www.we-online.com/en CAPACITIVE DIGITAL ISOLATORS 23 www.power-mag.com Issue 3 2024 Power Electronics Europe SiO2. The signal is transmitted across the isolation barrier using a modulation process known as on/off keying. The onchip oscillator is used to modulate the input signal, which is driven by a Schmitt trigger. The modulator generates a differential signal that is transmitted across the capacitive isolation lines. Figure 4 shows the basic structure of a capacitive digital isolator. The demodulator, on the secondary side of the isolator, performs the functions of amplification, filtering and reconstruction of the input signal. The signal delay and signal distortion are minimal. Finally, a buffer routes the signal from the demodulator output to the overall output, whereby the buffer amplifies the signal to the required level. Figure 5 illustrates the internal structure. Digital isolators are manufactured using standard CMOS technology, which means they use familiar and proven materials and processes. The transmitter and receiver capacitors are mounted on a lead frame. The capacitors themselves, shown in gray in Figure 4, are located between the two horizontal contacts shown in red. The dielectric material between the electrodes or capacitor plates serves as a galvanic isolation barrier. The isolation thickness achieved by the process is in the range of a few tens of micrometers. In digital isolators, SiO2 is used as the isolating material in the capacitor because its much higher dielectric strength of 500 V/µm means that it requires considerably less space for the isolating gap. Other common insulation materials, such as polyimide, have a dielectric strength of only 300 V/µm. The two capacitors are electrically connected with a bonding wire so that two capacitors are connected in series, as shown in the block circuit in Figure 4. To protect the entire structure, the die and leadframe are molded using a standard IC assembly process. Reliability and safety Digital isolators are designed to protect people from dangerous voltages. They must therefore meet the highest safety and durability requirements. The digital isolators of the CDIP and CDIS series from Würth Elektronik have been certified by the VDE in Germany according to the latest and most demanding standard DIN EN IEC 60747-17 (VDE 0884-17):2021-10 “Magnetic and capacitive couplers for basic insulation and reinforced insulation”. But what do the terms “basic” and “reinforced” actually mean for the safety of a person? The standard itself only gives a rather abstract definition here, see IEC 60747-17:202X: So, when do you use basic or reinforced isolation? Simply put, it comes down to “single fault condition” and “normal operating conditions”. Reinforced insulation provides protection against electric shock even under single-fault conditions in normal operation. Basic insulation is only effective in normal operation, i.e. without considering a single fault. Literature: [1] Uludag, T.: Reliable Galvanic Isolation, Simplified. Power Electronics News, December 2023, S. 6ff. [2] Digital Isolators WPME-CDIS from Würth Elektronik: https://www.weonline.com/en/components/products /DIGITAL-ISOLATORS-WPME-CDIS Authors: Timur Uludag graduated with a degree in mechatronics engineering from Regensburg University of Applied Sciences. He then worked for several years as a hardware engineer in the fields of switching power supplies and analog circuit design. Since 2015, Uludag has been working as Senior Technical Marketing Manager at Würth Elektronik eiSos in the MagI?C Power Modules business unit. There he specializes in the roadmap planning and market launch of new power modules. Dr. Heinz Zenkner studied electrical engineering with a focus on communications and radio frequency technology and holds a doctorate. He has been a publicly appointed and sworn expert for EMC for many years. In addition to numerous scientific publications, he is a frequent author of many works on EMC. Heinz has also worked as a lecturer at various universities, at the Chamber of Industry and Commerce and at numerous seminars. He has been involved in industrial electronics for many years, from the initial idea of a product through to series production. He is particularly interested in wireless energy transmission, for which he has developed his own theoretical and practical concepts. Figure 5: Block diagram of the digital isolator
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