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Methods to reduce transients and EMI noise in switching applications.

September 22, 2020


      Designing a power supply is a complicated matter. Nowadays, there are various sources of electric energy, and we cannot ignore effective management of these precious energy sources. Line power, solar power, battery power, Ethernet power and energy harvesting technologies are all sources of electrical energy that we can use. Power-receiving loads not only put forward the necessary voltage and current standards, but the semiconductors they use are becoming more and more sensitive. For this reason, the power supply must meet specific specifications. Its requirements are not limited to strict ripple tolerance, but also need to be able to Mitigating the effects of large and rapidly changing load conditions without introducing transients on the power rail and generating excessive EMI. For example, the current of FPGAs used in many computationally intensive applications in the field of machine learning can rise from a few amperes to more than 50A in a few microseconds.

   Technology to reduce transients and EMI.

     The causes of transients are diverse. As mentioned above, high-performance programmable logic and processors will bring significant load fluctuations to the power supply, and the power supply itself may also produce transients. Industrial motor control is another common source of transient noise. High-frequency, high-current dv/dt switching technology can cause larger transients. Unless the combination of filters and passive components can reduce transient amplitude, they may cause permanent damage to the high-side and low-side H-bridge drive transistors. In addition to voltage spikes, transients are also one of the main culprits of electromagnetic interference (EMI). In addition, the widely used high-efficiency AC/DC and DC/DC power switching topologies (such as buck/boost) will also generate EMI on the power rail. Depending on the length of the wire, EMI may be radiated and interfere with the operation of the terminal system and other nearby equipment. Although the use of conventional passive component filtering and foil shielding technology can eliminate induced noise and radiation noise to a large extent, it is not enough for some applications, because these applications require that the background noise must be reduced to an extremely low level. Level.

   For switching converters, to meet the trend of miniaturization, reducing the physical size of inductors and passive components is the only feasible way, but in this way, the switching frequency must be increased. Now, the operating frequency of the switch mode converter can range from hundreds of kHz to tens of MHz, covering part of the AM broadcast radio frequency spectrum. In order to meet these EMI challenges, converter manufacturers, especially those of automotive infotainment systems, are implementing spread spectrum technology to control the switching frequency. Texas Instruments TPS6281x-Q1 is an example of such devices. TPS62810 is an AEC-Q100 automotive-grade part, its default switching frequency is 2.25MHz, and it can be configured to vary randomly with an amplitude of ±288kHz around a certain switching frequency between 1.8MHz and 4.0MHz.

   Another innovative technology for EMI sensitive measurement applications is a DC/DC converter that can supply power to the load through a holding capacitor, thereby temporarily stopping the converter for measurement. TexasInstrumentsTPS62840 is an example of this technology, it has a STOP pin to stop the conversion, thereby eliminating any switching noise. See Figure 1.