Design Ideas: March 31, 1994

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.