Analysis and Design of a Flyback; Part 2, How to Model and Simulate a Flyback Converter

[@RobertBolanos]

Again, thanks for your comments. Request for TopSPICE simulation files can be made y emailing me at [e-mail]

[@derekwei2673]

I really like your video, i can get a lot of useful information. i am studying the Flyback circuit, hope there are some buck,boost circult analysis... i will recommend your video to my colleagues.

[@tomfang438]

Hi Robert: thank for you videos, It is too late for me to find this video, but at least i finally found it

[@mohitinch823]

equations shown in this lecture works for critical mode of conduciton .

[@RobertBolanos]

Yes. It is in boundary between DCM and CCM. So the inductor that you get is the for DCM. So if you use a lower inductance, you operate in DCM. But you use a bigger inductance than you calculate, then you operate in CCM. That was good of you to have noticed

[@tanvirfakir2356]

very great .. no one can summarize with deep.

[@jacobs7281]

great Video.Thank you very much.Can you add the design analysis calculations for output capacitor @5.12 before simulation starts

[@RobertBolanos]

Hi Derek, After I am done with the Flyback series, I will do a series of videos on the Forward converter which is a isolated Buck converter and will then cover the simple boost (which is a non-isolated flyback). I am glad that you find the videos useful. These videos were intended to train our younger engineers here where I work. I am glad other fellow engineers and students find them useful. Thank you for your comments. If you have any questions, you can post here or to [e-mail]

[@RobertBolanos]

Hi Jeevan, I use Topspice from Penzar development. They have a demo that can be downloaded and use to run some of the silumations

[@derekwei2673]

forget to say many thanks to your work!!!!! wonderful work!

[@SWECE]

Hi Robert, The Output voltage is +12V as opposed to -12V due to the voltage inversion by the flyback convertor, do you know why the spice simulation gives this value? Also, the Secondary is grounded, would it not cause the flyback convertor to not work as intended since they are supposed to be isolated? Thank you.

[@victorcastrellon4880]

Robert: Thank you for taking the effort to share with us your very valuable skills. Do you know if Topspice models includes the PWM models from Basso "Switch-Mode Power Supplies" ? Thank you Victor Castrellon

[@ttkoh123]

Hi Robert, Wonderful video, should have discovered it earlier. Your recording sound volume is inconsistent however, it's too soft. Thanks.

[@666aron]

Nice explanation! Clean and neat. Thanks!

Hi, Thanks for this useful video. Why do you multiply Nr*Nr*Lp to calculate Ls?

[@RobertBolanos]

The turn ratio determines the secondary inductance. Nr=Nsecondary/Nprimary. So the Secondary_inductance = Nr^2*Primay_inductance. I hope this makes sense.

@@RobertBolanos Thanks for answer. Yes I recalled the formula (Np/Ns)^2=Lp/Ls

[@ngangnangbrice6738]

Hello Sir Robert Bolanas, the video is so great, i really like it, but i need some informations about the circuit, concerning the snubber circuit. How did you get the values of R2 and C2? And what are the fonctions of C7 and R8? Thank you...

[@RobertBolanos]

Thanks Ngangnang! C7 is to simulate the parasitic capacitance of the mosfet modeled by S1. The resistor is a current sense resistor. The current flowing thru the resistor is the same current flowing thru the mosfet and the primary winding. It is a good point to attach a scope probe and monitor the current waveform. Also, if you use a current mode PWM, you connect this to the current sense pin of the PWM. I hope this makes sense.

[@ngangnangbrice6738]

@@RobertBolanos Thank very much sir Bolanos. You explanations are clear, but i still wish to know how you caculate those values. You have just taken randomly or there is way to calculate values of the 2 capacitors and the 2 resistors? Thank you in advance.

[@RobertBolanos]

@@ngangnangbrice6738 C7 is calculated from the mosfet datasheet. You look for Coss. For the Rsense resistor, you calculate at 0.7V/peak_current. The subbers are set by setting Csnubber=4*Coss, and Rsnummber=(Leakage/Coss)^0.5

[@ngangnangbrice6738]

@@RobertBolanos Thank you very very much for your explanations. I wish to know or understand a last thing. How can C7 simulate the capcitance of a real mosfet? Thank you...

[@RobertBolanos]

@@ngangnangbrice6738 I don't understand your question in "how C7 does it simulate the capacitance of a real mosfet".

[@nikulpanchal6655]

an extremely nice video for beginners in smps thanks robert bolanos sir i have one question can i use tina ti spice based simulator for simulating the same

[@RobertBolanos]

nikul panchal yes, you should able to use almost any simulators to simulate. if you email me at [e-mail], I can send you top spice files which you should be able to simulate on the demo version of topspice

[@nikulpanchal6655]

thank you so much for your help my email id is [e-mail] once again thank you for your help

[@YoussefBoksmati]

Very nice explanation Robert ;) Can you please tell me what is written in the bottom right corner so I can finish copying the schematics please?

[@yilmazkalafat8250]

Hi Robert, Thanks for your valuable tutorials about flyback design. I have a question regarding the " ground " symbol in your flyback circuit. Normally the primary and the secondary circuit parts are isolated by the transfomer, so they have their own "ground". In your simulation i see that you use the same grounding for both (primary and secundary) parts of the circuit. I am bit confused. Do we only combine it during simulation and in real life design we split it? I will try to simulate it using LTSpice. Thanks again for all your work, really appreciate it.

[@RobertBolanos]

Yilmaz Kalafat Yes, this is correct. SPICE only has one ground symbol and is harder to simulate with two different grounds. In addition, when first powering a power supply that you design, I do ground both the primary and secondary grounds together just to make sure I don't have a ground loop. Once I have the power supply working, I separate the grounds. This is a good question.

[@RobertBolanos]

Thanks Aron!! Robert

[@chopcooey]

How do you pick the frequency for your PWM in general? I believe that a higher freqency allows for a smaller transformer since less energy is stored at the same time during a cycle. However i am not sure how the SRF of the inductors come into consideration for the frequency selection. My guess is to stay away from SRF and its harmonics to avoid oscillations due to resonance. Im not really sure.

[@RobertBolanos]

chopcooey Hi Chopcooney. I try to keep the frequency between 100K to 200k. As you increase the frequency, your magnetic (transformer) and capacitors get smaller as the expense of switching looses. So it is a compomize between reducing the components and how much power you can dissipate or loose. As your switching loose due to the parasitice capacitance increase, your efficiency will decrease

[@JeevanKoshyJKK]

Hi Roberto. Thanks for your work. I would like to know what SPICE program you are using?

[@mohitinch823]

he is using TopSpice.

[@JeevanKoshyJKK]

@@mohitinch823 thanks boss. Better late than never.

[@shivanshu2959]

Hi , As mentioned in the schematic the input range is 24V to 34V . At 24V input the output is 12V . But as we increase the input to 34V the output also increases . So how do we regulate the output at 12 V . I have seen that the feedback loop plays an important role as the PWM controller can change it's duty cycle so that the output is always at 12V . But how the controller know that it has to be at 12V ? If you have any video regarding this please point me to it.

[@RobertBolanos]

After you have simulated the output filter, flyback transformer and the mosfet, you can put a PWM that will automatically adjust the duty cycle to bring the sample output voltage into regulation. Continue watching the videos as I built up to regulating the output

[@shivanshu2959]

@@RobertBolanos thanks a lot ❤️

[@fabianluttenberger7153]

is there also a way to calculate peak current of CCM?

[@victorcastrellon4880]

One more question. Does Topspice have a good library of controllers for switching mode power supply? Thank you Victor

[@RobertBolanos]

Hi Victor, yes, Topspice has a very comprehensive library of pwm model which include the average and switching models. It also has Basso, Copec and other average models. If you go to Penzar Development, they should have a list of the models in its library. Hope this ehlps

[@azdinator]

Hi Robert. Thanks for this great Job. Could you just tell me why you have chosen TopSpice ? it's not free. I suppose you can do the simulation with LTSpice don't you ? Regards.

[@RobertBolanos]

Hi Younes. That is good question. I like using Topspice because of it's large library of spice models and great convergence. When I say convergence I mean that I hardly get simulation errors. Topspice cost about 600 dollars which is affordable to most practicing engineers and they have great technical support. I have been using it for about 15 years and therefore a very happy customer. I highly recommend Topspice.

[@VIVEKSHARMA-bz4xu]

Hi .I have two questions . 1.does this syntax for topspice is valid for ltspice? 2.do we have any calculation for selecting output capacitor?

[@RobertBolanos]

The syntac such as #autoplot and #calc are not LTspice compatible. But other such defining K should be the same. For calculating the capacitor, they are shown later in the subsequent videos.

[@titeman9374]

Please can you send me the Top spice software you used in your simulation.

[@shivanshu2959]

How did you add the switch model in TopSpice ?

[@RobertBolanos]

The switch is in the library. I used the switch to simulate a mosfets transistor. It simplifies the simulation. This also makes the simulation run a little faster since a real mosfets model can be quit complicated. Once you have the simulation running, you can replace the switch with a mosfets. I hope this makes sense. "May you simulations always converge", Robert Bolanos

[@shivanshu2959]

@@RobertBolanos Hi , in your example when I used the switch model it worked perfectly and when I replaced it with an N-Mosfet the output came completely wrong. Any ideas how to fix this ?

[@RobertBolanos]

@@shivanshu2959 make sure you are using a mosfet model that has a low Rdson. If the RDson is too large the too much voltage will appear across the mosfet

[@shivanshu2959]

@@RobertBolanos Thanks a lot. You have really been very helpful.

[@tomfang438]

Can you tell me why you use R2 and C2, they function is ?

[@RobertBolanos]

They are snubbers

[@tomfang438]

@@RobertBolanos i got it

[@karastom2304]

Can you please give me your nice excel sheet of the fly back

[@RobertBolanos]

Hi Tom, regarding the 500W flyback, it would not be a suitable supply. Flyback work better under 100W. At 500W, the peak current would be very high. A forward converter would be a more suitable 500W converter.

[@RobertBolanos]

For the excel send email to [e-mail]

[@mahendrajoshi762]

Hello sir can you please share excel sheet of the flyback design

[@mahendrajoshi762]

@@RobertBolanos sir I already sended a mail you on [e-mail]

[@milan0091000]

@@RobertBolanos I want to know which software you use