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HV CAPACITOR dV/dt CAPABILITY estimation (or design)

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Newton Brawn

Thu Feb 17 2011, 09:37PM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Hi !

I have made several hi voltage glass capacitors 600~1200 uuF, 6000~10000Vac. They are in service continously for more than 3 years without failure.
The service is a continuos discharge of 6kV, 100~180 Apk, at rate of 120Hz, day and night, over a inductor of 1-2,5 uH, (as a tesla coil), air spark gap. Some calculations has shown that a suitable capacitor shall be rated 250 000 V/us (or more dV/dt capability).

As said, the capacitor is homemade. The dielectic is 6~10 glass plates (2X65X65mm), the plates are two strips of aluminum foil used in the kitchen, 0.020 thick, 43mm wide, 400~600mm long. Foils are inserted in zig zag between the glass plates.
The window glass that I have has relative dielectric constant of 8.8 ~ 9.2.
Always after the construction I proceed with the capacitance measurements with a capacitance bridge in order to check the design. Voltage tests are with gaps 3~6mm.
I do not have any problem with the capacitance calculations.

HOWEVER THE DV/DT CAPABILITY OF THE CAPS is cruel doubt:,
How can I estimated the capacitor dV/dt capability ?
Is the capacitor Dv/dt related to the aluminun foil??
Or it is associated with with the dielectric characteristics ??
Is any formula or rule to predict the dV/dt ??

I really will appreciate any information.

Thanks.

Newton.

Xray

Fri Feb 18 2011, 06:11AM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

I've been wondering about the dV/dt of caps too, but of those used in X-ray heads. In one particular example, I needed to find a replacement cap for the input of a CW multiplier. The cap is rated 2200pf at 6KV. The physical dimensions of the cap are approx 22mm diameter by approx 6mm thick. I searched through Digi-Key's website for a suitable replacement, and found a 2200pf at 6KV ceramic cap, but it has a smaller diameter with a thicker body. I then got to wondering what the difference is between a similarly rated cap that was large in diameter but small in thickness, and one that is small in diamter but large in thickness. Could it be the difference in their rated dV/dt? Could it be their current handling capability? Or is it something else?

Any capacitor experts out there?????

jpsmith123

Fri Feb 18 2011, 07:08AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

Wolfram

Fri Feb 18 2011, 09:35AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Xray wrote ...

I've been wondering about the dV/dt of caps too, but of those used in X-ray heads. In one particular example, I needed to find a replacement cap for the input of a CW multiplier. The cap is rated 2200pf at 6KV. The physical dimensions of the cap are approx 22mm diameter by approx 6mm thick. I searched through Digi-Key's website for a suitable replacement, and found a 2200pf at 6KV ceramic cap, but it has a smaller diameter with a thicker body. I then got to wondering what the difference is between a similarly rated cap that was large in diameter but small in thickness, and one that is small in diamter but large in thickness. Could it be the difference in their rated dV/dt? Could it be their current handling capability? Or is it something else?

Any capacitor experts out there?????

Capacitors in a multiplier for supplying a dental x-ray tube don't see much current (current is propotional to dV/dt), so I don't think you need to worry much about this, as long as the voltage rating is sufficient, and the capacitance is right. I've read through all the ceramic HV capacitor datasheets that I've been able to find, and not a single one had a current rating or a dV/dt rating, so I can't confirm this.

Steve Conner

Fri Feb 18 2011, 10:48AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, because I=C*dV/dt, a capacitor's dv/dt rating is just another way of stating its peak current rating, which is limited by what the electrodes can stand without vaporising or being ripped apart by Lorentz forces.

RMS current rating is something different.

Xray

Fri Feb 18 2011, 05:37PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

Steve McConner wrote ...

Yes, because I=C*dV/dt, a capacitor's dv/dt rating is just another way of stating its peak current rating, which is limited by what the electrodes can stand without vaporising or being ripped apart by Lorentz forces.

RMS current rating is something different.

Based on what you and Anders said, I probably don't need to worry about dV/dt rating since caps in a CW multiplier do not operate with extremely fast rise/fall time waveforms. The waveform used in most "DC" type of X-ray head is somewhat sinusoidal or in some cases, a weird looking square wave. I just became a little perplexed when I noticed the difference in the physical package size of the same rating caps from different manufacturers.

Proud Mary

Fri Feb 18 2011, 06:08PM

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

What about dielectric relaxation time? and dielectric absorption?

Aside from that, no very special circuits are required to produce the voltage and current needed by a conventional dental X-ray tube,
which we can take as a generic 75kV 7mA for a typical fixed installation at full power.

Newton Brawn

Fri Feb 18 2011, 06:11PM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Steve:

Thanks for the help!

You wrote :

"Yes, because I=C*dV/dt, a capacitor's dv/dt rating is just another way of stating its peak current rating, which is limited by what the electrodes can stand without vaporising or being ripped apart by Lorentz forces."

This bring my attention to the foil current density (A/mm2). For small transformers the aluminun allowed conductor current density shall not exeed 2Arms/mm2.
Also following your advise:
I= C*dV/dt >>> dt = C*dV/I
Assuming
C = 670uuF
dV = 6000V
Ipk = 158A

dt=670*10^(-12) * 6000 / 158 = 0.0254 us
repeating at each 1/120 = 8333us

The pulse duty cycle = 0.0254/8333 = 3.048*10^(-6)

As the rms current = Irms

Irms = Ipk * (duty)^(1/2)
Irms = 158*(3.048*10^(-6))^(1/2)=0.275A

And the capacitor foil area cross section S
S = 0.02X43mm=0.048mm2
Them the current density at foil will be=Irms/S
Irms/S = 0.275/0.86 = 0.321Arms/mm2.

This current density is lower tham 2Arms/mm2, so it apears that the foil withstand the peak current without vaporising.

The forces repelling the foils as calculated by Amper law, for 158Apk and 2mm separation between the foils, is in the range of 4-6 newtons/ linear meter. As the foils are sundwiched among the glass plates, secured by the plates, I think the foil will bee not ripped appart by the Lorentz forces.

Maybe the above assunptions are just speculations and not suitable for proper analyses of the situation.

If you and or any other person could provide more comments I will be happy.

Newton

Tony Matt

Sat Feb 19 2011, 01:05PM

Registered Member #3700 Joined: Sat Feb 19 2011, 12:59PM
Location:
Posts: 107

Hi Newton:

How about the voltage rating?

Newton Brawn

Wed Feb 23 2011, 03:50PM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Mary Proud:

To be honest with you I dont have any idea about the glass dielectric relaxation time and the glass dielectric absorption.

If you can advise me how the dielectric relaxation time and the dielecrit absortion affects a 1000uuF x 6000V glass capacitor I will be happy.

The stimated discharge pulse length is about 0.025 microseconds.

Thanks

Newton Brawn

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