transformer operated in flyback mode vs sine wave input - Page 1
Forum section: electronics / beginners

author: mercurial
author_rank: Regular Contributor
timestamp: April 26, 2023, 03:40:07 pm
content: I have a few questions regarding transformer operating modes1. If I operate a transformer primary in on-off mode, switch it on off with a square or pulse signal, does it make it a flyback mode of operation.2. In this case how are the kickback voltage of the transformer handled, do we just ensure that the secondary is loaded and the kickback transfers to the secondary, does the switch have to be rated multiple times the supply voltage for it to handle the spikes generated by the transformer primary.3. Are there any techniques to protect the switch from the kickback in case the secondary is not loaded?4. Does flyback mode produce more output voltage for the same turns ratio and given input, due to the kickback generated by the transformer primary.

author: Benta
author_rank: Super Contributor
timestamp: April 26, 2023, 07:25:15 pm
content: Homework?

author: mercurial
author_rank: Regular Contributor
timestamp: April 27, 2023, 05:38:47 am
content: Not really I was pulsing a transformer primary to create sparks across the terminals of the secondary winding and was wondering what is the most efficient way to do that,1. is flyback mode better, that is ramping the current up in the inductor and then cutting it off at the correct time so that it releases the energy into the secondary and creates a spark. (similar to automobile spark ignition system)2. Use a oscillator and oscillate the primary at high frequency (50Khz +) and get the high voltage on the secondary after rectification. (similar to mosquito zapper)

author: David Hess
author_rank: Super Contributor
timestamp: April 27, 2023, 11:07:09 am
content: Quote from: mercurial on April 26, 2023, 03:40:07 pmYes.QuoteThere are two different voltages on the primary side after the switch turns off. The first comes from the leakage inductance of the primary and is only limited by the impedance, primarily capacitance, across the primary. After the leakage inductance discharges, the second voltage is the output voltage reflected through the turns ratio of the transformer.QuoteThe switch needs to be protected from the leakage inductance spike whether the secondary is unloaded or not. The switch could have a high enough voltage rating to ignore it, or a snubber in some form could be could.QuoteYes.Quote from: mercurial on April 27, 2023, 05:38:47 amIt may not be the best way for every application, but it certainly works and is the easiest way.QuoteThat is how it is done. The output of the flyback transformer can also directly drive a capacitor-diode voltage multiplier for even higher voltage without increasing the voltage across the transformer.

author: MrAl
author_rank: Super Contributor
timestamp: April 27, 2023, 11:31:43 am
content: Quote from: mercurial on April 26, 2023, 03:40:07 pmHi,I think you always need a snubber of some type to protect the transistor because the transformer coupling will never be a perfect 1. That means even when the secondary is loaded that doesnt mean the primary is loaded enough to keep the voltage at an acceptable level. For example, if the turns ratio was 10 to 1 and the coupling was a perfect 1 and input was 10v, the output would be 100v exactly, but if the coupling was less than 1 (as in all practical designs) then the input could be 20v and the output 100v, or the input could be 100v and the output 100v, or the input 1000v and the output 100v, at least for a short time. During this short time there has to be something to limit the voltage across the transformer.This is actually the case with many converters that depend on transformer coupling to lower primary back EMF because transformer coupling is never perfect.Before you dive into this though you should really read up on converters and especially flybacks. There are many other issues that will come up such as feedback and control stability.

author: mercurial
author_rank: Regular Contributor
timestamp: April 28, 2023, 07:15:51 pm
content: Quote from: David Hess on April 27, 2023, 11:07:09 amI used a high voltage switch more than 10 times the dc supply voltage however I found that the transformer was getting damaged due to the high voltage spikes is there some protection needed for the transformer? I have seen a few circuits with a zener connected across the switch.

author: David Hess
author_rank: Super Contributor
timestamp: April 28, 2023, 10:17:55 pm
content: Quote from: mercurial on April 28, 2023, 07:15:51 pmProtection of the transformer may be required in higher voltage applications.

author: mercurial
author_rank: Regular Contributor
timestamp: April 29, 2023, 05:34:18 am
content: Quote from: David Hess on April 28, 2023, 10:17:55 pmDo you know what core material is preferred in flyback or single pulse flyback operation is it ferrite, or gapped cores or powder cores?

author: mercurial
author_rank: Regular Contributor
timestamp: April 29, 2023, 07:29:58 am
content: Hi,Can anyone say why ignition coil is not operated in step up mode instead of flyback mode.By step up mode I mean when the primary current is ramping up isn't the secondary voltage also increasing at the same time by the turns ratio? Is this turns ratio by itself not sufficient to breakdown the gap, as an alternative to the collapsing magnetic field.

author: MrAl
author_rank: Super Contributor
timestamp: April 30, 2023, 02:35:41 am
content: Quote from: mercurial on April 29, 2023, 07:29:58 amHi,Wouldn't you need many more turns for that?In theory if you apply 12v to a primary and you need 12000 volts out, that's a turns ratio of 1000 to 1, even if just for a short time. On the other hand, when you open circuit an inductance the voltage spike goes to infinity in theory and in practice it still goes up pretty high, much higher than the turns on the coil or any secondary turns would produce. This can be looked at as the volt seconds as a replacement for the energy. If you put in 1 volt second then you get 1 volt second out, regardless what the turns are, in theory. When you energize a coil with 12v for 1 second that is 12 volt seconds, and if you then disconnect it and have a 120v zener across the coil the voltage will jump up to 120v but only last for 0.1 seconds. That is because 12*1=12 and 120*0.1=12, both are 12 volt seconds. If you instead use a 1200v zener, the voltage will jump up to 1200v but only last for 0.01 seconds, because 1200*0.01=12 volt seconds again. Now of course a 12000v zener would cause it to jump to that voltage but then only last for 0.001 seconds. You can see how this works, we dont even have a secondary yet we are getting a very high voltage.There is also the required load current to consider too though, and just using an inductor alone probably wouldnt work as good, so we use an inductor and a transformer (ignition coil) to get a higher voltage. We see the jump in voltage combined with the step up turns ratio producing a high voltage that also has some significant energy too.

author: mercurial
author_rank: Regular Contributor
timestamp: April 30, 2023, 03:38:23 am
content: Quote from: MrAl on April 30, 2023, 02:35:41 amHmm., I seem to get it now, thanks for the explanation.I also read that they used to also add a cap in parallel with the switch to resonate it with the primary this way they get a stronger spark, not sure if they still do it with the electronic ignition systems that they currently have.BTW do you know what kind of core is preffered for these applications do they use laminated silicon steel or ferrite or powdered cores? Does the core have a gap in it?