Active EMI Filter Integrated Circuit Supports High-density Power Supply Design

Texas Instruments (TI) has introduced at APEC today the industry’s first stand-alone, active electromagnetic interference (EMI) filter integrated circuits targeted for high-density, switching regulator applications. The new technology will allow power system designers to reduce the size, cost, and weight of their EMI filter circuits while improving performance and maintaining compliance with EMI regulatory standards.  

The TPSF12C1 is designed for single-phase applications, while the TPSF12C3 supports three-phase commercial supply applications. AEC-Q100-qualified variants of both models are available for use in automotive applications.  
 

TPSF12Cx family of active EMI filter ICs. Image used courtesy of TI

EMI Becomes More Challenging

As switching regulators and other electronic circuits become more dense, susceptibility to EMI from adjacent circuits becomes increasingly problematic, requiring more sophisticated solutions to suppress interferers without expanding solution size or cost.  

TI’s new active filters are specifically designed to reduce common-mode (CM) conducted emissions in off-line power conversion systems. Common-mode conducted emissions are external interfering emissions generated from adjacent electronic systems that enter the circuit through conductive paths like power lines and signal cables. Without proper mitigation (EMI filtering), the interfering signals can create system errors and prevent circuits from operating as designed.  
 

Electrical systems are increasingly dense and interconnected. Image used courtesy of TI

 

EMI Filtering with Passive Circuits

Traditionally, EMI signals are suppressed using passive circuits formed from capacitors, inductors, and inductive chokes that attenuate interfering EMI signals at the desired frequencies.  

In a conventional EMI filtering configuration, CM chokes and Y-capacitors provide CM filtering, while X-capacitors and leakage inductances from the chokes provide differential mode filtering.   

While effective, a traditional EMI filter using only passive components can consume up to 20% of the total available board space in a typical power supply application.

 

Conventional passive EMI filtering circuit. Image used courtesy of TI

 

Active EMI Filtering

Using active EMI filters, like the TPSF12Cx, can reduce the size, weight, and cost of an EMI filter circuit without compromising performance.  

The TPSF12Cx works by sensing any high-frequency noise on the supply inputs through Y-rated capacitors CSEN1 and CSEN2. It then actively generates noise-canceling currents injected back into the supply lines through the Y-rated capacitor CINJ, along with some additional filtering circuitry to ensure stability.   

With a low impedance path for common mode noise at frequencies of interest, the TPSF12Cx provides 15 to 30 dB (more than a factor of 10) of CM noise rejection over frequencies ranging from 100 kHz to 3 MHz.  

 

OBC supply schematic with active EMI filtering. Image used courtesy of TI

 

By replacing the Y-capacitors used in the traditional passive filter configuration, the TPSF12Cx provides “capacitive multiplication” that lowers the required inductance values for the filter chokes, reducing their size, weight, and cost.

According to TI, active filtering can reduce required choke inductances by up to 80%, allowing for chokes that are 50% smaller than those used in purely passive filters. All without sacrificing EMI filter performance.  

Target applications for the TPSF12Cx include AC/DC power supplies, OBCs, UPS, and any power system where common-mode noise dominates and space is at a premium.

 

The TPSF12Cx injects noise cancellation signals into the supply lines. Image used courtesy of TI

 

Other Features

The TPSF12Cx operates over a supply voltage range of 8  V to 16 V and offers integrated device protection through UVLO and thermal shutdown protection. An enable pin allows the chip to be powered off when not used to save power.  

The new active filter circuits are offered in a SOT-23, 14-pin package and meet IEC 61000-4-5 surge immunity standards, which relaxes the requirements for additional protection components like transient voltage suppression diodes.