555 timer IC Flyback Transformer Inverter Driver Circuit

Flyback Transformer Driver for High Voltage Generation Projects with a 555 Timer IC

High Voltage Flyback Transformer Driver Circuit with 555 Timer IC

Flyback transformers are found in monitors, TVs or anything with a CRT, and are sometimes known as Line OutPut Transformers, or just LOPT. They are used for generating high voltage for the CRT, which is needed to create an electric field, which in turn accelerates electrons towards the screen, which finally excite phosphors and create the image you see. Flybacks are designed to work best anywhere between 15 to 150 kHz, so some experimentation is required to find the intended operating frequency. TV flybacks are generally designed for upper audio frequencies, which is the cause of the high pitched noise heard from a muted TV. (If you're over 40 you will need to confirm this with your kids.) The optimal operating frequency can have many harmonics, which will work as well as the actual optimal frequency to some extent. Since flyback transformers use a ferrite core they need vastly different operating conditions than an iron cored mains transformer. In fact flyback transformers aren't really conventional transformers at all, but coupled inductors which means they should be driven differently. Fyback transformers are generally either driven in "flyback mode", or some push-pull topology. The first two drivers on this page drive the flyback in flyback mode, while the last two use push-pull topologies. To obtain a high frequency variable duty cycle drive signal we can use the 555 timer. This simple driver circuit is quite efficient if tuned correctly, and in some cases quite powerful. It is currently set to run between 17-50 kHz, which should be a large enough range to sweep through any harmonics a flyback may have.

This is a pretty standard 555 astable design. All parts except the timer and mosfet are non-critical. Input power should be 12-16 volts, the current draw can reach a few amps. For the mosfet I used an IRFP450, though any mosfet with a breakdown voltage above 200V and "avalanche rated" will work. Make sure you use a mosfet and not a bipolar junction transistor, the symbol in the datasheet should resemble the one in the schematic. For a different frequency range you can use a 555 timer calculator or just experiment to find new capacitor and resistor values. As mentioned above this driver drives the flyback in flyback mode. What that means is that the mosfet is turned on by the timer, and current starts to flow through the primary winding. After some time the timer will turn off the mosfet again and the current will be forced to stop. However, this is not possible since the primary has significant inductance. The current then causes the voltage at the mosfet drain to increase in an attempt at allowing current to flow. The voltage will rise up to the breakdown voltage of the mosfet, where it stops (since the mosfet is avalanche rated this does no harm, and only produces heat in the mosfet). The voltage at the mosfet drain will potentially be equal to the breakdown voltage of the mosfet, meaning the primary voltage will be hundred of volts now. Due to the large turns ratio of the flyback the few hundred volts at the primary become several thousand volts on the secondary. Since some energy is avalanched in the mosfet, adequate heatsinking of the IRFP450 is required.

Flyback Transformer with self wound primary

I recommend winding your own primary for several reasons. For one thing you don’t have to worry about finding the built-in primary, and you can adjust the primary turns according to the drive voltage or desired output voltage. Also you don't need to worry about destroying the internal primary during the experimentation phase. The primary must be wound directly onto the exposed ferrite core. The number of turns varies, and is determined by operating voltage, on-time and core cross-sectional area. For general use, 3 to 10 turns should be right for this driver. Fewer turns mean higher voltage, but increased mosfet power dissipation. Start with 10 turns and remove them until the MOSFET gets too warm or the spark too big.

For those of you who have never seen a flyback transformer before, it may be a bit tricky to know where the primary, ground and other pins are. The ground pin can be found by finding the pin the HV arcs to the most. Simply take the HV lead and bring it near the pins on the bottom. The internal primary can be found by measuring resistance. It should be around 1 ohm. Some flybacks may have several winding which will appear to be primaries, in this case the real one can only be found by measuring inductance. A typical primary inductance is often 300µH.

high voltage spark 

flyback transformer high voltage spark

flyback transformer high voltage spark