20kV Flyback power supply

The 555 is wired as an astable and the capacitor is charged only through the 4.7Kohm trimmer (notice the diode) and discharged only through the 2.2 Kohm trimmer, making the duty cycle fully adjustable. The square wave is then fed into a totem pole made up of a 2N3904 and a 2N3906, which are cheap and easy to find. The totem pole ensures the gate is charged and discharged very fast (approximately 50nS). The IRF840 is a reliable and powerful power MOSFET, with a current capability of 8 A continuous and 32 A pulse, and an 800V drain-source voltage, protected by an internal zener diode. There is a snubbing network to ensure that voltage spikes are kept low (unless the insulation of the transformer starts to leak), protecting both transistors and the 555 IC. A 100-ohm resistor is a compromise between decay time and voltage spike. This is an efficient flyback driver for modern cylindrical rectified television flybacks. Many sites do not provide circuits for driving these transformers, simply stating that they are bad.

This circuit utilizes the 555 timer IC configured in astable mode, which allows for continuous square wave output. The charging and discharging of the timing capacitor is controlled by two trimmer resistors: a 4.7 kΩ trimmer for charging and a 2.2 kΩ trimmer for discharging. This configuration allows for fine-tuning of the duty cycle, which is crucial for optimal operation of the flyback transformer. The diode connected in the circuit ensures that the capacitor discharges effectively, preventing reverse current flow during the discharge phase.

The output square wave from the 555 timer is fed into a totem pole configuration consisting of two bipolar junction transistors (BJTs), the 2N3904 and the 2N3906. This arrangement provides a low-impedance drive to the gate of the MOSFET, allowing for rapid switching and minimizing delay. The fast switching speed, estimated at around 50 nanoseconds, is essential for efficient operation and minimizing energy loss.

The IRF840 MOSFET is selected for its robust specifications, including a maximum drain-source voltage of 800V and a continuous current rating of 8A, with a peak pulse current capability of 32A. The internal zener diode serves as a protective feature against voltage spikes that may occur during operation, enhancing reliability.

To mitigate voltage spikes, a snubber circuit is implemented, which typically consists of a resistor-capacitor (RC) network. In this design, a 100-ohm resistor is utilized, providing a balance between decay time and voltage suppression, thereby protecting sensitive components from potential damage.

This circuit is specifically designed for driving flyback transformers, particularly those found in modern cylindrical rectified television sets. The flyback transformer operates efficiently at a resonant frequency of approximately 15 kHz, with an ideal duty cycle around 90%. Adjustments to the off-time trimmer can lead to corona discharge at the terminals, which may produce audible sounds and ozone smell, indicating high-voltage operation.

For increased frequency range, a multiposition switch can be integrated to allow for different capacitor values, enabling operation at frequencies up to 200 kHz. However, caution is advised as higher capacitance values can lower the frequency, making the circuit suitable for other applications, such as ignition coils or charging high-voltage capacitors. The versatility of this circuit allows it to be adapted for various high-voltage applications, including the powering of devices like spark gap Tesla coils.The 555 is wired as an astable and the capacitor is charged only through the 4,7Kohm trimmer (notice the diode) and discharged only through the 2.2 Kohm trimmer, making the duty cycle full adjustable. The square wave is then feed in a totem pole made up of a 2N3904 and a 2N3906, which are cheap, and easy to find.

The totem pole ensures the gate being charged and discharged very fast (approx 50nS i think). The IRF840 is a cheap (i found it for 4euros) reliable and powerful power mosfet, it has current capability of 8 A continuous and 32A pulse, 800V drain source voltage, protecting internal zener diode. There is a snubbing network to ensure that voltage spikes are kept low (unless the insulation of the transformer start to leak) protecting both transistors and 555 IC.

100 ohm is a compromise between decay time and voltage spike. This is an efficient flyback driver for modern cylindrical rectified television flybacks. Many sites doesn't provide circuits driving these transformers, they simply say that they are bad. I don't agree. In fact I built this circuit. I spent a lot of time for finding resonant frequency (around 15Khz) and duty cycle. These transformers best work at around 90% duty cycle. You may notice corona breakdown at terminals and pfffff sound (as well as the ozone smell) when adjusting the off time trimmer to near 500-300 ohms. Of course it will work for other tipes of flyback as frequency and duty cycle have a large range. Frequency range can be increased using multiposition switch for other values of C3 capacitor ,for example 2 nF for 80KHz-200000KHz, but didn't found flybacks with so high resonant frequencies, in addition with higher values of c3 , eg 200nF, 2uF the frequency will drop making possible the use of ignition coils, and rectified power transformers @50Hz to charge high voltage electrolitic caps at 300-400V).

Unfortunately my ignition coil died because insulation breakdown (too long drawn arcs)... The 555 is wired as an astable and the capacitor is charged only through the 4,7Kohm trimmer (notice the diode) and discharged only through the 2.2 Kohm trimmer, making the duty cycle full adjustable. The square wave is then feed in a totem pole made up of a 2N3904 and a 2N3906, which are cheap, and easy to find.

The totem pole ensures the gate being charged and discharged very fast (approx 50nS i think). The IRF840 is a cheap (i found it for 4euros) reliable and powerful power mosfet, it has current capability of 8 A continuous and 32A pulse, 800V drain source voltage, protecting internal zener diode. There is a snubbing network to ensure that voltage spikes are kept low (unless the insulation of the transformer start to leak) protecting both transistors and 555 IC.

100 ohm is a compromise between decay time and voltage spike. I was able to power a small (20cm) Spark Gap tesla coil Using these dc rectified flybacks to charge primary tank capacitor. 🔗 External reference