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Interesting failure of buck converter
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ogden:

--- Quote from: mikeselectricstuff on May 30, 2019, 10:09:58 am ---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.

--- End quote ---

Synchronous switcher then, definitely. http://www.ti.com/product/LM76002 May blow your BOM, but efficiency is way better than ST part.
MagicSmoker:
Well, honestly you ought to know better than to try to run a 50-55V rated part on 48V, and all just to shave less than a pound off the BOM... Given that the most common failure mode of a buck converter from overvoltage is a shorted switch, you are definitely playing with fire here, and perhaps not entirely figuratively, either.

Personally, I'd consider the MPS MP9846 or the Microchip (nee Micrel) MIC28514 just from a quick sort of buck regulator parts on Mouser that can handle at least 60V input and 3A output current.
mikeselectricstuff:
BOM cost isn't a huge deal, it's more that I'd already seen this part used in a reliable 48v application
Ice-Tea:

--- Quote from: mikeselectricstuff on May 30, 2019, 10:09:58 am ---
--- Quote from: ogden on May 30, 2019, 09:43:46 am ---
--- Quote from: mikeselectricstuff on May 29, 2019, 09:10:47 am ---Unfortunately there aren't many small 48V converters like this at sensible prices

--- End quote ---

You get what you pay for ;) I agree that 50V is way too slim margin for 48V supply, especially when transients can be there. Better add some 1$ to BOM for better regulator and sleep well ;) In your case I would use http://www.ti.com/product/LMR16030

--- End quote ---
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.

--- End quote ---

MP4559 might still be worth a look.
Fraser:
This thread is really interesting as it discusses component ratings. I come from a background where components used in mission critical applications were heavily over engineered. Even when a component had a rating of X Volts, it was a pessimistic value and the components would often have better capability, it was just not guaranteed. We often tested components to destruction and were impressed with the true headroom on a component rather than that indicated by the data sheet. Whilst it is true that it would be unwise to deliberately run a component beyond its maximum ratings in an important role, it might not be such an issue to run it at or near that rating if the manufacturer has already calculated a decent margin of safety to cover issues like transients. If transients are considered a risk them maybe a TVS can be used in the supply to the chip ? In important applications, some decent supply filtering and transient suppression is wise anyway.

If I were in Mike’s position I would sacrifice a few of the chips in a torture test to see at what voltage they fail and the failure mode. Such is useful information anyway as a non safe failure mode would either steer me away from a component or cause me to employ additional safety measures that might otherwise have not been required.

When there is little choice of components for a task, it is sometimes worth a little experimentation to see just what can be used safely without breaking the bank or having to completely change a design. This is, after all, what OEM’s do when prototyping, often to test reliability, but also to see how cheaply a design can be built and still meet the needs of the customer. Those who get it wrong do cause issues with premature component failures though.

Fraser
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