Electronic – Transformer for variable output off-line boost flyback converter

WARNING: This circuit can produce LETHAL voltages and is provided for information only.

You cannot just use any old transformer. A salvaged fluorescent tube transformer is very unlikely to work.

If you don't (or can't) wind your own transformer then at least go for a transformer you know is designed for the job.

(1) Car ignition coil

(2) A line (flyback) transformer from an old CRT display

Both are designed and made to generate very high voltages

Things you should notice in this circuit.

(1) The 555 is decoupled from the 12V supply through a 56 ohm resistor (R3) and 100uF + 0.1uF (C1,C2). Try to get these capacitors connected as close to the pins of the 555 as possible (pins 1 and 8).

(2) The circuit does not require the 2N2222 transistor to drive the Mosfet. (Neither does your circuit). The 555 is quite capable of driving the gate of a Mosfet which is essentially a small capacitor that needs a sufficiently large voltage change (0 -12V is fine) and not a lot of current to turn it on and off.

(3) It uses diodes (D3,D4 - rated at 600V,3A)) to protect the Mosfet from voltage spikes and reverse voltage.

(4) It uses a different Mosfet - IRF640. The main differences are a higher maximum voltage and a lower turn on resistance. The lower turn on resistance means LESS POWER (I^R) will be lost in the Mosfet and it will run cooler (although you still mount it with a heat sink).

An LM2586-12 is a 12V fly-back regulator chip - note pin 3 - it "sniffs" the dc output of the diode/capacitor and when it equals 12 volts the chip starts regulating the pulse width to maintain the output at 12 volts. If the 12V output rises a little high, the chip momentarily stops pushing energy thru the transformer.

Fly back designs are not like conventional transformers - in a fly-back design the circuit pumps energy into the transformer primary in one half cycle and that energy gets released by the secondary on the 2nd half cycle. The energy released produces a voltage that is load dependent and only in extreme cases of duty cycle requirements is the turns ratio altered to get a bigger/smaller output voltage. Typically, a 1:1 turns ratio produces input-output voltages that are the same value for a 50:50 duty cycle but this is load dependent too.

Try looking for an adjustable version of the LM2586 or maybe, as an experiment try potting down the feedback with a potential divider to "con" the chip that it is seeing 12 volts BUT before doing so, read the data sheet to see if they allow this.