LMR33640: Fly-Buck Converter with inverting power supply Problem

Hello

This device is not designed for fly-buck operation.

It is best if you can select a device with a FPWM mode.

I am not familiar with a dual winding inductor; you may wish to find a coupled

inductor that is meant to be used in fly-buck operation.

Do you have a ground plane under the layer shown ??

I don't see the ground connection for the VCC capacitor ??

Can you provide waveforms of the SW node and inductor current ??

Thanks

Hello Frank De Stasi
First of all thank you for your fast response to my problem. I drew this circuit inspired by the forum and www.ti.com/.../snva790 application note. I made Fly buck circuit for the first time and unfortunately it did not work. I would appreciate if you give a little more detailed information. I need all kinds of guidance.

Frank De Stasi said:

This device is not designed for fly-buck operation.

It is best if you can select a device with a FPWM mode.

I am a little bit confused, do you mention LMR33640DDDAR or Coupled inductor is SRF0703-4R7MCT ?

Frank De Stasi said:

I am not familiar with a dual winding inductor; you may wish to find a coupled

inductor that is meant to be used in fly-buck operation.

I m not familiar to coupled inductors too. I didn't know there were models of coupled inductors designed for fly-buck.

Frank De Stasi said:

Do you have a ground plane under the layer shown ??

I don't see the ground connection for the VCC capacitor ??

Yes pcb jas already ground planed and VCC cap tied to ground too.

Frank De Stasi said:

Can you provide waveforms of the SW node and inductor current ??

Do you mean i need to scope SW frequency ? Abouth the inductor current; I would like to let you know that the data I mentioned above with no Load conditions.
I would also let you know that even output from Inductors without any load. Input current is 0,9A at 12VDC

Would your suggestion for this circuit be to change the coupled-inductor or would you suggest me to change the IC ?
I am still looking for the most efficient and space-saving circuit that I can get 1A output both +6V and -6V in the input voltage range between 12-24VDC. How do you suggest the most optimal way?
Best Regards
Volkan

Hello Frank,
I would like to state a point. According to the Application Note; https://www.ti.com/lit/an/snva790a/snva790a.pdf named Designing an Isolated Buck (Fly-BuckTM) Converter using the LMR36520.

My design follows all the instructions in the application note. It uses LMR36520 and I used LMR33640. Can you help me understand why the device is not suitable for this operation ?

Frank De Stasi said:

This device is not designed for fly-buck operation.

It is best if you can select a device with a FPWM mode.

As you stated previously I think FPWM modulation is the key word.
LMR36520 and LMR33640 pin compatible devices. If i replace LMR33640 to LMR36520 does this mean that the circuit will work properly?
Thank you very much
Volkan

Frank De Stasi said:

I am not familiar with a dual winding inductor; you may wish to find a coupled

inductor that is meant to be used in fly-buck operation.

Hello again,
I am a little bit confused abouth the coupled inductor.
Isn't coupled inductors the same as isolated dual winding inductors in the same core ?
In some places it is called inductor array. Aren't they all the same?
I could not find any clear information about this on the internet.
Any thoughts appriciated.
B.R

Hello

Yes, you can use the LMR36520F

You may need to increase your inductance since that device is 400kHz

That is the first thing you should try.

If that does not help, then replace the coupled inductor with a single inductor

and be sure you get the correct output from your +6V.

Then we can continue from that point.

Thanks