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Re: [TCML] Driving a flyback off of mains gate drive woes.
Hi John,
That looks just about perfect, so I would run the 5V into the
photodiode through a resistor to limit to about 4 ma. Then tie the outputs
together and drive the MOSFET gate off of that? Here is where I get
confused, it says the device can isolate up to like 2000V, I assume it
cannot be driven off of that, as it says VCC 15-30V, so I would need a 25
ish volt supply for the IC, separate from the supply for the frequency
generator. Ok, so my only question then is if I feed my filter caps center
grounded so I got 240V from 120 input, and my MOSFET source pin is tied to -
120V potential (actually 170 till it is filtered but whatever) can I still
drive it off of that input, or do I need to feed a gate drive transformer to
relativize the voltages to the source pin of the MOSFET. Either way it
looks like this is the way to go with this chip, good current output and
good isolation, shame I'll need a separate power supply for my PWM as the
MOSFET driver, but whatever, I'll deal with it. Thanks for all of your help
guys!
Scott Bogard.
On Sun, Oct 9, 2011 at 11:31 AM, John Forcina <forcijo10@xxxxxxxxx> wrote:
> 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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