37. CHOICE OF POWER SUPPLY AND HEAT SINK

To calculate approximate power dissipated on mosfet you can use this formula:

P = ( Uin - Ud ) · I

Where:

P - power dissipated on mosfet

Uin - Input voltage

I - Current set by a potentiometer, flowing through the diode

Ud - Diode working voltage

Typical working voltage and PSU choice for particular laser diodes:

Diode

Diode Working Voltage Ud

Recommended PSU voltage

Infrared

1.8 - 2.5 V

3.3 V - 5 V

Red

2 - 3 V

5 V

Green

5.5 - 7 V

7.5 V

Blue

4.5 - 6 V

5 V - 7.5 V

To avoid producing additional heat, try to adjust the input voltage close to the diode working voltage. For example, for the green laser diode, 7.5 V PSU is the best typical choice. Very low voltage dropout on the driver's circuit allows you to select proper input voltage depending on the diode working voltage. You can drive your 445 nm M140 diode over 1.3 A using 5 V power supply which is actually around 5.1 - 5.3 V.

Notice that wrong settings such as too high input voltage or lack of proper heatsink will cause high power dissipation on the mosfet making it vulnerable to be burned. Be careful while choosing the power supply.

For high power, infrared diodes consider using low input voltage 3.3 V or 5 V because of large power loss on MOSFET while using 12 V. It can drive 808 nm diodes even powered by 12 V, but MOSFET will need a really big heat sink, like the one on the CPU in the computer.

Recommended heat sink:

Diode example

Wavelength [nm]

Power [mW]

PSU Voltage [V]

Heatsink

M140

445

500 - 2000

5 V

Small or without

M140

445

500 - 2000

12 V

Large or medium

NDB7875/ NDG7475T

445 / 520

2000 - 3000

7.5 V

Medium or small

NDB7875/NDG7475T

445 / 520

2000 - 3000

12 V

Large or medium

Mitsubishi/Oclaro

638

500 / 700

5 V

Small

Mitsubishi/Oclaro

638

500 / 700

12 V

Medium or large

Infrared

808 / 940 / 980

500 - 1500

5 V

Medium

Infrared

808 / 940 / 980

3000 - 5000

3.3 V

Medium or large

Infrared

808 / 940 / 980

3000 - 5000

12 V

Large (like the one on CPU)