The wide bandgap materials such as silicon carbide (SiC) and gallium nitride (GaN) for semiconductor switches are able to operate at high switching frequencies with steep rising edges and high voltage. These characteristics enhance the efficiency of switching power supplies but at the same time increase the challenges for EMI compliance. Optimizing for EMI compliance during development phase would reduce effort needs at later stage.

One source of EMI emission is the fast switching MOSFET bridge. A direct approach to reduce EMI is to modify the gate driving voltage of the switching transistors. This requires parallel measurement of the gate driving voltage, the output signal and the emitted radiation, as well as their spectrum.

https://cdn.rohde-schwarz.com/pws/general/pictures/rto_3/EMI-compliance-EMI-of-a-MOSFET-bridge_03_textimage.jpg

https://cdn.rohde-schwarz.com/pws/general/pictures/rto_3/EMI-compliance-EMI-of-a-MOSFET-bridge_03_textimage.jpg

(a)

(b)

Figure 1: Various drive signals of MOSFET bridge and their effect on emitted radiation. © IFE Graz University of Technology Austria

In Figure 1, the EMI emission of MOSFET bridge (red) is significantly reduced by optimizing the gate driving voltage (green). Observing the green waveforms, 1(a) uses a rectangular gate drive signal while 1(b) uses a cascaded two-level gate drive signal. EMI emission is observed in parallel with a near field probe. It clearly shows the amplitude of high frequency components in EMI signal (red) is reduced effectively.

 

The test solution

Rohde & Schwarz oscilloscopes have direct frequency and resolution bandwidth input and fast update rate. Combining with the R&S®HZ-15 compact probe set for E and H near-field measurements and R&S®HZ-17 compact H near field probe set (both 3GHz bandwidth), the source and transmission path of unwanted emissions on a PCB can be easily located.

 

To learn more on this EMI emission test and further optimization steps, click here.

 

 

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