Electronics > Projects, Designs, and Technical Stuff
STmicroelectronics ultra-low Vf Schottky
MustardMan:
I am very intetrested in a part that STmicroelectronics manufactured, and I don't know if they still do, or if there is a replacement.
The part is a ulta-low Vf schottky diode, with a quoted (datasheet) Vf of 120mV at 10A. I have not come across a diode with anywhere near this Vf (at any reasonable current, 1A being reasonable).
Most suppliers list this as an obsolete part, but Digikey lists it as still active (zero stock) with a minimum of 3000. I want less than a tenth of that number! And even if I order, they may come back and say no-dice.
It would appear that a TI part (the SM74611) has a lower Vf (claimed 26mV at 8A), BUT this is not a real diode, it is a so-called "smart" diode. Digikey has that part if you wish to look up how it works. Needless to say that although it is rather unique, it does not have the characteristics of a genuine diode.
Trying to contact ST has been a nightmare, and I still haven't managed to, to ask if there is an equivilant.
Is anyone aware of a replacement, or of another ultra-low Vf schottky?
Cheers, MM
exmadscientist:
Am I to assume you're talking about the SPV1001N30, or one of its close relatives? (You didn't give a part number.)
Because those aren't diodes either, they're MOSFETs with built-in driving circuitry, like the TI part.
In general the physics governing diode barriers does not permit the existence of such a thing, at least not as a true diode. For a Schottky barrier, the lower the forward voltage, the more the barrier leaks... until the barrier reaches zero volts, at which point the thing becomes perfectly leaky, because you've got a wire. The takeaway here is that forward voltage is fundamentally linked to diode characteristics. Advances in diode construction are about better approaching ideal performance, and figuring out how to trade off things we don't care about against things we do care about.
jbb:
I used the SM74611 to get myself out of a jam a while ago. It starts off with about 0.6V drop across MOSFET body diode, charges up an internal capacitor and then turns on the MOSFET as an ideal diode below 100mV. The capacitor runs down and the cycle repeats.
Average power loss is good but the fluctuations in forward voltage could be an issue for some applications.
I was curious, so I added a Schottky in parallel. The reduced voltage drop stopped the internal circuit from starting up, so it actually made the losses worse :D
If you need better behaviour than a standard Schottky, you can drop an ideal diode controller and MOSFET.
jbay:
Ideal diodes are usually good for slow or DC applications or perhaps for 60 Hz rectification, but generally are too slow for application in a lot of faster applications where you might want to use them, like for reducing losses in a DC-DC converter. So I can see why the OP would prefer a real diode.
Good luck!
BrianHG:
--- Quote from: MustardMan on August 31, 2020, 06:14:46 am ---It would appear that a TI part (the SM74611) has a lower Vf (claimed 26mV at 8A), BUT this is not a real diode, it is a so-called "smart" diode. Digikey has that part if you wish to look up how it works. Needless to say that although it is rather unique, it does not have the characteristics of a genuine diode.
--- End quote ---
That TI part is designed exclusively for solar cells. When switched on, it actually needs to open that switch for a small amount of time to charge an internal capacitor to drive it's internal mosfet's gate. It is designed to protect solar cells in the shade & pipe through the most power with the least loss.
Yes, that so called diode literally opens for around 3.5us 32 thousand times a second.
And you cannot cap smooth that thing out, it needs that open time's voltage drop to build a negative charge to drive it's mosfet gate.
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