说明：  以7805为例说明问题。 设I=350mA，Vin=12V，则耗散功率Pd=(12V-5V)*0.35A=2.45W 按照TO-220封装的热阻θJA=54℃/W，温升是132℃，设室温25℃，那么将会达到7805的热保护点150℃，7805会断开输出
(7805 as an example to illustrate the problem. Located I = 350mA, Vin = 12V, the power dissipation Pd = (12V-5V)* 0.35A = 2.45W TO-220 package in accordance with the thermal resistance θJA = 54 ℃/W, temperature is 132 ℃, located at 25 ℃, it will reach 7805' s thermal protection point 150 ℃, 7805 will disconnect the output)