Electronics > Power/Renewable Energy/EV's

The latest fad in SMPS's....Planar transformers?

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Planar transformers look good for when high power with low profile is needed, and even then only when f(sw) is high enough to make the low turn counts possible.
Planars suffer  "edge effect" w.r.t.  core gapping with eg flybacks.
For high currents, the only way to do planar transformers is literally to have the turns interleaved Pri/sec/pri/sec...etc etc
As a result, you end up with loads of unwanted interwinding capacitance......this  can give really high di/dt at fet turn on...not to mention hideous common mode EMC problems.

The mountings for high power planars also require custom  tooling set-ups, which just arent needed for custom wound "normal" transformers, eg ETD, PQ, etc etc.

Planars look pretty un-appealing, would you agree? I think they  really need an application which can handle the high interwinding capacitance.....preferably a resonant design aswell, because hard switched in combination with the high interwinding capacitance is going to give a nightmare common mode EMC problem.

I would say only use a planar txformer if you absolutely have to, wouldn't  you agree?

Also, for a high power planar design,  the planar windings cant be implemented in a normal PCB layout program, unless you are able to write eg python or C scripts for them.
There is also a need for custom mech tooling for lead frames when windings are high current.
Good luck to anyone trying to get quotes on planar transformers.
And good luck to anyone trying to get lead times for manufacture of a few pieces (or even large qtys) of planar transformers.
Then if you need to get it slightly modified, good luck on the time and money needed to achieve that.
All this isnt a great problem with "normal" conventional transformers.

I reckon planar transformers and hard-switched SMPS just dont go together (due to high interwinding capacitance and the resulting high di/dt noise, and the  common mode EMC problem that goes with it.)
Vicorpower modules use planars...but they are all resonant converters, and desperately need to be v v low profile.

The last sentence of this says it all...

Another point with planars, is that they can only really support low turn counts.....and so if you need a particular turns ratio with eg a full bridge, so that you can maximise the duty cycle, then you are not likely to be able to achieve this with a planar. With a planar  you simply cant make a small fractional increase in turns ratio like you can with a "normal" transformer.

And as the following says.....
Planar magnetics take up a great deal more circuit-board footprint than traditional transformers.
Planar devices require a printed-circuit layout and tooling for the magnetic core materials.
Were it not for the demand for higher frequencies, it’s somewhat doubtful that planar transformers would be considered as an alternative to traditional wound-wire magnetics.

The following deals with the "edge effect" of planar transformers...
...this is one of the ills from which the Planar transformer suffers....and can require custom cores to be tooled up for to  mitigate it....it is mitigateable, but you very often walk into jobs where they havent done the planar transformer correctly and are suffering it. The desire to shoehorn a planar into a design so as to augment the CV, is often so great, that one finds a badly implemented planar...often an offtheshelf planar, which is totally unsuitable due to the intracacies of planar.

So planars are disadvantageous with DCM flybacks...
As stated in page 23 of the following...

Planars are said to have the advantage of easly facilitating multiple interleave winding, giving extremely low levels of leakage inductance...the problem with this, is that a certain amount of leakage inductance in eg a Full or half bridge is actually beneficial, as it acts as a turn on snubber. In fact, quite large leakage inductance does no harm in Bridge converters, since the bridge current is AC, and balanced, and so the current  in the leakage inductor does not staircase upwards....it just shunts backward and forwards, harmlessly.
Then there's the active clamp forward converter, where a decent amount of leakage inductance is actually helpful, as it will facilitate ZVS in high load.
This is a big downpoint for planar transformers.......they actually need to be multiple interleave wound...which then results in the really low levels of leakage inductance......which really is not wanted at all.
(perhpas with the exception of flyback converters...but even CCM flybacks benefit from a bit of leakage L  for the turn-on-snubber effect.)

I don't think I would call it a 'latest fad', they've been around for years.

Clickbait as usual  ::)

Easily available from TDK, Siemens, Philips 35 yrs at laset.

Many papers and app notes on design and PCB layout.

Vicor among others use them



Many thanks for your kind consideration


Thanks, yes i have read huge numbers of planar literature. All of it is lauding the planar as the new messiah.
When you go into a job....you then find that you are working on a design that a designer has done where he/she has selected a planar txformer when it is not at all suitable.
They have simply selected it for their CV.
It is this area, where literature really doesnt exist.
Seek literature on "disadvantages of planar transformers" (of which there are a great many) and you find almost nothing.

It is realised that some people on this forum are genuinely concerned about the transfer of proprietary SMPS knowledge to places outside of the "western world", and therefore repeatedly seek to shut down any discussions such as this one...but please may i assure you, that..
1.....The "non western world" is miles ahead of the western world in all areas of SMPS design anyway. (so this isnt giving anything away)
2....The mainstay of missiles and military hardware etc ( the secret bits) is not in the power supply.

(AYK, point 1 above applies because the "western world" has outsourced virtually its entire power supply design and build industry to the Pacific Rim.)

So please feel free to discuss here-with, you are quite safe to do so, and you will not jeopardise anything.
(there are already 149 views and the post is only some 10 hours old)
Another point is that you often find yourself working on a SMPS with a planar transformer, but it has been implemented totally incorrectly.......for example, there are ways to do it which would end up with ridiculously high proximity losses.....and suffering edge effects........and often people dont get the full data on the transformer, since the design consultancy withholds this info......this is less than ideal when you need to work on it. The designer may also have forgotten to multiple interleave wind where it was necessary to do so, etc etc


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