Video Tutorial, Analysis, Design, and Simulation of a flyback Part 7 - Page 1
Forum section: electronics / projects

author: rbola35618
author_rank: Frequent Contributor
timestamp: December 31, 2012, 05:12:46 am
content: Hi EEVbloggers,Video Part 1Here is tutorial video on the analysis, design and simulation of a flyback converter. I show a step by step in how to analyze the flyback converter by breaking into "TON" and "TOFF" circuits and then use the volt-second-rule to derive a transfer equation. Voltages and currents are calculated and will be compared to the results from SPICE simulations (coming soon in part 2).Video Part 2.Second video shows the simulation of the flyback. I compare the simulation with the calculated values.Video Part 3In this video, I introduced a second method and show how to model the flyback transformer. In part 2, I used the coupled inductor model. In part 3, I show how to use the ideal transformer model and show that both are equivament.Video Part 4In this video, I talked about the parasitic in a flyback converter and its effects on the supply. I show how to measure the parasitic and come up with a coupling coefficient which is then used in SPICE to model the leakage inductance. I then show how to snubber the effects of the leakage inductance by adding a snubber.Video Part 5In previous videos I have assumed ideal components and made assumption that efficiency is 100% and that the dutycyle of the PWM will be 50% in order to symplify the design and make the presentation easier to understand. In this video, I revisit the parasitic associated with non-ideal components and show how they effect the efficiency of the flyback. I also examine how to account for the PWM's minimum dutycyle when calculating the primary inductance.Video Part 6In this video, I show how to wind a simple flyback transformer, calculate the turns required to avoid saturating the transformer, I then measure the leakage inductance and calculate the coupling coefficient which I use in my SPICE model. I then measured the parasitic self resonance capacitance and also include it in my SPICE model.Video Part 7In part 7, I test the flyback transformer and compare the measurement results with simulation results. I show the breadboard techniques that I use that allows easy installation of the transformer and Mosfet (generally, the pins and long leads will give you more leakage inductance but because this is low power, I can get away with it). I present the ringing in the drain due to the parasitic leakage ringing with the mosfet's drain to source capacitance. And finally, I show how to damp the ringing by using a snubber.Comments and questions are welcomed.Robert

author: user_ivo
author_rank: Contributor
timestamp: January 07, 2013, 04:37:53 pm
content: I have been following your videos about the flyback and after that, a quick search i understood that, there are two different mode of operation, continuous and descontinuous, in the previous video of analysis of flyback this mode of operation is take in account? or the flyback will operate in both modes ?Thank,Ivo.

author: rbola35618
author_rank: Frequent Contributor
timestamp: January 08, 2013, 02:12:01 am
content: Hi user ivo,The example that I presented is for a discontinuous fly back. In the next video I will show that the current in the primary (flux) actually goes to zero current. The is done by making sure the the transformers gets reset and setting the proper turn ratio. I am hoping to have the follow by this Thursday.Thanks for the question.Robert

author: rbola35618
author_rank: Frequent Contributor
timestamp: February 06, 2013, 05:17:28 am
content: Second video.In this video, I briefly go over the design equations, values and then show how to simulate the flyback converter. I then compare the simulation results with the calculated values from my design example.Questions and comments are welcomed.Robert

author: user_ivo
author_rank: Contributor
timestamp: February 08, 2013, 12:59:08 am
content: Hi,I tried simulate the flyback but using the LT Spice instead, and by mistake I loaded the output with 50 Ohm instead of 24 like the video and I realize that the output value was bigger than 12V, reducing the load for 24 and i got the same result as the video. My questions is this behavior is normal, the transfer function Vo= N*(D/(1-D))*Vin only is valid if I load the output in this case with 0.5A.Thanks,Ivo

author: rbola35618
author_rank: Frequent Contributor
timestamp: February 08, 2013, 05:08:06 am
content: Hi User_ivoThis is a very good questions. Yes it is normal. It is called peak detecting. As the Rload resistance is increased, then the Vo=N*(D/(1-D) no longer applies. There is another equation that is in the Abraham Pressmen book that expresses Vo but in terms of Rload. The equation and will allow you predict the voltage. I will post the equation tomorrow if time permits.A flyback is different in that it stores the energy and then releases it. But instead of the Vo being more dependent on the equation (Vo=N*(D/(1-D)), it actually is more dependent or driven by the secondary current. It is a little complicated to explain here because there is a reset of the transformer that also comes into the picture.Try running a simulation of a 24 ohms and then a 100 ohms Rload. Look at the voltage from Vin-drain.During the time the mosfet is on, VLprimary = Vin-drain you will get Vin which is 24V. At Toff since the Vo for a 24 ohm load is and the Nr of the tranformer is 0.5 when have a reflected to the primary inductor VL=Vo/Nr which is -12V/(0.5) = -24V. This satisfies the volts second rule. But now if you run a Rload of 100 ohms, Vo will increase to about 50V. If you you now reflect this back to the primary, you should get Vo/Nr = 25/0.5 = -50 volts. Now the voltage second rule still applies. Let work this out,RL=100ohmsVo=25VTon = 5usToff = I don't know at this timeVL_Ton = Vin = 24VVL_reflected=Vo/Nr = 25V/Nr = -50v at is reflected backVL-Ton*Ton = VLreflected*Toff, Solve for ToffToff=2.4us, This means that because you have a higher voltage refected back to the primary, the transformer gets reset which means that it released all of its energy sooner.I will make another video where I will explain and show how the transformer get reset and also the effects on the output voltageStay tune for the next video.Robert

author: Marco
author_rank: Super Contributor
timestamp: February 08, 2013, 05:39:23 am
content: Quote from: rbola35618 on February 08, 2013, 05:08:06 amI don't see the value in thinking about flybacks in this way, needlessly confusing ... rarely are you designing power supplies for known loads.The basics of a discontinuous flyback transfers are simply that the energy stored in the primary inductance at cut off is transferred to the secondary, with secondary current starting at Np:Ns times the primary current at cut off and the primary and secondary acting as plain inductors at any other time. Output voltage is far less fundamental, best to not dwell on Vo calculations for known loads too much. You should be able to calculate them, but making a point of that early on distracts from the fundamental difference between normal transformer action and flyback. Better to just go straight to control loops.

author: rbola35618
author_rank: Frequent Contributor
timestamp: February 08, 2013, 06:16:50 am
content: Hi Marco,I agree, it can be confusing. You are correct that a discontinous flyback strores the energy in its primary and then releases it thru the turn ratio thru its secondary. I also agree with the rest of your feedback, I appreciate your feeback because it indicate to give me what I need to explain better.In my opinion, you need to know what is your maximum loads, input voltage range, ie minimum input voltage,and capacitance load so that you can compensate it properly.In the next video, I will explain it the way you mentioned and show the simulation. I like to show the effects of the leakage inductance. I am building this flyback (circuit, and winding my transformer) in my lab and I would like to compare calculated values, simulations and prototype measurementsKeep those comments-feedback comingRobert

author: rbola35618
author_rank: Frequent Contributor
timestamp: February 09, 2013, 02:48:43 pm
content: Video Part 3In this video, I introduced a second method in how to model the flyback transformer. In part 2, I used the coupled inductor model. In part 3, I show how to used the ideal transformer model and show that both are equivament.Comments and questions are welcomed.Robert

author: rbola35618
author_rank: Frequent Contributor
timestamp: February 16, 2013, 06:13:57 am
content: Video Part 4In this video, I talked about the parasitic in a flyback converter and its effects on the supply. I show how to measure the parasitic and come up with a coupling coefficient which is then used in SPICE to model the leakage inductance. I then show how to snubber the effects of the leakage inductance by adding a snubber.

author: rbola35618
author_rank: Frequent Contributor
timestamp: February 17, 2013, 01:06:19 am
content: In time I will. These video were originally intended for my fellow junior engineers as a way of mentoring them. After thr flyback, the next series will be a two swtich forward converter. After that soft switching techniques.Thanks for the comments.Robert

author: rbola35618
author_rank: Frequent Contributor
timestamp: March 06, 2013, 04:40:53 am
content: Video Part 5In previous videos I have assumed ideal components and made assumption that efficiency is 100% and that the dutycyle of the PWM will be 50% in order to symplify the design and make the presentation easier to understand. In this video, I revisit the parasitic associated with non-ideal components and show how they effect the efficiency of the flyback. I also examine how to account for the PWM's minimum dutycyle when calculating the primary inductance.Comments and questions are welcomed.Robert

author: rbola35618
author_rank: Frequent Contributor
timestamp: March 21, 2013, 12:44:33 pm
content: Video Part 6In this video, I show how to wind a simple flyback transformer, calculate the turns required to avoid saturating the transformer, I then measure the leakage inductance and calculate the coupling coefficient which I use in my SPICE model. I then measured the parasitic self resonance capacitance and also include it in my SPICE model.

author: user_ivo
author_rank: Contributor
timestamp: April 17, 2013, 03:29:11 pm
content: Hi Robert.Thank for the videos. Just a little comment, in Part4, in the formula to determinate the ringing frequency you forgot to include the sqrt. In the rest of the video you include the sqrt in the formula, so is easy for us to notice i guess, its just for put you aware of this, in the case you haven´t notice yet.I 'm looking forward for the next videos.

author: rbola35618
author_rank: Frequent Contributor
timestamp: April 17, 2013, 03:47:11 pm
content: Hi User_ivoThanks you for catching that error. I guess I had a senior moment againI will try to see if I can make an annotation on the Youtube video. I'm lucky to have smart people like yourself checking my math.Hoping to get Part 7 out next week. I suffered a micro-stroke about 2 months ago that affected my motor skills, speech and balance. I am slowly recovering.Again, many thanks for spotting the error and pointing it out to me!Sincerely,Robert Bolanos

author: rbola35618
author_rank: Frequent Contributor
timestamp: September 19, 2013, 06:00:18 am
content: Video Part 7In part 7, I test the flyback transformer and compare the measurement results with simulation results. I show the breadboard techniques that I use that allows easy installation of the transformer and Mosfet. I present the ringing in the drain due to the parasitic leakage ringing with the mosfet's drain to source capacitance. And finally, I show how to damp the ringing by using a snubber.