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Oh no! Not Another
CFL Inverter . . .
Design by BRANKO JUSTIC
Words by STEPHEN DAVID
Yes, another one. But they just keep getting better and better!
This highly efficient 12V DC-DC converter runs multiple compact
fluorescent lights (CFL) for use in recreational vehicles, camp-sites,
remote homesteads or anywhere remote when you need efficient
lighting. As a bonus, it can even run a small colour TV set.
T
his one’s a beauty. It will drive
multiple CFLs without the need
for heatsinks on the MOSFETs.
Fit heatsinks and you can run even
more, up to a total of 100W.
However, the real bonus is that you
don’t have to wind the transformer – a
tricky job at the best of times. This kit
comes with a pre-wound transformer
so all you have to do is to solder it into
the PC board.
As pictured, the prototype inverter
is running four 11W CFL tubes and
the MOSFETs are barely warm. And
to do that it is drawing just 3.2A from
a 12V battery so it is a pretty efficient
device.
It’s compact too, housed on a PC
board measuring 83 x 50mm and
76 Silicon Chip
standing about 58mm high because
of the on-board transformer.
The circuit is quite simple, as shown
in Fig.1 and is based on an ST SG3525
regulating pulse-width modulator (IC1)
which drives a couple of Mosfets and
CAUTION: DANGEROUS VOLTAGES
The output from this inverter is around
350V DC and is thus as dangerous as
mains voltages. Handle with care –
never work on the circuit with power
applied. The finished project should
be installed in a plastic case.
the push-pull stepup transformer.
The on-chip oscillator is set by the
4.7nF (.0047mF) capacitor at pin 5
and the 3.3kW resistor at pin 6, giving
a frequency of about 85kHz. This is
divided by two by an internal flipflop
and the flipflop’s complementary outputs drive buffers which can source or
sink in excess of 200mA.
The buffer outputs at pin 11 & 14
drive two MOSFETs, Q1 and Q2, with
the high output current capability being ideal for rapid charge and discharge
of the MOSFET gate capacitances,
resulting in fast switching times. This
results in high efficiency and little
power loss in the MOSFETs.
The MOSFETs drive the push-pull
step-up transformer. This has four
turns on the two primaries and 120
turns on the secondary, giving a stepup
ratio of 30 times. The transformer is
connected to a bridge rectifier consiliconchip.com.au
Fig.1: an SG 3525 pulse width modulator chip running at 85kHz drives two MOSFETs and a stepup transformer to
develop 350V DC to drive compact fluorescent lamps (CFLs) or a small colour TV set.
sisting of four BA159 1A high speed
diodes and a filter capacitor of a mere
1nF (.001mF). The resulting DC voltage
is about 350V.
The output voltage is not regulated
unless it exceeds 370V, as set by the
resistive feedback network (220kW,
270kW, 6.8kW) connected to pin 1, the
inverting input of the internal error
amplifier. This samples the output
voltage and compares it to a 5.1V internal reference (pin 16) connected to
pin 2, the non-inverting input of the
same internal error amplifier.
The reason for not regulating the
output voltage to a lower value is to
avoid reducing the efficiency of the
converter. In any case, regulation is not
necessary in this application because
CFLs can operate over a wide voltage
range with a little variation in light
output.
The 68nF capacitor at pin 9 provides
a “soft start” facility. The capacitor is
charged by a 50mA constant current
source inside IC1 and this causes the
duty cycle of the output signal from
pins 11 and 14 to slowly rise from
zero to 50%.
The soft start is needed because of
the relatively large filter capacitor in
an electronic ballast or a CFL.
Under-voltage protection is provided by the SG3525 by virtue of the
fact that it will stop operating below
8.5V. This avoids the possibility of
inadequate gate drive to the MOSFETS
siliconchip.com.au
which would lead to over-dissipation.
However, note that this does not
protect any SLA battery from overdischarge.
Construction
Assembly of the PC board is pretty
straightforward as there is only a handful of components. The component
layout is shown in Fig.2.
Mount the resistors and diodes
first, making sure that you install the
diodes the right way around. Then
mount the small capacitors and the
electrolytic which must have the correct polarity.
The transformer can be soldered in
next. Its leads are already soldered to
pins, so it simply drops into the appropriate place on the PC board.
It’s efficient and it’s easy to build, thanks in large part to the pre-assembled
transformer. Watch the polarity on the semiconductors and electro.
September 2004 77
Then install the two MOSFETs and a
socket for IC1. When you insert the IC
into the socket, make absolutely sure
you install it the correct way around
(otherwise smoke will escape when
you turn it on!).
For up to (say) 50W output (4x
11W CFLs), MOSFET heatsinks aren’t
needed. Higher output (such as driving the TV set below) will need small
heatsinks on each of the MOSFETs.
Fit four PC pins for the external
connections and then you attach the
leads for the battery and the output
socket (or sockets).
Carefully check your work and
then connect one CFL to its socket.
Then connect the 12V battery. The
CFL tube should light instantly. If
not, turn off and carefully re-check
your work (which you should have
done before!)
Note that while the output of the
inverter is polarised, it does not matter
which way around a CFL is connected
as its electronics contains a bridge
rectifier.
SC
Fig.2: component overlay for the PC board.
Kit availability
A kit of parts for this project, including the prewound transformer, is
available from Oatley Electronics, for $24 including GST (Cat K111A).
Postage and packing is $8 for up to eight kits.
Contact Oatley at PO Box 89, Oatley, NSW 2223. Phone (02) 9584
3563. email sales<at>oatleyelectronics.com; web: www.oatleye.com
Operating a colour TV from the inverter
As part of the development of this kit, Oatley Electronics (who own the design copyright) have used this
circuit to run a 240VAC 30cm colour TV, as seen below.
This “Prima” brand set was purchased at a supermarket
for only $99 (truly!), much cheaper than an equivalent
12V TV. Therefore it makes sense to use this circuit
with a 240VAC small colour TV, doesn’t it?
Don’t attempt to do this, however, if you are not
experienced with switch-mode supplies and TV set
circuitry in general.
The only proviso is that the TV’s degaussing coil
must be disconnected because it is important that it
The CFL inverter board mounted on the inside of the TV set
case via cable ties (the other board you can see is part of the
tube drive). It connects to the TV’s main PC board at the same
place as the normal mains input connects, taking advantage
of the fact that the switch-mode supply immediately rectifies
the 240V AC mains (to about 350V DC). NEVER run the TV set
from the mains and the inverter at the same time. And note
our comments about disconnecting the degaussing circuit.
does not get the 350V DC from the CFL driver circuit.
Then from time to time (especially if the TV is moved),
the degaussing circuit will have to be reconnected and the
set temporarily run from 240VAC to make sure the picture
tube is degaussed, otherwise the tube will develop purity
problems.
Oatley Electronics are currently developing a 12V Degausser which should be ideal for use with this setup. More
details when they become available!
78 Silicon Chip
siliconchip.com.au
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