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Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Well, awhile ago i helped someone on youtube make his 555 flyback driver work, and i figured i would post my suggestions/thoughts ( ) of how to make it more efficient/not dead. Hehe. It is all very simple stuff, simple to put together that is, and i am sure most people out there know how to do it, but someone may run across this that needs help.
Well, flybacks sometimes have high voltage kickback that can kill your 555 and mosfets. First, remember you should always have a mosfet rated 4 times your power supply voltage anyway, for example a 50 volt source could use an irfp250 mosfet, which is 200 volts and 30 amps.
First thing you can do, add a 10 ohm resistor from pin 3 of the 555 to the gate (or base for transistor) of the mosfet. It's purpose is to reduce parasitic oscillations, which will (hopefully) be transferred into heat.
Second is the best, use a snubber across the primary of your flyback. The snubber is very simple, it is just a 10ohm resistor in SERIES with a 10nf capacitor. Its purpose is to reduce or eliminate the high voltage kickback/spikes.
Third is decoupling capacitors. For this just use a .1-1uf capacitor right across pin 1 and 8 of the 555, as small leads and as close as possible. Its purpose is to eliminate any parasitic oscillations caused by inductance and capacitance in the circuit. Parasitic oscillations will cause the mosfet to switch when it shouldn't and sometimes at high frequency, which will increase switching losses which means more heat.
Last is a resonance capacitor. Don't thank me for this one, all thanks to Jan Martis, AKA Dr. Kilovolt, AKA jmartis2 on youtube. This is just a high quality capacitor right across the primary. Use this and the snubber if possible. The capacitor should not be polarized, and of exceptional quality. Around .22uf is good, but you should experiment with it. It may not really help with spikes or anything, but it will reduce heat in the mosfets or transistors. It will improve efficiency, you will get greater current, and less heat.
The same stuff applies for the ignition coils, but without a snubber it has more hv kickback than a flyback(from my experiences). This is caused when the mosfet stops conducting. When it does, the magnetic field collapses, which is coupled into the hv secondary, which gives us the high voltage. But unfortunately, some of it is coupled straight back into the primary. This is inductive kickback, and it will kill 555's and mosfet fairly quickly. Sometimes i could only run mine for a few seconds and it would die due to hv inductive kickback.
This is all just stuff i have learned over the past year or two since i have been into electronics. Mostly learned from this forum, actually.
Update: Another way to protect the 555 is some fairly high wattage zeners across the supply rail. This will clamp the voltage to 12 volts, so any hv kickback will be dissipated into heat, not blown silicon. ALL credit goes to Myke. Really a very genius way to protect the 555, best i have heard so far.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Arcstarter wrote ...
Well, flybacks sometimes have high voltage kickback that can kill your 555 and mosfets. First, remember you should always have a mosfet rated 4 times your power supply voltage anyway, for example a 50 volt source could use an irfp250 mosfet, which is 200 volts and 30 amps.
They always do have a kickback The flyback spike usually reaches around 5-10x the supply voltage so I'd suggest to have a FET rated at 10x supply. If it is rated less, it will catch the spikes by non-destructive avalanche but will get hot accordingly. I've measured the spike with the quasi-resonant capacitor at around 5x the supply voltage so here I suggest a FET rated 7x supply V.
Arcstarter wrote ...
First thing you can do, add a 10 ohm resistor from pin 3 of the 555 to the gate (or base for transistor) of the mosfet. It's purpose is to reduce parasitic oscillations, which will (hopefully) be transferred into heat.
yeah. I've found it's not needed at all with 555 as it does not produce enough current to ring any dangerous oscillations.
Arcstarter wrote ...
Second is the best, use a snubber across the primary of your flyback. The snubber is very simple, it is just a 10ohm resistor in SERIES with a 10nf capacitor. Its purpose is to reduce or eliminate the high voltage kickback/spikes.
This may sometimes help but from my experience it helps just marginally and the 10ohm resistor gets smoking hot. Again, most FETs are avalanche rated so they will snub any spikes by themselves.
Arcstarter wrote ...
Last is a resonance capacitor. Don't thank me for this one, all thanks to Jan Martis, AKA Dr. Kilovolt, AKA jmartis2 on youtube. This is just a high quality capacitor right across the primary. Use this and the snubber if possible. The capacitor should not be polarized, and of exceptional quality. Around .22uf is good, but you should experiment with it. It may not really help with spikes or anything, but it will reduce heat in the mosfets or transistors. It will improve efficiency, you will get greater current, and less heat.
Important note: When you do this, you need to slow down the turn-on of the FET(@ ) to avoid damaging it by dumping the leftover cap energy into it quickly. Besides the advantages that Arcstarter listed, this driver can also produce very high voltages from a diode-split xfmr (I had arcs jumping a 14cm gap between 2 nails, powered by a single xfmr)
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Dang, maybe next time i will leave this stuff to the professionals...
Well, i have not tried the snubber personally, but just yesterday i told this guy on youtube to use it( name is Zezimashock) and he said that it made his mosfets stop dying completely. His supply voltage is 35 volts and the mosfets are only 200 volts. So, his snubber did not reduce performance, i guess it doesn't heat too much, otherwise he probably would have told me, and he hasn't had a mosfet death yet, and before his mosfets would die almost instantly.
All that stuff i listed is just stuff that i sent to others with 555 flyback problems. I have only sent it to two people acually, and it either helped or it completely fixed. But like i said, i am not a professional, so don't listen to me, listen to people like Jan.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Arcstarter wrote ...
Well, i have not tried the snubber personally, but just yesterday i told this guy on youtube to use it( name is Zezimashock) and he said that it made his mosfets stop dying completely. His supply voltage is 35 volts and the mosfets are only 200 volts. So, his snubber did not reduce performance, i guess it doesn't heat too much, otherwise he probably would have told me, and he hasn't had a mosfet death yet, and before his mosfets would die almost instantly.
All that stuff i listed is just stuff that i sent to others with 555 flyback problems. I have only sent it to two people acually, and it either helped or it completely fixed. But like i said, i am not a professional, so don't listen to me, listen to people like Jan.
The stuff you listed could really help, I'm just saying there are better ways of doing some of the things
Well, I'm not sure about the snubber as I've never really needed it, it could decrease the dV/dt on the FET drain and if his 555 supply was poorly decoupled, it really could have helped. From my quick calculations the loss on the 10R resistor will be a few W at most so it seems it really doesn't hurt to include that "snubber" (it's really not a snubber, more like a "slow-downer" )
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
Look at the schematics of any crt TV or monitor and what you will find protecting the transistor that drives the flyback are just a capacitor across the primary winding and a fast diode across the transistor. The combination of these two elements limits the voltage pulse across the transistor, and returns to the power supply most of the energy that is not transferred to the secondary circuit. When the normal primary winding is used, the capacitor is of about 10 nF or somewhat less. If you wind another primary, what allows operation from a lower voltage power supply, the capacitor shall be scaled up in the same proportion in which the primary inductance is scaled down. Another tip is to ground the circuit. Otherwise the production of positive high-voltage at the high-voltage end of the flyback transformer tends to be followed by negative high-voltage where the ground should be.
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Help for anyone with ne555 flyback/ignition coil driver problems.
) of how to make it more efficient/not dead. Hehe. It is all very simple stuff, simple to put together that is, and i am sure most people out there know how to do it, but someone may run across this that needs help.
The flyback spike usually reaches around 5-10x the supply voltage so I'd suggest to have a FET rated at 10x supply. If it is rated less, it will catch the spikes by non-destructive avalanche but will get hot accordingly. I've measured the spike with the quasi-resonant capacitor at around 5x the supply voltage so here I suggest a FET rated 7x supply V.
)