Hi,
I'm testing a simple flyback converter using the STCH03L. All seems as expected, however I can not seem to get the IC to detect an overvoltage condition. To conduct the test I am simply shorting out the optocoupler. When I look at the ZCD pin the reflected signal clearly goes full scale. i.e., clamping at both -60mV and +3V. At the point of demagnetisation the peak that occurs just before the aux winding collapses is hard up against the upper clamp. The slope of the collapsing current takes just over 1uS before the signal passes the +60mV demag. trigger point. This repeats continuously, but the IC does not trigger a fault condition.
Attached are captures of the conditions.
Single cycle OVP
OVP across several cycles
Normal operation several cycles.
I am using a transformer with Pri = 800uH Aux 4.6uH Sec 5uH. Therefore the turns ratio between sec:aux is approx 1:1.
I calculated the Rzcd to be ~ 58K. and I have ~58k in the Rovp position.
Because the sec:aux is 1:1 and I am running Vout at ~4.3V, I am powering the IC from an independent supply not the aux winding. However even simply allowing the IC to start and stop from the HV supply, does not trigger OVP, so this does not seem in any way related to the Vdd supply.
As a last ditch attempt to make the IC trigger OVP. I actually tied the ZCD pin to +12V through a 100K resistor. Thereby constantly forcing the ZCD pin over the 2.5V threshold. It still would not trrigger OVP.
Any ideas?
Thanks
Aidan
