12v flourescent lamp inverter.htm

12 v cfl lamp drawing

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      <DIV class=article>
      <P>Fluorescent tubes use far less energy than incandescent lamps and 
      fluorescent tubes last a great deal longer as well. Other advantages are 
      diffuse, glare-free lighting and low heat output. </P>
      <DIV class=wpimg style="WIDTH: 302px"><A 
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      <DIV class=wpcaption>Fig.1: two switch-mode circuits are involved here: 
      the DC-DC inverter involving IC1, Q1 &amp; Q2 and the fluoro tube driver 
      which converts high voltage DC to AC via IC3 and Q3 &amp; Q4 in a 
      totem-pole circuit.</DIV></DIV>
      <P>For these reasons, fluorescent lighting is the natural choice in 
      commercial and retail buildings, workshops and factories. For 
      battery-powered lighting, fluorescent lights are also the first choice 
      because of their high efficiency.</P>
      <P>The main drawback with running fluorescent lights from battery power is 
      that an inverter is required to drive the tubes. Inverter efficiency then 
      becomes the major issue.</P>
      <P>There are many commercial 12V-operated fluorescent lamps available 
      which use 15W and 20W tubes. However, it is rare to see one which drives 
      them to full brilliance. For example, a typical commercial dual 20W 
      fluorescent lamp operating from 12V draws 980mA or 11.8W. Ignoring losses 
      in the fluorescent tube driver itself, it means that each tube is only 
      supplied with 5.9W of power which is considerably less than their 20W 
      rating. So while the lamps do use 20W tubes, the light output is well 
      below par.</P>
      <DIV style="CLEAR: both"></DIV>
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            <DIV class=breakoutTitle>Warning:</DIV>This circuit generates in 
            excess of 300V DC which could be lethal. Construction should only be 
            attempted by those experimenced with mains-level voltages and safety 
            procedures. </TD></TR></TBODY></TABLE>
      <DIV style="CLEAR: both; MARGIN: 0px"></DIV>
      <P>Our new fluorescent inverter drives 36W or 40W tubes to full brilliance 
      and has the option to dim the tube down to about 80% brightness. So not 
      only do you get full brightness when you want it but you can dim the tube 
      down when full brightness is not required and you want to conserve power 
      drawn from the battery.</P>
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      <DIV class=wpcaption>Fig.2: this is the internal schematic for IC1. The 
      TL494 switch-mode controller.</DIV></DIV>
      <P>Built on a long thin PC board, the inverter fits easily into a standard 
      36/40W batten.</P>
      <P>Drive for the fluorescent tube is controlled with a specialised IC 
      which provides filament preheating before the tube is ignited. Once the 
      tube is alight it monitors the tube current to maintain constant 
      brightness. This current feedback control also provides for the dimming 
      feature.</P>
      <P>By the way, this project is quite similar in concept to the fluorescent 
      inverter described in the November 1993 issue of SILICON CHIP. This 
      earlier circuit is now superseded.</P>
      <H3>Block diagram</H3>
      <P>Fig.1 shows the general arrangement of the fluorescent inverter. The 
      12V supply is stepped up to 334VDC using IC1 &amp; IC2, Mosfets Q1 &amp; 
      Q2 and transformer T1.</P>
      <P>IC1 is the well-known Texas Instruments TL494 pulse width modulation 
      controller. The internal functions of IC1 are shown in Fig.2. It contains 
      a sawtooth oscillator, two error amplifiers and a pulse width modulation 
      comparator. It also includes a dead-time control comparator, a 5V 
      reference and output control options for push-pull or single ended 
      operation.</P>
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      <DIV class=wpcaption>Scope1: These two waveforms show the gate drive to Q3 
      and Q4 when the fluorescent tube is at full brightness. Top trace is the 
      gate drive to Q4, a nominal 12V peak-to-peak signal. Lower trace is the 
      gate drive to Q3, which is from 0-334V plus the gate voltage when switched 
      on. The small step in the top of the waveform is when the gate goes to 12V 
      above the 334V supply. </DIV></DIV>
      <P>Oscillator components at pins 5 and 6 set the operating frequency and 
      for our circuit this is around 100kHz. This frequency was selected to 
      enable use of a relatively small toroidal core for the transformer. The 
      PWM controller generates variable width output pulses at pins 9 and 10, to 
      ultimately drive the gates of Mosfets Q1 and Q2 via the CMOS buffers in 
      IC2, a 4050 hex buffer package.</P>
      <P>Mosfets Q1 and Q2 drive the centre-tapped primary winding of 
      transformer T1. The centre-tap of the transformer's primary winding 
      connects to the +12V supply while each side of the primary winding is 
      connected to a separate Mosfet. Each Mosfet is driven with a squarewave so 
      that when Q1 is on, Q2 is off and when Q2 is on Q1 is off.</P>
      <P>With Q1 on, 12V is applied to the top half of the transformer primary 
      winding. Similarly, when Q2 turns on, 12V is also impressed across the 
      lower primary winding. The resulting square waveform on the primary is 
      then stepped up by the secondary winding. High speed diodes rectify the AC 
      output from the transformer T1, while a 470nF 630V capacitor (C4) filters 
      the output to provide a stable DC voltage.</P>
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      <DIV class=wpcaption>Scope2: These waveforms are identical to those in 
      Scope1 except that now the frequency is much higher, at 65kHz, to dim the 
      fluorescent tube. Notice the "dead time" between Q4 being switched off to 
      Q3 switched on. This prevents high current pulses which would destroy the 
      Mosfets if both were on at the same time.</DIV></DIV>
      <P>A portion of the DC voltage output (called the error voltage) is 
      returned to IC1 for feedback control and the pulse width modulation is 
      varied to maintain the 334V output.</P>
      <P>The high voltage DC from the inverter is applied to the fluorescent 
      tube via Mosfets Q3 &amp; Q4 and an LC network consisting of L2 and C1. 
      Mosfets Q3 &amp; Q4 are switched alternately by the ballast driver IC3, an 
      L6574 fluorescent ballast driver, made by SGS-Thomson. The resulting 
      squarewave signal is applied through inductor L2 and capacitor C1 to the 
      fluorescent lamp. The inductor is included to provide AC current limiting 
      while capacitor C1 blocks DC current flow.</P>
      <P>During the starting phase, Q3 and Q4 are driven at a very high 
      frequency and this provides a current flow through L2 and C1, the top tube 
      filament, through C2 and the lower tube filament and then to ground via 
      the current sense resistor R1. This current is limited to a low value by 
      the impedance of L2 and it heats up the lamp filaments so the tube start 
      easily. After about one second, the drive frequency is lowered to the 
      series resonant frequency of L2 and C2 and the resulting high voltage 
      across C2 fires the tube. Once the tube is fired, the drive frequency is 
      further reduced to provide full tube brightness.</P>
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      <DIV class=wpcaption>Scope3. These are the gate drive signals to Q1 and Q2 
      when the fluorescent tube is driven to full brightness. Frequency is 
      around 100kHz. Note the "dead time" between one Mosfet turning off and the 
      second Mosfet turning on.</DIV></DIV>
      <P>As you might expect, there is a fair amount of circuitry packed into 
      the ballast driver IC; its internal workings are shown in Fig.3. An 
      oscillator section comprises the VCO (voltage controlled oscillator) and 
      the current sources set by resistors Rign and Rpre at pins 4 and 2 
      respectively. Frequency during starting is controlled by resistor Rpre in 
      conjunction with capacitor CF at pin 3. This sets the maximum frequency. 
      Once the tube is started, the frequency is set by Rign and capacitor CF. 
      An op amp at pins 5, 6 &amp; 7 can be used for frequency control.</P>
      <P>The duration of the tube filament preheat is set by capacitor Cpre at 
      pin 1. The enable inputs at pins 8 &amp; 9 can be used to reinitiate 
      starting if the tube does not fire or to shutdown the circuit if a tube is 
      not installed.</P>
      <P>The gate drive for the Mosfets is interesting. Mosfet Q4 is driven 
      directly via the low voltage gate (LVG) driver at pin 11. When pin 11 goes 
      high, Q4 is switched on and when pin 11 is low, Q4 is off.</P>
      <H3>High side switching</H3>
      <DIV class=wpimg style="FLOAT: right; WIDTH: 302px"><A 
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      <DIV class=wpcaption>Scope4: This waveform shows the firing cycle of the 
      fluorescent tube and is an attenuated signal of the actual tube voltage. 
      The voltage is initially high and then drops once the tube has 
      fired.</DIV></DIV>
      <P>Mosfet Q3 requires a special gate driver to allow it to drive the high 
      voltage (HV) supply. The special gate driver comprises the bootstrap 
      diode, level shifter, high voltage driver (HVG) and capacitor Cboot 
      between the source of Q3 and Vboot. When Q4 is switched on, Q3 is off and 
      so capacitor Cboot can be charged from the supply at Vs via the bootstrap 
      diode and Q4 (to ground).</P>
      <P>Thus Cboot will have the supply voltage across it. When Q4 is switched 
      off and Q3 is switched on, the entire gate drive section for Q3 is pulled 
      up to the HV supply and the gate drive is higher than this by the Vs 
      supply stored on Cboot. The gate drive circuit (HVG) thus maintains its 
      supply from Cboot. The bootstrap diode is now reverse biassed and plays no 
      further part in the operation. </P>
      <P>When Q3 is switched off and Q4 is switched on, Cboot can be topped up 
      via the bootstrap diode again. The capacitor value needs to be 
      sufficiently large to prevent the HVG driver supply from drooping as it 
      needs to charge the gate capacitance of Q3. </P>
      <H3>Circuit details</H3>
      <DIV class=wpimg style="WIDTH: 302px"><A 
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      <DIV class=wpcaption>Fig.4: the full circuit of the fluorescent inverter. 
      IC3 is the clever component, varying the tube drive frequency between 
      100kHz and about 30kHz to preheat the filaments, ignite the tube and then 
      maintain the tube current at the correct value.</DIV></DIV>
      <P>The full circuit of the fluorescent inverter is shown in Fig.4. IC1 is 
      the TL494 PWM controller. Its frequency of operation set at around 100kHz 
      by the 4.7kW resistor and 1nF capacitor at pins 6 and 5 respectively. </P>