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.---CarlHi 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 onthe FET you're driving and the rise/fall times you want. -MikeOn 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 from120 to 240 volt operation, super high power capacity, but there arelimitations 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 bythis 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 theInformation 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 lookingfor.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 thatmakes a difference. 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