5 Rail Isolated FLY-BUCK DC-DC Design, Schematic, Detais with DIY Toroidial Transformer

Another rather old Design I made way back in 2018 out of the necessity for multiple Isolated Power Rails. I Design Power Supplies and other Power Electronics and I almost always need 1 or more Isolated Bias / Auxiliary Power rails for Controller Electronics, Voltage / Current Displays, etc. So I Designed this 5 Rail Isolated DC-DC Fly-Buck Converter with a Customized(DIY) Toroid Transformer to be more Flexible.
It is a Max 16Watt, 20-24V Input and up to 20V Output Fly-Buck Converter with 4 Fully Isolated Rails and 1 Input Ground Referenced one (the Buck Rail) that is also the Regulated one. The 4 other Rails are Tracking(more or less) the Primary regulated one through the tight Coupling of the Windings.
Bcs of the so-so Regulation of the 4 Isolated Rails with the Primary Rail(Bcs of the Fly-Buck concept) it is intended to be used with an LDO Post regulator and LC+Ferrite Bead Filtering for sensitive analog Circuits.
!!! W A R N I N G !!!
!!!! Do NOT use this Circuit in any MAINS related Application !!!!
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When using the Transformer for High Voltage Rail Isolation use ONLY:
1) TIW (Triple Isolated Wire)
www.block.eu/fileadmin/2c9ee4...
or
2) FIW (Fully Insulated Wire)
www.elektrisola.com/en/Fully-...
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WÜRTH Designing Buck Converters with Isolated Outputs:
www.we-online.com/components/...
TI AN-2292 Designing an Isolated Buck (Fly-Buck) Converter:
www.ti.com/lit/an/snva674c/sn...
Please don’t forget to like and subscribe. Thanks for watching...
!!! Thank you to all my subscribers !!!
#electronicscreators
#flybuck
#powersupply
#fly-buck

[@kensmith5694]

I have made basically the same circuit but with a very different core. I kind of like the EFD cores because you wind on a plastic former then put the core onto it.
I have also done it with the SEPIC style DC-DC converter. This nice thing about the SEPIC style is that by a bit of cleverness in the transformer design, you can make the input ripple current just about zero. Here is how:
Draw a normal SEPIC converter but use the "shared core" design.
The thing about transformer models is that you can model a real transformer as a perfect K=1.0 transformer with an inductor in series with the primary.
When you do this, you will find that the perfect transformer in the model has a slightly different inductance.
If you add perhaps one or two extra turns to the primary, you can make the ratio as a 1:1 for the model.
When you do this you will notice that the ripple current on the primary side is zero.

[@gregandark8571]

Nice and beauty.

[@KonstantinGrigoriadis]

Thankyou!

[@migsvensurfing6310]

Nice made. I guess the post linear regulation will also filter out a lot of the switching noise. Not a bad solution.

[@KonstantinGrigoriadis]

Thankyou, Yes.

[@kensmith5694]

One noise that is hard to get rid of is the noise that gets onto the secondaries via capacitive coupling. A common mode choke helps on this.

[@BjornV78]

Hi Konstantin, again a very nice circuit and neat design.
Never heared of a "Panta filler" before, and Google doesn't show any results with that, i used the english subtitles, but i think something whent wrong in converting to text and it is misspelled by TH-cam? Grtz

[@KonstantinGrigoriadis]

Hi Bjorn, thankyou! its called Pentafilar, from Penta 5 :-), Cheers, Konstantin

[@BjornV78]

@@KonstantinGrigoriadis thanks, i'm gonna study that type off transformer. Out of old habit, it always use LM78xx regulators to build small linear powersupplies, and if i need 2 different voltages like per example 5V and 12V, i placed a LM7805 after the LM7812 and use a big smoothing capacitor and a 100nF capacitor for reducing transients on the input and output. When i think off switching powersupplies, i always think they are noisy, but i think with a good design they can also be very quiet. Grtz

[@KonstantinGrigoriadis]

Bjorn, its just a 5 winding Transformer, coupled coil, where the 5 wires are tightly twisted together to ensure a tight coupling. The Design is all about Isolated Rails.Cheers

[@LimbaZero]

Interesting circuit. I need to do some testing.
I hope you don't use those isolated section on live side circuits. Usually between mains and secondary you need good isolation or it will be hazardous to user.

[@KonstantinGrigoriadis]

Hello, Please read the Info in the Description about the Wire to be used :-)

[@LimbaZero]

@@KonstantinGrigoriadis Thanks for the info. Also found application note of this. Thanks.

[@KonstantinGrigoriadis]

Hello, No Problem, what application note y mean, for the Flybuck ?

[@sanjikaneki6226]

how does it perform with no or light load on the buck side of the coupled inductor?
Also why there is no safety zener on the auxiliary outputs?

[@KonstantinGrigoriadis]

Hello , without the Light Load the The Outputs go Higher when Open (what is expected), what safety Zeners ? for what ? I never seen in ANY FlyBuck or Flyback Design with "Safety" Zeners....

[@kensmith5694]

@@KonstantinGrigoriadis You haven't seen my designs :) I usually one just low enough that I don't fry the linear regulator when the isolated output is unloaded.

[@sanjikaneki6226]

how do you deal with the compensation depending on the number of outputs you have?

[@KonstantinGrigoriadis]

Hello, Compensation is done for the Primary Output, this Isolated Outputs are interned to power Light Biasing Loads NOT Large Fast Switching ones!

[@sanjikaneki6226]

@@KonstantinGrigoriadis thx, i was considering driveing a date driver from it and maybe some op amps that should be light enough , right?
then again regulation is not a problem if it goes down a bit

[@KonstantinGrigoriadis]

@@sanjikaneki6226 This was more or Less the Reason i Designed it. But be careful with the Gate Driver , if you mean you want to use one of the Isolated Outputs to for the SUPPLY of a Gate Driver, this Depends, what Current the Drivers must Deliver to the Fates of the Power Devices you use! The 4 Isolated Outputs are Designed to deliver Maximum 200-300mA each.

[@KonstantinGrigoriadis]

Hi Again, If you use the Design to isolate between Low & High Voltage, use the correct Isolated wire for the Transformer !!! A Normal Lacquer Wire is NOT enough!! See in the Video Description about the Wires that MUST be used when Isolating Low-High Voltages. Cheers, Konstantin

[@kensmith5694]

@@KonstantinGrigoriadis In the US the terms to look for on the wire are things like "high voltage" or "600V" or some other voltage rating.