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Constant current sink with large inductance has voltage spikes

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mpegjohn

New Member
Hi,
I am designing an inductance bridge for iron cored inductors of around 10H 100mA

I need a constant DC current of upto 100mA, to do this I have a stabilized 100V PSU, that feeds the top of the inductor.

The circuit is a standard current sink using a high voltage Mosfet.

Looking at the drain of the Mosfet, I can see quite large voltage spikes, that seem random, but quite large.

If I replace the inductor with a resistor, all is calm and there is no noise to speak of.

So it does seem to be the fact that this is an inductor. I have tried putting the inductor in a diecast box, to see if it was some form of pick up, but this made no difference.

Any ideas?

John
 

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alec_t

Well-Known Member
Most Helpful Member
Welcome to ETO.
With 10H, any current glitch is going to cause substantial spikes. At the very least a reverse-biased diode across the inductor is advisable to protect the FET.
 

mpegjohn

New Member
Thank you for the reply.
I am unable to use a diode across the inductor, as it will eventually be in a bridge. I intend to put a TransZorb across the Mosfet eventually.
Why should there be a current glitch though? The PSU is fine.

Regards,
John.
 

ronsimpson

Well-Known Member
Most Helpful Member
What is "Ictrl 0-10V"?
If there is any noise in that or the connection you will have this problem.

Why you see noise on the inductor but not the resistor:
I think you have some small amount of noise in the system. Maybe there is a pot or long wires.
If there is 0.1% noise on "Ictrl" then there will be 0.1% (more or less) on the resistor and that might be hard to see.
The 10H inductor is a constant current source, in the AC world. If you have 100mA and, because of noise, you change to 100.1mA, then the voltage across the inductor must change until the current reaches 100.1mA.

You have a capacitor across the op-amp. Increase it size by 10:1 and the spikes you see will change.
You should also have a cap from Ictrl to ground. (o.1uf)
 

MikeMl

Well-Known Member
Most Helpful Member
You have built an oscillator. Note what LTSpice predicts as the input V(in) jumps from an initial value of 5V to 6V. You have a pole in the right half plane...

D5.jpg

The "initial conditions" solution finds a stable bias point at V(in)=5V; the jump from 5V to 6V causes a perturbation that begins the oscillation. In the real circuit, the tiniest bit of intrinsic noise would cause the oscillation even at an input of 5V...

Obviously, I took a WAG at what you are using for the opamp and the opamp supply voltage. Both of the those would effect the tendency for the circuit to oscillate. Note that the average inductor current is as you expect, 60mA :D
 
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mpegjohn

New Member
Thank you all,
I will put a 0.1uF on Ictrl to gnd.
I will also try increasing the cap across the op-amp to say 5nF
 

MikeMl

Well-Known Member
Most Helpful Member
The missing resistor makes no difference.
This one is marginally stable (under-damped, but finally settles to the correct value. Limiting the dV/dt of the input would help a lot...

D5a.jpg
 

MikeMl

Well-Known Member
Most Helpful Member
One more try. This one limits the dV/dt at the input, and I tweaked the compensation some more to improve the settling time.

You still need to post a complete circuit showing the opamp and its power supply details... The 15V supply V2 I show at the opamp is insufficient to drive the inductor to 100mA. It would take a minimum of ~18V.

D5b.jpg
 
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Mikebits

Well-Known Member
Could the inductor be acting like an antenna and just pick up outside noise like from nearby computer? Just wondering. I was thinking maybe some of the spike was just emi and fools ya into thinking it is the circuit?
 

Tony Stewart

Well-Known Member
Most Helpful Member
Hi,
I am designing an inductance bridge for iron cored inductors of around 10H 100mA

I need a constant DC current of upto 100mA, to do this I have a stabilized 100V PSU, that feeds the top of the inductor.

The circuit is a standard current sink using a high voltage Mosfet.

Looking at the drain of the Mosfet, I can see quite large voltage spikes, that seem random, but quite large.

If I replace the inductor with a resistor, all is calm and there is no noise to speak of.

So it does seem to be the fact that this is an inductor. I have tried putting the inductor in a diecast box, to see if it was some form of pick up, but this made no difference.

Any ideas?

John
Hi JOhn I have a few questions .

Do you have any more specs?

1. Do you expect to be able to measure L vs Idc saturation?
2. Do you want to measure linear L?
3. Do you know how to measure flyback inductance from V=LdI/dt?
4. Did you know that good RLC meters use a CC sine wave at different f?
5. Have you researched any working designs yet?
6. Are there any other parameters like SRF , Q, Remanence that are very important?
7. Do you know the value of H vs I and loss factor for the Core steel? Any specs on the steel?

info on MOSFET Avalanche mode

cheers,

Tony
from Toronto
 
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spec

Well-Known Member
Most Helpful Member
Hi,
I am designing an inductance bridge for iron cored inductors of around 10H 100mA

I need a constant DC current of upto 100mA, to do this I have a stabilized 100V PSU, that feeds the top of the inductor.

The circuit is a standard current sink using a high voltage Mosfet.

Looking at the drain of the Mosfet, I can see quite large voltage spikes, that seem random, but quite large.

If I replace the inductor with a resistor, all is calm and there is no noise to speak of.

So it does seem to be the fact that this is an inductor. I have tried putting the inductor in a diecast box, to see if it was some form of pick up, but this made no difference.

Any ideas?

John
Hi MJ,

The symptoms you describe sound like the onset of break-down, possibly due to a combination of voltage, temperature, and current.

You do not state what the inductor characteristics are, particularly the inductor resistance, and you do not state the value of the resistor that you replaced the inductor with, so I will assume that the inductor resistance is zero and that the substitute resistor had a high enough resistance to drop a significant voltage and thus lower the voltage on the drain of the NMOSFET. The later could possibly explain why the symptoms disappeared when you replaced the inductor with the resistor.

100V is quite high, as is 100mA at that voltage and would mean that the IFRH5015 would be dissipating 0.1A * 100V =10W.

The IFRH5015 would require a substantial heatsink to keep it's junction temperature below the specification sheet limit of 150 Deg C at 10W dissipation.

But the main concern is the IFRH5015 secondary break down characteristic. It seems that you would be exceeding the limits as you can see from the safe operating area graph extracted from the IFRH5015 datasheet.
2016_11_23_Iss1_ETO_IFRH5015_SOAR.png

If my assumptions about the inductor are essentially true, you would need an NMOSFET with a much better SOA and I would advise a VDSmax of 250V upwards. Also, if my theory is correct the IFRH5015 would be damaged and should not be used. It is very unwise to use a device on its limits, especially as there are other factors, like junction temperature, rate of current and voltage change, etc, that need to be factored into the data sheet figures.

In general, I would also advise checking your circuit to ensure there is no arcing, especially from the NMOSFET case to heatsink, and ensure that any insulating washer, and thermal compound, is OK for your voltage.

I would be inclined to reduce the voltage to 50V, fit a new IFRH5015, fit a heatsink, if one is not already fitted, and see if the spikes vanish.

spec

DATASHEET
http://www.infineon.com/dgdl/irfh5015pbf.pdf?fileId=5546d462533600a40153561abe001e9c

 
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spec

Well-Known Member
Most Helpful Member
Another possibility is that you may be seeing the spikes on the inductor as a result of the IFRH5015 Drain/Source leakage current, which can be 250uA at high junction temperatures. The leakage current can be very noisy and the high reactance of a 1H inductor would give massive voltage gain, possibly giving, or adding to, the voltage spikes you are seeing.

Just a thought.:)

spec
 

mpegjohn

New Member
Hi everyone,
Thank you all for your help.

Last night I took MikeMI's proposa, and after a bit of messing around I arrived at the attached circuit.

This is fantastic, so quiet.

The Mosfet used is an IRF730 not a IRFH5015, unless they are equivalent.

To answer Tony's questions:
1. Do you expect to be able to measure L vs Idc saturation? ->Yes
2. Do you want to measure linear L? -> Yes
3. Do you know how to measure flyback inductance from V=LdI/dt? -> Yes I do, but this does not apply here
4. Did you know that good RLC meters use a CC sine wave at different f? -> This is for an Owens bridge, bur could be used in an RLC meter. The meter would need to be AC coupled.
5. Have you researched any working designs yet? -> My idea originates from
http://diyaudioprojects.com/mirror/members.aol.com/sbench/owens3.html
However, this was not good enough when I came to use it, as the ripple and changes on the PSU were quite large. So I regulated the supply, and decided to try to tame the Mosfet
6. Are there any other parameters like SRF , Q, Remanence that are very important? -> Later maybe.
7. Do you know the value of H vs I and loss factor for the Core steel? Any specs on the steel? -> The steel is M6 0.35. I do have the graphs for the steel. However this particular choke is one I bought off the shelf.

I am looking at the HP 42841A Bias source as inspiration
http://file.yizimg.com/420922/2014051310240121.pdf


John
 

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spec

Well-Known Member
Most Helpful Member
Hi MJ,

Whoops:banghead:, I took the PMOSFET number from post #5. I didn't notice that you had used the IRF730, which will be fine on secondary break-down grounds, as you no doubt already knew.

spec
 
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