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gdt problem

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the_anomaly

Mon May 28 2007, 12:04PM

Registered Member #19 Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168

Ok so Iâ€™m trying to finish building my first sstc and Iâ€™ve come across a few problems. Iâ€™m basing my driver off Richie Burnettâ€™s â€˜small table top coilâ€™. I changed it bit to build it with the components I have (replaced the transistors with UCC chips)

. Its fine up until I get to the gate driver transformer. I get a high pitch sound and the 10 ohm resistor in series with the GDT gets hot in seconds. Without the GDT connected to the UCC chips I get this waveform: UCC wave

But when I connect the GDT the waveform I get out of the secondary is: GDT wave

And then several seconds later the resistor in series with the GDT is smoking. I canâ€™t see anything Iâ€™m doing different to the rest of you guys but something isnâ€™t right. Does anyone have any ideas? Iâ€™m sure the core is ferrite because itâ€™s dark grey in color and was packaged in some white plastic that pulled apart.

Sulaiman

Mon May 28 2007, 12:20PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

There are not enough primary turns on your GDT for the frequency that you're operating at.
the core is clearly saturating, you need at least three times as many turns.

Your drive waveform is also not 50% duty which will need to be adjusted but this is not the reson for the 'bad' waveshape.

Bennem

Mon May 28 2007, 12:27PM

Registered Member #154 Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 260

I agree with Sulaiman,
plus you need a cap on the UCC output to your GDT primary.

Marko

Mon May 28 2007, 12:57PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Generally, your TL494 output connection is *wrong*.

Each output (pins 9 and 10) need to be connected to their own UCC with a pull up/down resistor.
This will give a symetric waveform with variable deadtime. But still, a DC block capacitor is required to fix little imperfections wich may cause saturation of the core.

Regarding your voltage feedback, I don't think it will work that way. You'l just have heavily unstable frequency.

And, why is the pin 3 of upper chip connected together with input?
Enable pins should be tied to Vcc if you are not using them.

Also, by using SG3525 you could simplify things further as you could directly drive mosfets with it's powerful output stage.

But still, I actually wouldn't recommend using a VCO at all.
Just take a look af final version of steve's SSTC5.
It will definitely save you a lot of trouble and possibly blown mosfets.

Just examine this, take look at pics, you'l surely be happy.

ragnar

Mon May 28 2007, 02:12PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Firkragg, how can you use a DC-blocking capacitor on the output of the TL494 if you're using a centre-tapped primary and pushing it from both ends (from the complementary outputs)... you can't, right?

The DC-blocking capacitor is only possible in half-bridge (etc) arrangements... This means my ferrite will eventually get stuffed, because in a particular TL494 circuit, I've used a centre-tapped primary. (makes it easier to keep both MOSFET sources grounded... no gate-driver transformer, etc)

Hmmm

Marko

Mon May 28 2007, 04:41PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Firkragg, how can you use a DC-blocking capacitor on the output of the TL494 if you're using a centre-tapped primary and pushing it from both ends (from the complementary outputs)... you can't, right?

Take a look at nick's schematic, it _is_ a halfbridge, of UCC's. Maybe I was unclear, the cap surely needs to be on UCC37322's output, in place of that 10 ohm resistor, nowhere near TL494.

I only mentioned that outputs of TL494 are wired wrongly on schematic since we want symetric waveform all the time.

Steve Conner

Mon May 28 2007, 05:05PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I'd just like to add that if you can hear a high pitched sound, your frequency of operation is obviously way too low for a tabletop Tesla coil. So rather than putting more turns on your GDT, turn the frequency up.

the_anomaly

Mon May 28 2007, 07:38PM

Registered Member #19 Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168

Ok I've put more turns on (30 still 1:1) but maybe I will unwind them and try increasing the frequency. I put some filter caps on the ucc chips which helped greatly improve the stability but at the start of the top and bottom of the wave there is what I think is called ringing (It looks like a giant voltage spike but when I zoom in it turns into a damped sine wave). I tried few different caps ~100n up to 110uf (and got rid of the resistor) but none of them seem to damp it at all.

wrote ...
And, why is the pin 3 of upper chip connected together with input?

If I connect it I wonâ€™t get it to swing the voltage below zero for the off time. As it is it goes +12volts down to -12 volts and then back again.

Marko

Mon May 28 2007, 08:06PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I apologize for one thing I missed out - if you are going to 'amplify' output of a TL494, you need both drivers to be noninverting and connected as I explained before.

If you are not using enable pins they should be on supply voltage. Leaving them floating is generally bad.

Your current design is, I would say, all-wrong with you having to manually set the duty cyce (as far as I see it's not even close to 50% on your scope shots), and the transformer needs exactly symetrical waveform on it's primary in order not to saturate.

Even after you fix that, you should put a few-uF capacitor on output of one UCC, in place of that 10ohm resistor.

The DC blocking capacitor ensures that no DC component gets into GDT primary and possibly causes it to 'walk' into saturation over a number of cycles (wich can happen even with tiny imbalance of duty cycle).

Transformer needs constant volt-seconds on it's primary - so if it is pinged witha square pulse on one side it must also get exactly the same pulse with reverse polarity to reset the core. Deadtime between these pulses can be, it's not important, but you should still keep it small to ensure zero current switching for the devices.

Regarding frequency, I'm pretty sure that you are safe by keeping it over 100 or so KHz.
30turns looks like a lot for SSTC frequencies, but I don't know your core dimensions.

I hope I helped somewhat...

Danielle

Mon May 28 2007, 08:52PM

Registered Member #632 Joined: Mon Apr 09 2007, 01:09AM
Location:
Posts: 267

This isn't your problem yet but you probable want to synchronize the PWM with the interrupter to prevent turnning off in the muddle of a wave.

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