Ferrite toroid core as a low pass filter

Thread Starter

myil

Joined May 2, 2020
37
Hi Everyone,

I am currently working on an inverter. I need to filter the output to obtain pure sine wave. I have some ferrite toroid cores. I would like to know if it's possible to use them as a low pass filter. Does it specifically have to be iron powder core?
 

WendellB

Joined Feb 14, 2020
22
Hi Myil,

Perhaps too late for a reply but since you have recieved none, here is mine.

One question can be answered directly. Yes you can use ferrite toroids for low loss inductors in genereral. Ferrite core losses are measurably lower than powdered iron. So your on the right track.

The main issues in using an LC low pass filter in an inverter that has a voltage feedback loop to regulate the output voltage
are these:

(1) First the coil design on a ferrite core because of the DC current passing through the coil requires that the ferrite be physically
gapped to handle the DC flux produced. I usually purchase ferrite cores and specify they be cut in to so I can gap them usually
in a range of 1 to 10 mils; in two places. And use just one layer of copper wire to avoid large proximity effect losses.
Also keep the wire a quarter of an inch away from the gaps to avoid flux fringing loss in the copper wire.
(2) The voltage feedback loop gain and phase will both be affected considerably and so the feedback loop will have to be
adjusted to accommodate the effects and produce a stable loop. And it can be done.
(3) You mentioned you want the load ripple to be a pure sine wave.

To meet your goals you should specify them and model your circuit, hopefully in LTspice to view your inverters performance.
So if you can post a running LTspice schematic then others including I can use it to complete your task.
Then if you can provide the physical size of the toroidal cores and their material specifications we have almost all that
is needed to finish your project.
Finally, if you can define what you mean by a pure sine wave we will have a real performance specification to meet.

I am willing to help, provided you can provide what was requested here. I have been designing switchmode power
converters some with low pass filters to reduce load ripple, Since 1980

My home email, if that will help is: (Email address removed by moderator - forum policy. The address was sent by private message.)

Best regards

Wendell
 

Papabravo

Joined Feb 24, 2006
13,711
Hi Myil,

Perhaps too late for a reply but since you have recieved none, here is mine.

One question can be answered directly. Yes you can use ferrite toroids for low loss inductors in genereral. Ferrite core losses are measurably lower than powdered iron. So your on the right track.

The main issues in using an LC low pass filter in an inverter that has a voltage feedback loop to regulate the output voltage
are these:

(1) First the coil design on a ferrite core because of the DC current passing through the coil requires that the ferrite be physically
gapped to handle the DC flux produced. I usually purchase ferrite cores and specify they be cut in to so I can gap them usually
in a range of 1 to 10 mils; in two places. And use just one layer of copper wire to avoid large proximity effect losses.
Also keep the wire a quarter of an inch away from the gaps to avoid flux fringing loss in the copper wire.
(2) The voltage feedback loop gain and phase will both be affected considerably and so the feedback loop will have to be
adjusted to accommodate the effects and produce a stable loop. And it can be done.
(3) You mentioned you want the load ripple to be a pure sine wave.

To meet your goals you should specify them and model your circuit, hopefully in LTspice to view your inverters performance.
So if you can post a running LTspice schematic then others including I can use it to complete your task.
Then if you can provide the physical size of the toroidal cores and their material specifications we have almost all that
is needed to finish your project.
Finally, if you can define what you mean by a pure sine wave we will have a real performance specification to meet.

I am willing to help, provided you can provide what was requested here. I have been designing switchmode power
converters some with low pass filters to reduce load ripple, Since 1980

My home email, if that will help is: wendell.boucher@gmail.com

Best regards

Wendell
Great post Wendell. You should be reluctant to include your actual email address in an open post. There are web crawlers that will harvest explicit email addresses for the purpose of loading your inbox with spam, You should fuzzify it in some way or ask the TS to PM you for further contact.
 
Last edited:

Thread Starter

myil

Joined May 2, 2020
37
Hi Wendell,

Thank you for your reply. That is the project I would like to use for my bachelor thesis. It would be really nice to have a voltage feedback loop. But, I don't have enough knowledge to design it. Is it possible for you to help with designing a feedback loop?

Kind regards
 

WendellB

Joined Feb 14, 2020
22
Hi Myil,

Yes I can. For starters please look at a YouTube video. The first one of 7 I have created with the
title "Modern Switchmode Power Supply Design, Closing Feedback Loops" In YouTube, simply
type the title above to fetch the video. The free demo version of SIMETRIX/SIMPLIS is used to
demonstrate all known ways, when the video was made, to close feedback loops. The video is
on a friends website for he asked for and got my permission to post it. More of my videos on
my YouTube channel are all educational technical presentations about power conversion
circuitry and their magnetic components. My most recent of them on my channel is entitled:
"Calculating transformer Kcoupling coefficients using inductance measurements"

However, I do use LTspice, as well as the SIMPLIS demo software since it is both free and
very practical, to model my designs and if I am to help you then I would appreciate it you could
create the circuit you have in mind, send it to me as an LTspice file and I will help you to meet
your specifications best I can.

I am a fully retired Senior Electronics Engineer, Practicing Power Electronics design since 1980.
Having fun helping others to realize their ambitions in the field of power electronics which
keeps my mind active as well.

BELOW IS A CALCULATED FEEDBACK LOOP GAIN AND PHASE BODE PLOT PRODUCED
BY THE FREE DEMO VERSION OF SIMPLIS ELEMENTS. THE PHASE MARGIN AND GAIN
MARGINS ARE QUITE ACCEPTABLE.

1589984093504.png

BELOW IS THE ACTUAL CIRCUIT THAT PRODUCED THE BODE PLOT ABOVE. THERE IS AN INNER CURRENT
FEEDBACK LOOP AND AND OUTER VOLTAGE FEEDBACK LOOP.

1589984275202.png



Best regards

Wendell
 
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