Flyback converter at TOP-258YN

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This is my first project with TOP drivers, so I have a few problems with that. I count for your's help.
I performanced the flyback converter with TOP 258YN driver (photos at attachment). My desired parameters are output voltage: 12V, output current 8A. But I can't achive the desired parameters. Maximum current is 1,23 A but voltage go down to 6,5V.
In attachment are results of my converter. My converter works in current limit but I don't know why.
One thing I can add the circuit which will help in switch off transistor (based on diode, capacitor and resistor), this circuit can reduce value of oscilating.
What do you thing about value of resistor in transoptor loop?
Could you help me in my problem, please?
Thanks for help.

PCB design48.69 KB
Schematic.png28.45 KB
Drain current2.37 MB
Drain-Source voltage1.96 MB

Hi Mati,

Thank you for using PI product on your design. We need more information about your design to fully understand your problem. Please see below:

1. What is your input AC voltage when you power-up the board?
2. What is the transformer inductance and turns ratio you used?
3. Is the output regulating at 12V with no load condition?
4. Are you Switching ON the SWITCH (S1) on your operation? If yes what is the value of the resistor? Can you try to power up with the switch open?
5. What is the zener voltage of your VR2?
6. Why did you choose to use 91 ohms for R11 and 220 ohms for R12?



Sorry for my late answer, but for a few days I didn't see any response.
1. AC Input voltage is 230V
2. Transformer primary winding: 260,7uH, leakage inductance: <5uH
3. Converter was in load condition at screens which I showed in previous notes. Output current roughly 1,23A. Is that answer for your question? Because I don't understand exactly your question.
4. Yes, switch S1 is on postion ON during operation. Switch S1 is connect with potentiometer (value 10kohm). I can't power on converter with open switch S1.
5. Zener voltage is 10V +/-5%.
6. Because I have same value in reference schematic. Have you got any equation to calculation these resistors?
Thank you advace for your help.
Best regards,

It would appear that your Lpri is not 260uH as seen in the drain current waveform, what are the primary turns and air gap? on what core?

Also the sec loop is too big (open) for 8A out ( 32A pk) you need to use both halves of the o/p diode, also the pri side power tracking is too open as well.

You need to heatsink the pri device and the o/p diode quite well.

Check you have the Tx the right way round ( pri and sec swapped ) ...

good luck ...

Hi Mati,

What I mean for question 3 is when you power up the unit and you remove the load (no output current), is it properly regulating to 12V? Thank you.

Best Regards,

Hello Easy Peasy,
Transformer parameter:
- primary turns: 43
- core: ETD29-K-H-5.0, Phenolic T375J
- no air gap
Yes I use heatsink to diode and TOP switch.
What does mean too open for you?
Best regards,

Hello PI-SevenOfNine,
I performed tests only with some load. But now output voltage don't achieve 12V, it is roughly 11,5V. I have to change value of resistor in Optocoupler circuit.
Best regards,

Hello Mati,

Thank you for the information. When you tried to changed the resistors, did you achieved the required 12V output? Thank you.

Best Regards,