The HF transformer: Facts you may have missed

An intuitive explanation of the operation and design of the HF transformer, including a discussion of some key issues such as the effect of a short, DC current injection, the possible need of an air gap and flat magnetics.

[@rulmismo]

I did some review on my basic knowledge on magnetics recently related to a job issue related with changing voltage requirements in some inductive components. After some hard work of remembering why things work in all this stuff, of N, L, voltages permeabilities and whistles, I wish I had seen this lecture before. It is clear when somebody master the topic and how to explain it, you don´t need fancy animations, but a whiteboard and a clear understanding. Will save the channel for other videos.

[@sambenyaakov]

👍🙏😊

[@DazHarkz90]

I absolutely love your videos. You had a significant effect on my degree, and you are continuing to help now I am working as a power engineer. The work I do pushes HV technology to overcome the use of herbicides in agriculture, so looking after the planet. Thank you Sir - from the UK

[@sambenyaakov]

Thanks for warm note and good luck with your work.

[@keithking1985]

My second video of yours to watch and it answered some questions I had about transformer gaps. Thank you very much for your knowledge.

[@sambenyaakov]

👍

[@petegaslondon]

Dohh yeah - I keep thinking I know high frequency power - then I watch YOUR stuff and realise theres a bunch of things I forget - so, you CAN go gapless but your SYMMETRY has to be real good to avoid DC flux - JUST like in our old AUDIO transformers !

[@sambenyaakov]

Thanks for exchange.

[@petegaslondon]

@@sambenyaakov Hi Professor - I actually had a question - supposing you had a half bridge, where the primary windings ground goes to the 'centre tap' of two input filter capacitors, but there's no DC path... I keep thinking it shouldn't be possible to magnetise a core if that's the case - no net DC current? Even if the pull-up and pull down pulse widths were unequal or some asymmetry in the rectifiers? Do I still have to carefully observe symmetry in a case like this, or am I inherently ok? (My guess is the worst that will happen is the voltage at the node will deviate from mid-rail, but i can't damage the core? But i thought I'd ask) Apologies if you covered this elsewhere as I've no doubt missed some of your prev lectures ( and I feel much more confident about knowing my magnetics after catching up on some! ) Hi from London

[@khalidaboushiwa6322]

In the process of watching the HF Transformer ...Thank you sir ! you'er the Best! stay safe ..

[@sambenyaakov]

🙏

[@child_of_god_]

Thanks sir, much appreciated. An indepth yet practical transformer calculations

[@sambenyaakov]

👍🙏

[@Chris_Grossman]

This is an excellent concise review. Thank you.

[@sambenyaakov]

Thanks

[@yunusemreozdemir1342]

I have looked at your video about HF transformers. These videos are really helpful but when designing or calculation part, units of elements(B-->Tesla or mT, Ae--> mm^2 or cm^2) do not exist. If you can, please share these units with us in the comments. Thanks for your lecturing.

[@sambenyaakov]

What do you mean don't exist? Tesla millimeter square?

[@swh19]

Hi Professor, on page 9 [14:25] you mentioned that shorted output will not cause the core to operate in the saturation while large input current is induced. However, under this circumstance the input voltage is still AC with very large AC current as well. Is that possible when exceeding a certain threshold of input current, this would bring the core into the saturation?

[@sambenyaakov]

If the core is fed by a voltage source, it fixes the magnetic flux density B. The current at the output is balanced by the current in the input and the net change in B is zero.

[@cyruschady2053]

Thanks.

[@sambenyaakov]

👍

[@sujoybha]

Regarding introducing an air gap to reduce the "u" and increase the DC saturation current of the core, will it not also increase the area enclosed in the hysteresis curve and thereby increase core losses? I feel that the increased hysteresis loss is not so important for inductors but crucial for HF transformers. Please clarify. Also thanks a lot for this entire series of lectures. They help a lot.

[@sambenyaakov]

No, a gap does not increase hysteresis losses.

[@sujoybha]

@@sambenyaakov Interesting, thanks.

[@Rajat0607]

Sir, could you pls create a video on how to use dowells equations practically to estimate AC resistance in flyback transformer

[@sambenyaakov]

Good subject will try

[@mohamedhadjhafsi9441]

thank you for the valuebale informations that you are proviing us with, i have a question how can you find the number of turns/b max of a transformer from the nameplate ?

[@sambenyaakov]

You can't

[@TSulemanW]

Hi , Prof Sam, if the direction of primary winding is clockwise and the second winding is the same there is no current flow at the secondary winding. is this correct. thank you.

[@sambenyaakov]

Direction of winding only changes polarity. In any winding direction the secondary winding picks up the induced EMF due to change magnetic flux caused by primary winding.

[@AMMCElectronics]

Hello professor! I have a doubt with the wire size, I'm agree that we need to dimension it regarding the power losses that we think acceptable, but why everyone uses AWG table of current for dimension the wire size on their HF transformers? e.g. if you have 3 turns with a current of 10A and you use a 0.6mm diameter you will have 0.1V drop aprox but in the table 10A is for 2mm wire, why?. Do you have any recommendations to select the right wire size in HF transformers? thanks so much!

[@sambenyaakov]

Hi, on the one hand you are mentioning HF transformer an then you talk about 2 mm wire. Please clarify.

[@AMMCElectronics]

​@@sambenyaakov I'm just want to understand if with HF transformers you can use an smaller wire diameter due the short length of their coils, which mean less resistence. For example, AWG 22 (0.62mm diamm) is rated for 0.92A, so if you need 10A you will implement a multicore of 11 AWG 22 in parallel aproximately to avoid skin effect, etc. So, my question is: it is necessary? because the drop voltage ir really short with short wires in HF transformers... so my point is: if you use only two wire of AW22 @ 10A for about 6cm of lengh wire (0.003 Ohm), the drop voltage is 0.03V aprox and I think that it should work anyway, isn't it?. do AWG wire table is a good reference for wire size selection? Thanks in advance!

[@sambenyaakov]

@@AMMCElectronics If Litz wire is used the reliant parameter is the copper cross section from which resistance per length is calculated. O.3W seems to be reasonable assuming that the power lever is much higher.

[@theoldbigmoose]

Professor are your slides available for download anywhere? Your sessions are priceless!

[@sambenyaakov]

Sorry. Not available. Thanks for interest.

[@taimurzaman7322]

Thats a nice explanation. i am working on Dual active bridge, it works good, but after few seconds the hardware start heating up. After that i add in series an inductor to transformer primary, which decrease the overall efficiency but the over heating problem resolved up-to some extent. What are your views professor?

[@sambenyaakov]

Hi, It is difficult to tell without the details.

[@JorgeSilva-em8pf]

Dear Zaman. I suggest you to study the subject: "staircase saturation" on "push-pull like" converter. B. Regards.

[@taimurzaman7322]

Thank you Silva, I appreciate your suggestions :)

[@jigyasakumari9477]

Did u make solid state transformer in simulink ??

[@AS-zs7jw]

hi prof. Sam Ben-Yaakov i'm designing a HF transformer, but it gives me output voltage more than the turns ratio, i have designed HF transformer for 50% pulse duty at 30 khz, but running it at 25 % duty , if if applied 30 volts DC to fullbridge converter with HF transformer having 30 turns ratio, it gives distorted pulse at secondary with amplitude 2000 (not square or quasi square as is at primary side) rather than 900 as expected from turns ratio. please answers this question.

[@sambenyaakov]

Test the resonant frequency of transformer. May be too low.

[@AS-zs7jw]

​Thank your sir for your reply, i will test it, does too low "transformer resonant frequency" means that my transformer have high leakage inductance and high intera winding capacitance? can you please share any literature how to reduce intera winding capacitance ? @@sambenyaakov

[@AS-zs7jw]

@@sambenyaakov sir, please make a video lecture on winding capacitance and leakage inductance of a high frequency transformer, to get correct waveform at secondary side with squarewave signal at primary side.

[@Aaaaaasssssss382]

@@AS-zs7jw hi, you can go to chapter 1 page 69 [Marian_Kazimierczuk(auth.)]_High-Frequency_Magnet and you will find much details. good luck

[@audiokees4045]

I Have look at your presentations about LCC versus LLC very informative but still I have not yet the full clue of how to calaculate the transformer, I did see on LLC it is not that difficult but LCC is different because it do drive as a current spource, so the impedance of the primairy needs to be known? I have done a transformer for a normal hardswitch half bridge, set the frequency on 100 khz, the point where the output power of the LCC is max, and I did get a respons who like to be not bad, when set 110 to 250 Khz it did regulate in LTspice. (after first set the feedback correction up). So after ead much papers, the N of the transformer is known but do not now how to start right with tranformer calculation, that is why I did try the normal half bbridge aproach. For LLC it is not difficult transformer is part of resonance tank while for LCC the capacitor bridge is the impedance point for the power transport, so miss induction valeu,s. these are into the mH region while LLC is in uH region.. Have a nice day.

[@sambenyaakov]

Why is this different from full or half bridge PWM converters?

[@audiokees4045]

@@sambenyaakov THanks for the answer. I have multply outputs, 350 volts, 2 x 130 volts and 12.6 and 6.3 volts, for getting N what of the voltages I can use? the feedback is coming from the 12.6/6.3 because it is filament voltages and need to be stable. feedback did work fine in sim. Primairy induction for full bridge is higher then for a half bridge. that is why I do use the calculation for a half bridge because I also do us this, I use then the calculated transformer for match on the LCC tank output, I did put Cp also on the 350 volts 0.10 amps output winding, it has already some capacitance there. I did now test some, and I get regulation, even with open outputs I get stable outputs. Only the high voltage get lower with open outputs and get higher when I do full power. I did calculate and a full bridge is 22mH and a half bridge is 5.54mH as the calculator says . Have calculate the tank with a normal LC calculator and set as I did read it is better for efficency the Cs/Cp to 0.1 Peace from a old paper mentioning design of the LCC, so here I see I just need to have a high enough inductance to get it right? That is why the inverter now works on sim, play with the Q and resonance of the tank helps a lot, I use just a simple online calculator for resonance and q, I set the low frequency just above that and give enough room for regulation, and it did work with open and closed outputs. When open I get a big feedback hobble and it then settle, the PID is not yet complete right. You have to understand I am a audio amp guy, and autodidact learning, never see a university, just a lot of shoes in the factory I did work from 15 years of age. Piece who says, sufficiently large inductance, so calculate it just for a normal half bridge, seems to work. but winding capacitances do matter, and I have one high voltage output so set Cp there. But I can be wrong afcourse, all with all, I enjoy the learning but it is slower with mine 66 years of age brain. As an example, the magnetising inductance of the transformer, in certain cases, may be assumed sufficiently large to be removed from the model. Alternatively, the inter-winding capacitance of a high-voltage transformer may be significant, thus requiring either the parallel-resonant or series-parallel resonant topologies to fully exploit the transformer parasitics, as opposed to reducing the capacitance's impact by the addition of further inductance. I do now watch your movies, also for hf transformers, the ExcelentIT software do well for transformer calculation incuding multply outputs. have a nice sunday.

[@sambenyaakov]

@@audiokees4045 Thanks for explanation

[@audiokees4045]

@@sambenyaakov I have downloaded some of your video and lie on the couch with a beer (hot here) to see it on tv, and was very informative. You have to now I am a audio amp man, and even do class D who is also switching reaching now 0.0012 procent distortions with current and for low frequencies voltage feedback and 5 level switching., it is more easy to simulate, and have more experience with it, smsps is quite new, so some things I do understand oke, some not like the transformer calculations who is just N1/N2, in your video you explane some of the ideas I did mention, i need to look at the output load reflected to the primairy, that is why U use an ac resistor in the resonance tank Cr and Lr (Cs Ls and Cp for LCC). U did also mention series resonance with only CS and Ls it has less part stress also I did see in sim. I have now do the LCC, have a Cs/Cp of 0.1 did give the lowest amps drawed from primairy for max output. I have use the transformer calculator ExcellentIT (free download) to calculate the transformer, if I did warch your presentations right, the trannsformer needs to mede such it has the less possible load om the resonance tank in lcc. but do fit on the core, when do full bridge get with etd 29 4.36mH prim 350 volts out = 7.19mH = M 0.60 and with half bridge I get 1.090mH primary and 350 volts out = 3.59mH = N =0.300. it is precise this what I have not learn much about, I presume that LCC or series resonance use capacitors for regulate output voltage in a current source setup, and the LLC has the transformer included into the multiresonant setup so when I calculate that with a tool, (yes for these I have nice tools from infinion or others but not for LCC or other series resoance types. (older ones). Do go on with your fantastic work maing world greener. ( I am against windturbines though), I am a thorium fan. Excuse me for english errors, here is the link to the transformer calculator. 320volt.com/en/gelismis-smps-transformator-tasarim-programi/

[@audiokees4045]

@@sambenyaakov The voltage on the transformer, half the input voltage for half bridge and full for full bridge, give higher inductance. But I think I have some idea with the LCC, because Cp is a current source acting capacitor with his reactance on frequency I just need to optimize the transformer such that it fit with Cp, in fact inductance X needs higher then X Cp to transfer power efficiently, If I am right this is..