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Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Being a novice in this field myself, allow me to relate what I have learned from the 2 Steve`s
First I measured the secondary resonant frequency, let`s say it is 55kHz, and call it Fressec. Next, I used fanTC to calculate the coupling of the coils, let`s say it`s K=0.24
FanTC ->
Then I got familiar with the fact, that a mutually coupled coil system like this, has 2 resonant frequencies, called the lower and the upper pole. The frequency of the lower pole is Fressec / sqrt (1+K) The frequency of the upper pole is Fressec / sqrt (1-K) In the case of the example I have shown above, the upper pole turns out at 63kHz and the lower pole at 49kHz.
A good starting point would be to tune the tank circuit to the lower pole, that is to 49kHz, ant then run the oscillator at the frequency of the upper pole, that is 63kHz.
Get your scope, and use a differential probe to display the voltage at the output of your bridge, and a current monitor on the leads to the primary coil. Now pray you have equal phase shift from both transducers, and check these 2 signals against each other to see if you really have zero voltage swiching. Getting reliable results from measurements is an art form in its own right, so use a grain of salt here.
Hope this helps, or that at least it is not misleading.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Part Scavenger wrote ...
Ok, thanks, I guess that's good.
How do I know if I'm in tune, and whether or not my IGBT's are getting stressed?
Well, being in tune is kind of a misnomer. There is a wide range of tuning that will still provide good results in a DRSSTC. However, it has been speculated that tuning to the lower pole (remember, there are two pole frequencies with a coupled resonant system) is better since it both reduces operating frequency (lower switching losses), and also makes any delays in the feedback less critical since the period is longer.
The only way to determine if the IGBTs are being stressed is to measure the current in your primary. You should be able to construct a simple current transformer and couple it to your oscilloscope to measure the peak current.
Registered Member #79
Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673
I'd like to say that I'm VERY happy with the controller circuit and the H-bridge! I just can't seem to get the coil tuned though. I am putting though 200A and getting miniscule sparks. I think I need to add another turn to my primary if my calculations are correct to tune to the lower pole. I should be able to get the high pole though.
I guess I am getting ZVS, because I have had no problems. Please take a look at these scopeshots. Anyone forsee any problems?
You guys were right about the unloaded gate drive. Here's what I was freaking out about on the other forum:
Now it looks like this:
Here's a shot of a ground strike at 200A
Something I've noticed. When you're way out of tune, ground strikes cause the primary current to get lower. When close to tune, it skyrockets. I guess that it begins to transfer energy much more effieciently, but I don't see why that would cause the out of tune strikes to lower the current.
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DRSSTC tuning
I should be able to get the high pole though.
