UCC25600: UCC25600 - open loop LLC operation , i.e. always resonant condition

Raja Saha1

Prodigy 40 points

Part Number: UCC25600
Other Parts Discussed in Thread: UCC21520

Hi All,

I am using isolated DCDC converter with LLC topology, where primary side has full bridge & secondary diode bridge.

I am thinking to use UCC25600 as controller for this LLC.

I always want to open in open loop, i,e, always at resonant switching condition, to get the maximum loss benefit.

I am thinking to use the below type schematic with UCC25600:"-

GD1 & GD2 are the pulses to the two leg of LLC .

Any thought on this, will it work?

I have seen application of UCC25600 with closed loop LLC, but here I want to make it work as in Open loop.

Is the Schematic section is correct, for open loop or always resonant condition operation for LLC?

Please give your feedback.

Regards

Raja

over 1 year ago

0 Manikanta Pallantla over 1 year ago

TI__Genius 13245 points

Hi Raja,

Connecting a simple resistor will make the controller to operate at a fixed frequency.

GD1 and GD2 are ground referenced signals. Also, they have very less gate drive current sourcing/sinking capability. They needed to be connected to external gate drivers to drive the two legs of the full bridge LLC.

Regards

Manikanta P

0 Raja Saha1 over 1 year ago in reply to Manikanta Pallantla

Prodigy 40 points

Hi Manikanta,

Thanks for this reply.

This seems to me inline, what we have thought.

And yes GD1 & GD2, are fed to UCC21520 TI gate driver to drive those MOSFET. I use 4.7V Zeners to clamp them to 5V. The following is the final schematic..

I hope these are fine..

Pls comment, if you find something change required..

Regards

+1 Krystian Plaskota over 1 year ago in reply to Raja Saha1

TI__Expert 4065 points

Hi Raja,

I recommend some output resistance and ferrite beads on OUTA and OUTB to help protect your FETs and minimize ringing.

Please make sure all decoupling caps are <5mm away from the driver pins.

I recommend a gate to source resistor of 5.1k-20k Ohms which should reduce risk of dv/dt induced turn on.

Aside from that the schematic looks fine.

Here is some additional information for when and why to use ferrite beads:

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

Regards,

Krystian

Power management

Power management forum

UCC25600: UCC25600 - open loop LLC operation , i.e. always resonant condition