AN3400 datasheet(7/78 Pages) STMICROELECTRONICS

7 / 78 page

AN3400

Circuit description

Doc ID 018840 Rev 1

7/78

working frequency and, consequently, the lamp power are influenced by the sizing of both

L1 and C2 components. Capacitor C2 has also the function of putting the base current in

phase with the collector current. Resistance R2 of the series R-C network has the function

of regulating the current level to provide to the bases of both transistors acting on the power

lamp. In the proposed driving solution, the two capacitors C3 and C4 are charged with

opposite polarity to each other and assuming the same polarity of the base-emitter voltage

of the relevant bipolar transistor. In the regular functionality, when a capacitor goes through

a charging phase, before or during the conduction time of the relevant bipolar, so the

opposite capacitor is discharging contemporarily with a long constant time due to the series

with the breakdown resistance R4. So, this resistance R4, inserted between the bases of

the two transistors, creating an alternative path for the discharge of each capacitor, also

avoids that the oscillation is blocked during startup phase when one of the two capacitors is

fully charged. Regulating the discharge time of each capacitor, this resistance allows to fine

tune the working frequency and consequently the power supplied to the lamp. Two

capacitors C3 and C4 are not totally discharged during a forcing voltage source period but

maintain always a residual charge passing from the discharging to the charging transient.

Therefore, current across the breakdown resistance R4 flows always in the same direction

going from the PNP to the NPN base because the two capacitors maintain always the same

polarity during a period of the voltage signal across the C2 filter capacitor. A breakdown

resistance with a too high value can increase the working frequency but also can bring the

reverse biased base-emitter junction of the inactive device in breakdown condition. And vice

versa, a too low value of the breakdown resistance could cause re-conduction peaks in the

PNP transistor at the high temperature when the emitter-collector voltage reaches the

maximum value. In particular only the PNP device can show the phenomena above

mentioned because it has a lower base-emitter breakdown voltage and it has a higher low

current hfe than the NPN one. Setting the resistance R4 in order to guarantee a VBEoff value

in the range -4 V to -7 V, it is possible to avoid breakdown phenomena for the base-emitter

junctions of the bipolars and, at the same time, anomalous re-conduction effects due to

unexpected disturbances.

AN3400 Datasheet(PDF) 7 Page - STMicroelectronics