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Help with diode dissipation - Arduino UPS

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Mr.B:

--- Quote from: Peabody on April 23, 2020, 09:38:56 pm ---...If the leakage current is 420µA when on battery...

--- End quote ---

How can it be that high?
Current flows through the 10K resistor only when the USB 5v is available.
When the 5v is removed the gate of the FET is pulled low.
At that point, the only current flowing through the diode is leakage current, therefore no heating effect on the diode.

Mr.B:
Here is the exact circuit I have used countless times:

Peabody:

--- Quote from: Mr.B on April 23, 2020, 11:14:15 pm ---
--- Quote from: Peabody on April 23, 2020, 09:38:56 pm ---...If the leakage current is 420µA when on battery...

--- End quote ---

How can it be that high?
Current flows through the 10K resistor only when the USB 5v is available.
When the 5v is removed the gate of the FET is pulled low.
At that point, the only current flowing through the diode is leakage current, therefore no heating effect on the diode.

--- End quote ---

But the instant before USB was disconnected, all the load current was flowing through the diode, so the diode may be very hot.  The mosfet has to turn on so the battery can power the load.  But if the diode is still hot enough that leakage current is high, gate voltage may stay too high.

Peabody:

--- Quote from: Mr.B on April 23, 2020, 11:25:53 pm ---Here is the exact circuit I have used countless times:

--- End quote ---

What would be the maximum current you would put through that circuit?

Mr.B:
In this particular application, only 300mA. Generally powering ESP8266 that needs almost 300mA when transmitting.
The MBR230LSFT1G diode is rated 2A continuous.
The SI2305DS-T1-E3 FET is rated 3.5A continuous.
The MCP1802T-3302I/OT regulator is rated 300mA continuous.
However, it is your choice of components that dictates the maximum current.
In your OP you mention 1A in the last line of the post.
Selecting a 3.3v LDO regulator that can do 1A is trivial, but maybe not in a SOT-23-5 package as in my circuit.

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