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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Firkragg wrote ...
On just 25 kHz slow switching isn't that big problem, but you'l certainly get more HV by making them faster.
my findings are exactly the opposite: lowering the frequency results in higher power and voltage from flyback, but the fets start burping and it squeals awfully. With high frequency (~50k) the fets are cool and the arcs short and thin
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I said nothing about the frequency, but about transistent speed (edges of waveforms, especially falling wich looks slow, will dramatically improve performance in flyback driver).
When I used slow, crappier transistors like 2N3055 i go maybe third of spark length I could get with 2SB863.
IRFP250's are pretty fast but you aren't using it if your drive was limited like NE555.
With faster (by switching times, not the frequency) drive, especially turnoff, you can get maximum out of your mosfets (their own delay limit), maximum dI/dt on primary.
Increasing Frequency is totally different thing, it's normal you get puny arcs that way with unchanged number of primary turns. Lowering it will increase input current until the core saturates and your mosfets meet an ultimate end.
Your current 25kHz is good to flyback, you don't need to change it (don't listen to my 200kHz ramblings).
I just wanted to point out few things that would make even scarier arcs, but if you feel 'don't touch if it works' about this circuit and you don't want to mess up your nice PCB then maybe it's better to do this in another project.
One more important thing you can add is duty cycle control on NE555, as this is simple and neat way to control the output power and tune for maximum Vout for minimal input power.
You also said nothing about the airgap, I actually even doubled it in my flyback converter and it responded with even more HV.
This may, I guess, cause more voltage peaks on your transistors so maybe it's better not-to-touch it for now.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Damn, I've just fried my hand-wound flyback it arced over externally so I will try to clean it and pot it somewhat in hot glue
Firkragg wrote ...
One more important thing you can add is duty cycle control on NE555, as this is simple and neat way to control the output power and tune for maximum Vout for minimal input power.
ok, ya got me there. how do you change the duty cycle independently on frequency with 555???
Firkragg wrote ...
You also said nothing about the airgap, I actually even doubled it in my flyback converter and it responded with even more HV.
I have a mega airgap in there, maybe half a milimeter, this really allows power to flow (and I will try to make it even bigger what happens...)
Firkragg wrote ...
This may, I guess, cause more voltage peaks on your transistors so maybe it's better not-to-touch it for now.
the transients are slowed down by the .1uf cap, so the fets have sufficient time to turn off before the voltage rises too much ,also it forms a L-C circuit, that allows the primary to "ring" to high peak voltages
J.M.
Edit: WOAH! Doubled airgap, went from 10 to 12 primary turns. The voltage from the poor "patched" hand-wound flyback seems to be over 25kv and the arcs are longer (though a little thinner). The fets also seem to be a little colder for the same power. operating frequency ~30khz,
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
the transients are slowed down by the .1uf cap, so the fets have sufficient time to turn off before the voltage rises too much ,also it forms a L-C circuit, that allows the primary to "ring" to high peak voltages
Um, maybe we are misunderstanding continuously because of my poor language.
I was trying to explain how would higher-peak-current gate drive help output, and also keeping fet's much cooler. Simple bipolar totem pole or using stronger oscillator IC (like TL494) would help a lot.
The capacitor you are talking about is another story, and now it seems to be hindering performance rather than helping. It's way too big.
D-S caps are used to snub the spikes that occur on primary. If it's horribly big like your 100nF one, it kills the inductive kick wich IS what we need in flyback operation. Voltage spike gets lowered since cap 'shorted' it for a while.
By killing it we reduce the energy transferred to secondary in the cycle, and lowering the voltage output.
Another great disadvantage is that this snubbed energy must go somewhere, and when mosfet opens again with next cycle it thorts the cap into itself , wasting it as heat and getting stressed.
While at low frequencies this isn't a big problem, in etc. SSTC's at MHz range even mosfet's parasitic output capacitance of few hundreds of pF can become a killer.
For your driver I'd choose this cap to be about 10nF or less. I never used snubbing or any protection on my flyback converters (140V transistor, 30V input) and I never killed the switch by overvolting.
TVS's can be a great protection if you fear overvoltage, and a small cap is optional.
I hope this cleared things a bit.
ok, ya got me there. how do you change the duty cycle independently on frequency with 555???
Take a look at this circuit , just take look at 555 circuit.
There is also an option of astable wich uses pin3, best seen at Steve Ward's DRSSTC interrupter. This circuit may make problems if overloaded by direct gate driving.
(see interrupter).
U used such circuit with my driver, with totem pole on output and it worked fine.
Exscept NE555 you have lots of IC's, some even with inbuilt high-current gate drive like TL494. You can get it from dead ATX supplies.
Damn, I've just fried my hand-wound flyback it arced over externally so I will try to clean it and pot it somewhat in hot glue
You can wind a secondary using normal 'hook-up' wire if you have enough of it. Don't wind it like normal flyback (turn-to-turn with insulation between each layer), but rather look how are solid state MOT's and some small HV transformers wound.
They usemultiple insulated gaps in secondary former and lots of turns in each. PVC insulation is much stronger than varnish, I have some thick-ish PVC insulated wires that withstanded direct torture by 15kV OBIT (just a single wire!). Altough they are rated much, much more
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Firkragg wrote ...
D-S caps are used to snub the spikes that occur on primary. If it's horribly big like your 100nF one, it kills the inductive kick wich IS what we need in flyback operation. Voltage spike gets lowered since cap 'shorted' it for a while.
sorry, but the cap was included by "evolution" -trying different cap sizes, leaving the cap out, and finding what works the best. With no cap the output is poor blue arc and the fets are boiling in less than minute. (MAYBE they were avalanching)
Firkragg wrote ...
Take a look at this circuit just take look at 555 circuit.
you can not control the duty cycle independently on frequency
Firkragg wrote ...
I have some thick-ish PVC insulated wires that withstanded direct torture by 15kV OBIT (just a single wire!).
by obit maybe, but not at flyback frequencies. Corona digs through the insulation really fast. I also have 0.2mm PE foil that withstands 40 kv from DC flyback.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Nice work Jmartis It's the first time I've seen someone take time to find the proper size of snubber capacitor. (I guess other people have done it, but I never noticed.) This should make the circuit work much better than with no cap or one chosen at random.
The idea is to choose the cap such that the primary voltage kicks up to just short of the MOSFET's rating, to give the maximum output voltage without avalanching. I think you can get a ballpark figure for the capacitance using (V/I) = sqrt(L/C).
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Steve Conner wrote ...
Nice work Jmartis It's the first time I've seen someone take time to find the proper size of snubber capacitor. (I guess other people have done it, but I never noticed.) This should make the circuit work much better than with no cap or one chosen at random.
The idea is to choose the cap such that the primary voltage kicks up to just short of the MOSFET's rating, to give the maximum output voltage without avalanching. I think you can get a ballpark figure for the capacitance using (V/I) = sqrt(L/C).
Thanks Mr. Conner , but how do I (easily) measure the inductance of the primary without L/C meter?
The strange thing I've noticed, is that when it is hot in the room, the fets are warm in few seconds, but if it is 5C less, they are cold even after about 30 secs. in both cases they seem to heat up to some temperature (~60C) and then stay at this temp. (thats with estimated power ~150-180W)
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I think you can get a ballpark figure for the capacitance using (V/I) = sqrt(L/C).
It's hard to know L without a meter, but if you 'tried eveything' then I guess it's OK. Frequency is fairly low nad cap gets enough discharging time while mosfet is OFF. (too big one would blow the MOSFET.)
Snubber cap seems to be
The idea is to choose the cap such that the primary voltage kicks up to just short of the MOSFET's rating, to give the maximum output voltage without avalanching.
In my evil 1337 flyback driver I used no snubber (for maximum performance) and 140V bipolar transistor never got hot or avalanched (died), with input voltage just about the same Jmartis used. But that probably cant' be compared since I got used to Uber high frequencies and original DC flyback.
60 degrees isn't that bad actually, but it could probably get better with stronger drive.
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Flyback driver
it arced over externally so I will try to clean it and pot it somewhat in hot glue
(and I will try to make it even bigger what happens...)
, wasting it as heat and getting stressed.
