Solid State Relay Heat Sink Calculations

Heat sink size calculation:

(Max Heat Sink Temp - Max Ambient Temp) / Watts=_____ size needed

Maximum heat sink temperature minus maximum ambient temperature divided by the power dissipation (use the chart for power dissipation at desired current).

For example, use a:

• SVDA/3V25 running at 20Amps in a 45C ambient

• From the chart, at 20Amps it dissipates 23Watts

• A 25A unit can have a 85C heat sink

• (85-45 ambient)=40C temp rise is allowed

• 40C/23W=1.74C/W heat sink rating or less (less temperature rise per watt is better)

• Therefore, the recommended heatsink would be part number: S505-HEATSK-DIN (rated at 1.6°C/W) or any equivalent heat sink that is LESS than 1.74. Remember, the lower the heatsink value, the better it dissipates the heat. The relay must be connected to the heatsink using an appropriate thermal conduction grease or thermal pad.

De-rating calculation for a known heat sink:

(Max Heat Sink - Max Ambient Temp) / heat sink rating=_____

Maximum heat sink temperature minus maximum ambient temperature divided by the heat sink rating (use the chart for current at calculated power dissipation).

For example, use a SVDA/3V10 in a 60C ambient with a 2.0C/W heat sink. 90C - 60C=30C heat sink temperature rise is allowed. 30C divided by 2.0C/W=15W. From the table, full load current of 10A only dissipates 11W. Thus, a SVDA/3V10 mounted on a 2.0C/W heat sink can switch 10A at 60C.

Power calculation in place of the charts:

(0.9 x Irms x Vo) + (Irms² x Rt)=Power.

For example, use a SVDA/3V25 for a 21 amp application. (0.9 x 21A x 0.80V) + (21² x 0.021 Ohms)=24.4Watts need to be dissipated.