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Author Topic: Magnetic CARA - Proof of Concept  (Read 17093 times)

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...and D1 and Q1 should prevent C2 from getting charged when S0 is pressed.
Is D1 ok and polarized correctly?

Right, like can be seen in the below drawing, all that is red highlighted is at the same potential.

I just measured D1 with my Fluke 179 DMM in the diode test setting, it measures 0.9V in the forward direction only (kind of high,  right?)
and it is polarized the way it is in the diagram.

I have some other diodes i can try.

Regards Itsu
   

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I just measured D1 with my Fluke 179 DMM in the diode test setting, it measures 0.9V in the forward direction only (kind of high,  right?)
For a SiC Schottky - yes.
For a  Si Schottky the forward voltage drop should be around 0.2V
For a normal Si diode the forward voltage drop should be around 0.6V
   

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Hmmm,  thanks,    i just found 2 other scs106AG diodes, one new from the shipping bag, and they all measure a forward voltage of 0.9V.

I will try that new one first, else swap to some other diodes.

http://www.farnell.com/datasheets/1384101.pdf

Regards Itsu

   

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I just found 2 other scs106AG diodes, one new from the shipping bag, and they all measure a forward voltage of 0.9V.
Yup, SiC diodes are great but they have a high Fv.
That 0.9V is correct according to your datasheet.
   

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Don't try this at home!!    It will smash your pot cores!    :-\

I was doing some tests with just lifting the upper pot core half with my hand a few mm.
After 5 or 6 slams,  this was the result.

I could take some voltage / current measurements now,  but the current only shows up when leaving the 30V PS connected.
When removing the 30V (leaving the cap filled) there is no current seen, i guess the energy in the cap is not enough
to supply enough current to be visible on the scope.

Video here: https://www.youtube.com/watch?v=B7Nl2gLteZ8&feature=youtu.be

Regards Itsu
   

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Sorry this happened to your core :(

The pulse appears to have too long of a duration in respect to the L/R constant Tau - the red current waveform on your scope does not exhibit any ramping up (but it should), it just shoots up instantaneously (probably up to the V/R limit), which suggest that the time scale is too small.   I don't mean the pulse repetition rate is wrong.
When you get the time scale right, then you will see the waveform across the CSR have the shape of the yellow dashed line depicted below ( when powering directly from the power supply ).  
However, when powered only from the C1 cap, the waveform across the CSR will have a shape resembling a quarter cycle of a sine wave.




Also, regarding the yellow trace: Through what is C2 discharging after it becomes charged?  The scope probe?  D1 reverse leakage current?

Finally, the stray inductance of the long connecting wires (and their large loop areas) might attenuate short pulses in your setup.  Also, stray inductance is the well known cause of spikes.
   

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Sorry this happened to your core :(  

No problem,  i have still another one and will order some more.

Quote
The pulse appears to have too long of a duration in respect to the L/R constant Tau - the red current waveform on your scope does not exhibit any ramping up (but it should), it just shoots up instantaneously (probably up to the V/R limit), which suggest that the time scale is too small.   I don't mean the pulse repetition rate is wrong.
When you get the time scale right, then you will see the waveform across the CSR have the shape of the yellow dashed line depicted below ( when powering directly from the power supply ).  
However, when powered only from the C1 cap, the waveform across the CSR will have a shape resembling a quarter cycle of a sine wave.

Thanks, i will try to use a bigger cap (i have 4 of those 0.47uF caps) and try to use a smaller pulse

Quote
Also, regarding the yellow trace: Through what is C2 discharging after it becomes charged?  The scope probe?  D1 reverse leakage current?

Good question, it was first because the DMM (voltmeter) was across it, but now i need to test to see how long the charge remains without D1, without the probe etc.

Quote
Finally, the stray inductance of the long connecting wires (and their large loop areas) might attenuate short pulses in your setup.  Also, stray inductance is the well known cause of spikes.

Right, i will tidy up the circuit,   thanks.

Regards Itsu
   

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Quote
Quote
Also, regarding the yellow trace: Through what is C2 discharging after it becomes charged?  The scope probe?  D1 reverse leakage current?

Good question, it was first because the DMM (voltmeter) was across it, but now i need to test to see how long the charge remains without D1, without the probe etc.

Its the scope probe, when i leave it on, the 30 V drops to 0 within 10 seconds, else it hold the charge for a long time.
Both scope probes (Owon and Tek) cause this.

Regards Itsu
   

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New setup on a pcb, now using 4x 0.47uF capacitors parallel as C1.
Also some switches S0 / S1 (across C2) are added.
First testing the MOSFET signals, with minimum (width) pulse input from the FG, see screenshot 1

Blue is the FG signal
Yellow is the gate signal
Purple is the Drain signal

Video here:  https://www.youtube.com/watch?v=xF0wC3I-GSc&feature=youtu.be

Lateron i used the same setup as above, but now measured the known points (across csr and C2), see screenshot 2
This shot was taken with Switch S0 activated!! (no current seen when S0 is open after charging C1)

Purple is point C to A (C2)
Yellow is point B to A (CSR)


Regards Itsu
   

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Finally for today, i used my current probe (only probe attached), just above the 0.1 Ohm csr.

The first screenshot is with the 30V PS connected (s0 switch activated), and current controller set to 5A/div.

The second screenshot is without the 30V PS connected (s0 switch deactivated), but C1 caps charged, current controller
set to 100mA/div.

This to me shows 2 completely different signals which i cannot explain.
It looks to me that with the S0 switch deactivated, we miss something (a completed circuit?) which cause
only a floating ringing signal instead of a polarised current.

Last 2 screenshots show the comparison between the current probe (green) and the voltage across the CSR (yellow).

3th screenshot, again with 30V PS activated, current controller set to 5A/Div.
4th screenshot with 30V PS deactivated (C1 charged), current controller set to 500mA/Div.

Regards Itsu
« Last Edit: 2015-02-08, 21:03:53 by Itsu »
   

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This to me shows 2 completely different signals which i cannot explain.
Seems like a difference between the transient response of a series LR circuit vs. LCR circuit.  The latter (L1+C1+R1) is periodic when underdamped.  When S0 is closed then C1 stops participating, leaving only L1 and R1 in the circuit. (C2 participates only during the first ¼ of the cycle, anyway).
We should be looking at only ½ time period of the LCR cycle of oscillation (70μs/div if the time base of those scopeshots was set to 4ms/div).  Anything longer than that does not interest us.
« Last Edit: 2015-02-09, 15:08:26 by verpies »
   

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Some further testing.

Still the same circuit, see diagram
C1 = 1.88uF
C2 = 9nF
D1 = Sic diode SCS106AG
Q1 = IRFP260N
U1 = UCC37321
L1 = 6mH (2 halfs clamped),
     170uF (upper half removed)

Screenshots are:

yellow: FG signal
Blue:   gate signal
purple: Drain signal
green:  current at R1 (controller set at 500mA/Div.)


1st screenshot @10us, upper half pot core removed
2nd screenshot @20us, upper half pot core removed
3th screenshot @10us, both halfs together
4th screenshot @20us, both halfs together
5th screenshot is same at the 4th, but now the current controller is set at 50mA/Div.
There is also a ramp up current, but mostly obscured by the ringing signal


Video here:  https://www.youtube.com/watch?v=q6A44p2Xzrs&feature=youtu.be    


Regards Itsu

« Last Edit: 2015-02-10, 22:52:35 by Itsu »
   

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L1 = 6mH (2 halves clamped),
         170μH (upper half removed)
What immediately jumps out at me is that the inductance with the two pot core halves clamped is 35x higher than with one half of the pot core removed.

This means that the L/R time constant is also 35x longer with the two halves clamped and because of that the pulse width of the signal generator and the time base of the scope should also be increased 35x when viewing the activity with the two halves of the core clamped together.

What worries me is the current flowing through R1 after the MOSFET opens.  There should not be any.  All of the energy accumulated in L1 should be transferred into C2 through D1 in ¼ of the cycle formed by L1C2 oscillation, after the Q1 opens.
At 250V Q1 is breaking down from drain to source.  The current flowing through L1 is approximately 1.9A when Q1 opens.  This means that C2 (9nF) must become charged to 260V in order to absorb all the energy that this 1.9A current in L1 (170μH) represents, because V = iMAX *SQRT(L / C) and Q1 can withstand only 200v.  Poor MOSFET :(

Solution to Q1's D-S breakdown: Increase C2 or increase V(BR)DSS of Q1 or decrease +V1A so iMAX does not exceed 1.4A



« Last Edit: 2015-02-11, 20:25:04 by verpies »
   

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Nice analysis, hopefully we get some answers.


I made some changes to the circuit,

first of all i changed the D1 diode for an IDH12SG60C
secondly i used another coil, this is a single coil on a single bobbin, it measures:

no ferrite:           3.8mH
bottom ferrite only:  11mH
Both ferrite halfs:  110mH

Now i do not see any difference in current with the upper half removed or not.
Also the current trace is different, no ramp up signal, only a short peak see screenshot.
I had reinstalled the old coil and then the ramp up signal (with upper half removed) is back, so its due to this new coil

Video here: https://www.youtube.com/watch?v=9EiqM7ror-I&feature=youtu.be     (uploading still another 20 minutes).

Regards Itsu
   

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Secondly I used another coil, this is a single coil on a single bobbin, it measures:

no ferrite:           3.8mH
bottom ferrite only:  11mH
Both ferrite halfs:  110mH
So, assuming the series resistance of this coil's circuit is 10Ω (including R1 and RDS(ON) of Q1), then the time constants (L/R) of these LR circuits will be, respectively:
380μs for the 3.8mH inductance
1.1ms for the 11mH inductance
11ms for the 110mH inductance

These are the times needed for the current to reach 63% of the V/R limit - which is 3A if a 10Ω circuit is supplied with 30V.
You are however pulsing it with only 10μs, so during that time, the current through the coil will only reach:

2.6% of 3A (or 78mA) for the 3.8mH inductance
0.9% of 3A (or 27mA) for the 11mH inductance
0.09% of 3A (or 3mA) for the 110mH inductance

Thus, it is not surprising that you are not seeing a clear current ramp at this pulse width and time scale.
To see it well, the pulse width has to match the time constant L/R ...at least approximately.
   

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Right,

i tried to go down on the pulse, but hit this "flip over" bug of my FG (at 14Khz), so will have to try one of your solutions.

Regards Itsu
   

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I don't know if these thoughts are exactly applicable to your experimental system, but here they are anyhow, just in case. If it's too off-topic or mistaken please ignore, I'm just trying to come up to speed on a couple of the projects here.

You could try extracting some energy from the moving core by electromechanical means, like having it strike or bend a piezoelectric element as it moves. This can be surprisingly effective as I found out while playing around with my "MescalMotor" bipolar linear pulse motor.

I like the spring suspension mentioned on the previous page. The mechanical resonance frequency can be set by varying the spring constant and/or the moving mass. You could match the electrical and mechanical resonances and get maximum amplitude of the core movement and maximum power transfer to the piezo element bending.
Or you could use the driven core motion to drive another coil-core set and extract electrical power from that, ala QEG parametric oscillator principle.


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"The easiest person to fool is yourself" -- Richard Feynman
   

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Yes, I was planning to tackle the measuring and converting the kinetic energy of the core, once we obtain a stable circuit for driving and recovering the EM energy from L1.

So far driving the S1 & S2 switches as a MOSFETs from a common voltage level still presents a difficulty ...and I have doubts about oscillations of the drain voltage after Q1 opens and D1 stops conducting as well as the negative/reversing current through R1 after Q1 opens.
« Last Edit: 2015-02-12, 19:55:40 by verpies »
   

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I went down with the pulse time (using pulse instead of square wave) to 500Hz (2ms) and now the current ramp up is visible again.
The MOSFET voltage was down to 15V to avoid destroying the MOSFET.

Both pot core halfs are together (110mH @ 9 Ohm).

yellow: FG signal
Blue:   gate signal
purple: Drain signal
green:  current at R1 (controller set at 10mA/Div. so same is what is showing in the screenshot)

Regards Itsu

   

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Lowering the pulse time even more (150Hz (6.66ms)) see screenshot 1 shows that the core gets saturated
When going to 100Hz (10ms) the current peaks to 1.8A, see screenshot 2


Regards itsu
   

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Lowering the pulse time even more (150Hz (6.66ms)) see screenshot 1 shows that the core gets saturated
Good analysis.  This tells us how much amp*turns that core can withstand.
That's for two core halves clamped together, right?

When going to 100Hz (10ms) the current peaks to 1.8A, see screenshot 2
Yup, you can begin to see the asymptotic approach to the V/9Ω limit
   

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Good analysis.  This tells us how much amp*turns that core can withstand.
That's for two core halves clamped together, right?

Right.

Quote
Yup, you can begin to see the asymptotic approach to the V/9Ω limit


Finally for tonight, i changed the MOSFET for a IPI90R500C3  (900V, 24A pulsed, Rdson 0.5 Ohm)
which should be able to handle the 500V pulses see screenshot (30V on the drain, @ 500Hz (2ms) pulse)

Regards Itsu



   

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Everything looks like it's supposed to, until the Q1 turns off.

What is going on after that?  All of the energy should be permanently trapped in C2 behind the D1 diode after ¼ of the L1C2 cycle !
So where is the energy for these oscillations coming from ?

Is Q1 getting turned on by the Miller capacitance?
What is the value of the "R Sink" gate resistor ?

These questions need to be answered if we are to have a decent control of this circuit.

   

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I think it looks like it is supposed to, after Q1 turns off too.

I believe that the oscillations are between the inductance and the remaining capacitances in the circuit, like the mosfet's own capacitance and the parasitic capacitance of the wiring. The energy comes from the turn-off spike (which comes from the input energy in the first place) and it is the _same_ energy being sloshed back and forth between inductance and capacitance. That is, each separate spike represents the same bit of energy, not new energy, and it is dissipating at a rate that results in the decreasing amplitude of the spikes over time. A little of the energy is lost in heat and radiation with every "slosh" so the spikes decrease. The current reversals show the direction of the "slosh": from inductance to capacitance, or the other way around. The voltage of the sloshing is clipped on the bottom by the diode blocking so the sensed voltage at the probe doesn't go below the zero baseline. 

That's what I think anyhow. You might compare the case with the diode shorted by a short jumper and see if the drain voltage trace ringdown gets more symmetrical around the zero baseline.


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"The easiest person to fool is yourself" -- Richard Feynman
   

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Everything looks like it's supposed to, until the Q1 turns off.

What is going on after that?  All of the energy should be permanently trapped in C2 behind the D1 diode after ¼ of the L1C2 cycle !
So where is the energy for these oscillations coming from ?

Is Q1 getting turned on by the Miller capacitance?
What is the value of the "R Sink" gate resistor ?

These questions need to be answered if we are to have a decent control of this circuit.



The ucc37321 MOSFET drivers "R sink/source" gate resistor used is 10 Ohm.

Regards Itsu
« Last Edit: 2015-02-13, 09:16:53 by Itsu »
   
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