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Hello,
I'm designing an AC/DC power supply 115Vac@400Hz to 32Vdc and in primary side I’m facing startup issues with UCC28780RTE.
in the power on phase I get in SWS pin around 3.01V and for VDD, 1.83V, and it doesn’t allow the primary side components to get supplied.
I've used the excel calculator provided by TI to configure the UCC28780 for our application.
I’ve checked all the components in the board and all were according to schematic.
I've obtained the following curves during startup:
startup sws (red) & hvg (yellow):
startup Vdd(yellow) & SWS (red):
startup vs(yellow) & SWS(red):
Hello Marcos,
Thank you for your interest in the UCC28780 ACF controller.
This kind of start-up behavior is not normal for the UCC28780, but it is not clear whether the UCC28780 is causing the start-up issue or is a victim of it.
You mention that all components are "according to schematic", however you have not provided the schematic diagram.
It is possible that you have included a sub-circuit that is intended to operate from primary-VDD which suddenly draws a burst of bias current when VDD reaches ~3.5V. This current overcomes the current supplied into SWS and pulls down VDD to the point where the sub-circuit turns off and VDD starts charging up again.
Without the schematic to examine, I can only conjecture this possibility.
Please check your design for unwanted loading on the primary VDD during charge-up of the VDD capacitors.
Regards,
Ulrich
Hi Ulrich,
Good Day. I send a friendship request with regard to your query. Can you help to check it out? Thank you very much.
Best Regards,
Ray Vincent