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Why doesn't my inductor give more kick.


H_man
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Hi, I built the circuit shown below (sorry, I don't have a drawing package to hand). I was hoping to get voltage spikes from the inductor when I had a high duty cycle for the pulses. Unfortunately I just got the inverse of the pulses (which wasn't so surprising), but I thought I'd get more of a kick from the inductor given the fast rise and fall times of the pulses!

Details: Power Supply was giving 15V, Inductor was 0.1mH, Pulses were at under 10Hz and also 15V peak to peak. I had an oscilloscope attached where I have written +V and Gnd.


        |------------- Inductor---------------____________ +V
        |                                                  |
        |                                                  |
        |                                                  |
    Power          Pulsing _|'''|_|""|_------Mosfet
    Supply        Unit                                |
        |                                                    |
        |                                                    |
        |________________________________ |____________ Gnd

Thanks

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Mosfets have a built-in zener diode from drain to source. If you use an IRF540 Mosfet then the zener clamp voltage is about only 100V.
The rapidly rising voltage at the drain feeds back through the Mosfet's internal capacitance to the gate, trying to turn the Mosfet back on and slowing down the voltage rise unless the gate drive has a high current capability. When the voltage rises slowly then it doesn't rise as high.

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That is how a switched-mode stepup power supply works, but it doesn't stepup the voltage very high. For a higher stepup, a transformer is used with this coil as its primary winding, then the secondary winding has many more turns. An auto-transformer with a single, tapped coil can also be used like a car's ignition coil.

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AG

You may want to review power MOSFET construction and operation (i.e. Miller Effect, and body diodes) and give that explanation another go.

What?
Mosfets have a fairly high drain to gate capacitance that is amplified by The Miller Effect. If the gate's source impedance is high then the negative feedback caused by The Miller Effect is strong.
The body diode of a Mosfet has reverse-biased avalanche breakdown like a zener diode. That is the drain-source voltage rating of the Mosfet.
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Mr. Klampfer

On occasion, some of  "your theories", only "loosely" follow fact, and are not 100% correct... for example:

That is how a switched-mode stepup power supply works, but it doesn't stepup the voltage very high. For a higher stepup, a transformer is used with this coil as its primary winding, then the secondary winding has many more turns.



This statement is 100% incorrect. A boost converter (what you call a "stepup" power supply or is know more commonly as a non-isolated flyback) and a isolated flyback converter, which has a "transformer" (really a coupled inductor), are considered to be the same, and you can generate the exact same voltage levels from either. You stated it can't step-up the voltage very high with out a transformer... that's just plain wrong.

The same applies to your MOSFET response above
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Hi Indulis,
I was talking about a step-up power supply that uses a single Mosfet and only a coil, not a transformer. The circuit is about the same as H_Man's circuit and they don't step-up the voltage very high. Cell phones use them to light a string of LEDs. Silicon Chip magazine has a couple of circuits to replace a 9V battery with two AA cells.

I was also talking about most Mosfets and precisely the IRF540, that has a zener body diode in its symbol.

He said his inductor had a low value of only 0.1mH, so since its value is known then I didn't think it was just a couple of turns of wire wrapped around a nail.
You are correct, it might be saturating and limiting the voltage spike.
Maybe he used a 2N7000 tiny low current Mosfet. Who knows?

post-1706-14279143118718_thumb.png

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I'd be cautious calling it a zener!!


I was talking about a step-up power supply that uses a single Mosfet and only a coil, not a transformer


What you said was...

That is how a switched-mode stepup power supply works, but it doesn't stepup the voltage very high. For a higher stepup, a transformer is used with this coil as its primary winding, then the secondary winding has many more turns.


Sounds like you used the "transformer" word to me.

A non-isolated flyback (or if you like "... a step-up power supply that uses a single Mosfet and only a coil...") can generate a voltage just as high as a converter with a transformer (or coupled inductor).

Let's do one better, here are the question's... in "theory" (in a ideal electrical world) how high does the inductive voltage spike go??? Why doesn't it get there, and what are some of the things that limit it??
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To make a high voltage spike from this single coil and Mosfet, you need a high magnetic field in the core, good coupling of the coil to the core and a high di/dt (fast rate of change).

He might not have a high magnetic field in the core due to core saturation, not enough current or not enough inductance.

The di/dt might be limited by a high resistance feeding the gate of the Mosfet, causing its total gate charge to slow down the rate of change. Then the Mosfet will ramp instead of quickly switch.

The voltage spike is limited by the avalanche breakdown of the Mosfet, insulation breakdown or breakdown of air between wires.

The resistance of a load hasn't been mentioned. If it is too low then the voltage spike won't happen.

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To clarify a few points..

The Mosfet used was a 2SK1358 which claims a drain-source voltage of 900V
and a pulsed drain current of 27A.

The power supply driving the pulsing circuit was also connect round to the inductor.

I had an oscilloscope connected across V+ and Gnd, I assume the resistance of the
oscilloscope is in the Mohm range so wasn't worried about losing my pulses that way as audioguru mentioned in his last post.

Would a capacitor somehow be of help?

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Would a capacitor somehow be of help?

most likely, no.

The only way it might help is if a large value one was connected across the ppower supply to smooth out the power voltage, that way, the voltage will be more consistent as the power supply drains.

other than that, I cannot see a capacitor as a helpful device.
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I think the input cable to his 'scope is arcing at about 100V and is arresting the voltage spike.


Even my old Eico 5MHz kit scope I built over 30 years ago could do 1KV, so I doubt and newer scope is limited to 100V.

Since you didn't want to give these a go...

...so with one end of the inductor hanging in space, will there still be a spike??


answer: Yes there will be a spike. The di/dt is from the current going from some level to zero. Again it's... V=L*di/dt As time (t) approches zero, V will approach infinity.

Actually, you don't want a load!!!! Why not?? (this is a semi-trick question)


Answer: A load will not alow the current to stop flowing quickly, and you want it to be as fast a s possible.



If you "plug-in" the numbers, you'll see that 10Hz is way too slow. 10Hz has a period of .1 sec and for a 50% duty cycle, the "on-time" is .05sec  If we rearrange the equation... so  i=(V*t)/L  you get (15*.05)/.1x10^-3=7500A Just a wild guess, but I'd bet that the 15V source can't supply that much current. It's a good bet the inductor is saturated!! To see where the inductor starts to saturate, place a small value resistor in series with the inductor (sub 1 ohm). If you look across the resistor with your scope, you should see a voltage ramp whenever the MOSFET is on. The ramp should be linear. If it starts out linear and then starts to "tail-up" then you are on the edge of saturation. If you have saturated your inductor, you now just have a piece of wire in series with a "on" MOSFET across your power supply... in other words a short!!! The Rds on of your 900V MOSFET is around 1.1ohms, so to a 15V supply that's a pretty good short... around 13.5A worth. I wouldn't think your supply can suport 13.5A!! Increase the switching frequency so that the voltage ramp across the resistor on longer tails up and see what you get then. If you know how much current your 15V supply can source, you can calculate what the minimum frequency is that you have to switch at.

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The bloody thing works now, Hurray  ;D ;D ;D ;D ;D

I changed the the Mosfet to an IRF830 (don't know if that helped).

I just connected it up this morning after going over the circuit last night and it has come to life.

The input pulses are 13V and output spike are >150V. I am now going to make a bigger inductor for bigger spikes  :D

Thanks to everyone for their suggestions, I found it reassuring to know that my circuit wasn't nonsense.

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