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"Thumper"

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Finn Hammer
Fri May 01 2009, 08:05PM Print View
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Bislev, Denmark
Posts: 628
As you all know, good old Dr. Spark gave me a Ward DRSSTC driver board, of the 2005 edition. I have populated it, and it is now driving a nice little 40n60 bridge.



With a 1.5µF cap and associated inductance, it is resonating at 32kHz. I look at it as my breadbord where I am going to test out features that I think might be good to have in a DRSSTC driver board.
Starting out with something that already has proven it´s worth seems like the best approach.

Big bricks have delays close to a microsecond, so it would seem like a good idea to initiate the switching of the bridge well in advance of the tank current's zero crossing. I am indebted to Richie Burnett for the method I use to predict this crossing:
Using a 0.01V/A current transformer (50turns, 0.5ohms burden) adding a bit of inductance in series with the burden, the voltage of the combo is made to lead the tank current. A small 10turn coil, 3/4 of an inch in diameter is enough to extract a signal that is 2µS ahead of the current:





This signal is fed into pin 2 of a 311 comparator, and the output looks like this:



You will see, that I now have a perfectly good clock signal that leads the current by a couple µS.
This generously advanced signal forms the basis of the final clock:
Passing it trough 2 schmitt inverters, interspaced with a suitable RC time delay, the final clock signal can be made to move from "well ahead" of CZC:





To much too late. Not that this last timing is of interest other than to show that it is possible:




So far so good, but does it work?, is it possible to make the bridge switch with this clock, instead if the original, "brute force" diode clamped signal.

Absolutely! And this small video shows how it is possible to advance and retard the clock relative to CZC, and how this effects the voltage spikes produced by the bridge.
Notice that I am unable to achieve perfect switching on both edges simultaneously. I assume that adding a deadtime generating circuit will take care of that. I think this will come in handy, when I start to work with the big bricks.

So far so good, enjoy the video:


Cheers, Finn Hammer

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Dr. Spark
Sat May 02 2009, 04:23AM
Registered Member #290
Joined: Mon Mar 06 2006, 08:24PM
Location: Arizona, USA
Posts: 1663
You just cannot leave it alone…….your such a pain in the butt; however, would love to hire you in R/D dept……..and keep you in the back so do not scare/alarm the customers! Hee hee

Going to finalize the new Driver Board (naming it H3W_2009 rev 0) , so hurry up and get me all modifications proven…..! Just kidding, no hurry as still stuffing the last of the old boards and one is for Quadzilla Pulser?

Cheers,
Ch


BTW Nice Skull Indeed!


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Finn Hammer
Sat May 02 2009, 06:01PM
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Bislev, Denmark
Posts: 628
Dr. Spark wrote ...

You just cannot leave it alone…….your such a pain in the butt; however, would love to hire you in R/D dept……..and keep you in the back so do not scare/alarm the customers! Hee hee

Going to finalize the new Driver Board (naming it H3W_2009 rev 0) , so hurry up and get me all modifications proven…..! Just kidding, no hurry as still stuffing the last of the old boards and one is for Quadzilla Pulser?

Cheers,
Ch


BTW Nice Skull Indeed!





ROFL, you are funny indeed!.
But no confirmed mods before making sparks all day.
This mod is not going to break anybody's budget. Here is the burden board, showing the 0.5 ohms non-inductive resistor, and the nifty inductor which advances the trace for current zero crossing predict.
The small 10ohm resistor on end, is across the inductor, to damp out oscillations which I am not sure are real or measurement error. Don't do any harm, though.




There is one strange thing I don´t understand:
The 311 has an open collector output, and when the signal is taken from the collector, the responce is 4 times faster then whan the signal is taken from the emitter. (Assuming same input signal slope)




It is real enough, I can see it on the scope.

Anybody able to explain this strange behaviour? After all, it is the response of the same device. Only thing I can think of, is that we are seing the current being pumped into the base-emitter region, and so the difference of the curves shows this current, and thus also the turn on delay of this output transistor.

Anybody?

Cheers, Finn Hammer
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Steve Ward
Sat May 02 2009, 06:49PM
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1022
Finn, the common collector (or emitter follower) has lower inherent gain because of the negative feedback implied by the rising emitter voltage, so it slows itself down.

Ummm... could you PLEASE get a decent comparator for this?

The issue i always had with this type of circuit (phase lead) is that i figured it always had some limit on dynamic range. But, i suppose there are comparators out there that will take a pretty big signal swing and still work fine with 10's of mV level stuff.

The other issue i worry about is that you could accidentally over compensate and start switching too early, which is a lot nastier than switching late.

The un-even switching times bother me too, i suspect it has to do with your delay circuit.

Anyway, i look forward to your results for a fully implemented system... surely it cant be *this* easy .
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Steve Conner
Sat May 02 2009, 07:59PM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6725
I'd be worried about the little air cored coil picking up magnetic field from something that it shouldn't.
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Finn Hammer
Sat May 02 2009, 08:43PM
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Joined: Sat Feb 18 2006, 11:59AM
Location: Bislev, Denmark
Posts: 628
Steve Ward wrote ...



Ummm... could you PLEASE get a decent comparator for this?


I can't claim to know what makes a comparator decent. I just grabbed 311 because I have them, they take + - 15V and seem fast enough. I don't really want something that is so fast it rips the surroundings apart . And the open collectod output made interface to 5V logic easy.
Do you have one in mind you would like to suggest ?

Steve Ward wrote ...


The issue i always had with this type of circuit (phase lead) is that i figured it always had some limit on dynamic range. But, i suppose there are comparators out there that will take a pretty big signal swing and still work fine with 10's of mV level stuff.

I am pretty surprised that it kicks in at only 5V into the bridge, and still syncs well.
Steve Ward wrote ...


The other issue i worry about is that you could accidentally over compensate and start switching too early, which is a lot nastier than switching late.

I guess only an experiment can answer that.
Steve Ward wrote ...


The un-even switching times bother me too, i suspect it has to do with your delay circuit.


Absolutely. And the unequal turn on and turn off delays of the 311.



I am going to use this circuit which has the advantage that I can adjust both transitions separately: This will take care of clock 50/50 duty cycle. (Disregard component values)




Steve Ward wrote ...


Anyway, i look forward to your results for a fully implemented system... surely it cant be *this* easy .


You are probably right . After all the name of the game is "sparks all day"
That´s the acid test.

Cheers, Finn Hammer
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Steve Ward
Sat May 02 2009, 09:59PM
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1022
I was just thinking you should use a faster comparator, the LT1016 comes to mind. But, perhaps i remembered incorrectly because i thought the 311 had a delay in the uS range, not the 130nS range, so maybe its not that big of a deal, but the less compensation required, the better, id think.

Stable oscillation at only 5V input is good, thats about what my systems normally require to get going at all, and maybe arent even fully stable until about 20-30VDC on the bridge. Of course, the sensitivity could be improved for lower voltage/current systems so its all relative i guess.

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Finn Hammer
Sun May 03 2009, 11:35AM
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Joined: Sat Feb 18 2006, 11:59AM
Location: Bislev, Denmark
Posts: 628
Steve McConner wrote ...

I'd be worried about the little air cored coil picking up magnetic field from something that it shouldn't.


Maby this is the time to pull out that sheet of µMetal to solder a nice guarding sheath.

Btw. I fitted the delay network where I can shift each transition independently and recorded another video, where I show the improvement in voltage overshoot/oscillations possible by fitting the current prediction circuit.



It does exactly what I have always wanted in a controller: give me an opportunity to act. To remove transition overshoot/ringing, with my diddle-stick, instead of having to mess around with gate resistor diode networks.
Gate resistors are 20ohms, to calm things down, btw.

I spent a lot of time figuring out why the comparator went into oscillations at times. It appears that inbetween bursts, both inputs are at same potential, or close.
What would be a good solution to this?
I could introduce a few, say 5mV, hysteresis, but how to do that in a comparator with + - supply?
I just realize that I don't understand the function of the balance pin.
I could need a basic text on comparators, if any of you can help me with that, I*d appreciate it a lot.

Cheers, Finn Hammer
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Bennem
Sun May 03 2009, 12:09PM
Registered Member #154
Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 258
Hi Finn,

Great work!
How are you adjusting your prediction circuit to eliminate the spikes?
is it the 'differance signal between the original drive signal to the the predicted signal'?
Have you got a diagram to help understand it better?

Mel
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Finn Hammer
Sun May 03 2009, 03:56PM
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Joined: Sat Feb 18 2006, 11:59AM
Location: Bislev, Denmark
Posts: 628
Thanks, Bennem.

Here it is:



1K on bal to B+ solved oscillation on input, gues it shifts the reference of one pin a bit.
If you want to try yourself, run the coil as it is, and monitor pin4 on 74HC109 on Wards troller, to assure it matches output of zero predicter.
When you have same signals, connect predicter to pin 4 and disconnect pin 12 on 74HC14 to allow swing on flipflop.
Enjoy!

Cheers, Finn Hammer
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Bennem
Sun May 03 2009, 06:37PM
Registered Member #154
Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 258
Hi Finn,

Thanks for the info,
Do you think you could get the same results with a high Freq Res coil?
I'm going to try it with my small 300Khz coil as i have one of Steve's
older version circuits in that coil.

Mel
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Finn Hammer
Sun May 03 2009, 07:43PM
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Bislev, Denmark
Posts: 628
Bennem wrote ...

Hi Finn,

Thanks for the info,
Do you think you could get the same results with a high Freq Res coil?
I'm going to try it with my small 300Khz coil as i have one of Steve's
older version circuits in that coil.

Mel

It certainly is not designed for that. But I can't see why not, for different reasons, though.
For the low frequency coils the reason is the long shut off delays of the bricks, mainly.
But in a high frequency coil, the delays are smaller, but so is the frequency, so the delay to period ratio might be comparable.

All I can say, give it a try. Worst case scenario: the thing stops to switch, no big deal, just rewire and make sparks all day!

I am pondering a new version.
First of all, there was the problem with oscillation due to equal input voltages. I can solve it by referring the one side of the burden a few millivolts away from - input, the other to + input. Better it by adding a bit of hysteresis.
This can not be done with an open collector output, so I have chosen a totem pole output comparator, AD790 which has some good features. Built in hysteresis, 500µV, will accept up to +- 18V supply, fast, but not too fast, 40nS etc.







Bias the - input for adjustable offset. And maby looking a bit more nice in a way. Have to wait for the comparator, though.

EDIT:
Woops. Having read up on the datasheet of AD790: It has a 5Vlogic output totem pole. What a wonderfull comparator!, that means I can forget about the interfacing transistor. This is looking good indeed.



Cheers, Finn Hammer
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Bennem
Mon May 04 2009, 08:04AM
Registered Member #154
Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 258
Cool.....I look forward to seeing your results!!
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GeordieBoy
Tue May 05 2009, 11:36AM
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Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 866
Hi Finn,

Great work. You can actually vary the amount of phase-lead in the feedback signal from the CT just by making the inductive part of the burden impedance variable. i.e. Use a tiny adjustable pot-core or I.F. transformer type coil that has an adjustable ferrite slug. You should then be able to adjust the inductive part of the burden impedance to get the amount of phase-lead that you want.

Steve W is right about the dangers of too much phase-lead in the feedback path. Present DRSSTC's with no phase-lead compensation switch a load current that has leading power factor. To put this another way, the load current has already passed through zero and changed polarity before the message gets through the controller and tells the IGBTs to actually switch over. Funnily enough, this is the way it should be for IGBTs....

If you add too much phase-lead into the feedback path at the operating frequency you will end up with the IGBTs switching in advance of the load current passing through zero. This is generally bad for slow IGBTs as they have stored charge which will then be swept out in the form of current tailing. The further the IGBT switches ahead of the current zero, the higher the instantaneous current will be at turn-off and the greater the tail-current losses will be.

So in short it is always better to let the IGBT's collector current fall smoothly to zero, THEN turn off the IGBT when there can be no current tailing. Then, only when the first IGBT has been allowed sufficient time to turn-off, the opposing IGBT can be turned on.

The holy-grail of timing that you should be aiming for is this:

1. Turn off the current carrying IGBT as soon as possible after its collector current has fallen to zero and the load current has started to build in it's co-packaged free-wheel diode. (The device can be turned off with a very rapid slope because current has already commutated to the co-pack diode some time ago!)

2. Wait sufficient time for this IGBT to regain blocking ability before trying to turn on the opposing IGBT.

3. Turn on the opposing IGBT with a controlled slope so that the rising load current is now commutated from the other device's free-wheel diode to this IGBT in a smooth and controlled manner.

If all of these things are performed as quickly as possible after the current zero-crossing then the load current should not have had time to rise very much from its zero crossing. Therefore turn-on losses should not be very high, despite the controlled turn-on slope. Note that the controlled turn-on slope may actually reduce turn-on losses in the IGBTs due to forced reverse recovery in the opposing device's co-pack free-wheel diodes. Again, it is advisable to start the turn-on process as early as possible. The longer the load current is allowed to flow in the free-wheel diode the higher the current magnitude will rise, and the worse the reverse-recovery spike will be when the diode turns off.

The phase-lead network I told you about with the L+R burden impedance is great for compensating for inherent delays in the control and gate-drive paths. However, you should be careful not to introduce overall net phase-lead for the reasons stated above. It is also important to realise that the amount of time-lead in nanoseconds is dependent on the amplitude of the current signal being sensed. That is to say that the output of the comparator will switch further ahead of the current zeroes if the current being sensed is large, than if the current being sensed is small. The L part of the burden impedance basically adds a "differential" component to the otherwise proportional output from the CT. So the output is no longer proportional to the sinewave load current being sensed, but is instead proportional to the sinewave load current plus a certain amount of the differential of this sinewave load current. The more rapid the slew of the load current, the earlier the comparator will switch and the more "time-lead" is introduced.

I hope this helps explain the details of how an IGBT inverter operates with a series resonant load. Have fun experimenting.

-Richie,
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Dr. Spark
Tue May 05 2009, 04:37PM
Registered Member #290
Joined: Mon Mar 06 2006, 08:24PM
Location: Arizona, USA
Posts: 1663
Designing Si for the last 25 years at Intel, have learn one thing on hardware design. Kiss method is the best!

The fewer chips, the more reliable in the long run as less chance for something to fail.

Issue with high powered DRSSTC is RF!

PLL’s, adjustable lead-times, etc are just asking for RF to muddle logic and timings.

Have built many DRSSTC with old design and never had an IGBT fail due to driver board.

Same IGBT’s in FATBOY for 5 years now. Built in hardwire delay will always work, RF will have a hard time getting into driver and IGBT’s will run ICE cold (Fatboy does).


Just an old farts 2 cents!


Spark on,
Ch



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Finn Hammer
Tue May 05 2009, 07:32PM
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Location: Bislev, Denmark
Posts: 628
Richie,

I don't plan to shut any device off before current zero crossing (CZC)
But when the turn off delay is the longest of the switching parameters, up in the µS region, That´s why I thought it was prudent to initiate the turn off before CZC.
I read your advice about allowing the IGBT to recompose itself before opening the other one, and this is where the adjustable deadtime circuit comes in. Supported by generous gate resistance, to slow down the turn on. (I don't so easily forget the hardship this reverse recovery current caused us in the CCPS thread.

I had not thought about the effect of increasing current, creating a larger advance in timing, and this means that the design has to be optimised for the end of the burst. Maby the deadtime window can be long enough to accomodate the differences here.

Chris, I also read your objections, and you obviously have a point. Fatboy is hard to beat, and why mess with a proven design.

The 1968 Shelby Mustang GT500 KR was a very successfull design in it's time.




But it didn't stop further development effort, and the well deserved success of the Ward controller should not do so either.
Your many years in a craftmans environment must surely also have taught you another thing: Apprentices ask around with the masters that care to share insight, then they go and do what they want themselves.
But we take every bit of detail you said that can go into this new design, be it bypassing, tuning or basic circuit ideas.
I bet you did the same back in your rookie years.
And like I stated early in this thread, hell I can always go back to the basic controller as it is.

I got the AD790 comparator, and it works really well, the internal hysteresis, 500µV did the trick, but biassing the negative input a couple millivolts is probably going to be needed.
The offset here has to be greater than any voltage induced in the burden coil, and this can only be determined in a running coil. I*l look into guarding the coil, and of course orienting it perpendicular to the main source of mutual induction: the primary coil.

I also got a bit of work done on the over current detector.
Here I am also going another route, but not much:
I always felt that it was not too much to ask, that it was a straightforward procedure to set the maximum current trip point. This is a place where I don't want to experiment, but rather would demand a well defined voltage point to set the trip current.

I do so by using a precision rectifier instead if a diode bridge,
I know, Chris, you are going to puke now, but really, it is just one op amp more instead of the diode bridge.
This rectifier rectifies a current transformers output, and the result is fed into a comparator, as already, only this way, the trip voltage is well defined. Like incorporating a measurement instrument in the controller.

I do read your warning about RF, the enemy. I will fight it by your own measures: Decoupling and layout.
No power pin on any device is going to be more than 3mm away from it's decoupling cap, there are going to be analogue ground planes separated from digital ground plane, decoupling and bypassing galore!

Allow me to scite a great piece of inspiration I read the other day: (it is prolly a classic already)

"The breadboard is both the designer's playground and proving ground. It is here that Reality resides, and paper (and computer designs) meet their ruler. More than anything else, breadboarding is an iterative procedure, an odd amalgam of experience guiding an innocent, ignorant, explorative spirit. A key is to be willing to try things out, sometimes for not very good reasons. Invent problems and solutions, guess carefully and wildly, throw rocks and see what comes loose. Invent and design experiments, and follow them wherever they lead".

Jim Williams wrote AN72 in 1998, and subtitled it:
Guidance to put civilised speed to work.


With that guidance, and that of yourself: honorouble list members and friends, I intend to persue this idea to success, or to it's pitifull failiure.

Cheers, Finn Hammer
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Dr. Spark
Tue May 05 2009, 09:22PM
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Joined: Mon Mar 06 2006, 08:24PM
Location: Arizona, USA
Posts: 1663
Hi Finn,

In no way am inhibiting or disapproval of the great R/D, as would love to be closer and work togeather on this, just ensure when firing up @ 10kva, that all is stable.

Run all my electronics open, right under the primary with no issues just to prove how solid the coil is.

Am on hold on the new driver board until all your experimentation is complete, and may have a bypass jumper to allow phase adjustments on the fly for future audio modulation schemes.

So keep up on the great work sir, as nice to see a worth while post again!

BTW nice little picture of FATBOY eating everything in its path until was shut down for eating the 20 foot ceiling insulation at ¾ power. Maybe we can meet for another FATBOY session?

Cheers,
Ch





This was fun working on FAYBOY togeather!














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Dr. Spark
Tue May 05 2009, 09:26PM
Registered Member #290
Joined: Mon Mar 06 2006, 08:24PM
Location: Arizona, USA
Posts: 1663
Yes a double post sorry, first one in 200 years, a spark picture of what is to be is a must indeed!

Spark On,
Ch


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GeordieBoy
Tue May 05 2009, 09:45PM
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Location: Doon tha Toon!
Posts: 866
Hi Finn,

All of what you are doing sounds good to me.

Advancing the drive signal to compensate for delays in the control and gate-drive electronics sounds like a sensible approach. As for how much this will increase efficiency, I am not sure for a pulsed application like a DRSSTC. I can say with some certainty that it won't noticeably increase the spark length because efficiency will likely already be in the 90's of percent, but it may reduce the device heating considerably and also reduce reverse recovery problems.

Regarding the phase-lead's dependancy on current amplitude: I would optimise the circuit so that it switches with best performance when current is maximum. This is the normal situation when designing a resonant switched mode power supply. Maximise efficiency at full-power because this is when it matters most. Poor efficiency at low power doesn't imply as much dissipation so is generally not a problem.

Regarding the concerns about interference pickup of that little air-cored coil: I would replace it with a pot-core or IF transformer type with a semi-enclosed bobbin and adjustable core. This will be less susceptible to strong magnetic fields. It may also be wise to orientate it's axis orthogonally to the TC's primary and secondary axes to minimise any remaining coupling.

-Richie,
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Steve Conner
Wed May 06 2009, 10:11AM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6725
Hi all

Having an advance that changes with current may be a good thing, since the turn-off delays of IGBTs probably increase with current too.

My last word on the DRSSTC driver debate was the Mk2 PLL driver that I built.
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Finn Hammer
Sun May 10 2009, 05:31PM
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Joined: Sat Feb 18 2006, 11:59AM
Location: Bislev, Denmark
Posts: 628
All,
Project mooving forward, of course slower than wanted and predicted.
However, here is schematic of controller, only missing status led's:
Burden resistors are mounted on the terminals, so that they can be exchanged together with the current transformer.



And board layout:



This has been a satisfying job. In the course of drawing the schematic, I finally got the hang of using the supply symbols, which greatly simplifies the schematic.
The layout has separate grounds for analog and digital section, and 100nF bypass capacitors are surface mount cheramics on the back of the board.

Since I etch this board myself, and don´t have trough plating facilities, certain ground connections are pulled over to a via to connect to gnd.

Cheers, Finn Hammer
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Finn Hammer
Thu May 14 2009, 09:31PM
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Location: Bislev, Denmark
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Arrh! The joy of prototyping!

Knowing that to make the circuit work, a board has to be laid out, and produced. Early attention to chip power supply has to be taken care of.
To make a 40nS comparator spit perfect square waves, supply decoupling must be done as by the book!



I am the first to regret the shabby look of the etch, but (many good excuses...)It's the small surface mount 0.1µF caps I want to show off, and of course the blue wire shouting GOOF out loud. But never mind, even those many Bourns trimpots are not cheap, it is for sure: another board has to come after this one:



So many hours used to find out: does the idea work?
All the signals check out. I did the gate drivers for 5V since they are going to drive optocouplers ultimately, but since I have already cut traces, I am going to rewire them for 15V and drive the gate transformer for the "dummy"-bridge.
I made some invalid assumptions on the interface from the overcurrent protector too, that have to be worked out.
I have vacation now, and it is agreed that I don't work on the project for a couple days, untill then, I can smile at the memory of that precision rectifyer doing it´s task at 8Vin, and 31kHz:




Cheers, Finn Hammer
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Dr. Spark
Sat May 16 2009, 02:41AM
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Joined: Mon Mar 06 2006, 08:24PM
Location: Arizona, USA
Posts: 1663
Finn Hammer wrote ...

I have vacation now, and it is agreed that I don't work on the project for a couple days, untill then

Cheers, Finn Hammer


Yep been there for sure, “agreed” as if you had a choice, hee hee.



Great work sir, glad to see you are also enjoying the fun....next is the 12 footers singing.

Spark on,
Ch

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hvguy
Mon May 18 2009, 08:52AM
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Posts: 154
Great job with this thread, Finn!

I built this exact circuit, LM311 and all (fast comps too), about three years ago. I spent more than a month working with it. I too had good results on the bench and was able to easily adjust the system for perfect switching... BUT it ultimately proved of little use as the change in delay for from ~20ApK to 800Apk in my final test system was just too great and resulted in a phase change much greater than was needed. I tried many different R/L values and different CT ratios, etc. but could not find an ideal situation. This circuit definitely works great with a fixed or lightly varied current source, but with the dynamic load of a DRSSTC it just didn't cut it. It did work well in an SSTC feedback loop though... Ultimately I could not justify using it as the efficiency/performance gains where just not there. Also, since a very high speed comp was necessary to achieve perfect switching it was HIGHLY susceptible to any noise. I always had to run a full AL enclosure with feedthru caps and fiber controls when the system was under full power (~1kw). It was like trying to build a GHz RF amp

That being said, I'm not trying to burst the bubble on this one and I hope you come up with something I missed as this would be a really helpful addition to the classic DRSSTC.

It's funny, I remember around Y2K when I started experimenting with feedback based SSTC's (I believe I was the first or second (Jan Wagner?) to incorporate a feedback loop into a TC system) I accidentally created this situation. It was so bad, in fact, that it lead to me giving up on the CT idea and sticking with voltage feedback (oops!). It was actually Richie who helped me with my first open-loop SSTC so I'm glad to see he's involved with this thread... Good luck!

Oh, BTW, props on the scope; I love my 2014, can't beat the CF card storage and it never skips a beat even when inundated with Kw's of RF.
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Finn Hammer
Mon May 18 2009, 09:08AM
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Location: Bislev, Denmark
Posts: 628
Thanks, Aron

Your post sure ads to the excitement, but then, WTH, I actually started this idea based on a sample and hold circuit, which will not have any variation in delay due to different output from an inductor.
May have to pull that circuit into service in the end.

Cheers, Finn Hammer
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Finn Hammer
Tue May 19 2009, 08:47PM
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All,

Back from Paris, first on the list was to find out, whether the circuit works, (it does so, thank you) and whether the inductive component of the feedback current transformer burden, causes the feedwack signal to advance with rising current.
This does not appear to be the case:

Look at the video linked below:
Green is voltage out of bridge,
Cyan os current in tank, measured with Pearson model 101, and
Magenta is feedback signal, as measured across burden inductor and resistor.
I vary the current in the bridge from 50A to 500A
The 2 Magenta cursor lines help to convince you, that there is hardly any advance in the feedback signal within a 1:10 variation of current.


So now that this taken care of, I'l design a real kiss gate driver centered around the IXDD414 chip, which, since it is delivering it's drive through a 5.6ohm resistor, has more than enough omph:
Swinging up from -10V to +20V the current won't exeed 6A, but the IXDD can source and sink 14A.

Cheers, Finn Hammer
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Finn Hammer
Sat May 23 2009, 07:28PM
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All,

I admit to being a sucker for symmetry. So the gate driver for "Thumper" had to be designed with this in mind. I fried the 4 IXDD414 chips in short order, no good reason found, so the search for another gate driver started.
I want an optocoupler so that the gates get pulled all the way to -10V, even though there is deadtime. A gate transformer won´t do that.
The fast opto's I mentioned earlier demanded a low supply, 6V, and so I deemed them unsuitable for this project. I had a stick with 50 pcs. HCPL3120, and they look perfect for the job. The supply voltage is as high as 30V so it can straddle the gate driver supply, and swing the gates of the PNP - NPN totem pole output from rail to rail. Only thing to find was MOSfets with a 30V Gss rating. A prolonged datasheet search unearthed suitable candidates from Fairchild, both 30V gate voltage and around 15A current rating. FQP17P10 and FQP18N20
I am slowing the turn on of the NPN part, and slowing the turn off of the top PNP part with resistor/diode combinations, to avoid shootthrough.
The optocoupler needs 10mA to light the diode, but I am driving 110mA trough the supply cable, and shunting the 100mA. This way I divide the noise by 10, I hope?.
The propagation delay is 500nS, and this should not be a problem, after all, the current zero prediction scheme was developed with this kind of delays in mind. I may have to measure and select matched pairs for diagonal bricks.
It is the CM600 I intend to drive, but I have 1200A/1700V types in stock too =:-o


The schematic is here:



Getting more and more used to surface mount parts, the board layout looks like this:





I will etch a prototype tomorrow, and determine the values of the gate resistor/diode networks, even though this has to be done on IRF540/9530 since the fancy good ones from Fairchild are in the mail.

Comments, advice and even flames are welcome!

Cheers, Finn Hammer
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Finn Hammer
Mon May 25 2009, 08:08PM
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Joined: Sat Feb 18 2006, 11:59AM
Location: Bislev, Denmark
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Today, I have been looking at the gate driver, in particular why it is a good idea to keep cross conduction to a minimum in the totem pole output MosFets of the driver.

If you heep the schematic of last post in mind, I will show some scope shots of the cross conduction currents that happen unless the turn on and off of the output transistors are duly subdued.

In this case, I have chosen to mount 470 ohms gate resistors, and 1n5819 diodes, to tame the fet's.
This has caused the Cross Conduction to drop to:

On the falling edge of the gate signal:



A nice and calm one ampere for 50nS
If I short out the gate resistor, R6 in above schematic, this happens:




Cross conduction rises to 5 amps, and more important, ugly ringing is introduced.

Same thing on the rising edge, with 470 ohms in place:



And with R3 shorted:




It is the ringing that causes me the most concern.

Look at this rather benign gate voltage/current profile, with the resistors in place:



Then relish what the same situation looks like, if shoot trough in the gate driver exists:




This concludes my take on gate drivers: Keep them nice and clean, before attaching an IGBT.

Cheeers, Finn Hammer

One thing that bothers me: I cannot for the love of god yank more than 4 ampere into those gates, even though the gate resistor is 5 ohms. Go figure......
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hvguy
Tue May 26 2009, 12:37AM
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That video looked very promising, Finn. What was the current sweep range (dI)?

I'm glad to see people working with different GD arrangements. It seems everyone was stuck on GDICs for the beginning years of SS work. I too have had unexpected, costly, and seemingly unexplainable failures with GDICs. I stopped using them in most designs. I found the IXYS parts, while quite possibly the best available, where much more susceptible to failure if their rail voltage was higher than their signal input voltage, even though the DS says that should be fine. Their ENA pin also seems very likely to cause a failure if it is noisy.

I am a big believer in GDTs though; you just can't beat galvanic isolation and 0 insertion delay. I usually drive them with either a full bridge of N-CH FETs driven by a tiny GDT and SOGDIC, or a half bridge of P/N-CH parts. Everything is ultra compact SMT, as close to the IGBT as possible.

I'm really looking forward to seeing this system make arcs. Thanks for a good thread...
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Finn Hammer
Tue May 26 2009, 06:00AM
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Joined: Sat Feb 18 2006, 11:59AM
Location: Bislev, Denmark
Posts: 628
hvguy wrote ...

That video looked very promising, Finn. What was the current sweep range (dI)?

snip

I'm really looking forward to seeing this system make arcs. Thanks for a good thread...


The video showed a current sweep from 50A to 500A.

About gate drive: I am having a very hard time justifying my initial desicion to use opto's and separate gate drivers, when it would be so much simpler to build a nice little semi-high voltage bridge, and drive a step down gate drive transformer.
It is no easy task to configure the totem pole output stage of the driver and get both zero shoot trough as well as high current output ability:
When I slow down the turn on enough to keep shoot trough low, the turn on is prolonged so that the ability to deliver current within the time frame of turning on of the IGBT is impaired.
I will try the old trick with zeners on the gate, which you may recall from the days of the beefy driver.

(didn'd help)

What I _really_ need is a tiny NTC resistor, which could delay the ramp up to miller plateau, then turn low resistance there, to allow fast full turn on.

Well, this is fun anyway, and perhaps I should just make that bridge/gate tranny and get on with it.

Thanks for -your- work. It was the work of pioneers like yourself that got me started.

Cheers, Finn Hammer

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