[SOLVED] Selecting switching frequency for offline flyback converter

By theory you can use any switching frequency in flyback converter (to an extend). Some frequncies are more often employed than others. So what are those frequencies and what are the possible reasons for them??

Well switching frequency selection is important in DC to DC converter design. Its having advantages and disadvantages. So far I can’t sagest a particular frequency for SMPS, its depends up on your requirement.
High frequency in the range of 1-2MHz will reduce magnetic parts size and capacitor size. Magnetic cores, switching and control device are not commonly avilable. Disadvantages: efficiency less due to switching loss
Low frequency in the range of 100KHz to 300KHz , it will keep the magnetic part in normal size. Switching devices and controllers are redly avilable.

Low frequency in the range of 10KHz to 30KHz size of the magnetic part become big, High power switching device avilable low switching loss

francis29 said:

By theory you can use any switching frequency in flyback converter (to an extend). Some frequncies are more often employed than others. So what are those frequencies and what are the possible reasons for them??

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Well you might start out with a very rough initial idea of using some particular frequency, a design needs some starting point.

The next ting is to design the magnetics, and this is where it becomes really challenging.
Skin effect limits the wire size, but the wire must be a certain minimum size to carry the rms current.
So you may be forced to use a lower frequency to enable a certain larger wire size to work.

Design flux density comes into it as well. If you use a smaller core, that will require more turns, but more turns may not fit onto the core.
But if you use a higher frequency, it can work with less turns.
A higher frequency might mean higher core loss.

Another aspect is you might need say sixty turns, but only fifty five turns will fit in one layer.
All sorts of compromises in designing the magnetics.
But eventually after many many tries you will come up with a solution where the wire size, the number of turns, the core size and flux density are all practical workable numbers at some particular operating frequency. And that is what you end up using when testing your first prototype flyback transformer.

Many a novice starts out with the operating frequency as the very first design parameter, then finds out that the magnetics simply do not work, or gets too hot, or is very inefficient, or even ridiculously oversized.
All switching power supplies are built around a successful magnetics design, and the operating frequency is just one of many things you need to adjust to get an optimum overall design.

Common designs i have seen seems to be around 66KHz, 132KHz is there any particular reasons other than that mentioned above like EMI..

Hi,

Common designs i have seen ...

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--> show us the datasheets (because else we can´t see what you have seen)

Common designs ... seems to be around 66KHz, 132KHz

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--> as stated before: common designs have a huge range from low kHz up to MHz...

Klaus

I am referring Power integration's designs, UC3842 designs

Somebody has designed a practical circuit to demonstrate the use of a UC3842.
As mentioned in post #3 that requires winding a suitable transformer, and the designer has come up with a practical solution that involves using that particular chosen frequency.

If you give the same design problem to ten different design engineers, you will end up with ten different designs, and very likely ten different operating frequencies and ten different transformer designs. They will probably all work.

Some may be smaller, another may be much more efficient, one may be very low cost. They all work, but are different approaches.

So there is no one perfect answer right??
one more qstn.
Does this operating frequency has anything to do with creating EMI??

I don't pretend to be an expect in power supplies but, yes there is no perfect answer. As other have said different engineers will come up with different (probably equally valid) designs given the same specification.
As others have said it depends on what takes priority in terms of space, efficiency etc.

More specifically I have looked into an offline 240Vac (fairly low output power) flyback convertor recently. It became clear that as long as the transformer is well / appropriately designed the MOSFET switching losses dominate the design in terms of keeping the switching frequency low enough to have reasonably efficiency. For me that basically ruled out frequencies in the high 100s of kHz and above completely and I guess is why a fair few of the commercial units operate at about 40kHz-60kHz.

Does this operating frequency has anything to do with creating EMI??

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Yes, in offline designs, the conducted emissions graphs start at 150khz...so you are better off choosing a switching frequency below 150khz.
..then again, for the sake of switching loss reduction, its best to stay below 150khz anyway with an offline flyback.
I am finding low voltage dcdc's are often 200-350khz or so...sometimes more....but yes it depends on many things.
You only start learning about this when you dive in and do it, do it on the ltspoice sim first, then get your hardware proto going.

There is one other point worth a mention when designing magnetics for high voltage switching power supplies.

High voltage suggests a large number of turns, and depending on the winding geometry it may also end up having a lot of capacitance between turns or layers.
This can result in a self resonant frequency of the winding that can be unexpectedly low. And that can have a dramatic effect on how well the transformer and the rest of the circuit works.

The solution to this is to reduce the number of turns by increasing the core cross section, and lowering the operating frequency.

While operating frequencies are now well into the hundreds of Khz, even Mhz for some applications, where high voltages requiring a lot of turns become necessary, lower operating frequencies will usually give much better results.