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Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
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.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
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.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
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 .
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
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.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
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.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
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.
Registered Member #154
Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 260
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?
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
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!
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