EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: ocset on April 06, 2019, 06:51:25 pm
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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 (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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If its that thing with the gate transformer both providing isolated DC power to a totem pole transistor arrangement AND providing the base drive, it is straight off a TI app note from one of the TI PSFB designs.
It does need a reasonably stiff transformer driver however, no free lunch there, and PSFB obviously has the nice feature of having constant 50% duty cycle, you would want to give it some thought and modelling if running variable DF, but it might be ok if the thing does not get too extreme, not sure.
Regards, Dan.
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Thanks, i am aware of the one that you are referring to...
Its not that one, its a lot better than that one.......providing that there are no gremlins of course.
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Also, carefully consider its operating conditions during the start-up period.
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So we just have to wait around for someone to finally post a circuit that matches what you want to discuss? Weak!
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The gremlin i am hunting is one of oscillations perhaps coming through CDG when ON or OFF.
Also, what is the trr of the internal diode in a BS270 FET...datasheet doesnt say
https://www.onsemi.com/pub/Collateral/BS270-D.PDF (https://www.onsemi.com/pub/Collateral/BS270-D.PDF)
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Well... so what?
We can only guess, based on your non-description...
I've seen boosted drivers (see above), zener coupled (the gate sits around until pushed by extra drive voltage), direct transformer coupled (probably the most common case?), AC coupled + DC restore (the most common case for e.g. two-switch forward/flyback converters when not using bootstrap drivers), uhh what else...
Tim
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Ah yes, here indeed it is...
Going to do a gremlin search now.
Figure 7 in the attached.
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Ah yeah, like the zener coupled one then.
Dubious at best in the presence of noise. Leaving gates floating? No thanks.
Tim
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A slightly better approach with a FET: https://i.imgur.com/b6XzlhA.jpg (https://i.imgur.com/b6XzlhA.jpg)
from my thread with that damn HP power supply: https://www.eevblog.com/forum/projects/hp-0957-2183-power-supply-48v-1kw-how-to-turn-on-reverse-enginerding/msg2274825/#msg2274825 (https://www.eevblog.com/forum/projects/hp-0957-2183-power-supply-48v-1kw-how-to-turn-on-reverse-enginerding/msg2274825/#msg2274825)
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Floating un-driven gate yes, could fall victim to noise, yes.
I would be worried about disturbances coming through the CDG capacitance and turning it on or off, because the fet is essentially un-driven.
The lack of popularity of this method suggests some serious ills.
Thouigh when you consider all the ills of other pulse transformer gate drives, this 2_fet_secondary (as in post #7) method 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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I have not the foggiest idea of what it is we may or may not be discussing. Could some kind soul enlighten me?