Interesting failure of buck converter

[Jay_Diddy_B]

Efficiency of the TI part looks way lower - about 82% vs. 91% for the ST, based on nearest graphs in the datasheet - ST has a sync. rectifier fet across the diode.
For this application I'm very constrained for space to get rid of heat.

Hi,

I am not convinced that the ST1S14 has a sync. rectifier FET. The block diagram for the part shows a FET in the sync rectifier position:



But there is no mention of the characteristics of this FET on the datasheet.

The eDesignSuite tool gives diode dissipation that is consistent with no sync FET.



Rectifier dissipation = Vfwd x I x (1 - Vout/Vin)

Assume
Vfwd = 450mV
I = 1.3A
Vout = 13.2
Vin= 48

Rectifier Dissipation = 424mW

No evidence of a sync rectifier.

Regards,

Jay_Diddy_B

[ogden]

We often tested components to destruction and were impressed with the true headroom on a component rather than that indicated by the data sheet.

Oh, I know this "but we tested" very well. Problem with such "in-house" tests - they often are incomplete. When you test all voltage *and* temperature range extremes and their combinations, it appears that  margins are not that big and they are left for component manufacturing tolerances. If you do not test your every finished product in environmental chamber, in whole spec range, it's good idea is to *not* overrate components.

[David Hess]

We often tested components to destruction and were impressed with the true headroom on a component rather than that indicated by the data sheet.

Oh, I know this "but we tested" very well. Problem with such "in-house" tests - they often are incomplete.

Or your test run used golden parts.  Or the manufacturer changes the part in some way.  Or purchasing changes the part to one which is cheaper but has the same specifications.