Feedback CT theory

[Jesperb123]

Hi!
How does one decide what core size to use for the feedback CT and current CT? I know that the turns-ratio is important for determining the output current (which is passed through a burden resistor of appropriate size), but how do I make sure that I don't saturate the core?

I've seen this equation which I used when constructing the GDT:

But this doesn't seem very useful in regards to the feedback/current CT since the induced voltage in the CT is dependent on the magnetic field due to the primary current. The ring cores I have are made of T38 material with a saturation flux density of 0.430mT. Their AL value is 6070nH/N^2. As of now my primary CT looks like this:

There are about 21 turns on both cores for a 1:400 reduction (I'll have 400A peak current in primary circuit). I was thinking of using a 3ohm burden resistor.

//Jesper 

[Hydron]

You'll be very hard pressed to saturate a core in this type of configuration unless you pick a rather large burden resistor value. Core material shouldn't be at all critical, and with 3R burden and frequencies in the 10s or 100s of kHz I simply wouldn't worry about it at all.

Ideally you'd measure it with a known-good CT (e.g. a Pearson current monitor) to compare once the coil is running, but these cascaded CTs work _very_ well for our application, so realistically you can just rely on them working unless you use something awful as the core material (e.g. something with a distributed air-gap designed for use as an inductor). Your ring core choice should work just fine.

[Jesperb123]

Great! Then I'll use these cores. How many turns do you think would be appropriate for the feedback CT? I believe the UD 1.3b clips the input waveform at +- 5.3V so I assume voltage across the burden resistor should much be greater than this value (so the signal has a large enough amplitude even at low primary currents?)

Optional question : why is it that we're not going to saturate these CT's as compared to the GDT core?

Unfortunately I don't have any good CT's on hand, so I hope these will do!

[davekni]

Yes, the formula you quoted:  B = V * t / (N * Ae) applies to CTs too.  As Hydron said, your CT looks fine, so there is no need to worry.  However, it is great to see that you are interested in the theory.

For a two-stage CT, there are two cores to consider.  For UD1.3, the feedback CT has a square-wave voltage of roughly +-5.2V due to the input zener clamping.  "t" in the above equation is 1/4 cycle for a square wave and "V" is slightly over 5.2V due to winding resistance IR drop.  This applies to the second-stage core connected to UD1.3.  The first stage sees 1/20th the second-stage voltage (if the second stage is 20-turns) plus its own winding resistance.   Current in the first-stage winding is 20 times higher than in the second stage, so IR drop can be significant.  IR drop is sine-wave, so "t" is one radian rather than 1/4 cycle.  In your case, if peak current is 400A and both cores have 20 turns, there will be 20A peak in the first-stage winding.  If wire resistance is 0.2 ohms, that is 4V.  As Hydron said, you'll be fine.  It is all below GDT voltages.  (The typical 51-ohm burden resistor of UD2.7 can lead to higher voltages.)

400:1 total ratio should be fine.  I think UD1.3 input zener diodes are 5W rated, so can handle the resulting 1A current.

[Jesperb123]

Hi David! thanks for the thorough reply.

For UD1.3, the feedback CT has a square-wave voltage of roughly +-5.2V due to the input zener clamping.  "t" in the above equation is 1/4 cycle for a square wave and "V" is slightly over 5.2V due to winding resistance IR drop.

Why is it that "t" is only 1/4 cycle? Shouldn't it be 1/2 cycle if the dutycyle is 50%?

400:1 total ratio should be fine.  I think UD1.3 input zener diodes are 5W rated, so can handle the resulting 1A current.

So no burden resistor is used for the feedback CT?

If the current in the first stage is 20A peak I assume the first core needs thicker wire to handle the power dissapation (using cat 5 wire now). In addition to this, I was thinking of "cascading" the third core onto the first core (I only have three cores in total). Would this arrangement double the current in the primary core wires (40A)?

[Hydron]

Using a single "primary" core and two secondary cores works fine, I have done it myself (actually with 3 secondary cores!). It will not increase the current in the wire on the first core - the only effect would be a tiny increase in flux due to the reflected voltage of the extra core, certainly nothing to worry about.

As for current handling, it really depends on the duty cycle you are running. For a low-power DRSSTC without pulse-skipping etc it probably won't be an issue - there should be enough thermal mass in the wire to deal with the peak power, and the effective RMS current would be maybe 2-3A for duty cycle of ~5%, not tiny, but not enough to melt things either.