Process for Making a Flyback Transformer

Question

I am looking to make my own flyback transformer that can put out a DC voltage from 0 to -40 kV DC and a power of 400 watts, as most commercial flybacks do not go up to the specified power and typically do not provide a negative voltage. Thus, I was curious how I could build such a device and the what the required specifications for the components used in the transformer (i.e. the size of Litz wire and the amount of wraps on either side) would be to build a proper transformer.

Preferably, the voltage input for such a design should be 0-24 V DC with a transformer output that correlates with the input (basically resulting in an adjustable output based on an adjusted input if possible). The ripple of the -40 kV DC output should not exceed 350 V.

If the use of a single flyback transformer would be impractical for such power, how could I create an assembly of transformers that would collectively provide the needed power and voltage? Thank you.

Answer 1

I like to start such a project with a rough ballpark design. Looking at the primary side, you want 400 Watts at 24 volts, so your average current is about 16.6 amps. Round up to 20. So the current waveform is a sawtooth, with a peak of 40 amps.

Now let's pick a frequency, say 20 kHz. You want the current to ramp up at the rate of 40 amps per 50 microseconds with 24 volts applied, so the inductance of the primary is L=V/Idot = 30 microhenries. Now you just have to go shopping for gapped ferrite cores that will give you the power handling capability. If you go to www.mag-inc.com you will find design tables and all kinds of helpful stuff. (I don't work for them, but I have used their cores).

Once you have a core big enough for your power, you have to pick the number of turns and the gap size such that you get the right inductance without saturating the core at the amount of volt-seconds you'll be running. Then you pick the wire size so that the resistive losses are tolerable. Then you consider the flyback part of the equation. When your transistors turn off what voltage will the primary rise to? Here you're going to find out how expensive and slow transistors are at high voltage. Then once you know the flyback voltage you divide 40 kV by that and that will give you the ratio of the number of turns on your secondary to the number of turns on your primary.

Now look at the monster you've created. Will all the turns fit? Is it too big? If you go to a higher frequency the core gets smaller, the output filter capacitor gets smaller but the transistors start dissipating more heat. Also consider that you'll want to pot the transformer to keep the 40 kV from arcing, which makes it harder to cool your transformer. Also you'll have to have some kind of circuit adjusting the transistor duty cycle to regulate the output voltage. Finally you need a design for the output rectifier. You'll want to pay attention to the reverse recovery time of the diodes and make sure the dV/dt you're applying is reasonable.

Answer 2

40kV@10mA continuously isn't trivial. The only cheap (=mass produced) transformer in the same leaque is as far as I know the one made for old big CRT televisions to produce the needed acceleration voltge to the CRT tube, the original flyback transformer.

For cost optimization reasons the transformer was a part of CRT electron beam deflection system, returning the ray back to the beginning of a new line was "the flyback".

To get the wanted voltage , you still need voltage doubling and to get 400W, you need parallel transformers (=one outputs the flyback pulse and another collects the energy to the primary, both can output to the same circuit through their own rectifiers.

Start learning something about legacy televisions.

Answer 3

If you look at modern commercially available X-ray tube power supplies (50 kV for instance) you won't find a design that uses a transformer that generates the final voltage. What they do is use a transformer to generate maybe 8 kVp-p then use a cockcroft-walton multiplier to take the lowish AC kV produced by the transformer secondary up to 50 kV DC.

This means that the transformer secondary insulation (layer to layer and layer to chassis) is not wholly unfeasible for a ferrite core. The one I designed used 50 kHz as the switching frequency and a split primary driven by two MOSFETs controlled from a Linear Technology chip.

I am looking to make my own flyback transformer that can put out a DC voltage from 0 to -40 kV DC and a power of 400 watts

A flyback transformer topology isn't powerful enough for 400 watts - you need a push-pull configuration at the primary either from two MOSFETs and a split/centre-tapped primary or, a H bridge primary coil driver.

This means that from a 24 V DC power supply, the primary p-p voltage will be 48 volts and, with a secondary voltage of (say) 7968 Vp-p, you have a turns ratio of 1:166.

The implication of this is that with 6 turns on the primary, you need about 1000 turns on the secondary. This means that it can fit inside the core geometries of large commercially available ferrite cores along with all the insulation layers that prevent voltage breakdown.

Preferably, the voltage input for such a design should be 0-24 V DC with a transformer output that correlates with the input (basically resulting in an adjustable output based on an adjusted input if possible).

The only feasible way that I know of (other than buying an oil-filled old-fashioned X-ray transformer if you can get one) is to run the push-pull stage from a variable DC supply whilst the gate drivers were fed from a fixed supply voltage. The variable DC supply can be turned down to 0 volts resulting in zero output or turned up to 24 volts to give maximum HV output.

One final note about the cockcroft-walton multiplier after the transformer. It has to be designed very carefully and then resin filled to prevent flashover. The PCB should be slotted around each stage to prevent high-voltage-arcing along the surface (aka HV tracking) thus also allowing the resin to get to (and around) every component.