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555 Flyback Driver

Flyback Secondary Waveform
Board Layout
Board Layout

Here is a simple 555 timer based flyback transformer driver, with the PCB designed by myself for some HV experiments. Above is the Eagle CAD board layout.

The 555 timer is in astable mode, generating a frequency from about 22kHz to 55kHz, depending on the position of the potentiometer. The variable frequency is to allow the circuit to be tuned to the resonant frequency of the flyback transformer in use.

This is switched through a pair of buffer transistors into a large STW45NM60 MOSFET, rated at 650v 45A.

Input power is 15-30v DC, as the oscillator circuit is fed from an independent LM7812 linear supply.

Provision is also made on the PCB for attaching a 12v fan to cool the MOSFET & linear regulator.

Initial Board
Initial Board

Board initially built, with the heatsink on the linear regulator fitted. I used a panel mount potentiometer in this case as I had no multiturn 47K pots in stock.

PCB Traces
PCB Traces

Bottom of the PCB. The main current carrying traces have been bulked up with copper wire to help carry the potentially high currents on the MOSFET while driving a large transformer.
This board was etched using the no-peel toner transfer method, using parchement paper as the transfer medium.

MOSFET Heatsinked
MOSFET Heatsinked

Main MOSFET now fitted with a surplus heatsink from an old switchmode power supply. A Fan could be fitted to the top of this sink to cope with higher power levels.

Gate Drive Waveform
Gate Drive Waveform

This is the gate drive waveform while a transformer is connected, the primary is causing some ringing on the oscillator. The waveform without an attached load is a much cleaner square wave.

Flyback Secondary Waveform
Flyback Secondary Waveform

I obtained a waveform of the flyback secondary output by capacitively coupling the oscilloscope probe through the insulation of the HT wire. The pulses of HV can be seen with the decaying ringing of the transformer between cycles.

Corona Discharge
Corona Discharge
Arc Discharge
Arc Discharge

Corona & arc discharges at 12v input voltage.

Download the Eagle schematic files here:  Flyback Driver Eagle Files (891 downloads)

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