Comparison of EMI for single and dual output device with and without shield

AN2934

22/27

Doc ID 15394 Rev 1

6.1

Main switch waveforms

Figure 16 shows the MOSFET’s drain voltage and current waveforms for a minimum line of

85 V, and Figure 17 shows the waveforms at a high line of 264 V. Both measurements have

been taken at a full load of 12 V at 0.5 A.

6.2

Frequency jittering

Figure 18 shows the drain current and Vfb at maximum load. Jittering causes the drain

current and the feedback voltage to modulate with a triangular wave. If the power supply had

been operating at a fixed frequency, the drain current would be proportional to the output

power. If you compare three or more current waveforms, you can see that the middle one

stays still while the ones to the left and right tend to fluctuate from the middle. This indicates

that the switching frequency is being modulated.

Figure 18.

Frequency jittering

Figure 16.

85 V

Figure 17.

264 V

AM03430v1

AM03431v1

AM03432v1