Analysis of the advantages of switching power supply based on high voltage ceramic capacitor design

In modern power supply technology, whether it is a linear power supply or a switching power supply, aluminum electrolytic capacitors are essential key components. However, in the conventional AC-DC power supply design in the industry, aluminum electrolytic capacitors will cause problems such as poor reliability and short life under high and low temperature conditions. So, is there a device that can replace traditional aluminum electrolytic capacitors and improve the reliability and life of the power supply? This article focuses on the comparison and analysis of the advantages and disadvantages of high-voltage ceramic capacitors and traditional aluminum electrolytic capacitors.

1. Design defects of aluminum electrolytic capacitors

To achieve the conversion from AC to DC, the AC-DC power converter must first rectify and filter the AC voltage to form a stable and smooth DC voltage to power itself and external devices. Electrolytic capacitors are often the key components for rectification and filtering in conventional switching power supplies due to their large capacitance per unit volume, large rated capacity (can achieve farad level), and low price. Electrolytic capacitors are made of an aluminum cylinder as the negative electrode, filled with liquid electrolyte, and a curved aluminum strip inserted as the positive electrode. The electrolyte is very easy to leak and dry under extreme conditions such as high and low temperatures, which changes its electrical properties and eventually causes the capacitor to fail. Once the aluminum electrolytic capacitor fails, due to its violent reaction to form pressure, it will release flammable and corrosive gases, causing the AC-DC module power supply to fail.

According to the physical structure of aluminum electrolytic capacitors, the circuit shown in Figure 1 can be used as an equivalent, where CAK represents the ideal capacitance between the two electrodes; Rp is the parallel resistance, which represents the leakage current component of the capacitor; Rl represents the series resistance component of the capacitor lead end and the electrode part; L represents the equivalent series inductance component of the lead wire and the connection.

The performance of aluminum electrolytic capacitors mainly depends on the dielectric part, that is, the anode metal oxide film. In addition to being affected by the initial process, the electrolyte will continue to repair and thicken the oxide film during operation. As the anode metal oxide film continues to thicken, the capacitance value C in the equivalent circuit model of the aluminum electrolytic capacitor will continue to decrease, and the equivalent series resistance ESR will continue to increase. At the same time, the hydrogen produced by the cathode reaction accelerates the volatilization of the electrolyte. These are the main factors causing the degradation of aluminum electrolytic capacitors.

Therefore, although electrolytic capacitors have advantages that cannot be replaced by other types of capacitors, they still have defects such as large internal loss, large electrostatic capacitance error, large leakage current, and poor high and low temperature characteristics. Therefore, conventional AC-DC power modules designed with electrolytic capacitors have obvious disadvantages in terms of high and low temperature characteristics, reliability, and service life.

So, what will happen to the power supply product if electrolytic capacitors are not used in AC-DC power supply design? Can AC-DC power supply modules without electrolytic capacitors avoid the above fatal defects?

Recently, Mornsun has successfully designed the LN series of electrolytic capacitor-free AC-DC power modules that meet performance requirements by using valley-fill circuit design of high-voltage ceramic capacitors to replace and optimize the basic functions of electrolytic capacitors. This solves the problems of large size, short life, and poor high and low temperature performance of AC-DC power supplies caused by electrolytic capacitors.

2. Advantages of electrolytic capacitor-free products

Compared with electrolytic capacitors, ceramic capacitors have extremely low ESR and ESL, which can reduce the risk of damage caused by parasitic parameters; at the same time, because the electrolyte of ceramic capacitors is not easy to evaporate or solidify under extreme conditions such as high and low temperatures, the capacity is relatively stable and the electrical characteristics of the capacitor can be maintained for a long time, thereby greatly improving the high and low temperature performance and long-term reliability of power supply products.

1) High efficiency and environmental protection

The LN series adopts valley-filling circuit design, uses high-voltage ceramic capacitors to perfectly replace aluminum electrolytic capacitors, increases the conduction angle of the rectifier tube, and makes the input current waveform closer to a sine wave from a peak pulse, thereby greatly improving the power factor of the power supply (as shown in Table 1), improving the conversion efficiency of the power supply, being more conducive to environmental protection and energy saving, and significantly reducing total harmonic distortion. As shown in Figure 1:

In all the tables below, the old solutions are products using electrolytic capacitors, and the new solutions are new products using valley-fill circuits without electrolytic capacitors.

2) Improvement of product life

The power supply itself is a power device. During normal operation, the power loss is dissipated to the outside in the form of heat. The internal transformer, switch device, rectifier diode, etc. are all heat-generating devices. In addition to internal factors, most power supplies need to be used in a relatively high ambient temperature, which will cause the volatilization of the electrolyte and reduce the service life of the electrolytic capacitor.

Ceramic capacitors use ceramic materials with the most stable characteristics as dielectrics, especially Class I ceramic capacitors (NOP), which can achieve an operating temperature of -55℃~+125℃, and the capacitance change does not exceed ±30ppm/℃. When the temperature of the capacitor changes, the capacitance is very stable, that is, it has a temperature compensation function, which is suitable for circuits that require stable capacitance and high Q value within the temperature change range and various resonant circuits; Class II/III ceramics (X7R) achieve an operating temperature range of -55℃~+125℃, and the maximum capacitance change is ±15%.

From the characteristics of the dielectric of high-voltage ceramic capacitors and the electrolyte dielectric of aluminum electrolytic capacitors, it can be seen that ceramic capacitors can withstand more stringent environmental requirements, which is of great significance to the life and reliability design of power supply products, and can greatly improve the service life and reliability of power supply products.

Mornsun's electrolytic capacitor-free AC-DC power module LN series successfully replaces aluminum electrolytic capacitors with high-voltage ceramic capacitors by adopting valley-filling circuits. This can effectively avoid the poor high and low temperature performance of electrolytic capacitors caused by internal electrolytes; avoid the decrease in capacitance and shortened life of power products due to the volatilization of electrolytes; and even avoid safety issues caused by violent eruption or leakage of electrolytes.

3) Stable high and low temperature characteristics

At present, the rated operating temperature of most conventional electrolytic capacitors is 105°C, but because the electrolyte of electrolytic capacitors is easy to volatilize under high temperature conditions and the power supply itself generates a lot of heat, conventional AC-DC power supplies using electrolytic capacitors can only operate under 70°C environmental conditions. To increase the working temperature of the power supply, it is necessary to use more expensive and larger electrolytic capacitors, or to use derating to achieve applications under high and low temperature conditions. The following is the derating requirements of Jinshengyang's conventional AC-DC power supply products in high and low temperature environments as shown in Figure 3:

Mornsun's LN series can achieve high-temperature operation with little change in cost and volume, and can meet the conditions of -40℃ to 70℃ without any derating requirements. It can be used in high/low ambient temperatures and in situations where the reliability and service life of power supply products are high, such as street light control, LED and other industries.

4) High EMC characteristics

Mornsun's electrolytic capacitor LN series products fully consider different application scenarios and design requirements, and have comprehensively upgraded and optimized the EMC performance of the products. Through PCB design and multi-level EMC filtering inside the module, EMI meets CLSS B without any external protection devices, and the surge protection capability reaches level 4.

Conclusion

The successful development of the LN series of electrolytic capacitor-free AC-DC power modules from Mornsun proves that electrolytic capacitors can be used in AC-DC power design. By using valley-fill circuit design with high-voltage ceramic capacitors to replace and optimize the basic functions of electrolytic capacitors, the problems of poor high and low temperature performance, poor reliability, and short service life caused by the inherent defects of electrolytic capacitors in power modules can be perfectly solved.