This is only a preview of the March 1998 issue of Silicon Chip. You can view 43 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
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A nifty inverter
for compact
fluorescent lamps
Want to drive a number of compact fluorescent
lamps (CFLs) from a 12V supply? Here’s a low
cost way of doing it. The finished product fits
into a small plastic case yet will drive up to
three 11W CFLs at full brightness.
Design by BRANCO JUSTIC*
We have published a number of
inverters for fluorescent lamps in the
past but this is certainly the simplest.
It makes use of the internal circuitry
of CFLs and the result is a simple
low-cost inverter.
Before we go any further, perhaps
we had better explain what we mean
by a Compact Fluorescent Lamp or
CFL. If you visit the electrical section
of any large retailer or hardware store
you will find a range of CFLs rated at
between 10W and 40W. They have an
March 1998 23
Fig.1: the inverter consists of a CMOS oscillator with complementary outputs,
two Mosfets to drive the step-up transistors and a bridge rectifier with ultra-fast
diodes. The unsmoothed DC is then applied directly to the CFL which normally
runs from 50Hz 240VAC.
Edison screw base and run from the
240VAC 50Hz mains supply.
But you don’t have to run them from
240VAC 50Hz. They can be run from a
high voltage DC supply of about 340V.
How can this be?
Inside the circular plastic base of
the CFL is a bridge rectifier to convert
the incoming 240VAC supply to about
340V DC. This is then fed to an elec
tronic ballast circuit inside the CFL
to drive the folded fluorescent tube.
So because there is a bridge rectifier
inside the CFL, that means we can
power it from 340V DC (or therea
bouts) rather than 240VAC.
But the high voltage supply does
not even need to be smoothed DC.
It can be unfiltered DC, direct from
the inverter’s own bridge rectifier.
Fig.2: this is the voltage waveform at the drain of Mosfet Q5.
You can see that the waveform is close to 24V peak-to-peak.
Ignore the oscilloscope measurement of 32.2V peak-to-peak
because that includes occasional overshoots which are not
depicted in this waveform but are clipped by the 16V zener
diodes.
24 Silicon Chip
So the fact that the high voltage DC
does not need to be filtered means
there is a further saving because high
voltage electrolytic capacitors are not
required. Neat, huh?
But if the CFL has a bridge rectifier
in it, why does the inverter need a
bridge rectifier at its output too? Well,
it is necessary and we’ll explain why
later.
Circuit details
Fig.1 shows the inverter circuit. It
uses an oscillator to drive a pair of
power Mosfets and a step up trans
former. Its output is then rectified and
fed to the CFLs.
Only one 74C14 hex Schmitt trig
ger, IC1, is used in the circuit. IC1a
and IC1b plus frequency determining
components C1, R1, and R2 form a
simple two-gate oscillator that pro
duces a square wave output at about
20kHz. IC1e and IC1f are connected
in parallel to invert and buffer the
output of IC1b. Then IC1c and IC1d,
also connected in parallel, invert and
buffer the oscillator signal again. So
now we have complementary (ie, 180°
out of phase) signals and these are
used to drive complementary emitter
follower pairs, Q1 & Q2 and Q3 & Q4.
We could have used the outputs of
IC1e, etc to drive the following Mos
fets, Q5 & Q6, but the emitter follower
drive gives faster and cleaner switch
ing because it is better able to charge
and discharge the gate capacitance of
the Mosfets. By ensuring fast switch
ing, there is less stress and power loss
Parts List
1 PC board, 115 x 36mm
1 plastic utility box, 129 x 68 x
41mm
1 Edison screw socket
1 or 2 11W compact fluorescent
lamps (CFLs)
1 step-up inverter transformer (T1)
1 prewound inductor (L1)
3 rocker switches (S1,S2,S3)
Fig.3: this the voltage appearing between one side of the
transformer secondary and 0V. This is 320V peak-to-peak
and the full output is 640V peak-to-peak.
in the Mosfets and that leads to better
efficiency.
Because of the complementary
signals driving the Mosfets, each one
turns on alternately to drive its half of
the transformer primary winding. The
Mosfet effectively switches the +12V
across one half of the transformer and
transformer action in the other half
means that 24V peak-to-peak appears
at each Mosfet drain.
The transformer primary is 7.5 turns
centre tapped (ie, 3.75 turns each half)
and the secondary has 100 turns. So
with 24V peak-to-peak across each pri
mary half, the total voltage appearing
across the transformer secondary will
be 640V peak-to-peak.
This is confirmed by the waveforms
shown in Fig.2 & Fig.3. Fig.2 shows
the voltage waveform at the drain of
Mosfet Q5. You can see that the wave
form is close to 24V peak-to-peak.
(Ignore the oscilloscope measurement
of 32.2V peak-to-peak because that
includes occasional overshoots which
are not depicted in this waveform.)
Fig.3 shows the voltage appearing
between one side of the transformer
secondary and 0V. This is 320V peakto-peak, exactly as theory suggests. By
the way, this waveform was recorded
with two 11W CFLs connected. Note
that the waveform frequency was
recorded as just over 20kHz.
We mentioned overshoots in the
primary waveform and these are
clamped, to protect the Mosfets from
voltage punch-through, by 16V zener
diodes ZD1 and ZD2 plus diodes D1
and D2. The fact that two 16V zener
diodes are employed, explains why
the oscillo
scope produced a meas
urement of 32.2V peak-to-peak for
the overshoots.
The transformer secondary drives
a bridge rectifier consisting of four
ultra-fast 1000V diodes, D3-D6. The
rectified output is fed to the CFLs via
a low-pass filter consisting of R5 & R6
in parallel together with capacitor C5.
R5 and R6 also limit the peak current
when charging any load capacitance;
eg, the filter capacitor in the CFL.
Semiconductors
1 74C14, 40106 hex Schmitt
trigger (IC1)
2 C8050 NPN transistors (Q1,Q3)
2 C8550 PNP transistors (Q2,Q4)
2 2SK2175 N-channel Mosfets
(Q5,Q6)
2 16V 1W zener diodes (ZD1,
ZD2)
6 1NH42 ultra-fast diodes
(D1-D6)
2 1N4148 diodes (D7,D8)
Resistors (0.25W, 1% or 5%)
1 47kΩ
2 205Ω 2W
1 10kΩ
2 4.7kΩ
Capacitors
2 220µF 25VW PC electrolytic
1 0.1µF metallised polyester
(greencap) or monolithic
1 .0015µF metallised polyester
(greencap)
1 680pF 3kV ceramic
Miscellaneous
240VAC figure-8 or sheathed
twin cable, hookup wire, solder.
All parts for this project are available from Oatley Electronics who own the
design copyright. Their address is PO Box 89, Oatley, NSW 2223. Phone
(02) 9584 3563; fax (02) 9584 3561. The prices are as follows:
Earlier on we raised the issue of
whether there was any need for the
inverter to have its own bridge recti
fier when the CFLs have an internal
bridge. The reason the second bridge
rectifier is required is that the invert
er runs at 20kHz and this is done to
enable high efficiency and a small
step-up transformer to be used. If the
20kHz output from the inverter was
fed directly to the CFLs, their bridge
rectifiers would immediately blow.
So we use ultra-fast diodes to do the
rectification and then the CFL internal
bridges can handle the unsmoothed
DC without problems.
PC board plus on-board components plus one 11W CFL ................$25.00
Building it
Extra CFL...............................................................................................$11
All the components for the CFL
inverter are accommodated on a
Where To Buy The Kit
March 1998 25
Above: the inverter board steps up
12V DC to drive one, two or three
compact fluorescent lamps (CFLs).
Make sure that all parts are correctly
oriented.
Fig.4 (right): this is the component
layout for the PC board and the
wiring to the switches. Both the stepup transformer (T1) and the supply
inductor (L1) will be supplied ready
wound.
small PC board which measures 115 x
36mm. It is then mounted in a stand
ard plastic utility box measuring 129
x 68 x 41mm. This has a panel on it
with three rocker switches, one for
each CFL to be driven.
Fig.4 shows the component layout
for the PC board and the wiring to the
switches. This project will be supplied
as a kit from Oatley Electronics and
both the stepup transformer (T1) and
the supply inductor (L1) will be sup
plied ready wound.
When assembling the board, install
the resistors and diodes first. Make
sure that you don’t mix up the diodes
26 Silicon Chip
and zener diodes otherwise the project
will have a very brief life. Then install
the four transistors, followed by the
capacitors. Again, make sure that you
install the transistors in their correct
positions and that the electrolytic
capacitors have the correct polarity.
Next, install the transformer and
inductor L1, followed by the two
Mosfets and the CMOS IC.
Finally, connect all the external
wiring to the rocker switches and the
Edison screw sockets for the CFLs.
Check all your wiring very carefully
and then connect one CFL. Connect
the inverter to a 12V power supply or
battery. The CFL should immediately
light up. As is normal with any fluo
rescent lamp or CFL, they will take a
couple of minutes to reach maximum
brilliance.
Be careful not to come in contact
with the inverter’s output. It bites!
*Branco Justic is Managing Director of
SC
Oatley Electronics.
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