Reduce EMI and power loss in dual-battery automotive systems

Article By : Timothy Hegarty

This article highlights a power-stage layout for lower EMI in automotive DC/DC regulator circuits that use a controller with external power MOSFETs.

The traditional 12V automotive lead-acid battery architecture has reached its usable power limit, given the growing load-power requirements of advanced automotive systems, the conversion from mechanical components to electrical functions for reduced weight, and increasingly strict Carbon Dioxide emission regulations [1-3]. To address this limit, automakers introduced a dual-voltage electrical system that combines a smaller 12V battery (for compatibility with existing systems) with a 48V Lithium-ion battery pack. The 48V system can run high-power loads including powertrain (electric super/turbochargers, regenerative braking) and chassis/safety (electric power steering, body roll stabilization).

This 48V/12V dual-bus architecture, shown in Figure 1a, is designed to improve the performance of conventional internal combustion engine vehicles with less of the cost/weight penalty incurred by installing a full hybrid drivetrain. An integrated starter-generator (ISG), belt starter-generator (BSG), or belted alternator-starter (BAS) supplies recuperated energy to the 48V battery and enables some level of power assistance, allowing fuel savings that were previously reserved for high-voltage hybrid technology.

Figure 1 An automotive 48V and 12V dual-bus architecture using two batteries and a high-voltage motor-generator (a); 48V battery voltage ranges interpreted from the LV 148 standard (b).

A DC/DC buck regulator [4] from the 48V battery feeds the conventional 12V battery in order to power customary low-voltage loads such as control units, ignition, lighting and infotainment. Alternatively, a bidirectional buck/boost regulator plus safety switches [5] enables both batteries to simultaneously supply the load if needed. The basic half-bridge switching cell is the same in both cases and is scalable to meet higher current demands by paralleling multiple phases.

This article presents an optimized power MOSFET and decoupling capacitor layout arrangement for such regulators. The objective is to minimize power-loop parasitic inductance and reduce voltage overshoot during switching commutation. Three advantages ensue: lower electromagnetic interference (EMI), lower switch voltage stress and better conversion efficiency.

48V battery voltage variation

Voltage levels and limits under different operating conditions for the 48V battery are defined in automotive standards such as LV 148 / VDA 320 [6] and ISO 21670 currently in definition (Figure 1b). The E48-02 dynamic overvoltage test in LV 148 specifies a maximum voltage on the high-voltage (HV) port of up to 70 V for at least 40 ms. The system must remain functional without any loss of performance during an overvoltage event.

For semiconductor suppliers, this means that everything connected to the 48V battery must withstand 70 V on the input. The automotive industry considers a 10-20% reliability safety margin , so systems and components on an unprotected 48V rail are typically rated for 100 V to meet this expectation [3].

Automotive DC/DC regulator EMI challenges
The low-frequency EMI spectral amplitude of a power supply is relatively easy to manage using a conventional EMI filter stage. However, a greater concern relates to the harmonic content from the high slew rates associated with the sharp edges of voltage and current during switch commutations. In addition to these voltage and current slew rates, overshoot/undershoot and the subsequent ringing of the switching waveforms also pose an issue.

Figure 2a shows a buck/boost regulator schematic with HV and LV ports designated BN48 and BN12, respectively. Figure 2b shows the switch-node voltage waveform when operating in buck mode. The switch-node voltage ringing frequency ranges from 50 MHz to 250 MHz, depending on the resonance of the power-loop parasitic inductance (L_LOOP) with the MOSFET output capacitance (C_OSS) and inductor parasitic self-capacitance (C_EPC). Such high-frequency content can propagate by near-field coupling [7] and is difficult to attenuate with conventional filtering. Synchronous MOSFET body-diode reverse recovery imparts similar negative effects, exacerbating the ringing voltage as the diode recovery current flows in the parasitic loop inductance.

The energy stored in the power loop parasitic inductance, before MOSFET commutation, is responsible for the switch voltage spike, and this energy dissipates during the subsequent damped oscillation. The energy lost per switching cycle multiplied by the switching frequency results in extra power dissipation and thermal management challenges. The EMI filter or snubber components to control the related emissions add further power losses, as well as increased cost. As mentioned previously, reducing parasitic loop inductance is the main technique to mitigate EMI and improve overall efficiency.

Figure 2 Synchronous buck/boost regulator power stage schematic (a); buck switch-node voltage waveform (b); equivalent circuits during MOSFET turn-on and turn-off transitions for buck operation (c); and boost operation (d).

[Continue reading on EDN US: Critical loops for EMI]

Timothy Hegarty is a systems and applications engineer in the power products solutions unit at Texas Instruments.

References

1. “Driving the green revolution in transportation,” by K.-H. Steinmetz, Texas Instruments (TI) white paper SSZY026, Sept. 2016.
2. “Transmutation of the automotive electrical system,” by Lou Frenzel, Electronic Design, Nov. 2018.
3. “Bridging 12 V and 48 V in dual-battery automotive systems,” by Jiri Panacek, TI white paper SLPY009, Nov. 2018.
4. TI LM5146-Q1-EVM12V synchronous buck controller evaluation module.
5. Bidirectional DC-DC Converter Reference Design for 12-V/48-V Automotive Systems, TI.
6. “VDA 320 – Electric and Electronic Components in Motor Vehicles 48 V On-Board Power Supply,” 2014 Edition, German Association of the Automotive Industry (VDA).
7. “The Engineer’s Guide To EMI In DC-DC Converters,” by Timothy Hegarty, how2power.com EMI guide landing page.
8. “Layout Considerations for LMG5200 GaN Power Stage,” by Narendra Mehta, TI application note SNVA729A, Sept. 2015.
9. “Evaluation of Switching Loss Contributed by Parasitic Ringing for Fast Switching Wide Band-Gap Devices,” by Zheyu Zhang, Ben Guo and Fei Wang, IEEE Transactions on Power Electronics, 2018 early access.

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