THD218DHI View Datasheet(PDF) - STMicroelectronics
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THD218DHI
Reverse Biased SOA
BASE DRIVE INFORMATION
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 of the dissipation, in the deflection
application, occurs at switch-off. Therefore 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 16 KHz and 32 KHz
scanning frequencies for choosing the optimum
negative drive. The test circuit is illustrated in
Figure 1: Inductive Load Switching Test Circuit.
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:
1
2
L
(IC)2
=
1
2
C
(VCEfly)2
ω
=
2
πf
=
1
√LC
Where IC= operating collector current, VCEfly=
flyback voltage, f= frequency of oscillation during
retrace.
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