Question about Diode in LM2596 based DC-DC converter

[tlovie]

I designed and built a LM2596 type DC-DC converter as a small battery charger for fun, and because I enjoy laying out circuit boards and tinkering.  During the design process I talked myself into adding a "protection" diode before the output to ensure that current can only flow in one direction.  After I made the boards and built the circuit, I've found that this diode gets fairly hot.  Now I'm questioning whether the D4 diode is actually doing anything other than making heat? 

This device is a battery charger that would step down the input voltage to 13.5v for charging a 12v lead acid battery, and I believe that I was thinking that D4 would protect against a reverse polarity connection to the battery.

[jpanhalt]

What is your charging current?
A 12V lead acid battery, depending on size, may well require 3A.  That is the maximum rating and can be expected to make that diode quite hot (see datasheet).

[tlovie]

What is your charging current?
A 12V lead acid battery, depending on size, may well require 3A.  That is the maximum rating and can be expected to make that diode quite hot (see datasheet).

The U2B opamp limits the current to 2.5A.  I'm just not sure if it adds any value to the circuit, or provides any protection at all.

[thm_w]

D4 will not protect against reverse polarity of the battery.
If I connect a battery backwards across the output, look at the path through D1 and D4.

Its only going to prevent backfeed into U1, when U1 power is off, which may or may not be an issue.

Maybe consider a fuse or resettable fuse on the output.