[TCML] Driving a flyback off of mains gate drive woes.
Carl Noggle
cn at q.com
Sun Oct 9 08:32:09 MDT 2011
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 at gmail.com>
>>>> 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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