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From: Joerg on 23 Apr 2010 11:06
Hammy wrote:
> 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.


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).

--
Regards, Joerg

http://www.analogconsultants.com/

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Use another domain or send PM.
From: John Larkin on 23 Apr 2010 11:34
On Fri, 23 Apr 2010 08:06:40 -0700, Joerg <invalid(a)invalid.invalid>
wrote:

>Hammy wrote:
>> 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.
>
>
>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

From: Hammy on 23 Apr 2010 12:00
On Fri, 23 Apr 2010 07:54:39 -0700, Joerg <invalid(a)invalid.invalid>
wrote:


>
>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.
From: Joerg on 23 Apr 2010 12:07
John Larkin wrote:
> On Fri, 23 Apr 2010 08:06:40 -0700, Joerg <invalid(a)invalid.invalid>
> wrote:
>
>> Hammy wrote:
>>> 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.
>>
>> 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.
>

That's a lot of stuff for a 50W-100W converter :-)


> The advantage of optos is DC coupling without duty-cycle problems. And
> not having to wind transformers.
>

Another option is to use a chip with proper bootstrap circuitry.

So far transformers have been good to me. The only problem is that
off-the-shelf gate transformers are IMHO way overpriced and the
reasonably priced ones from China are custom runs and have MOQs. Which
for a product design doesn't really matter. It seems the usual COTS
vendors don't really realize how much of their lunch is being eaten.
Maybe time to invest in Rinminbi, if I just knew where ;-)

--
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 12:24
Hammy wrote:
> On Fri, 23 Apr 2010 07:54:39 -0700, Joerg <invalid(a)invalid.invalid>
> wrote:
>
>
>> 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.....
>

If you don't turn off the menu then the side screen will be stored in
the image file.


> 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?
>

It does work on XP, it runs on a laptop on the lab bench here and that's
an XP machine. However, MS screwed up backwards compatibility in .NET
and you must load its version 1.1 environment. Freewave did not install
correctly with version 2.0 but .NET 1.1 can coexist peacefully with 2.0.

It's really nice, I take measurements for clients and then send the
results over the LAN into the client directory on the office server.
Then head to the big PC to compile the reports. Ok, it doesn't transfer
my cup of coffee along :-)

>
>> 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.
>

Oops ...


> 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.
>

With a flying source that's a problem because the Instek connects the
probe shield to PE. So you get ground loops and tons of ringing. Plus
you neighbors listening to AM radio won't be very happy about that. You
can get battery packs for many Instek models but it's very expensive. I
never checked whether one can hack into this and connect one's own
battery. Even then you'd at least need to wrap the probe through a nice
big ferrite a few times.

Maybe at your frequency range you could try a diff measurement with two
channels.


> 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.
>

With properly size caps it should be possible.


> 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.
>

Question: Since this is a flyback why is the source flopping about?
What's it doing? Maybe post the whole schematic if it isn't confidential.


> 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.


But you have to drive over 3500pF of gate capacitance. Make sure the
Rdson versus gate capacitance deal is somewhat optimized for the chosen
frequency and power level. Ideally the losses due to both effects would
be about on par.

It's pretty brazen that they advertize this as a 130W device. I wonder
what it's lifetime will be under that sort of load (in milliseconds,
from turn-on to a sharp popping noise).

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
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