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



John's suggestion is excellent. And thanks, John, for showing us a nice new part.



If you want to do this properly and isolate all your low voltage control
circuitry then you should look into using an optocoupler gate drive IC.  For
your application, i recommend something like this one
http://www.fairchildsemi.com/ds/FO/FOD3120.pdf  You can drive the diode from
a 5v logic source, you just need to calculate what value resistor you need
to source the proper amount of current to the photodiode input.  The gate
drive section needs to be powered by a separate transformer isolated
supply.  This insures that your microcontroller ground will stay separate
from your mosfet output stage ground.

On Sun, Oct 9, 2011 at 10:32 AM, Carl Noggle<cn@xxxxx>  wrote:

Hi, Scott,

I forgot to mention that the capacitor from the upper input lead (to the
center of the diodes) should be AC rated for high current, such as a
motor-start or a motor-run cap.  These are available from Grainger, etc.
  Polypropylene caps may work, too--you will have to check the ratings.

---Carl






  Hi Carl,
     Interesting, so I don't need a transformer, that certainly makes
things easier since I already need 12V for the frequency generator.  There
is one thing though, If I use a rectification configuration like is used for
the PVM400 driver, to get 240V out of 120 using two filter caps, tie center
to ground, that floats the ground of the MOSFET source pin below ground by
120V, so will a driver on a separate power supply grounded to a true ground
still work?  I really don't know how they work or hook up, does it reference
the MOSFET ground somehow so regardless of its voltage it is OK, or will it
burn out using that configuration?  Thanks.

Scott Bogard

On 10/8/2011 11:44 AM, Carl Noggle wrote:

Hi---

Since you are using an N-channel MOSFET, the source is grounded and most
drivers will work fine.  No need for a xfmr--that will slow the transition
times and increase the dissipation in your device.  Just make sure the
driver will handle your gate capacitance and will output enough voltage to
turn the FET on completely.  Most drivers will do that.  You will need a
separate low voltage supply for the driver.

The advantage of a half- or full-bridge is that the FETs are
self-protecting against voltage spikes by their body diodes.

---Carl




  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>
<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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