While not really helpful in determining if that board is good or not (I have no idea aside from the title is flat out wrong...it's not a RAMPS board), power dissipation (that 300 watt rating) isn't useful for how you're using it. P(TOT) for that transistor is computed by the (max operating temperature - ambient) / Thermal resistance junction-case max, or (175°C - 25°C)/.5°C/W = 300 W. You're not going to get anywhere near 300 watts out of it with how it's mounted. At that power rating, you'd need a hefty heat sink and possibly active cooling, or only operate it for extremely short periods unless you wanted to free the magic smoke that is contained inside the casing.Quote
ChrisT88
The heated bed has a beefy 300 watt mosfet:
For gauging how hot it's going to get, which is the Achilles heal for the reference RAMPS design, you want the R(ds) value to be as low as possible. R(ds) is the resistance between the drain and source, the main path current is flowing. As current increases, the resistor heats up due to that resistance value. That transistor that you linked to has a R(ds) value of .0095 ohms with a max of .011 ohms. The recommended replacement for a RAMPS board TO-220 package is a IRLB8743PBF which comes in at about 1/4 that at .0025 ohms (.0032 ohms max). For the record, the reference RAMPS design calls for a STP55NF06L MOSFET power transistor with a R(ds) of .014 ohms (.018 ohms max).
For the math behind the numbers, presuming typical R(ds) values, 12 volt heated bed and 11 amps current (since most beds run 10-12 amps...):
P = I^2 * R
STP75NF75 - 11 amps^2 * .0095 ohms= 1.1495 watts
IRLB8743PBF - 11 amps^2 * .0025 ohms= .3025 watts
STP55NF06L - 11 amps^2 * .014 ohms = 1.69400 watts
Take each of the numbers above, multiply by 62.5°C/W, and add to an ambient temperature of 25°C. That will give you how hot each transistor will get
STP75NF75 - 1.1495 watts * 62.5 + 25 = 96.84375°C (~206°F)
IRLB8743PBF - .3025 watts * 62.5 + 25 = 43.90625°C (~111°F)
STP55NF06L - 1.69400 watts * 62.5 + 25 = 130.875°C (~276°F)
As you can see, your resistor and the reference design are going to run pretty hot (although within normal spec), while the middle one will run barely warm.