Some short calculation for the required heatsink, worst case:
Rds_on (high/low side): 0.1R
Imax: 14A
Tambient: 30°C
Rth_jc(Top): 0.36°C/W
Power dissipation per channel:
Pd_channel = I²R = Imax²*2*Rds_on = 14A²*2*0.1R = 196A*0.2R = 39.2W
Power dissipation total:
Pd_total = 2*Pd_channel = 2*39.2W = 78.4W
Tcase = Tjunction - Rth_jc * Pd_Total = 125°C - 0.36°C/W * 78.4W = 125°C - 28.224°C = 96.776°C
Rth_heatsink = (Tcase - Tambient) / Pd_total = (96.776°C - 30°C) / 78.4W = 0.85°C/W
Rth_heatsink includes thermal conductivity of heatsink grease.
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Lets do the calculation again for Irms (sinus power):
Rds_on (high/low side): 0.1R
Imax: 14A
Tambient: 30°C
Rth_jc(Top): 0.36°C/W
Power dissipation per channel:
Irms = Imax*sqrt(2) = 14A*0.707 = 9.9A
Pd_channel = Irms²R = Irms²*2*Rds_on = 9.9A²*2*0.1R = 98A*0.2R = 19.6W
Power dissipation total:
Pd_total = 2*Pd_channel = 2*19.6W = 39.2W
Tcase = Tjunction - Rth_jc * Pd_Total = 125°C - 0.36°C/W * 39.2W = 125°C - 14.112°C = 110.888°C
Rth_heatsink = (Tcase - Tambient) / Pd_total = (110.888°C - 30°C) / 39.2W = 2.1°C/W
Rth_heatsink includes thermal conductivity of heatsink grease.
Much better.
As music is normally not pure fixed frequency sine wave the maximum temperature will likely not reached even at full power.
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Lastly, for normal use, with an estimated programm factor of 0.5 there will be:
Rds_on (high/low side): 0.1R
Imax: 14A
Music programm factor (mpf): 0.5
Allowed max junction temperatur: 90% from Tj = 0.9*125°C = 113°C
Tambient: 35°C
Rth_jc(Top): 0.36°C/W
Power dissipation per channel:
Irms = Imax*sqrt(2) = 14A*0.707 = 9.9A
Iprog = Irms * mpf = 4.45A
Pd_channel = Iprog²R = Iprog²*2*Rds_on = 4.45A²*2*0.1R = 19.8A*0.2R = 3.96W
Power dissipation total:
Pd_total = 2*Pd_channel = 2*3.96W = 7.92W
Tcase = Tjunction - Rth_jc * Pd_Total = 113°C - 0.36°C/W * 7.92W = 113°C - 2.85°C = 110.15°C
Rth_heatsink = (Tcase - Tambient) / Pd_total = (110.15°C - 35°C) / 7.92W = 9.5°C/W