BU208A
BASE DRIVE
The Key to Performance
By now, the concept of controlling the shape of the turn–off
base current is widely accepted and applied in horizontal
deflection design. The problem stems from the fact that good
saturation of the output device, prior to turn–off, must be as-
sured. This is accomplished by providing more than enough
IB1 to satisfy the lowest gain output device hFE at the end of
scan I CM. Worst–case component variations and maximum
high voltage loading must also be taken into account.
If the base of the output transistor is driven by a very low
impedance source, the turn–off base current will reverse
very quickly as shown in Fig. 3. This results in rapid, but only
partial collector turn–off, because excess carriers become
trapped in the high resistivity collector and the transistor is
still conductive. This is a high dissipation mode, since the
collector voltage is rising very rapidly. The problem is over-
come by adding inductance to the base circuit to slow the
base current reversal as shown in Fig. 4, thus allowing ac-
cess carrier recombination in the collector to occur while the
base current is still flowing.
Choosing the right LB Is usually done empirically since the
equivalent circuit is complex, and since there are several
important variables (I CM, I B1, and h FE at I CM). One method is
to plot fall time as a function of L B, at the desired conditions,
for several devices within the h FE specification. A more infor-
mative method is to plot power dissipation versus I B1 for a
range of values of LB.
This shows the parameter that really matters, dissipation,
whether caused by switching or by saturation. For very low
LB a very narrow optimum is obtained. This occurs when IB1
hFE
ICM, and therefore would be acceptable only for the
“typical” device with constant ICM. As LB is increased, the
curves become broader and flatter above the IB1. hFE = ICM
point as the turn off “tails” are brought under control. Eventu-
ally, if LB is raised too far, the dissipation all across the curve
will rise, due to poor initiation of switching rather than tailing.
Plotting this type of curve family for devices of different hFE,
essentially moves the curves to the left, or right according to
the relation IB1 hFE = constant. It then becomes obvious that,
for a specified ICM, an LB can be chosen which will give low
dissipation over a range of hFE and/or IB1. The only remain-
ing decision is to pick IB1 high enough to accommodate the
lowest hFE part specified. Neither LB nor IB1 are absolutely
critical. Due to the high gain of Motorola devices it is sug-
gested that in general a low value of IB1 be used to obtain
optimum efficiency — eg. for BU208A with ICM = 4.5 A use
IB1 1.5 A, at ICM = 4 A use IB1 1.2 A. These values are
lower than for most competition devices but practical tests
have showed comparable efficiency for Motorola devices
even at the higher level of IB1.
An LB of 10
µH
to 12
µH
should give satisfactory operation
of BU208A with ICM of 4 to 4.5 A and IB1 between 1.2 and
2 A.
^
[
[
TEST CIRCUIT WAVEFORMS
IB
IB
IC
(TIME)
IC
(TIME)
Figure 3
Figure 4
TEST CIRCUIT OPTIMIZATION
The test circuit may be used to evaluate devices in the
conventional manner, i.e., to measure fall time, storage time,
and saturation voltage. However, this circuit was designed to
evaluate devices by a simple criterion, power supply input.
Excessive power input can be caused by a variety of prob-
lems, but it is the dissipation in the transistor that is of funda-
mental importance. Once the required transistor operating
current is determined, fixed circuit values may be selected.
4
Motorola Bipolar Power Transistor Device Data
ONSEMI [ ON SEMICONDUCTOR ]
