What is Wrong with my Flyback Driver? - Page 1
Forum section: electronics / projects

author: nshin31302
author_rank: Contributor
timestamp: June 12, 2015, 12:25:58 am
content: So, I soldered a 555 timer flyback driver circuit using pad per hole perfboard. I double checked my connections/soldering and printed the flyback driver circuit and highlighted what I soldered on the circuit as I soldered my project. I checked if the MOSFET worked with a transistor tester. When I powered the circuit, with 12 volts with a computer ATX power supply, nothing happened. No sparks came out of the flyback connections. However; I began to realize that the MOSFET was heating up rapidly. Then when I quickly unplugged the wires of the circuit, I realized that the wires were also hot. I built a few flyback circuits before, on breadboard, perfboard, and PCBs. However with all the circuits, all that would happen is that the MOSFET and wires would heat up. On some circuits, there would be a small arc coming from the flyback, but in all the circuits, I would smell something burning, which would be my MOSFET, and if I left it on long enough, the wires would melt. I have a fairly large heat sink, and the MOSET heats up to the point at which it can no longer be touched only in a few seconds. What is wrong with the circuit?

author: moffy
author_rank: Super Contributor
timestamp: June 12, 2015, 12:32:16 am
content: You need to post a circuit diagram and preferably some photos of what you have built. Have no idea based on your description how to advise.

author: flynwill
author_rank: Regular Contributor
timestamp: June 12, 2015, 01:11:58 am
content: Do you have a 'scope?My first guess would be that due to some mistake the 555 is not oscillating. Without access to a 'scope debugging is going to be tough.

author: nshin31302
author_rank: Contributor
timestamp: June 12, 2015, 01:18:22 am
content: Here is the schematic.Instructables Pagehttp://www.instructables.com/id/Audio-modulated-flyback-transformer-driver/?ALLSTEPSI also used this schematic for some of my flyback drivers.URL to page.http://uzzors2k.4hv.org/index.php?page=flybacktransformerdriversPictures of setup:

author: nshin31302
author_rank: Contributor
timestamp: June 12, 2015, 01:22:25 am
content: Quote from: flynwill on June 12, 2015, 01:11:58 amNo I don't. I'm a beginner so I don't have one yet.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 12, 2015, 01:31:52 am
content: A few things come to mind:Increase the primary turns to at least 10 turns. I've got something similar at home and anything less on my particular transformer will saturate the primary causing heating of the MOSFET. Yours may be different.You want the current to go from your power source, straight into your primary, into your Mosfet then ground (or the reverse if we're talking electron flow) This means short, heavy gauge wire. Powering the FET and primary via traces on your board will cause large ground loops and could upset the 555.At 100khz the gate will pull a fair bit of current on its own (limited by the gate drive resistor) - make sure the driver transistor pair is up to the task, it wouldn't hurt to add a bypass cap as close as possible to the pair. And keep the wire to the gate of the mosfet short too. Also, make sure you have the NPN and PNP in the correct place. Its not your typical use of a NPN/PNP here.Try dropping the frequency a bit, bring it down to audio range with a larger RC cap, you'll at least hear if its oscillating that way. The transformer will likely give out a higher voltage too as its closer to its design freq.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 12, 2015, 01:35:21 am
content: Also, looking at your component locations. Put the RC components as close to the IC as possible. On the IC legs if you need to. This includes the decoupling cap. It will have very little effect if its halfway across the board jumpered with wires.Oh - that is not how you should have the Pots configured either... That is likely your main problem.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 12, 2015, 03:52:04 am
content: Definitely get rid of those lead lengths. Put the transistor right by the driver transistors, and put a big (1000uF+) bypass cap from supply to ground. You also need a clamp diode or TVS on the transistor, to protect it from flyback voltage. You might happen to find a P6KE200, or something along those lines, in a junked switching power supply, or you can at least use an RC snubber like this.Also, this drive circuit is better (it also works for large MOSFETs like the IRFPxxx), and the 555's duty cycle control may be preferred. (Definitely DON'T do what the first schematic in this thread shows: when the upper pot is at zero, pin 7 is shorted to +V and the pot or the 555 will burn out.)Tim

author: BennVenn
author_rank: Regular Contributor
timestamp: June 12, 2015, 09:46:07 am
content: This is pretty much built from the first schematic you posted, as Tim said you need to keep in mind r1&r2 at its lowest value will stop the 555 oscillating which will probably kill or at least overheat your 555/mosfet/wiring.I'm using an IRFB4110 which would normally be rated at 120amps at 100volts though these were sourced from eBay and are counterfeit. Breakdown voltage has been measured at ~120v, and I've seen them fail anywhere between 30Amps up to the full 140amps.I've put a 12v zener between Gate and Source, this is due to the Drain-Gate capacitance that will destroy the gate by coupling HV spikes from the transformer. I've destroyed a few FET's in this application before - haven't lost one since adding the zener. Proper board layout will negate this.Overall, this flyback driver has lasted around 4 yrs on and off use. Usually run @ 30vdc and the pots adjusted to keep the pulse width low enough to prevent the mosfet overheating. Will run all day without overheating with no load on the output (ion-craft stuff)

author: Zero999
author_rank: Super Contributor
timestamp: June 12, 2015, 05:50:43 pm
content: I don't see how the BJT MOSFET driver gains you anything?The emitter followers reduce the voltage so the MOSFET can't turn on and off as quickly.You'll probably get better results by omitting it and driving the MOSFET directly off the 555 timer.A MOSFET driver IC would be ideal but if you don't have one, try this.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 12, 2015, 09:49:04 pm
content: You are switching a capacitive load, the .6v loss either side of the supply is outweighed by the peak current delivered to the gate. The fet is fully on by the time it hits 11.4v and off by 0.6vThe OP's gate drive circuit is a tried and tested method for switching high gate capacitance FETs. The circuit you posted only adds complexity with very little to gain.

author: diyaudio
author_rank: Frequent Contributor
timestamp: June 12, 2015, 11:03:30 pm
content: Quote from: nshin31302 on June 12, 2015, 12:25:58 amOnly the switching waveforms can help you solve problems like this, do you have an oscilloscope?

author: Zero999
author_rank: Super Contributor
timestamp: June 13, 2015, 07:50:48 am
content: Quote from: BennVenn on June 12, 2015, 09:49:04 pmIt doesn't though. A 555 timer already drops more than 0.6V from either rail and adding emitter followers just makes it worse.Look at the internal schematic for the 555 timer.http://www.ti.com/lit/ds/symlink/lm555.pdfIt has a Darlington driver on the high side so it will drop 2V or more and adding another emitter follower increases this to 2.7V or more. The 555 is better at sinking current but it can still drop 1V and adding an emitter follower increases it to 1.7V.The base resistor on the emitter followers doesn't do you any favours and just drops more voltage.It also depends on the MOSFETs you use. Some may take a long time to turn off with this set up, especially logic level MOSFETs. The IRL540 can have a threshold voltage as low as 1V so may never fully turn off.http://www.vishay.com/docs/91300/91300.pdfThe correct way to do this is with a MOSFET driver which can switch close to the negative rail.

author: BennVenn
author_rank: Regular Contributor
timestamp: June 13, 2015, 08:15:55 am
content: It depends if you are using the CMOS variant of the 555. N+P channel fets are driving the output pin.I agree the correct way to do this is with a dedicated MOSFET gate driver IC or discrete circuit, a calculated snubber, the primary winding wound on the same side of the core as the secondary or better still, use the stock primary winding at the designed B+ voltage.There is a lot that can be improved but the idea of these types of circuits is a kind of bare minimum flyback driver with moderate reliability. These transformers were never designed to draw arcs, or run at 5+ times their designed power rating. For most HV enthusiasts this circuit is completely adequate. What the schematic doesn't inform the builder is the need for correct component placement, lead lenghts etc...If you follow the schematic exactly and have an idea of how to lay the components out correctly this circuit WILL work, every time. An oscilloscope will probably be necessary to debug the circuit but if you build it as hundreds of others have there will be no need for one - it will just work.

author: Arjan Emm
author_rank: Regular Contributor
timestamp: June 13, 2015, 05:13:47 pm
content: If everything so far failed, i would go back to the 2n3055 2 resistor driver.It works amazingly well. The only thing that can go wrong is the polarity of the coils. It either doesn't work, or it has a little spark , or it really works very well. Just swap it around. Nothing dies at 10-12V input.It will give you a good idea what a flyback can do.Google 2n3055 flybackdriver or something like that..

author: nshin31302
author_rank: Contributor
timestamp: July 02, 2015, 04:37:47 pm
content: Found this thread:http://4hv.org/e107_plugins/forum/forum_viewtopic.php?49027.0