TDA8143 Datasheet, PDF (5/9 Pages) STMicroelectronics – HORIZONTAL DEFLECTION POWER DRIVER

TDA8143
APPLICATION INFORMATION
The conventional deflection system is shown in
Figure 3. The driving circuit consists of a bipolar
power transistor driven by a transformer and a
medium power element plus some passive compo-
nents.
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 4
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 convenctional net-
work cannot guarantee the saturation ; for this
reason, generally, the driving circuit must be care-
fully designed and is different for each deflection
system.
The new approach, using the TDA8143, over-
comes these restrictions by means of a feedback
principle.
As shown in Figure 4, 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 β is the minimum current gain of the transitor.
This method always ensures the correct base cur-
rent and acts time independent on principle.
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 5.
Conventional driver systems inherently result into
a stable condition with a constant peak current
magnitude.
Figure 3 : Conventional Horizontal Deflection System for TVs
V_CC +
DRIVING CIRCUIT
I_B
I_C I_D
HORIZONTAL
TRANSFORMER
YOKE
DEFLECTION CIRCUIT
V _IN
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