THD218 datasheet(4/6 Pages) STMICROELECTRONICS

4 / 6 page

Reverse Biased SOA

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

100

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

negative base current IB2 must be provided to

turn off the power transistor (retrace phase).

Most

the

dissipation,

the

deflection

application, occurs at switch-off. Therefore it is

essential to determine the value of IB2 which

minimizes

power

losses,

fall

time

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 16 KHz and 32 KHz

scanning frequencies for choosing the optimum

negative drive. The test circuit is illustrated in

figure 1.

Inductance L1 serves to control the slope of the

negative base current IB2 to recombine the

excess carrier in the collector when base current

is still present, this would avoid 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.

THD218DHI

4/6

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