Accurate measurement of the ZCD/CS pin waveform is an important way to fine-tune the robust operation of the UCC28056. To better see what is happening, place the VDS and ZCD/CS pin waveforms on the same scale with the same ground reference, if your oscilloscope allows it. Otherwise, use a 100:1 attenuation probe and a 1:1 probe for the test.

If the VDS waveform is set to 100 V/Div attenuation, click on the ZCD/CS pin channel and select More from the menu. Then select the Fine Scale menu and adjust this channel to 250 mV/Div. The result is 400:1 difference between the scales of the two channels.

Now if the zero line is set at the same point for each channel, the two traces should have the same amplitude.

Since ZCD/CS is sensitive signal, use a low-capacitance probe (below 10 pF) with the smallest test loop between the ZCD/CS pin and ground. Follow Figure 3-1 when setting up the oscilloscope and probe.

Figure 3-1 Fine Scale Menu of the Oscilloscope and Probe Connecting

This method allows a clear check of how well these two waveforms are following each other. The waveform in Figure 3-2 shows the VDS and ZCD/CS together.

Figure 3-2 VDS vs ZCD/CS

To achieve a precise sampling of VDS from the ZCD/CS pin, follow the ZCD/CS Pin section in the UCC28056 6-Pin Single-Phase Transition-Mode PFC Controller data sheet to fine tune the amplitude of ZCD/CS to meet VDS.