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From: Jan Panteltje on 23 Apr 2010 05:23
On a sunny day (Thu, 22 Apr 2010 20:44:03 -0400) it happened Hammy
<spam(a)spam.com> wrote in <56r1t5phluuq8psdn9d8ho0vpudbe9dkg1(a)4ax.com>:

>
>
>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?

Use optos, and the transformer to generate a floating voltage
for a driver directly connected to the MOSFET?
From: Hammy on 23 Apr 2010 07:05
On Thu, 22 Apr 2010 18:08:57 -0700, John Larkin
<jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:


>
>What are the values of the parts? Something is resonating at 12 KHz.
>
>The burst has some very low-frequency DC components. Dump burst mode!
>
>John
>
I'm using 0.1uf caps and a 4.7k gate resistor the diode is just a
1n4148.

The burst mode frequency isn't specified in the data sheet so your
right I'm probably going to have to dump it. The controller is the
NCP1217D 100kHz.

The only reason I wanted to keep the feature is the supply may not
have a load on it all the time or a small load. The data sheet doesn't
say what the minimum duty is and I don't want the OVP kicking on and
latching the controller off when lightly loaded.

I guess I'll test the controller for minimum duty and ensure there is
a minimum load for it.
From: Hammy on 23 Apr 2010 07:12
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.

>> 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.
From: Hammy on 23 Apr 2010 07:48
On Thu, 22 Apr 2010 23:42:26 -0700, Robert Baer
<robertbaer(a)localnet.com> wrote:


> 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!

I have read through some decent DC/DC conversion app notes using PIC's
and for LVDC I might try one down the road. This is an offline flyback
for 90-140VAC in 100W out. It would get a little messy using one for
that.

The bridge rectifier is beefy and more then capable of handling the
turn on current surge the fuse and line filter help to reduce it. I
see your point though you can deal with a lot of things in software.
Adjusting the frequency (PFM) to suit the load demands, soft start and
less dissipative current sense etc.

The only thing is PFM usually isn't recommended for line powered
converters because it's difficult to design a line filter when the
converter can operate over a wide frequency range. Soft start can be
implemented on the secondary side with a couple of components I've
done it several times before you have to tweak it but it works.

I like PIC's and use them a lot but they have their places the
controller in a line powered converter isn't one of them.
From: Hammy on 23 Apr 2010 08:00
On Fri, 23 Apr 2010 09:23:59 GMT, Jan Panteltje
<pNaonStpealmtje(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.
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