Power Tips #157: Reducing conducted EMI in 48V automotive USB Type-C EPR designs

This tutorial examines conducted EMI behavior using an Extended Power Range (EPR) (≥100W) USB Power Delivery (PD) reference design.

Automotive electrical systems are moving beyond the traditional 12V rail toward 48V architectures. The higher bus voltage can reduce the required wire gauge, lower harness power losses, and reduce printed circuit board size by decreasing current for a given power level. At the same time, the transition introduces new design challenges, including higher component cost, additional creepage and clearance requirements, and electromagnetic interference (EMI) from high-power switching converters.

This tutorial examines conducted EMI behavior using an Extended Power Range (EPR) (≥100W) USB Power Delivery (PD) reference design from Texas Instruments (TI). The Automotive USB Power Delivery Reference Design with Two Ports 180W Maximum Each, 24V to 60V Input operates from a 48V source and supports two USB Type-C® ports, with each port capable of delivering up to 36V at 5A, or 180W. The measured results show how a combination of hardware changes, USB PD controller-based synchronization, and dithering techniques can take a design from failing Comité International Spécial des Perturbations Radioélectriques (CISPR) 25 limits to passing with margin.

Figure 1 shows the reference design’s architecture, in which the USB PD controller commands two DC/DC converters through I²C. Each power stage is a synchronous buck converter operating at a nominal 400 kHz switching frequency, while a single dual-port USB PD controller manages both channels.

Figure 1 This block diagram of TI’s automotive USB PD reference design features a USB PD controller and dual-port USB Type-C architecture. Source: Texas Instruments

CISPR 25 defines the conducted emissions test configuration in detail but does not specifically define how to incorporate a USB Type-C load. Using a previously approved test setup for a USB Type-C application, Figure 2 shows the output cables, resistive loads and supporting equipment, along with their integration into the standard automotive test configuration. The intention here is to clearly showcase the measurement conditions and confirm that they are clear and reproducible.

Figure 2 Setup pictures for the conducted emissions test showcase dual-port operation in a conducted EMI chamber. Source: Texas Instruments

During conducted emissions testing, Port A operated at 36V and 5A, while Port B operated at 5V and 3A. Both loads were strictly resistive in order to not affect the EMI testing common from electronic loads.

Several early design choices improved the likelihood of meeting the conducted emissions limits. For example, we selected a 400 kHz switching frequency because CISPR 25 has a frequency gap between 300 kHz and 530 kHz. Placing the fundamental switching frequency noise within this gap reduces the possibility that the fundamental itself will violate a conducted emissions limit. Similarly, adding a common-mode choke helped attenuate common-mode noise at higher frequencies from 30 MHz to 108 MHz.

During testing, we made changes to address a lower-frequency resonance below the switching frequency. To move the resonance at 165 kHz to be well below 150 kHz, we added a 4.7 µF input capacitor across the input and increased the differential-mode inductor from 1 µH to 1.5 µH. The before-and-after scans in Figure 3 show the reduction in low-frequency conducted emissions. These hardware changes helped decrease the amplitude of noise at lower frequencies by approximately 30 dB.

Figure 3 These graphs show the conducted emissions scans before and after the EMI filter hardware changes. Source: Texas Instruments

For higher-frequency emission control, adding a 3.92 Ω bootstrap resistor to each DC/DC converter slowed the turn-on transition of the high-side field-effect transistor. Slowing this transition reduces switch-node ringing and resulting emissions in the 50 MHz-to-200 MHz range. There is a modest reduction in overall efficiency, however – approximately 0.3% to 0.5% at a full load.

Input filtering, component selection, switching behavior and power-stage implementation should first establish a strong conducted emissions control baseline. Firmware-based EMI techniques can then build on that foundation, providing the additional improvement necessary to meet the required limits.

The TI TPS26744E-Q1 USB PD controller provides SYNC outputs, which are clock signals used to synchronize the switching frequency of the two external DC/DC converters and help manage their EMI. There are two mechanisms involved. First, the two SYNC signals can operate 180 degrees out of phase, which helps avoid simultaneous switching of the two converters. Second, the controller’s ability to dither the switching frequency distributes that switching energy over a range of frequencies instead of being concentrated at a single frequency. The example synchronization clock signals in Figure 4 show both mechanisms.

Figure 4 Synchronization signals from the USB PD controller operate the dual-port buck converters out of phase and with dithering. Source: Texas Instruments

TI’s dual random spread spectrum (DRSS) EMI reduction technique combines controlled triangular frequency modulation with pseudorandom frequency variation. With a switching frequency of 400 kHz, this modulation spreads energy around the nominal frequency and its harmonics rather than allowing narrow, high-amplitude spectral peaks to dominate.

To isolate the effect of firmware, our measurements used the same hardware and loading conditions: Port A at 36V and 5A and Port B at 5V and 3A. Only the synchronization and dithering configuration differed.

With both SYNC and DRSS disabled, the dual-port design failed the conducted emissions limit by 10dB at 800 kHz. As shown in Figure 5, strong peaks occurred at the 400 kHz switching frequency and its harmonics, including 800 kHz, 1.2 MHz and 1.6 MHz.

Figure 5 The conducted emissions scan with SYNC and DRSS disabled shows failure at the resonance of the switching frequency. Source: Texas Instruments

Enabling the DC/DC converters’ DRSS while leaving SYNC disabled substantially improved the result. The remaining failures were approximately 2 dB at 800 kHz and 70 MHz (Figure 6).

Figure 6 The conducted emissions scan with SYNC disabled and DRSS enabled shows overall improvement but still failure at 800 kHz. An unknown resonance also appears at 70 MHz. Source: Texas Instruments

The best result came when enabling the TPS26744-Q1 SYNC function and DRSS together. Under the same dual-port loading condition of 180W on Port A and 15W on Port B, the design passed with approximately 3 dB of margin, as shown in Figure 7.

Figure 7 The conducted emissions scan with USB PD controller SYNC and DRSS enabled shows that this configuration passes with margin. Source: Texas Instruments

These results demonstrate passing EMI performance in a high-power 48V USB PD EPR design. In the TI reference design measurements, hardware changes improved lower-frequency behavior, while coordinated SYNC and DRSS optimization reduced the dominant switching frequency emissions and harmonics.

Overall, the measured performance changed from failing by 10dB to passing by 3 dB at 800 kHz. The primary takeaway is that combining practical hardware mitigation with controller-based synchronization and spread-spectrum techniques can provide meaningful emissions reduction in any 48V_IN power supply.

Sarmad Abedin is a systems engineer in TI Power Design Services, currently concentrated in automotive applications. He has been designing power supplies for over 15 years and specializes in DC/DC applications as well as low power AC/DC power supplies. He has a bachelor’s degree in electrical engineering from Rochester Institute of Technology.

Josh Mandelcorn has been an applications engineer in TI’s Power Design Services team for two decades, primarily focused on designing power solutions for data center and automotive applications. He has designed high-current multiphase converters to power core and memory rails of processors handling large rapid load changes with stringent under and overshoot voltage requirements. He previously designed offline AC-to-DC converters in the 250W to 2kW range with a focus on emissions compliance. He is an author or co-author on 17 U.S. patents related to power conversion. He received a bachelor’s degree in electrical engineering from Carnegie Mellon University.

Seong Kim is an applications engineer at TI, focusing on automotive USB PD and DC/DC converter solutions. With over a decade of experience, he has supported embedded and power designs ranging from wireless microcontrollers for Internet of Things to high-speed USB Type-C and USB PD systems in automotive environments. He is listed as an inventor on a pending U.S. patent related to USB PD. He has a bachelor’s degree in electrical engineering from The University of Texas at Dallas.

Power Tips #157: Reducing conducted EMI in 48V automotive USB Type-C EPR designs

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