Audio Modulated Flyback Circuit HELP / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

Audio Modulated Flyback Circuit HELP

1 2 next

Move Thread

LAN_403

fatboyslim

Sun Feb 01 2009, 08:35AM

Registered Member #1779 Joined: Sat Oct 25 2008, 11:05AM
Location:
Posts: 32

Hi

i was wanting to build the following circuit but am unsure about a couple of things.

1. Where does the number 3 pin connect from the 555 chip?
2. Am i able to substitute another MOSFET instead of the current one shown in the schematics.

Hope someone can help me PLEASE!

Fatboyslim Audiomodulatedflyback555IRFP250

Arcstarter

Sun Feb 01 2009, 09:08AM

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

Yes, you may use different mosfets. And what do you mean by pin 3? If you are asking what it does, it is the 555 output. Just make sure the mosfet is rated many times over the flyback power supply voltage, and quite a bit more current.

And please change your pic size, it is quite annoying :P.

Mads Barnkob

Sun Feb 01 2009, 09:45AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Look at google for 555 pinout if you need the pin number, or look its datasheet up at

look for NE555.

I think the rule of thumb with single mosfet flyback drivers is to use one that can handle 10x the input voltage. Its nescesary to handle reverse voltage spikes.

Another thing, deep linking from my homepage pisses my host off, so if you could download the schematics instead and upload them to the forum, it would be great :)

I'm currently working on a revised version of that article that will cover some more questions.

Dr. Dark Current

Sun Feb 01 2009, 09:57AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

If you use 10 ohm gate resistor, the FET will possibly heat less. It works even with no resistor at all.

MRacerxdl

Sun Feb 01 2009, 04:12PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Yeah, the resistor limits the gate current, with higher oscilating frequency setted at 555, more current the mosfet gate will drawn, and that can blow the 555. the gate limits the current to I = V / R (I think that applies for that)
so I = V / R => I = 12 / 10 = 1,2A, for 50 Ohm => 12 / 50 = 240mA
for low frequency the mosfet gate wont draw much, but I killed much 555 driving direct the gate.

Myke

Sun Feb 01 2009, 07:34PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

The MOSFET gate doesn't have any resistive connection to the drain and source. It's all capacitive. So you would use I=V/R where the resistance is the impedance of the capacitance of the gate (Z=1/(2piFC)) plus the gate resistor. That current shouldn't go over what the 555 can supply. If it does, you get smoke (or just a hot 555 if it doesn't die).

With a higher gate resistance, it takes longer for the gate to charge causing the MOSFET to spend more time in the non-saturation region. In the non-saturation region of operation, the MOSFET has more resistance than completely on and less than completely off. The current flowing through the MOSFET at this time is turned into heat so that's why you want to minimize that time.

I hope I got that right and said it in an understandable way...

Dr. Dark Current

Sun Feb 01 2009, 08:20PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Note that the gate driver only has to supply current peaks. The popular UCC drivers are "9A" drivers, and they would surely smoke if you tried to draw that continuously.

The 555 can probably supply an amp or two of peak current, and I have never observed any oscillations on the gate resulting from its direct connection to the 555.

The dissipation in the gate drive is calculated as P=C*V^2*f, where C is your gate capacitance, V is the voltage to which you charge it and f is frequency.
As an example, running IRFP250 MOSFET on a 555 at 20kHz, results in around 0.008W of dissipation.

fatboyslim

Mon Feb 02 2009, 03:15AM

Registered Member #1779 Joined: Sat Oct 25 2008, 11:05AM
Location:
Posts: 32

Thanks for that. What i meant about Pin 3 was that in the circuit it connects to a resistor and then to where. I thought mosfets had 3 pins. The resistor from pin 3 does not connect to anywhere i know.

Dr. Dark Current

Mon Feb 02 2009, 07:02AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

fatboyslim wrote ...

Thanks for that. What i meant about Pin 3 was that in the circuit it connects to a resistor and then to where. I thought mosfets had 3 pins. The resistor from pin 3 does not connect to anywhere i know.

Yes they do have 3 pins, the "bottom two" ones are usually connected together and called Source. The resistor connects to Gate and primary to Drain.

Electroholic

Mon Feb 02 2009, 10:11AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

Ah, i see what ur missing.

thats just the symbol for a mosfet, the line on the left is the gate. and the gate is insulated form the rest of the silicon, so its a line leading to nowhere.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob