our customer is using the UCC28631 in following system use case:
- Vin: 350 to 450 V
- Pout nom : 100W
- Vout nom: 24V
I can share the customer's schematics and design file (Excel) via e-mail. Also with some more details about the transformer.
With a smaller load (Dummy of 5kOhm), they see following problem:
" The switching frequency and duty cycle suddenly become too large, as if they stop being regulated and there is no apparent reason why does it happen. The scope plots (CH1:SD, CH2:I_PRIM, CH3:VDD, CH4:VOUT) shown below depict the behaviour that I try to describe.
The supply start and output is regulated to 24V. Suddenly the switching stops, the voltage starts to drop, and when the regulation picks up, the switching frequency is already 120kHz and the duty cycle is ~60%.
This “burst” of pulses continues, with a separation of 5 ms, increasing the output voltage until the OVP kicks in. The zoomed in scope plot show that the error code is 20, and also shows how the duty cycle and switching frequency change."
Different test points:
- Adding 680uF capacitance to the output also helps to decrease how often the “spurious behaviour” occurs, but it still does occur
- 50Ohm @ 200V works most of the time. Increasing the input voltage will make the problem appear again.
Could you please help us understand what issue might cause the problem?
Thanks and best regards
CH1:SD, CH2:I_PRIM, CH3:VDD, CH4:VOUT
- Rload = 5kOhm (applied externally)
- Vin ~= 120V (the input voltage is actually ramping to 350V but the supply already kicks in at 120V)