Flux density vs time in transformer core

Remenance depends on what you did with it earlier. If all you're doing is sending this 800G stuff through it, then it has to be less than that peak. Maybe 200G let's say. AFAIK, the hysteresis loop scales to whatever you apply to the thing.

I'm not sure if it remains biased (the center only moves when you hit saturation on one end or another, giving it something to "push against") or if it levels out after a few cycles. This can be phrased alternately: what is the smallest signal strength (in this case, magnetic signal strength) you can degauss a magnetic material with? Degaussing works best starting with a large signal then reducing slowly, so it's probably the former case.

If that's the case, then it may be that your core has a static bias of, say, +/-1000G or so on it, and your signal simply rides on that. It's no big deal either way, since DC flux doesn't matter.

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So you've got a waveform something like this? (Probably without the squigglies...)

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Now, that one you can measure directly. After 100us with the terminals held shorted, you'll still be carrying some current- it'll only have decayed through winding resistance. If current is small to begin with (as in the above waveform), then current will end up small and B will be back somewhere around Br, yes.

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Shouldn't be, sounds like DC is zero. If not, find what DC current is flowing and check if that's enough to saturate it.

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Hello,

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When there is no resistance in the wire, the magnetizing current through the primary will not change (with respect to 15V/2.5us pulse), as there is 0V across the coil. So flux will not change and remains the value at the end of the 15V/2.5us pulse. So when you give a same polarity pulse after the 100us pause, the flux will add.

In reality there will be some resistance. So you should determine the L/R ratio for the primary to find the current decay versus time. When L/R is large with respect to your 100us pause, there will be still current through the primary so there will be more flux (than Br) in the material.

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When the first two pulses (same polarity) does not show saturation, but next pulses will, you can be sure that L/R is to high so that there is still magnetizing current in the primary at the beginning of the pulse. In fact you are applying a DC component with unipolar pulses, where the average current is limited by wire resistance.

n

I checked the datasheet for the ferrite and my calculation gives flux change of 1100G (0.11T) after each 2.5us/15V pulse. The material datasheet says Br=3D1100G. So 2 same polarity pulses would result in about 3300G. That is rather high as, due to the core shape, you get non-uniform flux density (more flux a the inner side of the core).

Best regards,

Wim PA3DJS

remove first three letters of alphabet for PM.

The primary is driven with a dual FET driver with one output on each end of the primary. The drivers have an output resistance of about 2 ohms each. The primary has ~.2 ohms of DCR. I calculate the primary minimum inductance at 768uH so the magnetizing current should be di = E * dt / L = 15 * 2.5uS /

768uH = ~49mA. 49mA * 4ohms (driver impedance) = 200mV. so again di = .2V * 100uS(both ends low) / L = ~ 26mA... not reset. actually it will be current change as the 200mV drops as the magnetizing current also drops.

I fixed the problem by shortening the pulses to 1uS instead of 2.5uS. Now it only goes up by 19mA. But the voltage during the off time is now less....hmmm..

My original situation is as shown below. Dot and no dot ends are driven between 15V and 0V by the fet driver. The pulse width is 2.5uS at 50% dutycycle. It ends with the dot end high and the no dot end low. Then the

100uS gap is when both Dot and no dot end are held low. Then the first pulse after the gap is the Dot end going high for 2.5uS. (in the same direction as before the 100uS gap). I get the saturation at the end of that first positive pulse 2.5uS pulse on the dot end. Then the negative pulse kicks it out of saturation and then the next positive pulse shows a little bit of saturation then it is normal for the remainder of the cycles.

Did I fix it by going to 1uS wide pulses? The scope is happy but I'd like to better understand what caused it in the first place so it doesnt reappear during a condition I'm not currently testing.

Dot end _____ 2.5uS ____ ____ _____ 2.5uS _____ ____ ___|2.5uS|_____| |_____| |_________________________________________|2.5uS|_____| |_____| |_

No-dot end ___ _____ _____ _____ _____ _ |_____| |____| |____________________________________________________| |_____| |____|

thanks.

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Hello,

Is there any possibility to add in series two Shottky diodes anti- parallel. You will lose about 600mV out of the secondary (in case of

1:1 ratio), but gets about 400mV counter EMF to have a sure decay of the primary current. Based on 2.5us/15V, the current will be zero after about 2.5u*15/0.4 =3D 100us. So you can keep your 2.5us pulse in this case.

When in "pause" (100us period), the secondary output will show an output voltage equal to the diode drop of the schottky rectifier.

In case of your revised circuit, I would check at low and elevated temperature and pulse width extremes to make sure you are not working on the edge of the safety margin (as you mentioned). I do not have real experience with the P type material, so I don't the reduction in effective saturation flux density at narrow pulse width.

Best regards,

Wim

You might also experiment with capacitive coupling.

RL

Fixing the diagrams.

Saturates here

v _____ 2.5uS ____ _____ 2.5uS _____ ____ |2.5uS|_____| |________________________|2.5uS|_____| |_____| |_

No-dot end _____ _____ _____ _ |_____| |___________________________________| |_____| |____|

I was wrong, the core is the P41003 not the P41005. Yup thought about the back to back diode thing might give it a go it I find room.

Legg, are you talking about capacitvely coupling to the transfomrer primary?

. n

It's a transformer, right? Not just an inductor? So, to tell what the flux induction is, you need to SUM THE PRIMARY AND SECONDARY currents. The analysis presented is all on the primary side. The primary waveform doesn't have any obvious reason to saturate the core, so probably the secondary side has a net DC current component.

Capacitively coupling a drive transformer is one of the simplest ways of avoiding saturation and allowing full 2xBsat flux swing, if your drive signal cannot reverse in polarity.

There are issues, particularly when pulse trains are intermittent or interrupted. Scope it first.

RL

Yes, to get rid of the d.c. bias in your drive signal.

Your L/R time constant at the primary is over 380uS(!); that's why the primary current doesn't fully decay during your 100uS off-time.

Cheers, James Arthur

The secondary is a full bridge rectifier (4 diode) and ~600 ohms of resistance.