To reach the aforementioned goals, switching frequencies and slew rates continue to increase higher in modern power supplies. Historically, switching frequencies were kept at sub 100kHz levels, as increases in switching frequency correlate directly to greater switching losses in the system. However, techniques such as soft switching, and technologies such as Gallium Nitride FETs have allowed designers to push the boundaries of switching frequencies in the designers products higher. This drive has led to size reduction in the needed components across the system. This has also led to increasing edge rates for both voltage and current in systems. However, these frequency increases, along with the corresponding harmonics that come with the classical square and triangle waves used in the rectification process of the modern DC/DC converter have created scenarios where noise is becoming a far greater cause for concern. Furthermore, many ICs struggle with extremely high slew rates, which are intentional by design in power systems to minimize switching losses in the transition region of the FET.

Noise enters a circuit primarily in two ways: either through conduction, or through radiation. Conducted noise refers to noise that propagates physically through a system, for example, through wires, traces, or other conductive paths. One example of conducted noise can be a noisy low voltage supply node shared by multiple victim devices. This noise is typically on the order of 150kHz to 108MHz as per CISPR 25 automotive standards (standards for other product types can move these ranges). Radiated noise, also, radiates at much higher frequencies, and is defined by CISPR 25 automotive standards to be between 150kHz and 5.925GHz. Radiated emissions tend to propagate through the air, and require no physical contact with the device under test to necessarily influence change in the system.

The most probable radiated emissions coupling mechanisms from a nearby aggressor to a potential victim are either directly through the victim, parasitic inductive coupling, or parasitic capacitive coupling. Similar to mutual induction, parasitic inductive coupling occurs when a magnetic field is generated and couples into a trace or directly into a victim device. Parasitic capacitive coupling occurs when a neighboring aggressor emits an electric field that couples through a parasitic capacitive path to a trace or victim device. For example, a high speed GaN FET switching node adjacent to an unshielded signal trace can couple through parasitic capacitance created by the distance of the switching node and said signal trace. While less common, radiated signals can also conduct into the system through antenna effects due to poor layout or problematic areas on a PCB where traces effectively act as antennae and attract these signals into the trace. The antenna effect usually occurs with harmonics in the gigahertz frequency range where short trace lengths are ⅒ to ½ wavelength of the emitter frequency.

Figure 2-1 provides a visualization of these EMI types propagating from an aggressor to a victim mounted on a PCB. Most commonly for power supplies, the aggressor can be either the high-current loop of the inductor, or the high-voltage switching node of said power supply. In these situations, multiple victims can have degraded performance due to the susceptibility to EMI. This application note primarily focuses on how to mitigate the effects of conducted and radiated emissions on the TMCS112x and TMCS113x family.