WEBENCH® Tools/UCC28600: Designing poblems

Hello MM86,

Thank you for your interest in the UCC28600 flyback controller.

Certainly it is possible to design a 40-W, 24-V flyback converter operating from 40Vac to 265Vac using this controller.
The 40Vac low-end input is an unusual requirement, but it is not impossible to accommodate. A relatively large value of bulk capacitance will be needed to keep the minimum ripple valley voltage high enough to keep the transformer turns ratio reasonable.

It is not possible to determine the cause of your limited output power and voltage from the description you provided.
Do you have a physical prototype board operating (or partially operating)? If so, can you provide the schematic diagram of it, please?
Also provide the details of the transformer being used (inductance, turns ratios) and a copy of the Design Calculator Tool you used to generate the component values.

I do not understand what you mean by "I measured the feedback pin and it discharge the feedback capacity every cycle completely."
How are you measuring the feedback pin? Oscilloscope probe? Voltmeter? Something else?
What "cycle" are you referring to: the switching cycle or the ac-line cycle, or some other cycle? ( I can't imagine that the capacitance on the feedback node can discharge and recharge every switching cycle. ) If you can provide some waveforms, this can also help to resolve your issues.

Regards,
Ulrich

Thank you for your fast feedback. Attached you can find the excel document I used to calculate the values.

The transformator has the following values:

N_Pri/Sec=5

N_Pri/Bias =10

L_P = 109uH

The problem I mentioned above was, that the voltage at Cfb is getting down to zero every cycle. The following oszi screenshot shows this problem. It was taken by 66V and about 40W output power, but this problem I have by lower input voltages too.

CH1=C_fb

CH3=Output Voltage

CH4=Gate Voltage MOSFET

The trafo was coiled myself and it is sometimes noisy. Could the trafo be the cause for the problem? Is the circuit in principle correct? Can you say me, what I have to change, so that it works?

 

slvc104j - TI - Kopie.xls

Hello MM86,

Thanks for providing the additional information.

Looking at the FB signal behavior, this looks like a large-signal instability. I assume that VR in the schematic is a TL431 shunt regulator.
(If it is something different, please let me know.) The compensation cap Ccomp is only 200pF. Normally, I'd expect this value to be much larger (at least an order of magnitude or more), sometimes with a series resistance. This may be the main cause of the instability, but there may be other contributors.
I'll step through the design calculator and verify that the values make sense and that the schematic matches it.

The audible noise from the transformer is probably from the on-off nature of the unstable operation seen in the screenshot.
Also, hand-made transformers are usually "loose" and can vibrate more easily than varnished ones. Don't worry about your trafo yet.

The circuit arrangement looks correct, in principle, but various component values may not be correct.
I'll need some time to analyze this information. I hope to have a response in a day or two.

Regards,
Ulrich