UCC28704: UCC28704

Pradeep NG

Part Number: UCC28704

Tool/software:

Hi, I am planning to use UCC28704 fly back controller for a power supply design with 230VAC as input and 24V and 2 A (48W) as output.

When i gave these specification in WEBENCH, got the attached design. As per this the Rfbt is 120K and Rfbb is 34K. So as per the calculation the current into the VS pin is 21V/(120K+34K) = 136uA which is less than the

Ivsl(run)-220uA. Can you please help in resolving this ambiguity.

25 days ago

0 Mike O' 24 days ago

TI__Guru* 86266 points

Hello,

When the UCC28704 checks for input under voltage it is when the main FET is on. Rfbb will clamped to ground during this time through the UCC28704 with an internal FET. Only resistor Rfbt should be in circuit.

When the VS pin is drawing - 220 uA resistor Rfbt the resistor will have roughly -26.4 V across it.

You can calculate VIN UVLO Turn-on with the following equation based on the turn’s ratio.

Vin UVLO Turn-on = -(VRfbt)*Np/Na = 26.4 V*Np/Na

Could you double check the design’s input ULVO turn-on based on the transformers turns ratio and voltage across Rfbt?

Regards,

0 Pradeep NG 19 days ago

Prodigy 60 points

Out Np/Na is 6.67. Using this value, the Vin UVLO Turn-on is 176V. So, what is the ideal value of Vin UVLO Turn-on? To get the ideal value should i change the turns ratio or the value of Rfbt?

0 Mike O' 18 days ago in reply to Pradeep NG

TI__Guru* 86266 points

Hello,

You have calculated the typical UVLO turn-on of 176 V based on the equation previously mentioned above. If you need to adjust it, you are correct that you would adjust Rfbt. You will also have to adjust Rfbb afterword to get the correct output voltage for the new Rfbt setting.

Vout = (4.06 V/Rfbb)*(Rfbb+Rfbt)*(Ns/Na)

Regards,

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UCC28704: UCC28704