audioguru Posted May 13, 2012 Report Share Posted May 13, 2012 If three 2N3055 output transistors share the 5A of current then each has 1.7A and its minimum current gain is 40. Then the current in the BD139 is a max of 125mA. Its max voltage is 36.2V so its max power dissipation is 125mA x 36.2V= 4.5W.The max allowed chip temperature of a BD139 is 150 degrees C but 130 degrees is safer. Your ambient might be 30 degrees. The thermal resistance from chip to case is 10 degrees/W and for thermal grease is about 0.3 degrees/W. Then with a perfect heatsink the case will be at 30 degrees C and the chip will be (4.5W x 10.3) + 30= 76.4 degrees C.Your heatsink must have a thermal resistance of (100 degrees - 76.4 degrees)/4.5W= 5.2 degrees C/W or less. Quote Link to comment Share on other sites More sharing options...
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