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EDN Access -- 03.31.94 Low-cost converter drives fluorescent tube

-March 31, 1994

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Design Ideas:March 31, 1994

Low-cost converter drives fluorescent tubes


Steven C Hageman,
Calex Manufacturing Co, Concord, CA

In the last few months, several designers have published circuits for cold-cathode fluorescent-tube (CCFT) power supplies, and a specialized power-supply IC is also now available. However, a significant number of CCFT applications don't require the complexity and expense of a dual-FET resonant approach. Applications such as electronic night-lights, backlights for industrial equipment, such as gas pumps or signs, simply can't justify the cost of building a resonant supply. The low-cost circuit in Fig 1 produces a small, very reliable drive for many of the smaller CCFT tubes.

Initially, Q1's emitter drives the gate of MOSFET Q2 high, turning Q2 on. Current then ramps up through T1, which acts more like an inductor than a transformer. When the current through T1 and Q2 reaches approximately 0.62/RSA (approximately 1A for this design), Q3 turns on. The Q3's collector pulls the Q1's base and Q4s gate toward ground and causes programmable unijunction (PUT) Q4 to fire. When Q4 fires, it acts like an SCR and quickly drives the gate of Q2 to ground.

At this point, the energy stored in T1's primary winding transfers to the secondary and causes the tube to ignite. Q4 stays latched until T1 releases all of its energy to the tube. The current in T1 then reverses in a slightly resonant ring and flows back through Q2's body diode, causing Q2's gate to go slightly negative. Because of Q2's stray capacitance, the anode of Q4 then also goes negative, causing it to unlatch and release the gate of Q2 for another cycle.

With the component values in Fig 1, the converter oscillates at a frequency of approximately 30 kHz. The circuit runs with inputs from 10 to 20V dc and draws 170 mA at 15V. The value of R1 controls the brightness of the CCFT. Substituting a smaller value than 0.62V increases the energy in T1 and causes the tube to glow more brightly. Additionally, pulling the ON/OFF pin to ground turns off the circuit.

This basic circuit can drive larger tubes if you scale up the power by increasing the energy-storage capability of T1 (using a larger core) and by using a higher-power MOSFET. The design for T1 in Fig 1 is good for a maximum peak current of approximately 1.4A.


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