THD200FI Datasheet, PDF (5/7 Pages) STMicroelectronics – HIGH VOLTAGE FAST-SWITCHING NPN POWER TRANSISTOR

THD200FI
BASE DRIVE INFORMATION
In order to saturate the power switch and reduce
conduction losses, adequate direct base current
I_B1 has to be provided for the lowest gain h_FE at T_j
= 100 ^oC (line scan phase). On the other hand,
negative base current I_B2 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, t_s and t_f as a function of I_B2 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.
Figure 1: Inductive Load Switching Test Circuit.
Inductance L1 serves to control the slope of the
negative base current I_B2 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:
1
2
L
(I_C)²
=
1
2
C
(V_CEfly)²
ω
=
2
πf
=
1
√LC
Where IC = operating collector current, VCEfly=
flyback voltage, f= frequency of oscillation during
retrace.
Figure 2: Switching Waveforms in a Deflection Circuit.
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