Sunger, you said the driving board in the Dobot supplied laser is 3.5w output at 12v.
3.5w at 12v is 0.29 amps. In other words, one would need a diode that can accept 12v with a current of 0.29amps or 3.5w
The laser Ron is thinking of using to replace is 12v at 1-2 amps (12-24w), which is 3-7 times above the capabilities of the board you stated?.
As you say, some of that energy is lost as heat and active cooling, hence why your 3.5w diode outputs 500mw or 1/7th of the power consumed as light.
The one Ron linked is taking 12-24W and converting it to 3.5-4w of laser light, 1/4th-1/6th of the power consumed.
The issue here is, you (presumably) have stated 3.5w as the output power of the driving board, not the strength of light the laser can output. Ron has gone and grabbed a 3.5w laser in terms light output, which needs a 12w driving board.
1 Wavelength: 445nm~450nm
2 Output Power: 3500mW-4000mW
> 3 Working Voltage: 12V
> 4 Power supply: 1-2A