HUF76121D3S MOSFET replacement

[Brad O'Dell]

Hello everybody, I was repairing a socket 370 motherboard and needed to replace a noticeably damaged Harris HUF76121D3S (part is branded 76121D, also Fairchild has this part).  All I have easy access to is a RFP30N06LE.  Will this be a suitable replacement besides the package style (TO-252AA versus TO-220AB)?   starter data: both are N-channel enhancement mode logic level MOSFETs replacement has higher voltage rating (Vdss and Vdgr 60V vs. 30V) replacement has higher current rating (Id 30A vs. 20A) replacement seems to be faster however, drain-to-source ON resistance is more with replacement (Rds(ON) max. .047ohm vs. .030ohm).  Would this significantly affect circuit operation?  Also the HUF76121 Rds(on) was rated for the part maximum current of 20A, whereas the RFP30N06 Rds(on) was rated for that part's maximum current of 30A.  How would I recalculate the RFP device's Rds(on) with the lower current of 20A that will be used?   Thanks, Brad