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  1. #1
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    ingenious high side transformer isolated fet gate drive is just too good?

    Hello
    One of our contractors has just posted us a schematic with a high side transformer isolated FET gate drive design on it, in an offline power supply that he designed
    Put it this way, this circuit does not appear in here…..
    http://www.tij.co.jp/jp/lit/ml/slua618a/slua618a.pdf
    Its extremely ingenious, and yet, rather simple.
    What I cannot understand is why I can’t find it anywhere on the web.
    If you’ve seen this design youll know the one I mean. It appears to bypass many of the usual problems with transformer isolated gate drives.
    Do you know the origin of it?
    I am currently on a gremlin hunt of it, because this design is so brilliant that I suspect there must be a gotcha…do you know of any?

    He's got a working prototype, and it works well on the simulator at my end.
    I cant post the schem, but if you know this circuit, then you'll know exactly what i am talking about

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    take a stab in the dark

    https://www.google.com/search?client...w=1621&bih=858

    third item down - pdf fro toshiba

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    Never seen a transformer gate driver with active waveform shaping before? It's the natural answer to the problems involved with non-ideal transformer properties. But it has still limitations. You need to specify duty cycle range, frequency, gate capacitance, rise time to see if it serves your purposes.

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    IR had a nice ckt way back when in their printed app notes, two small fets that provided a semi latching on and latching off - very nice but needed re-freshing from time to time - relied on the Cgs holding the device on ( or off )

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    They have been around for quite some time (sufficiently long so that they are in text books)

    Its extremely ingenious, and yet, rather simple.
    With one transformer and three isolated windings, you can drive both high and low sides with one input.

    IMHO, engineering students have been weary of transformers (because of the magnetics, I guess) and they are therefore less visible in real life.

    But it has been see in many application notes (old and new) and the idea is an extension of the pulse transformer.

    That pulse transformers have not gone out of fashion suggests that they are still widely used.

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    Ah yes, here indeed it is...Easy Peasy knew about it as we tell from his post above.
    Going to do a gremlin search now.
    Figure 7 in the attached.

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    there'sa bit missing in fig. 6 fig 7 is the one ... nothing new under the sun ...

    - - - Updated - - -

    needs to be well damped ...

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    I like the scheme where you rectify the transformer to provide power to a 'proper' gate driver IC while also using the transformer signal as the input to the IC. This circuit is a 'clever' variation on that general idea.

    On the other hand when I've studied the problem I haven't come up with a reason to use a discrete transformer versus a modern isolated DC-DC plus an isolated driver IC.

    Mass produced royer type DC-DC's are just about as cheap as discrete gate drive transformers as far as I can tell for my applications.

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    When you consider all the ills of other pulse transformer gate drives, this 2_fet_secondary method (shown in post #6) seems too good to be true.
    It means a much smaller pulse transformer can be used.
    Long ON or OFF times are easily catered for.
    High duty cycles are no problem whatsoever.
    There is no series AC coupling capacitor there, so no LC oscillation with the primary inductance needing damping.
    It avoids the problem that some transformer gate drives have where the drive voltage reduces as the duty cycle increases.

    So why is this gate drive method not more often used?

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    Try it and you'll see - long ON times are not as easy as you might think at first glance ...

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    Thanks, another major point is the fact that this gate drive method (as in post #6) can use a much smaller gate drive transformer than other methods. Its very strange that such an advantageous pulse transformer drive method is so uncommon.

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    Um - very sorry to be a nay sayer - but the above is some way from the true performance of the GD circuit - if you build - or sim - you will see for your self ...

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    Here is a snip from the original IR app note text, note in fig 6 the diode is missing across the small fetClick image for larger version. 

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    If you power down you ckt for a short time - how do you know if the drive to a partic fet is still there or not ...?

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    Quote Originally Posted by Easy peasy View Post
    Here is a snip from the original IR app note text, note in fig 6 the diode is missing across the small fet

    If you power down you ckt for a short time - how do you know if the drive to a partic fet is still there or not ...?
    The diode across the small FET in figure 7 is the parasitic one inside the FET. I don't know why they sometimes draw it as a separate component.
    The problem with figure 6 is that the small MOSFET is reversed. Source should be to the left and drain to the right.

    If you power down the circuit for a while, you can't know how much gate charge/voltage will be left when you power on again.

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    Quote Originally Posted by asdf44 View Post
    I like the scheme where you rectify the transformer to provide power to a 'proper' gate driver IC while also using the transformer signal as the input to the IC. This circuit is a 'clever' variation on that general idea.

    On the other hand when I've studied the problem I haven't come up with a reason to use a discrete transformer versus a modern isolated DC-DC plus an isolated driver IC.

    Mass produced royer type DC-DC's are just about as cheap as discrete gate drive transformers as far as I can tell for my applications.
    Vicor used (maybe still does) a simple custom IC to get
    indefinite hang time from a pulse xfmr driven gate. A fat
    forward Schottky for charge, and once charged, a second
    "tickler" pulse applied through the same xfmr triggers a
    2-transistor SCR to discharge the gate (too skinny to
    give a spurious turnon). Seems maybe a bit noise sensitive
    but they never indicated a problem with it.

    I had a good look at the guts as I was supposed to do a
    redesign of it, before the program ran out of money. They
    also told me their "pulse transformer" was really just a pair
    of minimum spaced traces on the module substrate (like
    maybe more of a "transmission line balun" before that was
    a commonly understood thing).

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    Re: ingenious high side transformer isolated fet gate drive is just too good?

    The problem with figure 6 is that the small MOSFET is reversed. Source should be to the left and drain to the right.
    Well spotted - never trust an app note of a data sheet ...!

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