UCC28063: HVSEN pin open test

Hi Mason,

To be complete, HVSEN performs not only OV Protection, but also controls the PWMCNTL output. However, that is beside the question.

The reason that low-line stops switching, but not high-line, is because of Brownout.
When HVSEN voltage is brought below 0.8V, a special test mode for the UCC28063 is enabled. Part of this test mode is to make a lot of the timers in the UCC28063 much faster. This is done to make final test faster for each IC during production.
In particular, the Brownout timer is reduced from 400ms to less than ~2ms. Also, the internal re-start pulses (at 210-us intervals) are disabled.

Since the Brownout threshold at VINAC is ~1.4V, the VINAC signal (rectified sine wave) at low-line has voltage levels < 1.4V for > 2ms. So Brownout fault is triggered and gate-drive stops. Once the ZCD signals are lost, the switching operation cannot restart.
However at high-line, the time that VINAC voltage is < 1.4V is < 2ms, so there is no Brownout-fault and gate-drive continues as long as the ZCD signals are valid. This is the reason that switching operation is stopped only at low-line, during this HVSEN condition.

(Note: even when HVSEN is open, OVP is still provided by OV-detection at the VSENSE input.)

Regards,
Ulrich