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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.