Other Parts Discussed in Thread: UCC25640EVM-020, UCC256304, UCC28056, PMP
I've been having a lot of trouble getting the UCC256404 to start up and run.
Not being able to get a first QR SMPS version to work, I built a second, very standard, 24V 2A version, with the desire to replicate one of the reference designs (I couldn't copy it - as there was no information on the transformer type and turns). So I used the UCC256404 Excel spreadsheet calculator values.
To remove additional issues, I am powering the PFC and QR IC's VCC from an external 15V isolated supply (via a diode). I have shorted BW (pin-8) to GND and removed the diode between the Bias Winding and VCC.
The PFC is giving 388V. It appears fine.
My transformer uses "off-the-shelf" EPCOS parts from Mouser. ETD-44 core set with a 0.75mm gap Ferrite and 45 turns (Litz) of primary, giving Lm of 530uH. I've been experimenting with secondary windings and am currently on 5-turns (Litz) which just reaches the 24V output. I initially had 6-turns and found the output went above 24V to about 30V (the opto feedback appears to be ignored).
I have observed inefficiencies in the secondary coupling that is requiring another turn or two.
I observed the opto feedback switching hard high then low - but not having any impact upon the output. (With the UCC256404 removed) I tested the opto circuit successfully activating when the secondary is around 24V.
Adding a 1k load (24mA) to the 24V output results in a much faster output cap discharge rate versus no-load.
I have re-tested the values of all components used. The ICs are all new (not re-used).
I am getting 1 second start-up pulsing.
Considering the main "variable" might be my transformer, I consider ISNS and VCR to be the only values I could work on (change the capacitor value?).
Do you have any ideas on why it won't start?
I've notice that this is the latest evolution in the QR series. Is there a document describing how to get them going?
Attached should be some files