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haxor5354

Fri Jun 10 2011, 09:59PM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

is it true that in order to put out a very high voltage from a FBT, the primary has to be connected to a square wave power source?
the rapid drop in power inthe primary cause the magnetic field to collapse and induce a high voltage in the secondary?

also is there any flyback drivers that does true flyback mode other than the 555+mosfet method?
last time i tried to make a 555 flybak driver, the mosfet heats up quite a bit.
theres gotta be a more efficient way to drive a FBT in flyback mode.

Daedronus

Fri Jun 10 2011, 10:13PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
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Posts: 370

the 555 idea is fine, but
there are several reasons the mosfet will heat up.

First and most importantly, the 555 is not meant to drive mosfets, so the mosfet will only partially open, therefore you get I*R loses, so, heat.
Use a lowside mosfet driver powered by 15V power supply between the 555 and the mosfet.

Second, the back emf from when the field collapses appears as a voltage spike on all windings, not only on secondary, the mosfet might avalanche and conduct the back emf. You can try to use a higher voltage rating mosfet, or ignore the problem.

Last, use more then one mosfet in paralel. mosfets like to be used in paralel. And the lower ON resistance will get you more output power.

haxor5354

Fri Jun 10 2011, 10:17PM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

why isnt a 555 suppose to drive a mosfet? it still applying signals to the gate, doesn't it?

Adam Munich

Sat Jun 11 2011, 12:07AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

haxor5354 wrote ...

why isnt a 555 suppose to drive a mosfet? it still applying signals to the gate, doesn't it?

That's because mosfets are not perfect. Ideally they would let 0 mA flow through the gate, but since the world is not ideal the gate has capacitance. You need current to charge and discharge that capacitance, and when it is done at a high frequency that current can add up to be more than an an ampere!

Arcstarter

Sat Jun 11 2011, 02:16AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

also is there any flyback drivers that does true flyback mode other than the 555+mosfet method?

Well, i definitely suggest quasi-resonant. As someone recently stated on the forum, most TVs use a quasi resonant capacitor. It is still flyback, but a capacitor in parallel with the switching device is used like the shunt capacitor in a class E sstc.

Don't forget to use a mosfet rated at least 3.3 times the power supply voltage, or possibly even more. When the magnetic field collapses and generates the back EMF, it is a few times the supply voltage. That is why a single transistor driver can have fairly low input voltage and still get higher voltages when compared to a bridge. For example, a common TV often uses 80v for it's main supply, and the transistor used for switching the flyback normally seems to be in the 1200-1500v range. If you don't have any high voltage mosfets (something like 500v would work if you keep the input voltage fairly low) you could use the HOT from a TV, but you would need to drive the base with more current and less voltage (or just more current and a resistor).

As for driving a mosfet with a 555, it is definitely doable. A 555 has 200ma sink/source capability which is enough for mosfets with a small enough gate charge. A scope will tell you if a 555 can drive a mosfet. Just look for a clean squarewave. But, if your mosfet has too high capacitance gate (which it probably does), i suggest a simple current buffer in the form of a totem pole of transistors or mosfets if you don't have a gate driver.

But, why must it be flyback topology? A flyback transformer can be run with pretty much all of the most common topologies.

haxor5354

Sat Jun 11 2011, 02:24AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

Arcstarter wrote ...

But, why must it be flyback topology? A flyback transformer can be run with pretty much all of the most common topologies.

cuz I want MORE voltage.

Dr. ISOTOP

Sat Jun 11 2011, 09:23AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

You could always use, say, an H-bridge and up the primary voltage.
Alternatively, use a MOSFET or IGBT rated for lots of voltage (say, a 1200V IGBT) and drive it properly with a gate drive chip (such as the UCC37322/TC4422/IXDD630).
Or just use a brick

Daedronus

Sat Jun 11 2011, 07:36PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

In my experience in flyback mode 2 irfp260 in parallel driven by TC4420 will easily kill the diodes in a DC flyback and transform it into a AC one.

Unless it's a special winded flyback, probably imerrsed in oil, you don't really need a fancy driver to get the maximum voltage output.

You are probably better of adding a few CW multiplier stages to a AC flyback, if you want higher voltages.

Arcstarter

Sat Jun 11 2011, 09:16PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Well just because flyback generates higher voltage back EMF doesn't mean it is easier to generate high output voltage necessarily. I have used a halfbridge and H bridge driven by a 555 and something like 40-80v input, and the voltage is high enough to kill the diodes, cause breakout, and cause carbon tracks to ruin the flyback, especially if you find the resonant frequency of the flyback.

teravolt

Sun Jun 12 2011, 01:33AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

hi haxor5354 if you want bigger and better sparks I would rcomend that you use the zvs circuit. hear are some links on how to build and theroy.

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