THD200FI datasheet(5/7 Pages) STMICROELECTRONICS

5 / 7 page

In order to saturate the power switch and reduce

conduction losses, adequate direct base current

IB1 has to be provided for the lowest gain hFE at Tj

= 100

oC (line scan phase). On the other hand,

negative base current IB2 must be provided turn

off the power transistor (retrace phase). Most of

the

dissipation, especially in

the

deflection

application, occurs at switch-off so it is essential

to determine the value of IB2 which minimizes

power losses, fall time tf and, consequently, Tj.A

new set of curves have been defined to give total

power losses, ts and tf as a function of IB2 at both

32 KHz and 64 KHz scanning frequencies in

order to choice the optimum negative drive. The

test circuit is illustrated in fig. 1.

Inductance L1 serves to control the slope of the

negative base current IB2 in order to recombine

the excess carriers in the collector when base

current is still present, thus avoiding any tailing

phenomenon in the collector current.

The values of L and C are calculated from the

following equations:

(IC)2 =

(VCEfly)2

ω= 2 πf =

√

L C

Where IC = operating collector current, VCEfly=

flyback voltage, f= frequency of oscillation during

retrace.

BASE DRIVE INFORMATION

Figure 1: Inductive Load Switching Test Circuit.

Figure 2: Switching Waveforms in a Deflection Circuit.

THD200FI

5/7

THD200FI 데이터시트(PDF) 5 Page - STMicroelectronics