TDA8140 Datasheet, PDF (6/10 Pages) STMicroelectronics – HORIZONTAL DEFLECTION POWER DRIVER

TDA8140
During the active deflection phase the collector
current of the power transistor is linearly rising and
the driving circuitry must be adapted to the required
base current in order to ensure the power transistor
saturation.
According to the limited components number the
typical approach of the present TVs provides only
a rough approximation of this objective ; in Figure 5
we give a comparison between the typical real base
current and the ideal base current waveform and
the collector waveform.
The marked area represents a useless base cur-
rent which gives an additional power dissipation on
the power transistor.
Furthermore during the turn-ON and turn-OFF tran-
sient phase of the chassis the power transistor is
extremely stressed when the conventional network
cannot guarantee the saturation ; for this reason,
generally, the driving circuit must be carefully de-
signed and is different for each deflection system.
The new approach, using the TDA 8140, over-
comes these restrictions by means of a feedback
principle.
As shown in Figure 5, at each instant of time the
ideal base current of the power transistor results
from its collector current divided by such current
gain which ensure the saturation ; thus the required
base current Ib can be easily generated by a feed-
back transconductance amplifier gm which senses
the deflection current across the resistor Rs at the
emitter of the power transistor and delivers :
Ib = RS ^. gm ^. Ie
The transconductance must only fulfill the condi-
tion :
1
+
1
βmin
⋅
1
R_S
<
gm
<
1
R_S
Where βmin is the minimum current gain of the
transistor. This method always ensures the correct
Figure 6
base current and acts time independent on princi-
ple.
For the turn-OFF, the base of the power transistor
must be discharged by a quasi linear time decreas-
ing current as given in Figure 6.
Conventional driver systems inherently result into
a stable condition with a constant peak current
magnitude.
This is due to the constant base charge in the
turn-ON phase independent from the collector cur-
rent ; hence a high peak current results into a low
storage time of the transistor because the excess
base charge is a minimum and vice versa. In the
active deflection the required function, high peak
current-fast switch-OFF and low peak current-slow
switch-OFF, is obtained by a controlled base dis-
charge current for the power transistor ; the nega-
tive slope of this ramp is proportional to the actual
sensed current.
As a result, the active driving system even im-
proves the sharpness of vertical lines on the screen
compared with the traditional solution due to the
increased stability factor of the loop represented as
the variation of the storage time versus the collector
peak current.
Figure 5 : Waveforms of Collector and
Base Current
I_{C Off Phase On Phase Off Phase}
I _BIAS
Base Bias Current
I_C
Real Base Current
Ideal Base Current
t
t
t_S
I_D
I₀
t _don
I₀
dI_{0 =} I_S0
dt t _S
I_p0
I_S0
ON PHASE
I_n0
t
OFF PHASE
6/10
CONTROL
INPUT
t_S
t