As we all know, for reliable electrical performance and long service life, switching power adapter must select electrolytic capacitor with sufficient voltage and temperature margin. We are not familiar with the requirements of meeting appropriate ripple current rating. In order to better understand this requirement, we should first understand the basic structure of electrolytic capacitor.
In a typical Aluminium electrolytic capacitor, two aluminum foil strips are helically wound between layers of absorbing saturated electrolyte material. A very thin insulating dielectric film is used to form a capacitor at the interface between aluminum and conductive electrolyte, which is maintained by polarization voltage in the flowing electrolyte. If the electrolyte starts to dry, the resistance that can absorb the electrolyte separator will increase, and the dielectric will start to be damaged. At this time, the electrolytic capacitor will quickly fail.
To prevent the loss of electrolyte, capacitors should have sealed end caps, outgoing wires, and connecting wires. At high temperatures, the electrolyte tends to evaporate and pressurize the casing, and these seals as a whole bear a great deal of pressure. In addition, the loss of capacitors can increase at high temperatures, leading to loss of control. For long-term reliability, the temperature of capacitors has become a major concern. The combination of the following three main factors determines the internal temperature of the capacitor.
1. Environmental working temperature.
2. Heat dissipation design and ventilation environment.
3. Internal losses.
Environmental temperature is a content related to application and technical requirements, and it is basically not controlled by the designer. Heat dissipation design is often a major factor under the control of designers. The location, layout, radiator design, size and cooling method (forced ventilation or convection cooling) of high-temperature components have a greater impact on the temperature rise of electrolytic capacitor than internal losses. If it is necessary to maintain a good MTBF, the designer must always keep in mind the minimum thermal stress that the electrolytic capacitor needs to maintain. The internal loss of electrolytic capacitor is generally quite low, which is affected by voltage stress, temperature, and especially ripple current. To help the designers of switching power supply, the manufacturers of electrolytic capacitor specify the maximum effective ripple current rating as a general guide, which is usually obtained at 120Hz frequency and 85 ℃ or 105 ℃ air temperature.
At the testing frequency (usually 120Hz), the manufacturer establishes the rated values of these ripple currents by operating the capacitor under DC polarization voltage and sinusoidal ripple current stress. The referenced numbers are therefore based on the effective value of ripple current with low harmonic components, which will cause the maximum internal loss and temperature rise that can be determined inside the capacitor. The allowable temperature rise depends on the design of the capacitor, and its maximum level is usually 8 ℃. The actual allowable temperature rise caused by internal losses is not frequently referenced, but can be obtained from the manufacturer. It is important not to exceed the internal loss limit regardless of the operating temperature, as internal losses may increase under high ripple currents, which may result in thermal breakdown.
Regardless of the method used to establish the ripple rating and the size of the capacitor, it is recommended to measure temperature in the final application, as the final temperature rise is the result of internal heat loss caused by ripple current, proximity effects of surrounding components, and the combined effect of thermal design. Compared to internal heat loss, the thermal radiation and convective conduction of accessory components will generate a greater temperature rise in the capacitor.
Due to the influence of ripple current and peak operating temperature, the maximum temperature rise allowed for capacitors will vary depending on the type of capacitor and the manufacturer. In the example used here, in a freely ventilated environment, the maximum temperature rise allowed by the ripple current is only 8 ℃ (which is the limit used by manufacturers to limit the rated value of the ripple current). This rating is used for a freely ventilated environment with an air temperature of 85 ℃, while the shell temperature is 93 ℃. This method sets an absolute limit value during operation and does not consider the reason for temperature rise. The lifespan of capacitors will not last long at this temperature, and we recommend a lower operating temperature.
In most cases, we do not know the effective value of the equivalent current. Although this value can be calculated or measured at operating frequency, it is not always helpful in establishing the final temperature rise of the capacitor. In the switching mode, there are generally high harmonic components, and the capacitance loss will change with the frequency and amplitude of each harmonic (and the equivalent series resistance changes in a nonlinear way with the frequency). So the loss component is related to frequency, which is usually unknown. Therefore, the recommended final testing steps will be discussed below. In the final application, the rationality of the selection will be determined by measuring the temperature rise:
1. Measure the temperature rise of the capacitor under normal operating conditions far from the influence of other thermal effects (if necessary, connect the capacitor to a short twisted pair cable to stay away from the thermal effects of other components, or insert a thermal barrier between the heating element and the capacitor to isolate its effects). Measure the temperature rise of the capacitor caused by ripple current separately and compare this temperature rise with the manufacturer’s limit value.
If the temperature rise caused by ripple current is acceptable, install the capacitor in the normal position and make the Ac To Dc Power Adapter withstand the maximum temperature stress and load conditions. Measure the surface temperature of the capacitor to ensure it is within the manufacturer’s maximum temperature limit, and several samples should be tested.
The most important parameter for long-term reliability of electrolytic capacitor is undoubtedly the temperature rise of the capacitor in the working environment. At high temperatures, the loss of capacitors rapidly increases, which increases internal power consumption and leads to thermal breakdown. There is no suitable method here to replace the measurement of the performance and temperature rise of Ac Switching Adapter
Post time: Apr-17-2023