TRANSISTORS 



(1) Drives the collector more negative than the battery voltage. 



(2) Drives the base positive. There is then no emitter current, and only 

 a tiny leakage current I^^g) from base to collector, which may be neglected. 



(3) Develops a potential difference across the secondary winding which 

 rises rapidly until 'caught' at the steady output voltage V^ by the diode. 

 Secondary current then flows to charge the capacitor. During this period 

 the working point remains at C When the energy transfer is complete. 



£.n, 



Ry 



Current out 



of base, 



' 



vSltlge -. ^***'"y ^°'^^9e 



Secondary 

 current 



V 



Figure 45.41 



the base positivity disappears and the transistor begins to conduct from 

 emitter to collector again. The working point reverts to A and the cycle 

 repeats {Figure 45.41). 



The output voltage V^ depends on the step-up ratio «,. between L^ and L^,, 

 and on the value of the load 7?^. Because the phases of magnetic field 

 growth and collapse are quite separate in converters of this type, the growth 

 phase is independent of the collapse phase; that is, the device draws a 

 rather constant power from the battery, independent of the load resistance. 

 The output voltage and current are therefore related by a rectangular 

 hyperbola and the actual output voltage and current are determined in a 

 particular case by the load used {Figure 45.42). The power output of the 

 device is adjustable in practical circuits by the resistor Ry. 



Notice that, except during the transition period — quickly over — between 

 B and C in Figure 45.40, the P^ax hyperbola is never approached ; that is, 

 the collector dissipation is always low. This means that the efficiency is 

 good, and that the device can handle powers much greater than the maximum 



702 



I 



