UCC28070: Is UCC28070 can apply square or trapezoidal wave?

Hello Roy,

The UCC28070 cannot operate with a perfect square wave input, because the AC input is rectified and a rectified square wave would result in DC at the boost-stage input. The UCC28070 VINAC pin looks for zero-crossings to help detect changes in the peak input voltage.  A square wave would have virtually no zero-crossing and so the qVFF gain would be stuck at the lowest setting (it starts up at "high line" level 8 by default). Some dead time in the waveform would avoid this.

The UCC28070 can probably operate properly with a trapezoidal input wave, provided that the slopes of the trapezoid wave are similar to that of the sine-wave zero-crossings of 264Vac, 63Hz. Maybe even higher, such as 100Hz. There is a point where the slope is too steep for the zero-crossing detector to detect the rectified notch at the VINAC pin.  Noise filtering capacitance may "fill in" that notch and prevent the VINAC signal from falling below the 0.7-V zero-crossing detection threshold.

Finally, in order to follow the input voltage, the current loop bandwidth must be high enough to avoid significant overshoot when changing from fast rising edges to the plateau voltage.

Regards,
Ulrich