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Switching regulator controls CCFT

Switching regulator controls CCFT

You can control the current in a cold-cathode fluorescent tube and, hence, its brightness with a switching-regulator IC. Often used as a backlight, a CCFT requires an adjustable, high-voltage, ac-power source. EMI and tube-lifetime considerations dictate a sinusoidal waveform. A resonant circuit is best for generating sinusoids.
One of the simplest oscillators is the Royer type—a self-oscillating, current-fed, parallel-resonant power oscillator (Fig 1). The Royer oscillator comprises T1 and its secondary load, C1, Q1, and Q2. The effective inductance and capacitance of T1’s primary winding determine self-oscillating frequency. An auxiliary winding (T1’s pins 5 and 1) drives Q1 and Q2. The oscillator does not require capacitors at the emitters of Q1 and Q2 to work because L1 holds the current constant at Q1’s and Q2’s emitters. The circuit has a bypass capacitor at the input to the resonant circuit (T1’s center tap).

A current source, comprising L1, D1, and Q3, feeds the oscillator. IC1 and IC2 control this current source. D3 and D2 rectify the tube’s current. Half of that current develops a negative voltage across R1 and R2, producing a current that balances current from the 1.5V reference (pin 5, IC2). An increase in the error amplifier’s output increases the negative voltage by increasing the tube’s current and vice versa. Brightness varies from maximum to minimum as you adjust R2 from 0 to 2 k (ohms).

IC2 is a step-up switching regulator that normally employs both voltage and current feedback to control the MOSFET switch, Q3. Holding IC2’s FP pin (pin 3) below its REF (pin 5) confines this IC to the current-feedback mode.

IC2’s internal current-feedback controls comprise a comparator, which turns switch Q3 off, and a timer, which turns it on. Turning switch Q3 on causes its current to ramp up, which, in turn, causes the voltage at IC2’s CS pin to ramp up. When the CS voltage reaches 210 mV, IC2’s comparator turns switch Q3 off. Then, the current in Q3 quickly ramps down, and the timer holds switch Q3 off for 2 µsec before turning Q3 on again.

By sinking or sourcing current, the error amplifier, IC1, affects the maximum (peak) current through current-sense resistor R3.

At power-up, the circuit must start gradually to avoid secondary-voltage overshoots that might damage the transformer. Capacitor C2 provides a soft start by causing IC2’s internal, current-limited VREF to slowly approach its nominal 1.5V level. Because the current comparator’s voltage threshold (derived from the reference) rises slowly as well (toward 210 mV), the peak inductor current rises slowly, thereby eliminating overshoot at the transformer’s secondary.

The minimum gate voltage of Q3 sets the lower limit for the circuit’s input voltage. The maximum voltage depends on the regulator and op amp you use (12V for components shown). Input VBATT must be greater than 6.5V and one diode drop less than Q3’s absolute maximum drain-to-source voltage (30V). Raising T1’s turns ratio lowers the minimum voltage needed for VBATT.

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