LLC with synchronous rectifiers?

[cupoftea]

Hi,
The LLC converter is the only common converter that can utilise a synchronous rectifier controller in the secondary side, and that has no communication with the primary side, and has no current sensing in the synch rect FETs.

For example, the UCC24624..

https://www.ti.com/lit/ds/symlink/ucc24624.pdf?ts=1739009941824&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUCC24624

....Any other hard switched converter with synch rects, including the PSFB, needs current sensing in the synch rect fets. (if its SR's are controlled only by a "sec_side_only" SR controller)
The LLC doesnt need this...because there is far far less ringing on the secondary side with an LLC converter.
The drain voltages of the synch rect FETs are well behaved, and can be used as a reliable signal for turning on/off the synch rect fets.

Why is it that the UCC24624 datasheet makes no mention of this?.....they would massively increase sales if they did.

 

[Easy peasy]

a badly designed LLC with cross over current in the o/p rectifiers will need current monitoring

there are loads of fast uP controllers converters that calc the turn on and off times for the sec side rectifiers ( and they allow for GD delays ) - these do not need current sensing per se - but use the pride side CT or the shunt on the sec side to know the converter operating point - and Vout and Vin to make the calcs on the fly . . .

 

[dick_freebird]

There may be other ways to skin it. Like Vicor used (may or may not still) the switch node undershoot and servoed timing to "stick the landing" by turning off at the near zero
current that makes it so.

But the current was not sensed for timing, an easy proxy was. One transistor set up right. Super elegant.