OFF PHASE

CONTROL

INPUT

t

ON PHASE

t

dt

=

Figure 6

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

emitter of the power transistor and delivers :

The transconductance must only fulfill the condi-

tion :

1

1

+ βmin ⋅

1

< gm <

1

Where

transistor. This method always ensures the correct

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.

t

t

D

I

Base Bias Current

Off Phase

On Phase

Off Phase

Real Base Current

Ideal Base Current

Figure 5 : Waveforms of Collector and

Base Current

TDA8140

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