UCC21520: UCC21520A Vds oscillation coupling back to Input

Yang Wu

Part Number: UCC21520

Hi Team,

My customer is using UCC21520 and meet some issues that need your help.

Issue:

Customer found Vds oscillation, measuring PWM from the DSP and it’s good. But if measuring at INA of UCC21520, found similar oscillation on INA. So, we guess the oscillation on Vds affects Vgs (OUTA) and then coupling back to INA.

Questions:

1)Is the guess correct. If yes, How can the oscillation coupling back to INA?

2)What customer can do to eliminate the oscillation on INA? One proposal I think is add ferrite bead on Vgs.

thanks a lot.

Best regards,

Yang

over 2 years ago

0 Krystian Plaskota over 2 years ago

TI__Expert 4065 points

Hi Yang,

Noise coupling from the secondary side to the primary side does not happen through the isolation barrier of the part. It is possible outside the package through electro-magnetic fields depending on PCB layout and component placement. Any high current paths (e.g. DC-link bulk capacitors) can affect the primary side if shielding or careful layout is not observed.

My recommendation for minimizing gate oscillations is to add a ferrite bead as you suggest and also try to see if there is any way to optimize the layout. Minimize parasitic inductance and capacitance, minimize trace length, and ensure the decoupling capacitors are as close to the gate driver as possible.

We have an FAQ about ferrite beads and how to use them here:

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1037571/faq-ucc21520-how-should-ferrite-beads-be-used-and-selected-for-gate-drive-circuits

This appnote is a useful guide covering layout considerations that you can adopt if needed:

https://www.ti.com/lit/an/slua897/slua897.pdf

Regards,

Krystian

0 Yang Wu over 2 years ago in reply to Krystian Plaskota

TI__Expert 3330 points

Hi Krystian，

thanks for your suggestion. we can see how to optimize the layout first and reserve the PCB place for the ferrite bead. thanks.

Best regards,

Yang

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UCC21520: UCC21520A Vds oscillation coupling back to Input