UCC2897A: Self synchrounous rectification at secondary side.

Hi Aneesh,

Thanks for your interest in UCC2897A. You should not have a problem implementing self driven synchronous rectification in this design. Your efficiency will not be as high however. This application note provides further detail on the tradeoffs between self driven and IC driven synchronous rectification:

Best Regards,

Ben Lough

Hi Aneesh,

It is possible to get >90% efficiency at 70V input with self driven synchronous rectification but could be challenging. Here is an example of active clamp forward design that achieves ~90% efficiency at 72V:www.ti.com/.../PMP7032

It is difficult to say with absolute certainty how much your efficiency would drop if you implement self driven synchronous rectification and redesigned for a higher input voltage but you will have slightly lower efficiency at the higher input voltage .

As a reference, here is a 24V, 3A active clamp forward design using self driven synchronous rectification:www.ti.com/.../PMP20878

Best Regards,
Ben Lough

Hi Aneesh,

current sensing is accomplished through the CS pin on LM5026. Its looking at the primary side current through a current sense transformer and enabling cycle by cycle limiting when the pin is >500mV.

This works as an output current limit because the primary side current is proportional to the secondary side current. The exact relationship is derived in section 11.1.1 of this user's guide:www.ti.com/.../slyu036.pdf. There is some variation in the primary side current from min to max input voltage but the variation is small enough to provide a stable output current limit of ~33A. As a check, you can plug the LM5026EVM parameters into power stage designer to see how much the primary side current varies across the input voltage range:www.ti.com/.../POWERSTAGE-DESIGNER

Best Regards,
Ben Lough

Hi Aneesh,

The overcurrent limit is implemented as a discrete voltage threshold on the CS pin and the max voltage on the CS pin during one switching cycle occurs at the maximum of the primary side current. Because of this, the overcurrent protection on LM5026 looks at the peak primary side current instead of the RMS primary side current.

From the screenshots you shared, the difference between the maximum primary side current at Vinmin and Vinmax is 190mA. You will see a little variation in the overcurrent trip threshold because of this but overall the overcurrent threshold will be fairly consistent across the entire load range.

Best Regards,

Ben Lough

Hi Aneesh,

Yes, the over current protection scheme is based on the peak current.

Best Regards,

Ben Lough

Hi Aneesh,

A magnetics vendor will be able to answer your question in greater detail but in general, size and cost tend to be prohibitive factors. You are restricted to an "O" shape structure which utilizes much more space than an E core/bobbin equivalent. Winding a toroidal transformer is more difficult so there is some added cost in the manufacturing process. For reference take a look at this application note. You will find some information about toroid transformers on page 43:www.ti.com/.../slup132.pdf

Best Regards,
Ben Lough

Hi Aneesh,

The gate drive source and sink capability is determined by the internal gate driver for OUTA and OUTB. The source/sink capability of the internal drivers are fixed. Typically you want more (not less) drive capability to guarantee fast gate transitions to limit switching losses. An external gate driver is sometimes necessary when you operate at higher switching frequencies or you have larger gate capacitance because you have multiple MOSFET's connected in parallel.

Best Regards,
Ben Lough