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Re: SSTC, xfmr gate drive oddity
Original poster: "rheidlebaugh by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rheidlebaugh-at-zialink-dot-com>
What I am trying to say is the input must be biased at center potential to
turn the n chan on as the p chan is turned off. if the bias is not set
correctly to the incoming signal the results will be zero . Bias is
everything. If your input is not swinging each side of a common reference the
gates will not be turned on as the other is turned off giving a voltage
swing. The starting point must be 6v with no signal at the capacitor as you
are using only one power supply. The input must be biased at a mid point +.
> From: "Tesla list" <tesla-at-pupman-dot-com>
> Date: Tue, 04 Jun 2002 22:58:03 -0600
> To: tesla-at-pupman-dot-com
> Subject: Re: SSTC, xfmr gate drive oddity
> Resent-From: tesla-at-pupman-dot-com
> Resent-Date: Tue, 4 Jun 2002 23:18:45 -0600
> Original poster: "rob by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> Tesla list wrote:
>> Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>"
>> there haven't been very many questions about solid state coils lately...
>> :) Anyway, as I got around to tinker with SSTCs again lately, here's a
>> tough one... I hope someone can help out?
>> I've a mosfet buffer (complementary source follower with an n and p
>> channel mosfet) driving a small pulse transformer with a series capacitor,
>> i.e. a circuit like this:
>> 13.6V +-R-C-+
>> | | |
>> ||-+ drain | |
>> +--||<- n channel +LLLLL+
>> sig | ||-+ source =======
>> | | C1 =======
>> ------+ Y +-----------||---LLLLL--+
>> X | | |
>> | ||-+ source |
>> +--||> p channel |
>> ||-+ drain |
>> | |
>> GND GND
>> The signal at X on the mosfet gates is a clean square 0V<->13.6V signal
>> like it is supposed to be. When the xfmr is not connected, the sources'
>> node at Y follows 0V<->12V too.
>> But, as soon as I connect the xfmr, the square signal measured at Y "gets
>> smaller" in amplitude. The square wave centers around 1/2*13.6V = 6.8V,
>> but now goes only from 4V<->9V instead of 0V<->13.6V. I.e. peak to peak is
>> now only 5V instead of 13.6V. It is till a good square wave, though. But,
>> the 1:1 transformer output is now -2.5V<->+2.5V (and this is also seen on
>> the primary side of course). Not the expected -6.8V<->+6.8V.
>> Probably someone else has noticed this too in their gate drive setup?
>> Anyone have an idea where this comes from? It must be something simple but
>> it feels like banging my head on the wall, thinking&testing for 1 hour
>> already => no result.
>> The mosfets should be clamping point Y to 13.6V and 0V in turn, right?
>> But scoping at Y gives 4V and 9V levels for the square wave signal. Very
>> very odd.
>> Speefing up local RF decoupling from 330uF tantal to additional
>> 1000uF electrolytic didn't change anything. Also doubling the turns on the
>> toroidal ferrite core xfrm didn't help. Changing C1 from 330uF tantal to
>> 10uF electrolytic didn't have any effect either.
>> Sooo... I'm completely puzzled now...
>> Any ideas, tips, etc, would be highly welcome!
>> That is how to get the full +-6.8V voltage swings accross the pulse
>> transformer primary? Because +-2.5V is really on the weak side.
>> many thanks,
>> - Jan
>> high voltage at http://www.hut.fi/~jwagner/tesla
> I would recommend a UC2710 from TI. No sense fooling around with
> discretes when you can get 6A peak drive from a $5 part. My problem
> with the device is that is has so much drive capability that I needed to
> swamp the gate-source junction with a 15ohm resistor, otherwise there
> was severe ringing from the wires connecting to the FET. Digikey sells
> these drivers. Rob.
> The Numeric Python EM Project