A common choice that engineers face when designing a power system is the choice of DC/DC converter: linear or switched-mode? Generally speaking, switched-mode power supplies (SMPS) are superior, allowing for significantly higher efficiencies than linear regulators. However, SMPS pale compared to linear regulators in EMI.
A chart on linear vs. switched-mode. Image (modified) used courtesy of Advanced Conversion Technology
Texas Instruments (TI) is working to create solutions that provide the best of both worlds–– high efficiency and minimal EMI.
EMI and SMPS
Switched-mode power supplies offer a prime example of an engineering tradeoff, where the very behavior that makes them so efficient is what plagues them with EMI issues.
SMPS works by switching a semiconductor switch on and off to exploit the inductors’ transient behavior for boosting or bucking. While the switching process yields high efficiencies, it is also what directly causes EMI issues.
An example of a buck converter in the form of an SMPS. Image used courtesy of Components101
Ideally, the switch will have an infinite impedance when opened and an impedance of zero when closed. This perfect switch would allow for an instantaneous change between these two states.
Unfortunately, real-world switches experience rise and fall times as the state changes. These transitions cause EMI from SMPS, where the non-linear spikes in current cause harmonic components on the high-frequency SMPS waveforms. The resulting voltage spikes can cause EMI and RFI to exceed 100 MHz.
TI Integrates an Active EMI Filter
Generally, what designers do to mitigate the effects of EMI in SMPS is to use EMI filters, which are conventionally external from the rest of the SMPS IC.
Today, TI released a new family of synchronous DC/DC buck controllers claimed to be the industry’s first DC/DC buck controller to integrate an active EMI filter.
The integrated filter can reduce overall board size by up to 50%. Image used courtesy of Texas Instruments
This new family is headlined by the LM25149-Q1, which uses the active EMI filter and dual-random spread spectrum technology to achieve a conducted EMI reduction of 55 dBuV across multiple frequency bands. By integrating the filter, design engineers can reduce the size of the external EMI filter; according to TI, this allows engineers to reduce the overall area by 50%.
A buck converter utilizing active EMI filter with sense and inject capacitors, along with components for compensation. Image used courtesy of Texas Instruments
Another highlight of this family is the LMQ61460, which approaches low EMI by integrating internal bypass capacitors to reduce parasitic inductances in non-integrated solutions. The LMQ61460 operates between 3–36 V, tolerating up to 42 V for surge protection. Like the LM25149-Q1, this IC also benefits from a pseudo-random spread spectrum to minimize peak EMI in any given frequency band.
High Efficiency Without EMI Concerns
With more fields becoming electrified, it’s essential to minimize design tradeoffs across the board to bring the best possible electrical systems to life. TI is attempting to do just that by creating SMPS solutions that minimize EMI effects so that applications like EVs can benefit from the high efficiency without EMI concerns.
