impedance at 80MHz for a EE5.0 common mode choke

[szhighstar]

I am designing a commone mode choke, core is EE5.0, two winding,DCR of each winding is 2.05ohms MAX,inductance of each winding is from 0.34mH to 1.129mH,impedance is 1100 ohms MIN at 80MHz, DCR and L are not any question for the design, however it is very difficult to meet 1100 ohms MIN at 80MHz , would you please kindly advise how to increase impedance at high frequency! thanks!

 

[bob]

check this site
http://www.actown.com/download/Catalog/Oem_Design_Guide.pdf

I assume each of your inductors is tunable from 0.34 to 1.129 mh. If the coils are in series, the theoretical impedance at 80 mh = 1134 ohms. It seems that you need to increase the upper value of the inductance to some value like 1.20 mh or higher, which gives you a max value of 1205 ohms.

 

[szhighstar]

Bob,thanks for your comment, we test inductance for each winding, not series, we use high permeability core, inductance is not problem, it is main question about impedance, can you advise how to calculate impedance at 80MHz frequency? thanks!

 

[bob]

I am designing a common mode choke, core is EE5.0, two winding,DCR of each winding is 2.05ohms MAX,inductance of each winding is from 0.34mH to 1.129mH,impedance is 1100 ohms MIN at 80MHz, DCR and L are not any question for the design, however it is very difficult to meet 1100 ohms MIN at 80MHz , would you please kindly advise how to increase impedance at high frequency! thanks!

The basic formula for inductance is as follows:

To calculate reactance:
XL = 2piFL
2pi = 6.28
F = frequency in hertz
L = inductance in henrys

So for your problem, XL = 2pi x 80x10⁶ x 1.129 x 10^-6 = 567 ohms. If you are only using 1 coil, there is no way for you to have an impedance of 1100 ohms. If you put the coils in series you would have 2 x 567 = 1134 ohms in theory. I am not sure of what you are trying to accomplish so I am at a disadvantage as to what you need.

 

[gar]

120719-2339 EDT

Assume you had a distributed capacitance of 1 pfd and a choke inductance of 1 millihenry, then what is the self resonant frequency of this parallel combination. 10 pfd might be more realistic. 10 pfd at 80 MHz is about 200 ohms.

You need to experimentally measure the impedance of your choke vs frequency.

A resonant circuit at 80 megaHz might be 20 pfd and 0.2 microhenrys.

Get a Shure Brothers Reactance Slide Rule.

.

 

[szhighstar]

I test impedance by parallel two windings, I made some samples, some of them are good,some of them are not good, it seems capacitance effects the impedance,however I did not look for root reason.

 

[bob]

As you can see by the formula, the impedance of the coil goes up with frequency and the impedance of a capacitor goes down. At 80 mhz, everything will affect the circuit.
Here is more information on RLC circuits. You calculate the impedance as follows:
XL = 2piFL coil
XC = 1/(2piFC) capacitor

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/rlcpar.html
Are you trying to design a filter ckt? You said you are testing impedance using two parallel coils. You will a capacitance between the coils and the reactance for the 2 coils will be about 1/2 the reactance of 1 coil since they are in parallel. Give more details of what you are trying to do.

 

[szhighstar]

Dear Bob, good explanation from you, yes, I am making some samples to test, I select difference turns and difference wire diameter, difference wire type and difference core material, hope to find suitable materials and technology, I guess it maybe has a sutiable point, inductance and DCR and Capacitance are compatible in a point, so that creates high impedance.

 

[steve66]

The basic formula for inductance is as follows:

To calculate reactance:
XL = 2piFL
2pi = 6.28
F = frequency in hertz
L = inductance in henrys

So for your problem, XL = 2pi x 80x10⁶ x 1.129 x 10^-6 = 567 ohms. If you are only using 1 coil, there is no way for you to have an impedance of 1100 ohms. If you put the coils in series you would have 2 x 567 = 1134 ohms in theory. I am not sure of what you are trying to accomplish so I am at a disadvantage as to what you need.

mH is millihenries, so it should be 1.129 x10 -3

That should make the calc give yu a much higher XL. But as Gar mentioned, you will have to measure it due to the parallel capacitance.