of capacitors under high temperature and high pressure

The breakdown of capacitors is largely determined by their macroscopic structure and process conditions, as well as the resulting non-uniform electric field and non-uniform dielectric. The breakdown of capacitors often occurs at these weak points. The following main influencing factors are listed for analysis.

1. The influence of dielectric on capacitor breakdown:

ⅰ. When designing capacitor products, if the breakdown voltage of the dielectric is close to the working voltage, the capacitor is prone to premature failure under high temperature and high pressure.

ⅱ. When the medium is under the action of a uniform electric field, the medium's microscopic nature and poor quality, such as rough surface, pores, wrinkles, cracks, etc., will reduce the medium's ability to withstand electric field strength and cause the medium to break down.

The mechanism is that the free electrons in the capacitor medium collide with neutral molecules under the action of a strong electric field, causing them to ionize and generate positive ions and new free electrons. The rapid development of the ionization process forms an avalanche of electron flow, causing the medium to break down, causing the capacitor's tolerance under high temperature and high pressure to decrease.

2. The influence of the plate (aluminum foil) on the breakdown of the capacitor:

When the aluminum foil is cut, the end surface of the aluminum foil coil will be jagged and burred due to the blunt hob during cutting. Such coils are easy to pierce the dielectric film during winding, reducing the withstand voltage. Severe burrs will also cause the capacitor to break down after pressurization.

3. The influence of discharge path (margin amount) on capacitor edge breakdown:

电容器在瞬时过电压作用下，电容器不仅可能通过介质内部发生击穿，当极板边缘电场显著不均匀或放电路径（留边量）较小时，还有可能沿极板边缘发生表面击穿。如下图所示，“△L”为放电路径， “△b”为留边量， “d”为介质厚度。

As can be seen from the above figure, if the aluminum foil deviates or the discharge path (margin amount) is small during product design, it is easy to cause surface breakdown at the root of the lead. This is determined by the lead-out method of the capacitor lead. Under normal circumstances, the discharge path between the two poles at the root of the lead is ΔL = Δb + d, and without a lead end, the discharge path is ΔL = 2Δb + d. If d < < ΔL is negligible, the discharge path of the lead end is reduced by 1/2 compared to the discharge path without a lead end. In addition, the width error of the aluminum foil during slitting or the "snake-shaped" deviation of the aluminum foil during winding. As a result, the discharge path (margin amount) between one of the aluminum foil plates and the root of the lead on the other aluminum foil at that position is reduced, and the capacitor is prone to surface breakdown after pressurization, especially under high temperature and high pressure.

4. The influence of lead spot welding on capacitor breakdown:

During the winding process of the capacitor, the lead wire is spot welded directly on the aluminum foil. Therefore, improper adjustment of the pressure of the two electrode heads, the size of the spot welding current, the flatness of the spot welding platform, etc. during spot welding will cause burrs to form at the welding point between the lead wire and the aluminum foil, piercing the dielectric film and reducing the withstand voltage. Severe burrs will cause the capacitor to break down after pressurization.

5. Effect of heat sealing of outer film on capacitor breakdown:

The outer film of the capacitor core group is sealed by the heat of the heat sealer. If the heat of the heat sealer is not properly controlled, or the contact time between the heat sealer and the core group is too long, the outer film will be easily burned, affecting the withstand voltage of the capacitor, and in severe cases, it will cause the capacitor to break down after pressurization.

2. Control measures

1. Control of the influence of raw materials (dielectric, aluminum foil) on capacitor breakdown:

1. Establish a sound raw material management and inspection system to ensure that the quality of raw materials meets the requirements.

2. When designing the product, the thickness of the dielectric should be correctly selected according to the working voltage of the capacitor so that it can work under a larger field strength without breakdown.

3. Establish a sound management system and operating requirements for the equipment for cutting materials, especially the roller cutters should be checked and replaced regularly to prevent adverse effects.

(II) Control of capacitor edge breakdown:

In order to prevent capacitor edge breakdown, the discharge path between the two plates is increased when designing the capacitor. On the other hand, when the total discharge path of the capacitor core group is constant, the discharge path at the end with the lead wire should be slightly larger than the discharge path at the end without the lead wire. This can effectively avoid capacitor edge breakdown.

(III) Control of the influence of lead spot welding on capacitor breakdown:

1. Establish control documents for the lead welding process in the winding process, and clarify the welding state requirements of the lead wire and the aluminum foil. For the welding quality of the capacitor core group, the operator must conduct regular self-inspections, such as the first piece inspection before starting the machine. Inspectors' patrol inspections, special inspections, etc. Establish a complete quality assessment system to ensure that the lead welding state meets the requirements.

2. When welding the lead wire, the pressure of the two electrode heads and the spot welding current should be adjusted moderately. The spot welding platform should be flat and without grooves. The welding state of the lead wire and the aluminum foil should be flat and smooth on both sides without burrs. At the same time, the welding of the lead wire and the aluminum foil should be firm, and the lead wire should not fall off or the aluminum foil should not break. The flattening degree and length of the lead wire must be moderate.

(IV) Control of the influence of heat sealing of outer film on capacitor breakdown:

When heat-sealing the outer film, the heat of the heat sealer and the contact time between the heat sealer and the core group must be adjusted to avoid damaging the aluminum foil. In addition, the length of the outer film should be moderate. If it is too long, it will waste raw materials. If it is too short, it will easily burn the aluminum foil during heat sealing and cause the two plates to break through.

3. Conclusion

The above are only the main factors that affect the tolerance of capacitors under high temperature and high pressure. Of course, there are many other factors that will also affect the tolerance of capacitors. This requires us to strengthen quality management and control in many aspects during the production process. Continuously improve and enhance to meet the needs of the energy-saving lamp market with high-quality products.