Calculation of the allowable power dissipation of the Darlington tube. If the thermal stability of the bias circuit is high enough, then the allowable power dissipation of the device is: where P ′-the allowable power dissipation when the ambient temperature is T-Darlington When the maximum allowable junction temperature of the tube is 125 ℃, the allowable power dissipation of the Darlington tube is: in the case of No. 1 b. In the case of No. 2, the same reason can be obtained: under the above conditions, when the ambient temperature of the radiator When it is + 125 ℃, the power consumption of the power tube is 13.4W (or 14.2W). According to the analysis, the actual power consumption of the Darlington tube is about 20W when the ignition frequency is about 120Hz, so the radiator is under Linton Although the requirements cannot be met.
Through the above tests and calculations, it can be seen that if the ambient temperature of the ignition electronic component is + 125 ° C, taking the No. 1 radiator as an example, the temperature difference between the heat dissipation bottom plate of the housing and the environment is = 45 ° C, and the temperature of the heat dissipation bottom plate is 160 ～ 170 ℃. The temperature difference between the Darlington tube shell and the heat dissipation bottom plate is × 20 = 10 ℃, and the temperature of the Darlington tube shell is 175 ～ 180 ℃. The temperature difference between the tube junction and the tube shell is 20 = 194 ℃, at this time the tube junction temperature is 195 ～ 200 ℃, that is, the maximum allowable junction temperature of the Darlington tube is 175 ℃, in this case, the Darlington tube It can't work normally and reliably. According to foreign relevant information and test conditions of some domestic manufacturers, the maximum working environment temperature of ignition electronic components + 125 ℃ refers to the maximum temperature of the heat dissipation bottom plate, not the temperature of the environment where the heat dissipation bottom plate is located. The temperature of the tube shell of the Dayton tube is reduced from 160 to 170 ℃ to about 135 ℃. According to the temperature difference between the tube junction and the tube shell of 20 ℃, the temperature of the tube junction of the Darlington tube is about 155 ℃, and it is below 175 ℃. Working requirements, ignition electronic components can also meet their conventional performance and ensure their reliable operation.
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