Half bridge flyback driver
A flyback driver is a common amateur high voltage generator. It consists cathode-ray tube TV high voltage transformer driven by a high frequency inverter, in order to obtain HV output. The first and simplest driver designs generally consisted of a single silicon switch driving the transformer in flyback mode, which turns out as quite poor practice when the transformer is loaded by an arc (a problem I elaborated in my NE555 ignition coil driver project). Over time, experimenters have found other ways to push greater amounts of power through the flyback, generally up until it’s destruction. One very popular solution is “Mazzili flyback driver”, which is a circuit based on a powerful Royer oscillator with MOSFETs. Another common solution is the half bridge driver, which involves a half bridge of MOSFETs supplied directly from rectified and filtered mains. The MOSFETs are generally controlled by a PWM chip of some sort. And while I’m quite a fan of Royer oscillator and have used it in assortment of my other projects, here I’ll be discussing my own approach on a half bridge flyback driver design.
My chip of choice for the half bridge flyback driver is SG3525, for several reasons:
- It is a commonly available and low cost IC
- SG3525 posseses high drive power totem pole outputs, allowing it to directly drive MOSFET gates via a gate drive transformer
- Incorporated shutdown option makes it easy to implement over-current protection
The error amplifier of the chip was bypassed into a voltage follower in this application, and a single potentiometer was used as a means of PWM duty cycle (and power) control. Another potentiometer is used to adjust the frequency of the signal, which is generally set to 50-60kHz. A simple over-current protection scheme was implemented using a small current transformer (CT) that senses the half bridge output current. The signal from the CT is rectified and burdened by an adequate burden resistor, 3.3 ohms in this case. Voltage signal from the resistor is used to trigger a sensitive thyristor, which shuts down the SG3525 until the power is cycled. The thyristor takes about 1V to trigger, and the CT has a ratio of 30:1 , resulting in treshold current of around 9A which is adequate for the 14amp mosfets used. I’ve reconstructed the schematic as follows:
This circuit was built on a prototype PCB with MOSFETs mounted onto a heatsink underneath. The mains rectifier and fuse are separate from the board.
The arc length attainable with this circuit is primarily dependent on the resilience of the transformer itself. I had a great flyback that did well over 10cm sparks (which I considered a great result, at the time) but then I destroyed it after attempting to use internal primary and an air gap in the core (a total blunder for a half bridge topology). I never got a transformer that lasted longer since then, but I know some people have reported arcs lengths of excess 30cm from a single flyback. I doubt these lasted long!
I later added some changes to the circuit and put it onto a nice PCB. The over-current protection (some call it detection too, so OCD) circuit is now a combination of NE555 and a comparator, which turns off the drive for a set period of time and does not annoyingly require the power to be cycled every time the OCD triggers. Additional discrete totem pole drivers have been added to enhance the drive capability for larger devices like IGBT’s. I’ve also added an audio input which is AC coupled to duty cycle control, allowing for singing arc experiments. While it works, the audio quality is not very great, apparently because the SMPS control chip wasn’t designed with high PWM linearity in mind.
The final look of the PCB. Note that the power section is no longer on the same PCB; this allows me to use it to drive any of several half and full bridges I had around!
At the time I finished this driver I found myself out of flybacks (which are getting rare anyway, with decline of the cathode ray tube). However, it produced some killer arcs in my high voltage ATX power supply transformer hack.
A half bridge flyback driver is a neat project for any beginner HV enthusiast, which reflects much of the way a commercial high voltage power supply might be made. Since the leakage inductance of the transformer provides the current limiting, and there is never DC component on the primary, arcs can be safely drawn from the transformer without risk of saturation. The circuit may be used for fun on it’s own, or to power devices like Tesla coils or lifters. The topology can be easily upgraded to series-loaded resonant (SLR) converter which is capable for very high power outputs – this topology has been pioneered by Steve Ward in his capacitor charging power supply project.
Links and references
 SG3525 datasheet http://www.onsemi.com/pub_link/Collateral/SG3525A-D.PDF
 Steve Ward’s version of half-bridge flyback driver http://www.stevehv.4hv.org/FBD.htm
 Steve Conner’s low voltage version with interesting gate drive level shift http://scopeboy.com/tesla/flyback.html