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From: Tim Williams on 23 Apr 2010 09:00
"Hammy" <spam(a)spam.com> wrote in message
news:l9v2t5ttftnnn2e5n6maug7i3son7cosej(a)4ax.com...
> I'm using 0.1uf caps and a 4.7k gate resistor the diode is just a
> 1n4148.

0.1uF resonates with 1.5mH transformer inductance as 13.0kHz. Sounds like a
'duh' factor to me. ;)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


From: Hammy on 23 Apr 2010 10:34
On Fri, 23 Apr 2010 08:00:20 -0500, "Tim Williams"
<tmoranwms(a)charter.net> wrote:

>"Hammy" <spam(a)spam.com> wrote in message
>news:l9v2t5ttftnnn2e5n6maug7i3son7cosej(a)4ax.com...
>> I'm using 0.1uf caps and a 4.7k gate resistor the diode is just a
>> 1n4148.
>
>0.1uF resonates with 1.5mH transformer inductance as 13.0kHz. Sounds like a
>'duh' factor to me. ;)
>
>Tim

Yea I thought of that later. I have been known for a few brain farts
though;-).

The problem is the burst frequency isn't specified and it can vary.
They don't even give a frequency range just some scope shots at
various loads showing different frequencies. On semis usually pretty
good about data sheets but this ones pretty sparse on some important
stuff.

It is a nice feature though if your supply is going to be powered with
a small or no load at least it wont pump up past the nominal voltage.
From: Joerg on 23 Apr 2010 10:54
Hammy wrote:
> On Thu, 22 Apr 2010 19:49:35 -0700, Joerg <invalid(a)invalid.invalid>
> wrote:
>
>
>> Ah, another guy with an Instek scope.
>
> Joerg do you know how to take screen shots that show the measurement
> screen ? I'm assuming you can do it because they show pictures of
> screen shots in the manual with the measurement window.
>

Ahm, didn't you just do that? Not sure what you mean here. Mostly I use
their Freewave program but on the road also a USB stick. You can turn
the menu off with the little gray round button. If you want a totally
clean screen with just the plot, or an Excel-capable file you can also
do that via Freewave. Also from the bare scope, if you hit "save all" it
stores both formats on the USB stick.


>>> Here's a zoom of one of the burst.
>>>
>>> http://i43.tinypic.com/2eumxbk.jpg
>>>
>>> My primary and secondary are 1:1 ; inductance of 1.5mH.
>>>
>>> Is there anyway to clean this up?
>>
>> Looks like you haven't wound your toroid transformer in bifilar fashion.
>> That is essential for apps like this, leakage inductance must be
>> minimized, else this rings like crazy.
>
> Yep its wound Bifilar. Its also 5 turns past the minimum to avoid
> saturating for its worst case Duty and Drive level and I checked it
> using a current transformer.
>
> Its wound tight and all the way around the toroid.


But something is ringing. How are you measuring this? Are the caps sized
right? Your schematic is only partial so I can't see what happens south
of the FET's source. If the source is flying up and down that's a
challenge. This scope isn't that great for differential measurements at
high sample rates.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: Joerg on 23 Apr 2010 10:58
Robert Baer wrote:
> Hammy wrote:
>>
>> I wound a toroid to use as a HS gate drive transformer for a 100W two
>> switch flyback. I'm using the typical DC restore technique AC couple
>> the transformer.
>>
>> Here's my schematic;
>>
>> http://i42.tinypic.com/9gvyow.png
>>
>> I'm using a PIC and a FET driver to simulate the extremes of the PWM
>> controller duty cycle and drive level and it works well with the
>> exception of Burst Mode. Burst mode is when the controller modulates
>> the original gate signal at light load for anyone who doesn't know.
>> A picture is worth a thousand words.
>>
>> Here's my gate waveform at 90kHz 70% duty. A little sloping but
>> acceptable.
>> http://i39.tinypic.com/25kieyq.jpg
>>
>> Now here's where the fun begins I'm modulating the 90kHz to simulate
>> burst mode.
>>
>> http://i44.tinypic.com/qy5l69.jpg
>>
>> Here's a zoom of one of the burst.
>>
>> http://i43.tinypic.com/2eumxbk.jpg
>>
>> My primary and secondary are 1:1 ; inductance of 1.5mH.
>>
>> Is there anyway to clean this up?
> That is the one thing i hate about those stupid "controllers" that
> supposedly are good for DC-DC conversion.
> Why simulate?
> Toss the verdammdt controller and use the PIC directly; then you can
> have the program be as dirty or clean (or both) as you want...
> ..Namely slow non-giga-amp startup current spikes!


True PWM is next to impossible with a regular uC. That is because it has
a finite granularity of no better than one master clock cycle. At
contemporary frequencies that's not very good. For example, my last
design runs above half a megahertz. Try that with a PIC :-)

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: langwadt on 23 Apr 2010 17:13
On 23 Apr., 17:34, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Fri, 23 Apr 2010 08:06:40 -0700, Joerg <inva...(a)invalid.invalid>
> wrote:
>
>
>
> >Hammy wrote:
> >> On Fri, 23 Apr 2010 09:23:59 GMT, Jan Panteltje
> >> <pNaonStpealm...(a)yahoo.com> wrote:
>
> >>> Use optos, and the transformer to generate a floating voltage
> >>> for a driver directly connected to the MOSFET?
>
> >> It's an option I have some fast Avago gate drive opto's.
>
> >Optocouplers aren't very well suited to drive FETs. They can't muscle a
> >gate capacitance around within tens of nanoseconds and at a couple of
> >amps. Transformers are the better choice.
>
> >Size the caps right, and maybe optimize the transformer coupling some
> >more (twisting the wires before winding etc. unless you did that
> >already). Burp mode in a converter isn't that great with transformers
> >but there aren't many alternatives. In ultrasound we do this a lot,
> >transmit a sequence of fast power pulses across a transformer, followed
> >by a major pause (to wait for echoes).
>
> There are some fast optocoupled mosfet gate drivers.
>
> The home-brew way to do this is to use a cheap (ca $4) potted DC-DC
> converter to make a supply that floats on the fet source. Then use a
> fast logic-type optocoupler and a real gate driver chip up there.
>
> The advantage of optos is DC coupling without duty-cycle problems. And
> not having to wind transformers.
>
> John

There's also something like the ADuM6132,- supply,isolation and driver
in one.


-Lasse
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