Speaking of high frequency transformer stuff...

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Hi Tim,

Is the 1MHZ on the primary side - and getting into the secondary? Sorry, I work better with drawings. For gate drive transformer for inverters you normally want very good coupling and low capacitance. These contradict each other so that the design is a compromise. I've used some of the HP-Agilent-Avago opto-driver parts and I think they're rated at near 10kv/us. I don't do large inverters anymore, but when I did I always crossed my fingers when releasing to production and it has seldom failed me.

regards, Bob

One nice combo is a dc/dc converter to make 10 volts or whatever, and a fast logic coupler to get the gate drive signal up there, and then a proper gate driver chip floating with everything else.

Several people also make nice gate drivers with built-in isolators. Avago, IR, like that.

I've made my own transmission-line gate-drive transformers for short-pulse stuff, like 100 ns pulse widths/400 volt swing/5 ns risetimes. Longer pulses and high duty cycles become problematic.

Extreme case: use fiber optics. Megavolt isolation!

John

Ok. Picture the average half bridge MOSFET circuit:

At 1MHz, the FETs need local gate drivers (e.g. TC4420), so plant some inbetween the GDT and FETs. Add a DC-DC converter to power the drivers. Now, the GDT can be a higher impedance, since it only has to drive the TC4420's, not gates directly, which helps.

But now the problem is the pri-sec capacitance on the GDT and DC-DC conv. secondaries. The current flowing through that capacitance has to go somewhere. If it goes through the windings, you get coupling, which can lead to feedback and self destruction (I've seen the feedback before). The windings can be shielded, but that makes more stuff in the transformer = more leakage inductance. A tradeoff as usual.

Note that leakage in the DC-DC converter isn't a big deal -- I could filter and regulate that locally. But the GDT has to be fast and square (

Yup, that's the plan. But I'm not sure about transformer (and what design to use) or opto anything.

The HCPL-9000 does look pretty nice (and it's specified for 15kV/us at *1kV* common mode). The datasheet is suspicously quiet on what a "symmetric magnetic coupling barrier" is, but later on it states supply current is drawn in 2.5ns gulps. I wonder if the output state is stored in a flip-flop, and I wonder what its initial state is.

Perfectly acceptible If I were building a "valve house". ;-)

Tim

--
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Website: http://webpages.charter.net/dawill/tmoranwms

WTF... Si8233 would do quite perfectly, except there's no data? I must've learned something from Jeorg, cuz that doesn't make me too comfortable!

Oh well, I suppose one of those HCPL9000's with a TC4420 isn't so bad. Hmm, needs 5V though... zener will be good enough.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

I'd go along with that. There are optocouplers characterized for high common mode noise immunity (TLP759, HCPL4504) that might help.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

Not sure what you are concerned about but if it's the secondary winding capacitance to whatever you could make that winding out of coax and hang the shield onto the source of the upper FET. That's how we also get rid of E-field stuff leaking into RF transformers.

It doesn't have to be any fancy coax. Thin audio line might do if it can take the voltages and temperatures.

--
Regards, Joerg

http://www.analogconsultants.com/

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So, here's a possible optoisolated solution:

Notice the feedback circuit has time limited positive feedback, so shoot-through is impossible, high and low side on-times are equal (within component variation), and if it stops oscillating, it just stops (accordingly, something needs to be provided to kick it moving in the first place, which isn't shown here).

Not sure if everything is quite fast enough. There's a good ~100ns of delay in the loop, which is wholly 1/5th of the half cycle time. I'd rather not spend more bucks on an LT1016 or something like that. Incidentially, Mouser seems to have dropped LT from their catalog??

The timer is controlled for phase lock on the resonant tank, and either phase or supply voltage controls output power. (Someone mentioned PFC SEPIC recently, something like that could be handy here for generating 0-200V from a 90-265VAC input. No need for postregulation in an app like this, 120Hz ripple lets you know it's running.)

Regarding this thread, I've kept the application general, since 1MHz power transformers work just as well for induction heating as for generic switchers. Specifically, I am making another induction heater, but lessons learned here will do just as well anywhere else (like D from BC's 1MHz switcher with too much transistor-heatsink capacitance).

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

"Tim Williams"  wrote in message 
news:hlpo5u$re4$1@news.eternal-september.org...
> How would you tackle the problem(?) of 1MHz, 100V common mode on 
> transformers or whatever coupling you're using?  Think high side drive. 
> Is an ordinary gate drive transformer suitable, does it need special 
> construction or drive?  Or would it be better to use a big fat CM choke to 
> bring things down to earth?
>
> I don't think optical coupling would be very useful.  I've already had 
> poor results with 6N136's at 20kHz.  I know there are fancier parts 
> available, too, some with built in gate drivers.  I've seen it before 
> where fast edges will couple through the pri-sec capacitance of a GDT, 
> even when driven hard.
>
> Tim
>
> -- 
> Deep Friar: a very philosophical monk.
> Website: http://webpages.charter.net/dawill/tmoranwms
>

My favorite Intersil HIP4080 series of high-side/ low-side driver ICs easily goes to 1MHz. Check 'em out. No transformers, no optical couplers, nada. Ahem, HIP4081A in stock at DigiKey.

Rated at 80V (plus 15V swinging gate-drive), but hey, go ahead and push it to 100V if necessary, or adjust your turns ratio.

--
 Thanks,
    - Win

I built a servo amp with the 4080 over a decade ago. The chip is rated for 80 V, but the applications manager eventually admitted to me "Oh, you're doing REALLY good to get them to run at 59 V, none of our other customers ever got them to last above 54 V!" Oh, that was a REAL NICE admission! So, I eventually redesigned the whole thing to use the IR2113 half-bridge driver chip. They have been VERY reliable, way above the voltages I normally run at. The one gotcha is that the common point between the two transistors cannot be allowed to go negative, so I had to put an ULTRA-fast diode across the low-side transistor. (The body diodes in the FETS are incredibly slow to turn on.)

Unless the HIP4081A is on some improved process from the 4080 (I doubt it) you'll never get close to 80 V, even. Winfield, have you ever run the 4081A up above 40 V or so?

Jon

Whoa, I've been a fan of the HIP4081A for more than 15 years. The "A" variant, I must admit. In that time I never heard anyone 'dis them. My own designs use the parts (purchased repeatedly over a 7-year period) up to and over the maximum voltage spec rating, 80, 90 and 100V, and to and over 1MHz.

In my bench prototype testing I've had some ugly setups, creating substantial V = L dI/dt voltages, that get out of control in a real hurry (try 50nH*4A/10ns = 20V, be careful), yet I only had one failure, during testing, which quickly reverted to reliable operation after I tightened up critical wiring to reduce the current-loop area.

I have decades (OK, 1.5 decades) of successful 24-7 operation of my designs (the guys leave everything running continuously) using these parts at or near their limits. Recommended.

BTW, I'm also a fan of the IR half-bridge drivers, but they are relatively slow parts, meant for another kind of design. Their slow speeds mean you won't so quickly run into wiring issues, because for them L dI/dt has a much slower dt term. I use these to make elegant 400V pulse generators (posted on s.e.d.), but definitely not to run at MHz frequencies.

--
 Thanks,
    - Win

Just my 2ct: I haven't designed in Intersil parts since a couple years before I got my degree. And there's a reason ...

--
Regards, Joerg

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