Knowles – Looking Closer at Smoothing Capacitors in Electric Vehicles

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Posted by Simon Mao on Mar 9, 2022

In power electronics, rectification is the conversion of alternating current (AC) to direct current (DC). After the AC signal enters a rectifier circuit consisting of power diodes, the resulting raw rectified waveform yields a series of half sine waves with significant ripple. In order to minimize the pulsating DC voltage, a smoothing capacitor is placed in parallel with the load across the rectifier output. As the rectifier voltage rises, the capacitor charges and stores energy like a reservoir. Then when the rectifier voltage falls, the capacitor discharges, greatly reducing the ripple voltage.

rectification and smoothing

Figure 1. Rectification and smoothing effect on voltage (source: Electronics Club)

 

For electric vehicle (EV) applications, smoothing capacitors are commonly used in the on-board charger (OBC), which is responsible for converting AC power from the electric grid into DC power that charges the vehicle’s battery. As shown in Figure 2a, the process is broken up into several steps:

  • Step 1:The electromagnetic interference (EMI) filter protects the vehicle’s electronics by absorbing noise on the AC line.
  • Step 2:The rectifier uses power diodes to convert the AC voltage to DC voltage.
  • Step 3:Power factor correction (PFC) shapes the input current to a power supply, maximizing efficiency and reducing harmonics
  • Step 4:The produced DC voltage is adjusted up or down to provide the correct DC levels to the battery.

functionalOBC

Figure 2a. Functional block for the OBC subsystem

 

 

Simplified block schematic for AC/DC stage of OBC

Figure 2b. Simplified block schematic for AC/DC stage of OBC

 

In Figure 2b, the smoothing capacitor Csis placed after the rectifier and is used as the PFC input capacitor. In order to handle the 3.3 kW to 7.7 kW of grid power used for small vehicles, the capacitor must have a relatively high capacitance ranging in the microfarads range. We commonly recommend our large capacitance, small size multilayer ceramic capacitors (MLCCs), like the StackiCap 205V-2kV 0.1µF-1µF X7R with FlexiCap termination.

Smoothing capacitors play a crucial role in today’s EV applications wherever power rectification is needed, so it’s important to find the right component that meet AEC-Q200 qualifications and other automotive standards. For more information, contact us to discuss your project needs, or download our technical white paper Considerations for Selecting Automotive-Grade Multilayer Ceramic Capacitors in Electric Vehicles.