Preface
In modern power supply technology, aluminum electrolytic capacitors are essential key components in both linear and switching power supplies. However, in conventional AC-DC power supply designs, aluminum electrolytic capacitors can lead to poor reliability and short lifespan under high and low temperature conditions. So, is there a device that can replace traditional aluminum electrolytic capacitors while improving power supply reliability and lifespan? This article focuses on discussing and analyzing the advantages and disadvantages of high-voltage ceramic capacitors versus traditional aluminum electrolytic capacitors.
I. Design flaws in aluminum electrolytic capacitors
AC-DC power converters, to achieve AC-to-DC conversion, first need to rectify and filter the AC voltage to form a stable and smooth DC voltage to power themselves and external devices. Electrolytic capacitors, due to their advantages such as large capacitance per unit volume, large rated capacity (reaching the farad level), and low cost, are often key components in the rectification and filtering of conventional switching power supplies. Electrolytic capacitors are made by inserting a bent aluminum strip as the positive electrode into an aluminum cylinder containing a liquid electrolyte as the negative electrode. Under extreme conditions such as high and low temperatures, the electrolyte is very prone to leakage and drying out, causing changes in its electrical properties and ultimately leading to capacitor failure. Once an aluminum electrolytic capacitor fails, the violent reaction creates pressure, releasing flammable and corrosive gases, causing the AC-DC module power supply to fail.
Based on the physical structure of aluminum electrolytic capacitors, the circuit shown in Figure 1 can be used as an equivalent circuit, where CAK represents the ideal capacitance between the two electrodes; Rp is a parallel resistance, representing the leakage current component of the capacitor; Rl represents the series resistance component of the capacitor leads and electrode section; and L represents the equivalent series inductance component of the leads and connections.
The performance of aluminum electrolytic capacitors mainly depends on their dielectric component, namely the anode metal oxide film. Besides the influence of the initial process, the electrolyte continuously repairs and thickens this oxide film during operation. As the anode metal oxide film thickens, the capacitance C in the equivalent circuit model of the aluminum electrolytic capacitor continuously decreases, while the equivalent series resistance (ESR) continuously increases. Simultaneously, the hydrogen gas produced by the cathode reaction accelerates the evaporation of the electrolyte. These are the main factors causing the degradation of aluminum electrolytic capacitors.
Therefore, although electrolytic capacitors have advantages that other types of capacitors cannot replace, they still have drawbacks such as high internal losses, large capacitance errors, high leakage current, and poor high and low temperature characteristics. Consequently, conventional AC-DC power modules designed with electrolytic capacitors have significant disadvantages in terms of high and low temperature characteristics, reliability, and lifespan.
So, what would happen to power supply products if electrolytic capacitors were not used in AC-DC power supply designs? Could AC-DC power supply modules without electrolytic capacitors avoid the aforementioned fatal flaws?
Recently, Mornsun has successfully designed the LN series of electrolytic capacitor-free AC-DC power modules that meet performance requirements by replacing and optimizing the basic functions of electrolytic capacitors with valley-filling circuit design using high-voltage ceramic capacitors. This solves the problems of large product size, short lifespan, and poor high and low temperature performance caused by electrolytic capacitors in AC-DC power supplies.
II. Advantages of Electrolytic Capacitor-Free Products
Compared to electrolytic capacitors, ceramic capacitors have extremely low ESR and ESL, which reduces the risk of damage caused by parasitic parameters. At the same time, because the electrolyte in ceramic capacitors does not easily volatilize or solidify under extreme conditions such as high and low temperatures, the capacitance is relatively stable and can maintain the electrical characteristics of the capacitor 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 employs a valley-fill circuit design, perfectly replacing aluminum electrolytic capacitors with high-voltage ceramic capacitors. This increases the conduction angle of the rectifier diodes, making the input current waveform closer to a sine wave than a spike pulse. This significantly improves the power factor (as shown in Table 1), enhances the power supply's conversion efficiency, promotes environmental protection and energy saving, and significantly reduces total harmonic distortion. (See Figure 1.)
In all the tables below, the old scheme refers to products using electrolytic capacitors, and the new scheme refers to new products using valley-fill circuits without electrolytic capacitors.
2) Improved product lifespan
A power supply is a power device. During normal operation, power loss is dissipated to the outside as heat. Internal components such as the transformer, switching devices, and rectifier diodes are all heat-generating devices. Besides internal factors, most power supplies operate in relatively high ambient temperatures, which can cause electrolyte evaporation and reduce the lifespan of electrolytic capacitors.
Ceramic capacitors use the most stable ceramic material as the dielectric. In particular, Class I ceramic capacitors (NOP) can operate in an ambient temperature range of -55℃ to +125℃ with a capacitance variation of no more than ±30ppm/℃. The capacitance value remains stable with temperature changes, thus possessing temperature compensation functionality. They are suitable for circuits requiring stable capacitance and high Q values within a temperature range, as well as various resonant circuits. Class II/III ceramic capacitors (X7R) achieve an operating temperature range of -55℃ to +125℃ with a maximum capacitance variation of ±15%.
The characteristics of the dielectric material of high-voltage ceramic capacitors and the electrolyte material of aluminum electrolytic capacitors show that ceramic capacitors can withstand more stringent environmental requirements, which is of great significance to the design of power supply products' lifespan and reliability, and can greatly improve the lifespan and reliability of power supply products.
Mornsun's LN series of electrolytic capacitor-free AC-DC power modules successfully replace aluminum electrolytic capacitors with high-voltage ceramic capacitors by adopting valley-fill circuitry. This effectively avoids the poor high and low temperature performance caused by the internal electrolyte in electrolytic capacitors; avoids the problem of reduced capacitance and shortened power product life due to electrolyte evaporation; and can even avoid safety issues caused by violent electrolyte eruption or leakage.
3) Stable high and low temperature characteristics
Currently, most conventional electrolytic capacitors have a rated operating temperature of 105℃. However, due to the volatile nature of the electrolyte in electrolytic capacitors at high temperatures and the significant heat generated by the power supply itself, conventional AC-DC power supplies using electrolytic capacitors can only operate in environments up to 70℃. To increase the operating temperature of the power supply, more expensive and larger electrolytic capacitors must be used, or derating must be implemented to achieve applications under high and low temperature conditions. Figure 3 below illustrates the derating requirements of Mornsun's conventional AC-DC power supply products under high and low temperature environments.
Mornsun's LN series can operate at high temperatures with minimal changes in cost and size, and can meet the requirements of no derating in temperatures ranging from -40℃ to 70℃. It can be used in applications with high/low ambient temperatures and high requirements for the reliability and lifespan of power supply products, such as street light control and LED industries.
4) High EMC characteristics
Mornsun's LN series of electrolytic capacitor-free products have undergone comprehensive upgrades and optimizations to their EMC performance, taking into full account different application scenarios and design requirements. Through PCB design and multi-stage EMC filtering within the module, EMI meets CLSS B standards and surge protection reaches level 4 without any external protection devices.
III. Summary
The successful development of Mornsun's LN series of electrolytic capacitor-free AC-DC power modules proves that electrolytic capacitors can be eliminated from AC-DC power supply designs. By using a 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 lifespan caused by the inherent defects of electrolytic capacitors in power modules can be perfectly solved.
Original article address: http://www.mornsun.cn/news/NewDetail.aspx?id=268&channelid=132
