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Re: [TCML] Driving a flyback off of mains gate drive woes.

Hi Mike,
Hence my issue, for the moment I want a true flyback driver, not a half bridge. My issue is I can't find a MOSFET driver that will work Vcc on that high of a voltage, and from what I know of MOSFETs to run a driver off of a different voltage source, biases them on a different voltage, and they are stuck on or off no matter the 5 volt swing. From what I can see (did some perusing at work) several solid state Tesla coils use a gate transformer, so it is biased to whatever the MOSFET source voltage is, but I don't know where to get one of these, or how to make one, or if it will work on PWM over a range of duty cycles... I would imagine it would be easy, and I actually have an oscilloscope now to make sure it is working (finally) but I'm hesitant to just throw stuff together, I don't want to blow out the ARduino I'm using as a frequency generator, as it would take time and money to replace (project is kind of on a personal deadline, I'd rather get the correct part numbers.) If it makes a difference I am settled for the moment on IRFP250 MOSFETS, but plan on upgrading to IRF450s in the future. So I guess my question is how to make a gate drive transformer, and what MOSFET driver would be suitable for driving said transformer (assuming it is done that way) and how to wire up (external components) a MOSFET driver, since I've never done that before. Thanks again guys. 

Scott Bogard.

On 10/7/2011 5:37 PM, Michael Twieg wrote:

That schematic isn't even for a flyback supply.  It's a half bridge driver,
driven by a hi/low side bootstrapping driver.  For a simple flyback you will
need some kind of simple low side gate drive IC.  The IXDD414 is one option,
but it's overkill (also it's obsolete and very hard to find, unless you go
for the newer versions).  The requirements of the gate driver will depend on
the FET you're driving and the rise/fall times you want.

-Mike

On Thu, Oct 6, 2011 at 10:26 PM, Scott Bogard<sdbogard@xxxxxxxxx>  wrote:


Hi Dave,
     Yeah I saw that schematic, it kind of gave me the idea, switchable from
120 to 240 volt operation, super high power capacity, but there are
limitations with that chip I'd like to circumvent.  Namely that it is stuck
on 50% duty cycle (overall flyback duty cycle is independently controlled by
a second oscillator at a low frequency) and that you have to use the built
in frequency generator (I have a frequency generator, that I can program
frequency routines into.)  Basically I need some kind of plane Jane MOSFET
driver that I can mount to the high voltage rails like that chip, but that
accepts an external input 5V logic, I don't think such a thing exists, so
there must be some way to build one...  Can I make a BJT totem (I think that
is what it is called) fed by like a 3k resistor or will that pop under that
voltage?  Thanks in advance for your help guys, it is so much appreciated by
this solid state newbie!

Scott Bogard.


On 10/6/2011 8:20 PM, David Speck wrote:


Scott,

Below is a link to a full schematic for a flyback driver off the
Information Unlimited website.  No personal experience to know how well it
works, but it seems to be a close match to the sort of thing you are looking
for.

http://www.amazing1.com/**download/PVM500BASICSCHEMATIC.**pdf<http://www.amazing1.com/download/PVM500BASICSCHEMATIC.pdf>

Dave

On 10/6/2011 6:40 PM, Scott Bogard wrote:


Greetings all,
     Back up to my old tricks again.  After procuring a schematic for a
PVM 400 flyback driver, and noticing it runs off of mains voltages, I am
attempting to create my own version, with programmable adjustable duty cycle
and frequency.  What I don't seem to know how to do, is drive the gates,
what I have is an ATmega chip producing a nice 5 volt square wave, of
variable duty cycle and frequency (3-300 kHz) and some good flybacks.
  Obviously I cannot just plug that 5 volt signal into a bunch of MOSFET
gates, it would likely not have the guts to hold them open, even if it was
the correct ground, which it is not.  So basically what hardware is needed,
a gate drive transformer?  MOSFET driver?  Could somebody please point me to
some straightforward literature about biasing MOSFETS, and possibly provide
part numbers, not just reference "generic BJT."  I know it must be possible,
people have run solid state TCs off of MOSFETs for a long time, so what am I
missing...  Thanks in advance, I'm looking specifically to run it off of
about 320V DC (+-160V) low side drive (not half or full bridge) if that
makes a difference.

Scott Bogard.


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