DN32

Design Note 32
Issue 1 April 1996
A High Efficiency Constant Current Source for
Battery Charging Applications
FZT
949
68µH
2A
1k
2N3904
68µH
1N4148
OUTPUT
680R
1/2W
8
11
Vcc=20V +/-4V
C1 C2
1000µF
40V
12
47k
Comp 3
-ve
2
SB
340
Vcc
CT
5
RT
6
1000µF
35V
4k7
TL494
1 +ve
1.7A, 14-17V
0.1µF
1M
14
Vref
-ve
15
7 +ve
16
4k7
DTOC E1E2
Gnd
12k
1nF
180R
The 1000µF reservoir
0.1R capacitor can be omitted in
1W some applications, with no
degradation in efficiency or
performance.
Figure 1
TL494 Constant-Current Source.
This design note describes a 1.7A
constant current source for battery
charging applications. With an input of
20V, constant current can be generated
for loads from 5V to 17V - for charging
from 4 to 12 Ni-Cd cell battery packs.
An efficiency of better than 85% is
achieved using a very low VCE(sat) bipolar
transistor, the FZT949 or the FZT789A, as
the switching element.
The circuit uses the Texas Instrument
TL494 converter IC (Please refer to
Figure 1). A speed up circuit, using a
2N3904 and a 68µH inductor enhances
the FZT949/FZT789A switching speed Collector-to-0V, and drive waveforms
are shown in Figure 2.
N o h e a t s i n k i s n e c e s s a r y fo r th e
F Z T 9 4 9 / ZT X 7 8 9 A du e to t he h i gh
efficiency achieved. However, using the
FZT789A will result in a slightly higher
DN32 - 1
Design Note 32
Issue 1 April 1996
Figure 2
FZT949/FZT789A Switching speed-waveforms.
Upper Trace: Collector-to-0V; Lower Trace: IC Drive.
case temperature than the FZT949 - but
still well within the accepted operational
specification of the device. If the user
prefers to run the case temperature
lower, than the FZT949 is the preferred
device.
An industry standard switching
P-Channel MOSFET (BUZ271 - in TO220
outline) has been substituted for the
Zetex device but did not improve on the
e f f i c i e n c y - th o u g h i t d o e s c o s t
considerably more and is only available
in a larger package - TO220 compared to
SOT223 for the FZT789A/FZT949.
In conclusion, it has been shown in this
design note that with the selection of a
v e r y lo w VCE(sat) PNP Zetex bipolar
transistor, high efficiencies can be
achieved and at a much reduced cost
compared to a MOSFET based design.
DN32 - 2
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