Design/Build a flyback converter

[mxlemming]

I got a spare 5 hours to design a flyback and a few months later an evening and a half empty weekend to mess about with it.

Goal was not to make anything special, just to work out how flyback works, demystify it etc.

I used an NCP1377B control chip to allow up to 300kHz switching, a 250V MOSFET (also tried a 600V one but it makes no difference for my application) a self wound toroid and the cheapest opto isolator on JLCPCB. Topology was quasi resonant with a control winding and opto feedback.

Overall, I have concluded that buck converters are generally much more practical and efficient, but hey.


There's the schematic


There's the layout/render, 30mmx40mm. I chose to use a toroid at first, which turned out to be a bad idea.

Actually building it with a wound transformer on a ferrite core I had.


At light load it sits there pulse skipping

At high load, it is in QR mode (quasi resonant) and you can see the overshoot due to transformer referred flyback voltage.

Well, there we go. Built a flyback. It is interesting, but the power density compared to a buck converter is really low since 1) QR control means the inductor can only be used up to half current on average since it is driven with a triangle wave of current which includes the zero current state and 2) the duty cycle means it has discontinuous generation. Works happily with 90V in and 15W at 12V out.

Might be useful one day. I understand flyback converters now at least!

 

[ARod1993]

Yeah; the space I mostly see them used in is for low-power low-component-count isolated power supply designs for things like IGBT drivers (where the peak switching currents may be quite high but the average current draw is quite low). and very small (<20W) off-line converters for electronics. I'm actually currently looking at a funky variant of the design called a Fly-Buck (buck converter, but swap the inductor for a flyback transformer so you get isolated outputs) for an IGBT half-bridge driver design.

 

[mxlemming]

Yeah; the space I mostly see them used in is for low-power low-component-count isolated power supply designs for things like IGBT drivers (where the peak switching currents may be quite high but the average current draw is quite low). and very small (<20W) off-line converters for electronics. I'm actually currently looking at a funky variant of the design called a Fly-Buck (buck converter, but swap the inductor for a flyback transformer so you get isolated outputs) for an IGBT half-bridge driver design.

Seems to be the basis for most of the laptop chargers I have ripped apart (only about 3 or 4) with an up front power factor correction stage.

Fly buck is pretty useful, I have been considering using them as well. You can make a flyback/buck with 3 outputs (or more!)for a 3 phase bridge quite readily if you are willing to do some winding.

 

[mxlemming]

I would not advise using the NCP1377 like I did for anyone reading - On Semi list it as obsolete now. Wish I had realised that before using it for the design. Plenty of alternatives, but they all have some funny feature or other.

 

[ARod1993]

Seems to be the basis for most of the laptop chargers I have ripped apart (only about 3 or 4) with an up front power factor correction stage.

Fly buck is pretty useful, I have been considering using them as well. You can make a flyback/buck with 3 outputs (or more!)for a 3 phase bridge quite readily if you are willing to do some winding.

Yup! TI has a reference design for a four-output fly-buck that they use to drive an IGBT half-bridge that I'm looking at for work: https://www.ti.com/lit/ug/tidu478/t...00364&ref_url=https%3A%2F%2Fwww.google.com%2F
The nice bit is that the transformer is an off the shelf part so you don't have to wind it yourself if you don't want to For a three-phase two-level design like most ebike converters they also have a flyback-based design with four outputs from one transformer at https://www.tij.co.jp/lit/ug/tiduea...08725&ref_url=https%3A%2F%2Fwww.google.com%2F and a push-pull based design with three transformers at https://www.ti.com/lit/ug/tidu355a/...79022&ref_url=https%3A%2F%2Fwww.google.com%2F (those designs also provide bipolar supply power so you can hold the gate voltage on your switch negative when it's supposed to be off).