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Flyback Transformer Drivers

Flyback Transformer Drivers


The 555 Driver Basic

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.

Basic 555 Flyback Driver Schematic

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.

Winding your own 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.

Regular Flyback Transformer

Standard Monitor Flyback Transformer with new primary.

Pinout

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.

Pictures

Small Spark Another small spark

Sparks at 5v. This was before I knew anything about electricity, so 5V was all I considered safe for the driver.

Troubleshooting

Of course something can go wrong, so if you’re unlucky check these points.

  • If you hear a high pitched whine but you don’t have any HV, you’re close. This is caused by a false primary or wrong phasing. Try switching the primary leads, since the flyback is rectified by a single diode, which makes polarity is important. If the output is weak, the primary polarity may need reversing. Check to see if you have found the correct ground pin.

  • Nothing. Silence. This is what I hate the most. Basically just check that everything is wired correctly, if it is, check that all the parts are functioning. You can check for a frequency by replacing the flyback with a speaker. If you get a high pitched whine it’s alive. If you’re getting a drive signal out of the circuit, you are probably not connected to the correct primary. Wind your own if you fail to find the internal primary.

  • Always check that the 555 is still functioning.


555 Driver MKII

About 18 months later I decided to try this again, only this time I knew what I was doing. I whipped up this new 555 driver, which works quite well. The max voltage is 50V with an IRFP450, due to the primary energy becoming to much for the mosfet to avalanche without dying. If you want to power it from an even higher voltage, use a snubber or stronger mosfet instead. The additional circuitry simply isolates the 555 from the "power" supply, so it can be increased beyond the 16V rating of the NE555. Keep in mind standard 7812 regulators should only run from 30V maximum, so for 50v you need to cut out the 7812 and run the logic section from a dedicated supply.

555 Driver MKII.1

Make your own PCB!

I've designed a PCB for this driver which can be edited with ExpressPCB. A pdf of the copper traces and components layout is also included for those without ExpressPCB. flyback_driver_MK_PCB.zip. I recommend using a 5.45mm block connector for the IRFP450, in case it needs changing. As with the basic driver, heatsinking of the IRFP450 is required. I've used an old processor heatsink with a small 12V fan. If powering the fan the 7812 regulator will need a heatsink as well.

populated PCB  PCB view 2

Pictures

3 cm spark

24v input, 3 cm+ sparks

Fat Spark

24v in juicy spark

Dual Sparks

Dual sparks

7.5 cm long spark

70kV from 50 volt supply (driver was modified)


MORE POWER!!!

Tired of measly 2 cm sparks? Want more power? Try the Mazzilli ZVS flyback driver! This driver is capable of pumping upto a kW of power, so impressive arcs can be made. Normal operation from 12V only results in about 100W, but that's still several times what the 555 drivers will process. The circuit is has very few parts, is simple, and very elegant. If it weren't more deadly than the 555 drivers I would have made it my recommended newb driver. The arcs produced by this driver are very hot, the copper ground wire goes white quickly, and anything brought close to the arc in incinerated. The current in these arcs can kill, so be carefull. The primary winding is center-tapped, so two windings must be wound in the same direction. I usually intertwine two wires, and then simply wind one winding with the pair. The start of one wire and end of the other are connected and serve as the center-tap. The primary should be between 3 + 3 and 10 + 10 turns, depending on voltage. The amount of turns depends on supply voltage, and resonant frequency of the circuit. The highest voltage I’ve heard of people running the Mazzilli driver at is 100V. As you can imagine, the arcs were insane! The resonant frequency is determined by the total primary inductance and parallel capacitor. It is a simple parallel resonant circuit, so the resonant frequency is easy to find with the parrallel resonant LC formula.

Mazzilli ZVS Driver

Circuit designed by Vladimiro Mazzilli.

The circuit works by one mosfet turning on due to differences in the gate resistors or internal structure of the mosfet. Once on, the opposite mosfet will be held off by the fast diodes. The voltage across one primary half will rise up an fall again in a half-sine wave. Once at zero the mosfet that was on will be forced off, and the mosfet which was held off will be allowed on. The cycle repeats in opposite this time, before returning to where it started. The large inductor serves as a "current capacitor", providing constant current to the driver. Thanks to the resonant action of the circuit, it benefits from ZVS, or Zero Voltage Switching. This means that the mosfets switch on with no voltage across them, so while they transistion from off to on they won't dissiapte power. (P = I * V)

Pictures


White Arc

Arc at 25V supply voltage provided by my MOT PSU. Susposed to be 50V but it dropped to 25V when pulling an arc, I should have done a better job of rewinding the MOT!

Long Vertical Arc

Off-Line Flyback Driver (The wall’s the limit)

Running power through multiple transformers just to power another transformer seems a bit absurd, so I though it was time for a direct mains powered flyback driver. This driver rectifies mains and produces a 320V DC source. The circuitry from the TL494  to the GDT creates alternating pulses for the IRFP450s at any desired frequency and duty cycle. The IRFP450 half-bridge feeds a square wave at +/- 160V to the flyback primary. 160V at considerable current, stepped up to some kV and mA produces some impressive arcs. The driver itself is my "multipurpose inverter". Contrary to the 555 drivers this driver provides actual AC to the primary, and due to DC flybacks being half-wave rectified you'll likely have problems with saturation regardless. I've run DC flybacks offline several times, but they have all failed eventually, and they cause the driver to heat up. Unrectified AC flybacks are much more suitable for this driver, and cause very little heating of the mosfets. See my article on making HV transformers for details.

The frequency and number of turns are a matter of tweaking and design, but for safe testing 30 turns and 100kHz have worked fine for a wide selection of flybacks. The main issue here is saturation of the transformer, which prevents the core from becoming further magnetized. Practically speaking, it means the transformer inductance will drop suddenly when saturation occurs, causing incredible current draw from the inverter and most often destroying it. (Or with my multi-inverter, simply tripping the OC protection.) I've put a calculator in the multipurpose inverter spreadsheet for determining the minimum number of turns required to achieve a specific flux density. Use 0.25T as a starting point if you don't know the saturation flux value of the core.

Since you won’t be running the flyback transformer in flyback mode anymore you can remove the air gap to decrease the idle current drawn. Remove the metal bracket, and pull the core halves out. In-between the core halves is a thin plastic spacer which creates the air gap.

Boxed up driver

Entire driver assembled in case. This was before it was upgraded into the Multipurpose Inverter.


Huge Arc

Awesome arcing action.

   Another huge arc

Nice.

Some interesting things to do with HV arcs are coloring them with salts, or making magnet vortexes. Common table salt will give red/orange arcs, and boric acid will give green arcs. Check the Internet for flame tests to see which salts will give which colors. The effect is hard to capture since the camera saturates and can't see the green color. Arcs drawn with boric acid are much brighter than usual, and are uncomfortable to look at even with sun glasses on. Arcs drawn with salt (or iron, I'm not sure which is causing the arcs to change appearance) on the other hand, are actually pleasing to look at as they are dimmer than usual. Of course, coloring plasma with salts in more impressive with Tesla coils, since they aren't as bright.

Bright green flash from arc

This blurry picture is all I have to suggests that the arcs are green.

Another thing to do with DC arcs is to make a plasma vortex. Arcs consist of plasma which is easily affected by magnetic fields, hence the awesome vortex. See the 4HV thread for more info and videos.

Arcing to magnet, vortex effect visible

A magnet plasma vortex can be made with the round ferrite magnets found in microwave ovens.




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Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.