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DRSSTC - cannot get oscillating

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Marko

Mon May 29 2006, 07:00PM

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

Well I dragged my mess out this morning for first, low-voltage run.
It's a halfbridge one with steve ward's primary feedback circuit.

Voltage to halfbridge is filtered by one big 400V 1000uF cap and divided by two smaller 470uF 200V caps (there are also transzorbs, decoupling caps and mess).

I ran it on 30V (so I get 15V per igbt) but it just seems to be ''working'' on interrupter pulses, and secondary rings to some point where it lights neon bulb 5cm away.

Primary feedback CT cascade gives no signal at all, zero.

I really have no clue what could be happening, I tested the board bunch of times before and it seemsed to work perfectly with resistor-limited signal generator input.

Only thing I can guess is that primary current is simply way to low for such a cascade to feel it.
Output of CT is at zero voltage all the time, and I need some 3V to trigger 74HC14 input

But If I add more input I thing I'l just blow something up because it will continue to work on interrupter frequency.

And from what I figured out it really should get running at 30V in...

Dr. Shark

Mon May 29 2006, 08:20PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

What about using a higher input voltage but current limiting it (e.g. with a lightbulb), so you cannot blow up your IGBTs if it does not work? I am not a expert in current transformer, but I could well imagine that a transformer designed for 500A or so wont give much of a signal at only 5A.

Steve Ward

Mon May 29 2006, 10:19PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

The coil should oscillate just fine at 15V or better applied to the bridge (mine usually oscillates with only 5V or so). Its designed so that even a few A on the primary of the CT will give plenty of signal (its a very high gain system).

Did you use an oscilloscope to verify that the feedback is staying at 0V? Do you detect a short on the CT input when the CT is disconnected? Also, you may need to *reverse* the phasing the the CT, by either passing it through the wire the other way around, or reversing the output leads.

Also, each IGBT sees the full supply voltage (its not divided because the transistors are in "series").

HV Enthusiast

Mon May 29 2006, 10:21PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

What are the specifications of your current transformer? Assuming you have a 1000:1 transformer, you should have no problem getting a signal with even 5A of current. Remember, the transformer you have takes the sampled current and gives an secondary current equal to 1/1000th of the primary current. However, voltage is the inverse, and since you have no burden resistor in this circuit, the voltage can be quite high, hence the need for the clamping network. So, with even a 5A primary current signal, you should have ample voltage to drive your circuit.

In fact, you should get oscillation without even having power on the bridge circuit. This is what that initial buzzing sound you may get when you first plug in your circuit.

I suspect your polarity of your transformer may be reversed. Try doing that, and see if it makes a difference.

Marko

Mon May 29 2006, 10:31PM

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

Of course I tried to reverse CT.

And output is scoped and completely dead, with just some fluctuations at few hundred milivolts.

I don't see why should I get anything, even close to 5A trougt the primary when it is running just on interrupter pulses. I would probably blow it up if I increased the voltage more.

(Interrupter 'enables' , inverting UCC goes low and uninverting high, and this pulses the pri mary without feedback starting to oscillate.)

Also, each IGBT sees the full supply voltage (its not divided because the transistors are in "series").

IGBT will see 300V; but it will switch only half of it because of voltage divider on primary.

HV Enthusiast

Mon May 29 2006, 11:15PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Using a scope trace your signals. Use a signal generator to input a 5V square wave at the primary current feedback input and trace this through your circuit. I think you'll probably find out why its not working.

Steve Ward

Tue May 30 2006, 05:23AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

IGBT will see 300V; but it will switch only half of it because of voltage divider on primary.

Huh? When one IGBT is ON, the other is OFF, and has the entire supply voltage across it... end of story. I dont care about a "voltage divider" for the return of the primary circuit, its passing RF, so it all looks like a loop anyway (capacitors are low Z to RF).

There should be some noise at least on the output of the CT when the primary is pinged from the interrupter going HI.

I don't see why should I get anything, even close to 5A trougt the primary when it is running just on interrupter pulses. I would probably blow it up if I increased the voltage more.

Figure out the surge impedance of the primary, and use V=IZ, and that will give you exactly what the initial primary current would be from the very first pulse applied. My DRSSTCs have a surge Z around 6 ohms, so at 30V, thats 5A already.

What type of diodes are you using to clamp the CT? And what arrangement?

Marko

Tue May 30 2006, 05:46PM

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

Huh? When one IGBT is ON, the other is OFF, and has the entire supply voltage across it... end of story. I dont care about a "voltage divider" for the return of the primary circuit, its passing RF, so it all looks like a loop anyway (capacitors are low Z to RF).

I guess my last post was somewhat confusing :P
Never mind.

I tried to use antenna on feedback input, and got first light (or at least first oscillations).

With 30V input I got a 1cm streamer that could be drawn to a total of 2cm, when primary was around best tune. I also tried a signal generator (welcome back to Jimmy Hynes age

) but it was quite worse than antenna, and I needed double tuning...

I dug the whole bridge trying to debug the feedback, but it simply seems to be too weak.
CT's are completely fine,
All connections are good, and DRSSTC actually works (sort of 'works').

What type of diodes are you using to clamp the CT? And what arrangement?

Your arrangement, of course. two 5V zeners series with 4148's in reverse.
ANt that seemed to clamp the antenna signal very well.
I get a tiny streamer from the antena but it doesn't bother the circut at all.

HV Enthusiast

Tue May 30 2006, 06:13PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Sounds like you might have something out of whack. Check your primary / secondary resonant calculations again or measure the resonance and see if they are at least close together.

Reaching

Tue May 30 2006, 06:36PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

do you have clamping diodes on the secondary side of the CT, (i guess)? . if so, check the polarity of them, maybe they are wrong and let your signal going to gnd or so.

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