low Vf diode

I am looking for a very low Vf diode for a low power flyback. The switching frequency is low - max 30K dow to 7K, the peak current is low (200mA absolute max), the average current only in the order of

10mA, but the system efficiency is of paramount importance. With a load voltage of just 1.1V the diode Vf becomes a very significant item. Worst case (fault) diode reverse volts is 11.5V

Unfortunately a synchronous solution is not an option.

Any suggestions gratefully appreciated.

Regards

Reply to
RHRRC
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Forward drop isn't the only consideration for peak efficiency. I recall doing some careful work to select the best diode for a particular converter, and it wasn't clear from the forward drop alone which would be the winner.

That said, have a look at the available Schottky rectifier diodes. Larger-area (higher-current) and lower reverse rated diodes stand the best chance of having low forward drop, BUT at the expense of higher reverse current. Looks like you should be able to get down into the

0.25-0.3V region without going to synchronous. Maybe heat the diode to get the drop lower??

Cheers, Tom

Reply to
Tom Bruhns

Lord, you MUST be an old phart. We used to use that trick with the first germanium transistors to get a little more poo out of them. We figured we were STILL saving 90% of the filament power {;-)

Jim

Maybe heat the diode to

Reply to
RST Engineering (jw)

Here is one that would drop about .25 volts at the 200 mA peak current.

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Reply to
John Popelish

That's a nice part. Large-area Schottky diodes have lower forward drop as Tom suggested, and the high leakage-current penalty may not be so bad, e.g. 40uA at -3V, 25C, and 350uA at 11.5V, 50C for this part. But using a large-area diode has another hidden gotcha for low-power circuits: high capacitance. This part approaches 500pF at 0V bias, creating additional losses on each switching cycle.

I'm not sure why RHRRC rules out an active diode, it's not a very difficult matter to create a small bias supply to run the MOSFET.

--
 Thanks,
    - Win
Reply to
Winfield Hill

There's more than one kind of synchronous rectifier.

You might re-examine bipolar options at this voltage and current level. As long as base current ends up in the load, it can be quite efficient, and simple, at moderate frequencies.

It must indeed be a seriously energy-starved application if rectification efficiency at 11mW of load power is critical. If this isn't just an illusion, I'd be tempted to get my energy some other way.

RL

Reply to
legg

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