832 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1954 



power applications. Laboratory experience indicates that it is not de- 

 sirable to operate the junction of this diode above 65° to 70° centigrade. 

 The value of this critical temperature is not accurately kno^^^l on ac- 

 count of the difficulty in measuring the junction temperature inside of 

 the crystal. However, below the critical temperature, those changes in 

 characteristics which are associated with changes in junction tempera- 

 ture are reversible, that is, if the temperature is raised and then reduced, 

 the characteristics will shift back to values previously experienced at the 

 reduced temperature. Beyond the critical junction temperature any 

 change in the reverse characteristics is permanent and has the effect of 

 reducing the reverse resistance. In an operating circuit, this effect leads 

 to progressively greater permanent damage to the diode. Lowered re- 

 verse resistance allows more reverse current to flow, increases the re- 

 verse power dissipation and elevates the temperature of the junction 

 causing further reduction of the reverse resistance, and so on until the 

 diode no longer blocks. 



Thermal damage to the junction can be prevented by removing heat. 

 This method is employed "\Aith the diode under discussion by forcing air 

 through the cooling fins at a high velocity. The quantity of air needed 

 depends on the amount of heat generated in the junction, the efficiency 

 of the cooling fins and the temperature of the air employed for cooling. 

 In most Bell System applications, the maximum temperature of the 



<^ 100 



Fig, 



D 800 1200 )600 2000 2400 28 



LINEAR VELOCITY OF AIR FLOW IN FEET PER MINUTE 



(TO KEEP HOTTEST JUNCTION AT 65°C) 



4 — Junction rectifier forced cooling characteristics. 



