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High Voltage Magnetics: Chokes

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sngecko

Sat Nov 27 2010, 01:36PM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

I anticipate a 120Hz, <=250 Vp-p maximum ripple. Max current will be 160mA. Now, there is the initial switch-on inrush current, in the 10s of amps, and the attendant high-voltage banging, but that should be mitigated in other ways.

monokel

Sat Nov 27 2010, 02:40PM

Registered Member #2981 Joined: Thu Jul 08 2010, 01:47PM
Location: Germany
Posts: 35

I've created 2 diagrams for design of coils with cylindrical core.

These diagrams can be used in the following way. Assume you have specified an inductance L, a maximal allowed RMS value of a sinusoidal current through the wire i_RMS. Further you know the maximal B field B_max (about 0.3 T for ferrites). Then you can calculate the quotient L*i_RMS / (B_max*B_max). This quotient is at the ordinate of the first diagram. Choose a length / diameter value (H/D) (H=length, D=diameter) of the core. Select the curve of the first diagram belonging to this length / diameter quotient. Thus you can read out the length (H) of the coil. In the second diagram you can find the quotient L/(H*n*n) belonging to the quotient H/D. From this numerically known quotient L/(H*n*n) you can calculate the number of windings (n) because L and H are known. The wire diameter is then calculated by H/n. If this diameter is not reasonable for the current, you have to repeat these steps with other values of H/D.

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Edit: I have made a mistake. In the first diagram you have to use the maximal current i_max instead of the RMS current i_RMS. I have replaced the diagram with the correct one. If the current is sinusoidal, i_max=1.41*i_RMS. I have also corrected the values in the above posts.

With a coil length of 25 cm and a diameter of 1 cm, you need 1550 windings for an inductivity of 0.2 H. If the current is 160 mA (RMS), the maximal B field in the core will be about 0.37 T.

sngecko

Sat Nov 27 2010, 06:18PM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

It certainly seems like we're in the right ballpark... I see what you've done there monokel, but I'm not sure how to appropriately perform the calculations if the waveform is DC (20kV) with a ripple component (~250Vp-p).

klugesmith

Sat Nov 27 2010, 06:54PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

As others have said, the 20kV requirement is meaningless to the design of the choke, if the whole choke is HV insulated from the chassis. And OP said 250 Hz, not 250 kHz.
To get an idea of required size, look in catalogs like

Stancor C-2724
250 mH at 275 mA. DCR = 16 ohms. 1.44x2.38x1.25 inches, 0.35 lbs.
This would dissipate less than 1/2 W at 160 mA, so the HV insulation (plastic box?) needn't conduct a lot of heat.

Look up "filter choke" on ebay or online surplus stores; you can probably find something ready to go.

Proud Mary

Sat Nov 27 2010, 09:36PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Klugesmith wrote ...

As others have said, the 20kV requirement is meaningless to the design of the choke, if the whole choke is HV insulated from the chassis. And OP said 250 Hz, not 250 kHz.
To get an idea of required size, look in catalogs like

Surely, Rich, Vpk will appear across the choke over a complete cycle, so the winding insulation must be capable of withstanding that. When the choke's magnetic field collapses, the choke will have a very high voltage induced across it. Indeed, if this did not happen, the component could not be described as a 'choke' at all.

klugesmith

Sun Nov 28 2010, 03:14AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

You can't have Vpk (20 kV) across this choke without di/dt = 100 amps per millisecond.
We need to know more about the "big iron" power supply circuit. Schematic, please?
We know it's not "SMPS" and in my book that excludes boost converter (and flyback).
I assumed mains frequency transformer - rectifier - "C L C" pi filter, though the OP said "LC filter". What would be the point of putting a 200 mH inductor between the rectifier and the first capacitor?

monokel

Sun Nov 28 2010, 05:48AM

Registered Member #2981 Joined: Thu Jul 08 2010, 01:47PM
Location: Germany
Posts: 35

sngecko wrote ...

It certainly seems like we're in the right ballpark... I see what you've done there monokel, but I'm not sure how to appropriately perform the calculations if the waveform is DC (20kV) with a ripple component (~250Vp-p).

The only thing you have to eveluate is the maximal current (I_max). You can use it in the first diagram. But as Klugesmith already said, the max. current would be infinity if there was a voltage of 20 kV DC with a ripple of 250 Vpp at the choke. We cannot know what you want if you don't give us enough or false informations.

Klugesmith wrote ...

As others have said, the 20kV requirement is meaningless to the design of the choke, if the whole choke is HV insulated from the chassis.

Can you explain this a bit more ? Of course one need a HV insolation between the wire and the chassis. But if the max. voltage is about 20 kV then there is a high voltage between the layers of the windings. If there is no appropriate insolation between the layers, the insolation of the copper wire might get a defect.

radiotech

Sun Nov 28 2010, 07:15AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

I think the point could be clarified by considering the 20kV is
never across the layers of windings with the data given.
As an example, if the choke coil had a voltage drop of 200
volts, one end would be at 20,000 volts and the other end
(of the wire of the winding) would be at 19,800 volts.

If, however, the wire broke than you might get 20000
volts layer to layer.

monokel

Sun Nov 28 2010, 07:43AM

Registered Member #2981 Joined: Thu Jul 08 2010, 01:47PM
Location: Germany
Posts: 35

radiotech wrote ...

As an example, if the choke coil had a voltage drop of 200
volts ...

Of course. But without schematic it's difficult to verify if the voltage drop is really never higher.

Proud Mary

Sun Nov 28 2010, 07:50AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Klugesmith wrote ...

You can't have Vpk (20 kV) across this choke without di/dt = 100 amps per millisecond.
We need to know more about the "big iron" power supply circuit. Schematic, please?
We know it's not "SMPS" and in my book that excludes boost converter (and flyback).
I assumed mains frequency transformer - rectifier - "C L C" pi filter, though the OP said "LC filter". What would be the point of putting a 200 mH inductor between the rectifier and the first capacitor?

oh well...

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