gate drive transformer vs isolated supply fet driver

Hi,

I am trying to decide what is the best way to drive a 600V Hbridge, which requires fast (optimally 10ns or shorter) switching.

The two options I see are either a gate drive transformer, 1 primary, and two secondaries for driving each leg of the 600V Hbridge. Will the leakage inductance of the gate drive transformer may cause excessive gate ringing at this fast switching speed?

The second option is an isolated 18Volt supply with an optoisolated high current fetdriver IC. The required common mode resistance of the optoisolator will have to be 60kV/us at 10ns switching speed for

600Volts I think, the fastest opto I've seen so far is 15kV/us (HCPL-3120) I guess there are faster ones though?

Which of these two options would work best at this high voltage, fast switching?

cheers, Jamie

Reply to
Jamie Morken
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IIRC.. GMR isolators 110Mbps RF isolators 150Mbps Opto lost the race.

If I understand this right... Why not use a mosfet driver IC that has a bootstrap circuit for the high side? Example:

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D from BC British Columbia Canada.

Reply to
D from BC

I'd first look at the transformer method. I never had a problem sending rise and fall times under 10nsec across. In fact, sometimes the whole pulse wasn't much longer than that.

Two things to mind: You need to be or become familiar with multi-filar winding techniques. Also, make sure any cross conduction issues are taken into account.

If you can find a suitable transformer off the shelf and it's only one secondary consider using two of them.

As for optocouplers they usually don't produce much oomph and most are a bit sluggish for my taste. I'd imagine that you have to swing a substantial gate capacitance around.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

If you use it in a 600V application .... WHADDABANG!

AFAIK they aren't rated above 30V or so.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Yup...Was just a bootstrap example.. Some homework was required.

Here's a better example:

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600V ...Right at the limit for the op's app.

What next?.. It's not fast enough? :P

D from BC British Columbia Canada.

Reply to
D from BC

Hi,

Yes this is what I was wondering about, I guess the multi-filar winding is to minimize the leakage inductance as much as possible to prevent gate ringing?

For controlling all quadrants of a fullbridge using gate drive transformers I think these are the options:

  1. 1 transformer with 4 isolated secondaries (one per fullbridge quadrant)
  2. 1 transformer with 3 isolated secondaries, with one centertapped for the lowside fets, and the other two coils for the high side fets
  3. 2 transformers, one with a centertapped secondary, and another with dual secondaries.

Yes for the isolated supply+optocoupler option I was planning on using the optocoupler to drive a floating fet driver, ie. TC4452 12Amp fet driver.

cheers, Jamie

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Reply to
Jamie Morken

Yep, a bit sluggish, won't come anywhere close to his 10nsec. But it sure is an interesting part.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

It's to make sure the bandwidth is high enough so you get your >

Well, whatever you do, chart the turn-on and turn-off times on a piece of grid paper first. You don't want to have any overlap such as having both sides of one leg conducting at the same time.

Then you'd need a supply on that side as well.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

he

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Some additional tips:

1) Gate drive transformers are often used in conjunction with a PNP transistor on the secondary side to speed up off transitions. There's a nice TI/Unitrode App Note called something like "Gate Drive Techniques" that covers this and other bits. 2) If you're using a current sense resistor in any of your MOSFET drain circuits, transformer drive gives you the opportunity to drive the gate-source without having the voltage drop across the current sense resistor subtract from gate drive. Many app note schematics miss this point. 3) Beware of flux-walking in full-bridge circuits. This was discussed in another thread recently. Consider using a series capacitor in the primary circuit to absolutely prevent this possibility. Do not believe anyone who says that "current mode control prevents flux walking". It does, but only some of the time. Paul Mathews
Reply to
Paul Mathews

I'm not sure why these chips are not faster. It's only takes 1.5amps to charge 1000pF in 10nS. Perhaps the OP can find a faster chip or a high speed version based on the topology.

D from BC British Columbia Canada.

Reply to
D from BC

I used that IR halfbridge driver (ir21844) before to make the motor controllers for my homebuilt segway:

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Made two 3phase motor controllers with 6 of those parts they worked well.

cheers, Jamie

Reply to
Jamie Morken

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