Most transistor vendors will document a “junction-to-ambient” thermal impedance denoted by the symbol R_{θ J-A}, which is measured in units of °C/W. This value represents how much the junction temperature will rise above the ambient temperature surrounding the TO-220 package for every watt of power dissipated within the device. For example, if the transistor vendor states that the thermal impedance from junction-to-ambient is 62°C/W, this means that the 2.78W being dissipated within the TO-220 package will cause the junction temperature to rise 172°C above ambient (calculated as 2.78W multiplied by 62°C/W). Assuming the worst-case ambient temperature for this application of 50°C, the junction temperature will reach 222°C (calculated as 50°C plus 172°C). This far exceeds the rated silicon temperature of 125°C and will permanently damage the transistor, meaning a heat sink is required. Attaching a heat sink will significantly lower the junction-to-ambient thermal impedance. The next step will be to determine how low the thermal impedance path needs to be for safe and reliable operation.