The schematic has been independently verified by myself and fellow coiler Justin McMahan. Justin had some problems with duty cycle control, but has solved them and is using the circuit to power a "Illumastorm" plasma globe. The plasma globe is MUCH brighter than with the original circuit!
I designed and built this circuit on a breadboard, then re-built it on a PCB (directly off of the schematic shown below) and it works flawlessly. The schematic is good, and reasonable care has been taken with the PCB layouts shown below. They have been doublechecked but have not yet been constructed directly off this page, so therefore they are not fully tested. I made my PCB slightly differently than the one shown below.
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Stage 1: Generating Clean Power. The circuit needs a clean source of DC power to work properly, and this is managed by components T1, BR1, C1,2,3, and U1. Transformer T1 has a 120V primary, and when plugged into the mains produces 12VAC on its secondary winding. This AC voltage is then rectified by BR1 and filtered by capacitor C1. The resulting smoothed DC is then applied to voltage regulator U1 which provides a high degree regulation. The output of U1 is then applied to capacitor C3 which aids in regulation by smoothing out moderately fast changes in load current.
Stage 2: Forming a Variable Frequency (5kHz to 200kHz) Triangle Waveform. U2 and U3 are operational amplifiers arranged in a way that the circuit oscillates at a user-variable frequency. The output of U2 is a clean 50% duty cycle square wave, which is applied to the inverting input of U3 at pin 2. U3 then integrates the square wave, providing a perfect triangle waveform for the next stage of the circuit to use. The oscillation frequency is adjustable from 5kHz to 200kHz by potentiometer R4. ** Note - frequency can be decreased by using a larger capacitor in place of C5. A 470pF capacitor places the center frequency at around 180kHz. Maximum frequency in this application is limited to 250kHz because of the opamps used (LM318's) have a unity gain bandwidth of 20Mhz. However, they are still plenty fast enough to work at 200kHz.
Stage 3: Providing Duty Cycle Adjustment by Voltage Comparison. Voltage comparator U4 requires that its AC input signal ride on a DC bias. To do this, we use coupling capacitor C6 to get rid of any DC bias that is there. We then apply a calculated amount of DC voltage by using the voltage divider made up of resistors R6 and R11. The remaining signal is a triangular wave, riding on a DC bias voltage of 1/2Vcc. This is applied directly to the inverting input of U4. The IC now has an above-ground signal to accurately compare with another. The other input (noninverting) of U4 is supplied a user-variable DC voltage via potentiometer R12 to provide duty cycle adjustment. A pullup resistor is required for the output stage of U4 to work properly, and this is done with the 1K resistor R7. Finally, the output of U4 is a square wave who's pulse width is proportional to the adjustable DC voltage on pin 2. Therefore, the duty cycle may be quickly and accurately changed by simple adjustment of potentiometer R12, which may be mounted on a front panel for easy access. A .1uF high-frequency bypass capacitor may be required at the terminals of R12 if it is mounted more than 6" away from the circuit board.
Stage 4: Meeting the Requirements of MOSFET Gate Drive. Power MOSFET's possess a surprisingly large amount of capacitance between their gate and source terminals, and to quickly raise this capacitance to a specified voltage requires a large amount of peak current. The only way to supply this current is to keep the drive impedance as low as possible. Gate drive IC U5 (MAX4420) has an output impedance of 1.5 ohms, which is sufficiently low to quickly drive the MOSFET gate capacitance to the required amount. The output of U4 is applied to a voltage divider consisting of R12 and R14. This divides the voltage down to approximately 5V, which is then applied to the input of U5. The output of U5 is an extremely clean 12V square wave that may be used to drive a MOSFET gate through a small resistance.
** Note 1 - set all pot's to midrange when first testing the circuit.
** Note 2 - measure waveforms AT THE PINS of all IC's.
This circuit will drive a large power MOSFET through a 47 ohm resistor. Connect the source of the MOSFET to the circuit ground, connect the gate to the 47 ohm resistor. So: Pin 6,7 of U4 ---> 47ohm res ---> gate
There are many MOSFET's that will work here. I suggest using one with a Vds voltage rating of not less than 400V. Also, a drain current rating (Id) of not less than 8A. Also, keep in mind that the lower the resistance of the drain to source junction (Rds), the better your circuit will work. Rds(on) ratings can be found in the manufacturers data sheet. There are many sources to choose from, including Harris, International Rectifier, Motorola, and Fairchild Semiconductor.
Some suitable MOSFET's:
There are many options!
