Synchronous Buck with Schottky anti-parallel to Synch FET?

[Faringdon]

Hi,
Why do they bother with the SMA Schottky in this Synch Buck. (B360A-13-F)?
At the currents being switched , the internal diode in the Sync FET will have a lower vf than that Schottky, so its pretty much a waste of time?

D4 shows Schottky:
https://www.ti.com/lit/df/tidmb83/tidmb83.pdf?ts=1713876988568&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FPMP31210

B360A-13-F
https://www.mouser.co.uk/datasheet/2/115/DIOD_S_A0002833511_1-2542242.pdf

[NiHaoMike]

Some designs disable the low side drive at light load, under that condition synchronous rectification uses more power than it saves.

[__steven]

I remember reading that it is less so about the forward voltage drop and more about the reverse recovery characteristics of the diodes in question. Almost all IC's have deadtime between when the switches are turned on. During the deadtime, both switches are off, and the low side FET's body diode conducts. This can cause voltage spikes on the output during the body diodes reverse recovery. Sometimes a diode with better reverse recovery characteristics compared to the body diode is used to avoid this.

The datasheet specifies 22ns which seems pretty reasonable, and schottky diodes are pretty much a requirement for this. I could not find the original paper that I got this information from, but a quick google search brought me this:

How a Small Schottky Diode Minimizes Noise in Synchronous Converters:
https://ez.analog.com/ez-blogs/b/engineerzone-spotlight/posts/how-a-small-schottky-diode-minimizes-noise-in-synchronous-converters

You may be right however, if during inductor commutation the external diode has a greater voltage drop than the internal diode, the external diode may not conduct at all. I imagine that these diodes are primarily for lighter load conditions, less than 10Amps by looking at the datasheets.

Let me know what you think.

Steven

[Faringdon]

Thanks, i guess even an SMA Schottky , even if it has to conduct 17A for 50ns or so, it'll take some of the current out of the fet's own diode, and thus reduce reverse recovery   current, and hence reduce noise, and slightly increase efficiency.

[langwadt]

Some designs disable the low side drive at light load, under that condition synchronous rectification uses more power than it saves.

if the FET's body diode was good enough for normal operation it would also work when off