TI’s Active EMI Filters to Slash Size of Power Supply Designs

Today, electronic designs are increasingly impacted by electromagnetic interference (EMI). As devices become smaller and denser, they experience greater EMI because of the proximity between the aggressor and victim. One likely source of EMI is switched-mode power supplies, which are notoriously noisy. Now, with denser designs, the impact of these power supplies is becoming more pronounced.

To help address such EMI issues, Texas Instruments released two new active EMI filter ICs this week, both of which enable smaller and lighter EMI designs. In this article, we’ll talk about switched-mode power supplies as a source of EMI, active EMI filters, and the new solutions from Texas Instruments.

EMI issues stem from rapidly changing currents and voltages within an electronic circuit. Specifically, the surge current and peak voltage caused by dv/dt and di/dt during system operation leads to induced magnetic and electric fields, which can be conducted or radiated and interfere with nearby traces or electronic devices.

Based on this understanding, it is clear to see why switched-mode power supplies are such a strong source of EMI.

Switched-mode power supplies operate on a transistor's rapid-turning ON and OFF to manipulate the current through an inductor. By judiciously and rapidly turning this transistor ON and OFF, the circuit exploits the expanding and collapsing magnetic fields on the inductor, allowing switched-mode power supplies to either buck or boost an input voltage for a well-regulated output.

However, this rapid ON and OFF action of the power supply generates large discontinuous currents in the system. High speed and often high current switching results in radiated and conducted emissions because of the charging currents in the system and the voltage ripple on the output of the system.

One way to minimize the impact of EMI on switched-mode power supplies is to use active EMI filters (AEFs).

Unlike passive EMI filters, which use inductors and capacitors to create an impedance mismatch in the EMI current path, active EMI filters target sensing noise in the power supply and produce complementary noise to cancel out the overall EMI effects. To do this, AEFs consist of a sense amplifier in a negative feedback topology with a series of compensation and damping components in the feedback chain. Together, these components sense the noise voltage, amplify it, and inject a cancellation current into the system.

Because of their active nature, AEFs provide exceptional performance and significantly reduce area and volume compared to passive filters. In fact, compared to a traditional passive filter, AEFs can result in a nearly 50% smaller area and 75% less volume than passive solutions.

This week, Texas Instruments released two new standalone active EMI filter ICs designed to enable small and lightweight filtering of EMI caused by switched-mode power supplies.

Two highlights of the new family are the TPSF12C1-Q1 and TPSF12C3-Q1 for automotive applications. Uniquely, the ICs in this family fully integrate sensing, filtering, gain, and injection stages, as well as compensation and protection circuitry, into a single package. These features, combined with the AEFs' capacitive amplification, allowed TI to significantly reduce the size of EMI filters. TI claims as much as an 80% reduction in common-mode choke inductance values.

The company hopes these new products will help enable more space- and cost-efficient implementations of EMI filtering for a future of more lightweight vehicles and electronics.