# HS gate drive transformer?

H

#### Hammy

Jan 1, 1970
0
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?

J

#### Joerg

Jan 1, 1970
0
Hammy said:
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

Ah, another guy with an Instek scope.

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.

H

#### Hammy

Jan 1, 1970
0
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

H

#### Hammy

Jan 1, 1970
0
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.
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.

H

#### Hammy

Jan 1, 1970
0
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.

H

#### Hammy

Jan 1, 1970
0
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.

T

#### Tim Williams

Jan 1, 1970
0
Hammy said:
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

H

#### Hammy

Jan 1, 1970
0
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.

J

#### Joerg

Jan 1, 1970
0
Hammy said:
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.

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.

J

#### Joerg

Jan 1, 1970
0
Robert said:
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

J

#### Joerg

Jan 1, 1970
0
Hammy said:
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).

H

#### Hammy

Jan 1, 1970
0
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.

I meant the side screen which shows measurements automatically like
rise/fall, PP,AVG,RMS.....

Probably the only way to do it is with Freewave. The software doesn't
work on my XP machine but it works on my W2K pro machine?

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.

I have 0.1uf caps on the input and output. Tim's right the 1.5mH and
0.1uf caps are resonating with the burst frequency.

I'm not taking a differential measurement my probe tip is right at the
gate of the FET and the ground clip is right at the source.

I don't think its possible to do this without a floating supply and
using an opto to send the HS control signal to a driver. Even if I
disable the burst mode the controller could still go into burst mode
under fault condition like over current. To many chances something
could go wrong.

I suppose I could use an old UC384X it has none of these fancy
features and a 0.5 duty cycle clamp. Other then the unpredictability
of the burst mode it does work well.

I was also planning on using synchronous rectification a shottky
dissipates 4W but this FET would have less then 1W conduction losses.
I thought I would try something different;-)

http://www.nxp.com/documents/data_sheet/PSMN8R2-80YS.pdf

They are coming out with some nice FETS.

J

#### Joerg

Jan 1, 1970
0
John said:
There are some fast optocoupled mosfet gate drivers.

J

#### Joerg

Jan 1, 1970
0
JosephKK said:
Sounds like electrolyte dry out. See if you can find a way to "juice"
the cells again.

Strange thing is, the fluid level inside is ok. It's one of those
supposedly maintenance-free ones but one can pry off the lid assembly to
peek inside.

Why is it that this brand name battery is dead after 5-6 years, the gel
cell in my StatPower emergency kit also died after 5-6 years, while the
gel cell in my over 10 year old big \$10 Chinese flashlamp is still fine?

[...]

Oh yes, I remember Peak Meaningless Pretend Output from the 1960s. A
play value created by inventive misapplication of theory that cannot be
duplicated in the real world. And look at the thin cord that carries it
from the wall wart to the device, better than room temperature
superconductors it must be.

They probably rely on the fact that the majority of shoppers does not
have an engineering degree

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