60 optocouplers and photocouplers. Aside from these, it also applies to non-optical devices that perform similar functions in terms of isolation and signal control. The FluxLink technology developed by PI uses magneto-inductive coupling, where an isolation transformer forms the lead frame of the IC to provide internal isolation. Section 11 of the UL standard defines an important dielectric voltage-withstand test that is used to determine whether a device is capable of withstanding a potential surge equal to the rated dielectric voltage for 60 s without breakdown. This is commonly known as the High Potential (HiPot) test. The IEC 60747-17 standard covers magnetic and capacitive coupling for isolation, and it uses a test voltage that can be a transient, repetitive or continuous voltage under specified conditions. Passing criteria state that no external or internal flashover occurs during testing. For example, in the InnoSwitch3-EP, a 1700 V, high-voltage switch, driver, primary and secondary controller is combined with protection circuitry. Secondary-side regulation is achieved using the proprietary feedback mechanism for the control loop. This internal isolation eliminates the need for an optocoupler, which reduces component count and increases system reliability, but means that the InnoSwitch3-EP requires package safety isolation testing. The goal of this qualification was to demonstrate safety, not only during normal operation but also in the event of a catastrophic failure. Regardless of implementation, isolators must meet safety standards for robust galvanic isolation. They must also be reliable enough to outlast the equipment they are installed in, which, in the industrial world, can mean decades of use. Designers must ensure isolation circuits can withstand electrical stresses that can cause physical damage and reject data-corrupting noise from any number of sources. Therefore, the designer must carefully consider key isolator operating parameters, such as commonmode transient immunity, key timing parameters, such as propagation delay and pulse-width distortion and fieldrelated specifications, such as EMI and RF susceptibility. High-voltage insulation Insulation reliability directly affects the isolator’s ability to safeguard against user exposure to high voltage and it is of paramount importance. The insulator is the heart of the isolation barrier and key to maintaining system safety. It is very important that the insulation be uniform without voids, which can cause a localised breakdown. Insulator uniformity is a function of the insulator material and the fabrication process. The dielectric strength of the optocoupler’s injectionmolded plastic compound can vary by as much as 300% due to voids created during fabrication. In contrast, the CMOS digital isolator uses semiconductor oxide layers for its primary insulator. The CMOS oxide deposition process is tightly controlled and highly uniform, and the resulting variation in dielectric strength is only 20%. Each oxide layer has a breakdown voltage of 500 Vac RMS per micron. Higher voltages (such as 5 kVac RMS) are implemented by simply stacking oxide layers during wafer fabrication. The result is a higher absolute maximum breakdown voltage in a substantially smaller size than optocouplers, and insulator reliability that is independent of the packaging process. Optocouplers require current to bias the LED and some form of bias on the output side. The total input plus output current varies widely, depending on the type of optocoupler. When forward-biased, the optocoupler LED is low-impedance, and device power January/February 2025 | E-Mobility Engineering Isolation across an e-mobility platform (Image courtesy of STMicroelectronics)
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