Re: Flux density vs time in transformer core

On 19 mayo, 13:34, "mook johnson" <m...@xxxxxxxx> wrote:
<wimabc...@xxxxxxxxx> wrote in message news:ff161920-ad3d-41c9-ac3e-
3a72681e8...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

 <snip>

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.

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 ____       ____ <--------------100uS-------------------->
_____ 2.5uS _____       ____
___|2.5uS|_____|    |_____|
|_________________________________________|2.5uS|_____|     |_____|    |_

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

thanks.

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 = 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
.

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