Fenix HP12 headlamp repair - PCB component identification, schema

[Libas]

but what is the reason to use internal diodes in the transistor instead of use normal diodes?

[fzabkar]

There is no separate diode. The "body diode" is just an intrinsic feature of MOSFETs.

https://www.microsemi.com/document-portal/doc_view/14692-mosfet-tutorial
https://www.powerelectronics.com/technologies/discrete-power-semis/article/21861311/power-management-101-power-mosfet-charactertics

[Libas]

I mean why designer of this circuit didnt use simple diode instead of this transistor. I cant see any purpose this transistor...it doesnt switch anything. Or do I miss anything?

[fzabkar]

The body diode can have a much lower forward voltage drop than a regular Schottky diode. However, in the present case the voltage drop is 0.4V at 1A, so I can't see much of an advantage. :-?

What is the body-diode forward voltage of a power MOSFET?

Low voltage and low RDS(on) power MOSFETs are used in such synchronous rectifier modes since their forward voltage drop can be as low as 0.1V versus the typical Schottky diode forward voltage drops of 0.4-0.5V.

[Libas]

In my local store is available IRLML6402 as a replacement for PMOS C01C. Is that suitable replacement?

I have found info that "Silicon optimized for applications switching below <100kHz"   ....but original transistor is driven by  S-8520F18MC-BNDT2x which has according to datasheet "Oscillation frequency of 300 kHz".

https://www.infineon.com/cms/en/product/power/mosfet/p-channel/irlml6402/