UCC28634: High current in primary side of flyback , shunt resistor exploded

Amr Wael

Part Number: UCC28634

Hello , I am using UCC28634DR on a PCB Design for a fly back converter , the schematic for my PCB is attached Below ,After the fabrication when I connected the input voltage to the PCB , the 200 mOhm shunt resistor exploded. Tried it with two copies of the PCB. I have used Power bench designer for designing the PCB and I can't find anything wrong with the schematic , do you have any possible causes for this behavior?

12_Flyback.pdf

over 4 years ago

0 Amr Wael over 4 years ago

Intellectual 340 points

i would like to add the required specs of the design
VinMin = 90.0V
VinMax = 265.0V
Vout = 15.0V
Iout = 5.0A

0 Jaden Ning over 4 years ago in reply to Amr Wael

TI__Expert 7080 points

Hi Wael,

I have reviewed your schematic. and found there is no RC clamp network in the circuits. there is only a D1 diode parallel with primary winding .

that means the demagnetizing voltage is low , the transformer may be will saturation after several times switching cycle.

Could you please follow the reference design to re-design the schematic and PCB layout.

here is the link for EVM board design:

https://www.ti.com/lit/ug/sluuax9b/sluuax9b.pdf?ts=1628473293978&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUCC28630%253FkeyMatch%253DUCC28630%2526tisearch%253Dsearch-everything%2526usecase%253DGPN

Thanks.

0 Amr Wael over 4 years ago in reply to Jaden Ning

Intellectual 340 points

I am currently getting this pulse on the VDD , I think this means that the IC is going into the fault mode and auto-restarting , I think you are right that the secondary coil isn't getting enough energy from the demagnetization of the first coil. Will adding the RC clamping circuit solve this issue ?

0 Jaden Ning over 4 years ago in reply to Amr Wael

TI__Expert 7080 points

Hi Amr,

Without energy delivery to secondary and AUX sides , the VDD voltage will gradually reduced . before the VDD touch UVLO point , I believe other protection already happens , maybe is OCP , that let VDD sharply go down and auto restart . there is no doubt RC clamp circuit is necessary for flyback topology . and we are not sure if have any other problems.

Thanks.

0 Amr Wael over 4 years ago in reply to Jaden Ning

Intellectual 340 points

Hi Jaden ,

I suspect in the transformer too that it's not providing enough magnetization to the secondary coil could you please check if this transformer matches the requirements? I couldn't find the same transformer that is used in power bench design in stock anywhere.

This is the nearest transformer that I found and that I am currently using on my PCB

docs.rs-online.com/.../0900766b80106ef8.pdf

0 Amr Wael over 4 years ago in reply to Amr Wael

Intellectual 340 points

I don't think it's over current because I Monitored the current passing through the shunt Resistor Rcs Pin and it never exceeded 800 mV. It's actually always very low since the maximum current that I 've seen on Multimeter is only 0.25 Ampere

0 Jaden Ning over 4 years ago in reply to Amr Wael

TI__Expert 7080 points

Hi Amr,

this over current will happens at leading edge blanking time , it can't be captured by Multimeter , you have to use Oscilloscope to capture it.

Anyway , that is not the critical at present . please correct the RC clamp circuit then test it.

Thanks.

0 Amr Wael over 4 years ago in reply to Jaden Ning

Intellectual 340 points

Hey Jaden ,

Are you talking about R12 and C8 ? I couldn't find their values in the datasheet or the Link that you attached so any recommendations regarding their values ? Also do I need to add any other components beside those two?

Thank you very much in Advance.

0 Jaden Ning over 4 years ago in reply to Amr Wael

TI__Expert 7080 points

Hi Amr,

here attached a application note which shows how to design the RC clamp in section 9.

e2e.ti.com/.../0844.Clean-PSR-Sensing-App-Note-10-5-20B.pdf

Thanks

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UCC28634: High current in primary side of flyback , shunt resistor exploded