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4hv.org :: Forums :: General Science and Electronics
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Using optocouplers instead of GDT for half/full bridges?

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teravolt
Tue Aug 24 2010, 07:38PM
teravolt Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
the thing I do to speed up opto's is to drive the LED with a fet driver. I am guesing that since the driver can sink as well as source the LED have there charge removed or added. does that make since
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teravolt
Wed Aug 25 2010, 02:29AM
teravolt Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
hears how my driver looks for my cm600
1282703391 195 FT95065 Dscn0891 Driver Board
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Wolfram
Wed Aug 25 2010, 09:18AM
Wolfram Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
If I was doing this, I wouldn't bother with optocouplers. Magnetic isolator ICs are cheap, and are much faster. This one Link2 for example, has a typical propagation delay of 6 nanoseconds. In addition, it has logic level inputs and outputs.

If you are making a tesla coil or something else that requires 50% duty cycle, I would much rather use a gate drive transformer.


Anders M.
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Conundrum
Thu Aug 26 2010, 06:14PM
Conundrum Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
teravolt wrote ...

the thing I do to speed up opto's is to drive the LED with a fet driver. I am guesing that since the driver can sink as well as source the LED have there charge removed or added. does that make since

yes this is what I found.
Even at relatively low frequencies you can see the "rounding" and ringing on the scope, a parallel resistor or active drive gets around that.
However there is little you can do about the delays in the phototransistor, even with active base drive as found on some optoisolators this only gets you up to a few hundred kHz.

Interestingly there are a number of ways to "homebrew" your own magnetic isolators including the use of bare Hall effect sensors scavenged from broken VCR motors or floppy drives, and small handwound coils glued to a sheet of glass with the sensor glued to the other.
Well driven they can be used to ridiculous frequencies...

-A
"Bother" said Pooh, as the Borg assimilated the Wraith...
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teravolt
Thu Aug 26 2010, 07:36PM
teravolt Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
Hi Conundrum, I use the hcpl-2611 and I can get well beond tesla frequency applications with opto isolation also fiber optics are fast enough but the propigation delays can be a problem with fast fets and circuits work above 100khz. With big IGBT's I can usaly dial it out with dead time. like my cm600's usaly like it around 60khz but I can get them to 100khz for burst. even the fet drivers have a little delay in them so less electroncs from begining to end is better. It may be pausible to dial some of that out and do some voltage phase leading like Finn Hammer. I just have an afinity for my drivers because they are not frequency dependent to a point and can drive alot of current. Ido also think that transformers can be better for small teslas and other applications.
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