CCFL (cold cathode fluorescent lamp) is a type of fluorescent lighting techonology which were used mostly backlight of LCD screens and PC - car tuning. Nowadays, they are mostly replaced by LEDs. Still they can be used for general lighting if you have old LCD screen or TV from early 2000s and this project utilize CCFL tubes from old 17'' monitor.

The project is consist of 3 parts inverter, PWM controller and output driver.

Warning CCFL inverter produces dangerous high voltages, dont touch the output and do everything at your own risk.

Supplies

Inverter parts :

2x Power MOSFET IRFZ44N, RU6099 or similar (minimum 40-50V Vgs)

1x X2 or or similar high voltage or pulsed capacitor 100-470nF

1x 47Ω resistor

1x47uF electrolytic capacitor

2x 1n5919 schottky or similar high speed diode

2x 680Ω resistor

1x Power inductor

1x 1000uf electrolytic capacitor

1 Ferrite core with air gap (mine is salvaged from same LCD screen)

0.1mm enameled copper wire

0.4-0.6mm enameled copper wire

Output driver :

4x 3kV 30pf capactior

PWM controller :

1x NE555

2x diode (ı've used 1n4148)

1x potentiometer 5-10kΩ

1x 1kΩ resistor

1x 10nf capacitor

1x 470Ω resistor

1x 4.2nf capacitor

1x power MOSFET IRFZ44N or similar

1x 100uf electrolytic capacitor

Step 1: The PWM Controller

This is classical ne555 PWM controller which works in astable mode and drives a power MOSFET around 20kHz. First photo shows the assembled circuit and second photo is the gate signal of the MOSFET and 3. photo is the schematic of the PWM driver. It can dim the CCFLs from 8watt to 18watts at 12V. These tubes are around 4.5watt per tube so if your circuit consumes more than 18-20watt something is probably wrong and can destroy tubes.

The PWM transistor can heat up 20-30 degree above ambient temperature so a heatsink can be added.

Step 2: The Inverter

This inverter is known as ZVS or mazilli driver which produces sinusoidal voltage (sinusoidal voltage is neccessary for ccfl tubes according to Wikipedia: CCFLs exhibit a degradation in their current-to-light output efficiency in the presence of harmonics, so the resonant circuit is the one used to drive them.) My inverter does not create a perfect sine wave probably from homemade transformer.

In original mazilli driver gates of the MOSFETs are directly connected with the supply voltage but in this arrangment switching noise from the circuit returns back to power supply, to prevent this I add simple RC filter with 47Ω resistor and 47uF capacitor. This filter reduces noise from going back to power supply. Also adding bulk capacitors near switching elements leads less noise. The capacitor parallel to the primary winding is critical it must be rated at least 100V because it can see voltages 60-80V peak to peak at maybe few amps because of the resonance. X2 interference or special pulsed polypropylene capacitors can be used.

The inductor in series with with the transformer limits the current to primary and creates a constant current source for the transformer without it circuit is unstable and draws too much current. The higher inductance leads lower overall power.

Since the circuit switches at zero voltage (hence the name ZVS) the MOSFETs just warms up a little bit and there no need for heatsinking.

The 3. photo shows drain source voltage of the one the MOSFET. It is a half sinewave.

Making the transformer

Take your thin 0.1mm wire solder a thick wire to one end and secure the joint then wind first layer from 80-100 turns then put some tape over it and wind the other layer without breaking the wire. The wire is never cut off it goes with isolation, after making 8-9 layer, (by putting extra isolation in between primary and secondary) you can wind the primary winding using 0.4 - 0.6 mm wire make 5-10 turns with center tap.(Each shoulder has 5-10 turns). The number of turns in the primary found experimentally. Changing number of turns both change operating frequency and output voltage. More turns at the primary leads less voltage and lower frequency. Keeping above 20kHz is advisable to not to hear switching frequency. Advisable frequncy limits is between 20-50kHz. My inverter works between 32-21kHz depending on PWM.

Step 3: Output Driver

Since the voltage is sinusoidal capacitive current limiting can be used. These capacitors should at least 2kV and they can between 10-47pF depending on the frequency of the inverter. Lower the capacitance lower the brightness and vice versa.