Problem with UCC2894

Lokesha MH

Prodigy 80 points

Other Parts Discussed in Thread: UCC2894

Hello,

We are using UCC2894 for flyback converter.

Initial status of all pin are

LineUV >1.27, LIneOV<<0.1

Vref=5, Css charges to 4.5V and FB =5V, Rslope is as expected.

Problem arises when i slowly increase the Vin. At some value of Vin controller enters hiccup mode even before output reaching its regulation point that is rated o/p. We checked FB it is stiff at 5V, But Css Doesn't starts up, that is Css keeps charging and discharging (reaches 1.5V and discharges again to 0.5V) continuously . Why it is so?

We checked LINEUV and LINEOV both are proper.

Please support us in resolving this problem.

Thank you..!!

over 8 years ago

0 Billy Long over 8 years ago

TI__Expert 4015 points

Check the Vdd voltage, it sounds likethe bias cap is depleting before the output has come into regulation and the Aux winding voltage is high enough to supply the chip current. See equations 14-16 on the datasheet.

0 Lokesha MH over 8 years ago in reply to Billy Long

Prodigy 80 points

Hi, Thank you for answering.
Yes, we have checked VDD and it was constant at 14.5V. Also we are not powering UCC2894 from auxiliary winding, instead we have separate power supply. So the supply voltage is independent of auxiliary winding.

0 Billy Long over 8 years ago in reply to Lokesha MH

TI__Expert 4015 points

Can you attach your schematic for review. Is the SS pin discharge linear or exponential (resistive)?

Per the functional block diagram on the datasheet, the only things that should fire the SS discharge circuitry are Line UV, Line OV, Vdd UV and Vref UV. If all those signals are good, then the SS pin should not discharge.

0 Lokesha MH over 8 years ago in reply to Billy Long

Prodigy 80 points

Printing Print Schematic.pdfSS discharge is linear. And the schematic is attached here please note that the auxiliary winding is disconnected and device is separately powered.

0 Billy Long over 8 years ago in reply to Lokesha MH

TI__Expert 4015 points

I don't see anything egregious here, is this happening at about 34V on the input. Does it persist at higher voltages?

0 Lokesha MH over 8 years ago in reply to Billy Long

Prodigy 80 points

It happens at about 95VDC input. Yes this problem persists at higher voltages.
Initially we set LINEUV at 35V (LINEUV > 1.3V at 35V and above) and input was raised gradually, till 95V DC everything was proper,output also was increasing accordingly. But as soon as input hits 95VDC problem starts.
We again increased the LINEUV to 95VDC and started increasing the input still the same problem occurs. We confirmed same behavior on a different board as well.

0 Billy Long over 8 years ago in reply to Lokesha MH

TI__Expert 4015 points

Can you try testing it with R25 removed.

0 Lokesha MH over 8 years ago in reply to Billy Long

Prodigy 80 points

Yes we tried with R25 disconnected, no improvement was noticed.

0 Lokesha MH over 8 years ago in reply to Billy Long

Prodigy 80 points

Hello,
We found that with the soft start capacitor (Css) removed circuit works fine, but once we put back the soft start capacitor (Css) same problem arises. Can some one helpout as we are stuck ?

0 Billy Long over 8 years ago in reply to Lokesha MH

TI__Expert 4015 points

As I said previously, only the Line UV, Line OV, Vdd UV and Vref UV can activate the SS pin discharge, so I would focus on these pins.

Tie the Lin UV high, tie the Line OV low, bias Vdd with an external bias as you have, and I guess probe Vref to make sure it is ok. Are you sure that you don't have a component value error anywhere?

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Problem with UCC2894