ucc28910 voltage rises slowly at the output

UCC28910 has the minimum switching frequency and minimum peak current, which means there is a minimum power delivery in the system. When you have larger core, the parasitic capacitor is bigger, which also increases the minimum energy deliver to the system. In your case, you need to slightly decrease the pre-load resistor, so that the pre-load can absorb the energy.

which resistor are you talking about .. primary or seconday

It is the resistor in parallel at the output. It serves as a discharge resistor when there is too much input power than load can take.

but there is no much power at the output ... The output is not stable

HI,

There is not much power at the output its just that we have made a custom transformer with the following specs:-
Core:- EE20
winding 2-5 99 turns CW inductance:- 1.109mH
winding 1-4 21 turns CW inductance:- 53.6uH
winding 6-8 6 turns ccw
winding 7-9 6 turns ccw

The schematics is the same as the EVM only the transformer is custom made as per the above specs
We are getting 5V but the voltage is not stable it has ripples also when a 150mA load is connected it drops to 4V..
Please check and let me know what to do....

The resistor on the EVM, R12, 6.8k, is used to discharged and output during no load condition.
The output voltage rises slowly means the output voltage is out of regulation.
Here is what you need to do to find the root cause.
First, looking at your switching frequency, you can monitor the secondary side transformer winding voltage, because this gives you the minimum impact to the switching node. If the frequency stays the same, this means the control loop is saturated and you are running at minimum switching frequency. You need to decrease R12 value to keep the energy balanced. With the minimum switching frequency, there is minimum energy into the system, you have to consume it, otherwise the output voltage is going to rise to achieve the energy balance.
If the converter doesn't stay at the minimum switching frequency, you might have other issues. Maybe the converter is sensing the auxiliary winding load. In this case, your primary side loading (on the auxiliary winding) is higher than the output load. Still you can decrease the R12 value to make the output load heavier. Another option is to adjust R7 value, so that you can get more energy from the transformer leakage and keep the primary side loading low.
Please try reduce the R12 value to see if your problem is solved.
Good luck.

HI,
After adjusting R12 i do get a stable voltage but as soon as i connect the load the voltage drops ...
I think the switching frequency is less ....

how much the voltage drop?

1v drop

that looks to me your preload still too light that output voltage goes up a bit at no load. will the voltage keep constant after you add the load and change the load levels?

When i increase the load the volatge drop..

Even 50ma load drops the volatge... without load it remains constant

NO its not the preload problem ... As soon as i increase the load the swithcinh should increase to satisfy the current ..
But the load voltage drops .. It is not even supplying 50mA to the load.....

reminder

Hi, Aneesh,

  It becomes difficult to support through this.

  Is it possible to share some waveforms so that we can see exactly what happened?

  The switching node waveform and the auxiliary winding voltage would be where we start.

  By the way, is there any TI FAE supporting you or you just come to the forum for support?

  Thanks.

Bing

is there any Ti FAE who can support me.. it would be great ..

can you send me your company and city so that I can ask the local FAE to follow up?

Techsol
Address:- 1st floor, Plot no. 31, Sector 19C,
Vashi, Navi Mumbai - 400705
Landmark:- Satra plaza(Near turbhe railway station)
ph:-022-27841424

Hi Aneesh,I am an AE in Asia area. I am sure I can help you.
To have an efficiency communication, you can send email to me directly. Kening-gao@Ti.com
Here is my understanding on your case, I need you confirm it carefully. Any misunderstanding could lead us wrong way.
Schematic and BOM: Same as UCC28910EVM-526. it is a 5V/1.2A design.
Transformer:99:21:6
Issue: cannot output more than 50mA with stable output voltage
If the above description is right. Could you capture some waveforms and send it to me?
What we need is to test at 50mA Load, with AUX winding, VDD voltage, IPK , Vout in same screeen. in the screen, it would contain 2~3 switching cycles.
If possible, please also share your schematic and PCB layout file to me.
I will reply you asap when i got the waveforms.Br