Flyback Equation - where the equation comes from?

[berni80]

Flyback Equation

Hi,

Does anyone know where the equation 33 of this Application Note comes from?
It's the control-to-output transfer function.

https://www.fairchildsemi.com/an/AN/AN-4137.pdf

How can I obtain it?

Thanks in advance

 

[eem2am]

Re: Flyback Equation

well

other readers will correct me,

but i am sure that most SMPS designers just copy these equations from books or app notes....

because most dont derive this control to output function.

if you want , you can check it by buying a £12000 gain/phase analyzer and checking the SMPS when you have it built..........

...but you will have to build it in such a way, with connections so you can couple in the signal from the gain/phase analyser.

if you like living with risk, you can settle for pulsed load tests...transient tests to check smps stability....

but remember switchingpowermagazine.com article will proove to you that this is inadequate and prooves nothing about stability

 

[berni80]

Re: Flyback Equation

Hi,

now my major problem with this flyback is the poor stability.
It regulate the voltage, but the PWM is not continuous and the control goes in a burst mode.

but i am sure that most SMPS designers just copy these equations from books or app notes....

I suppose so, but I need to understand it correctly to get my flyback stable.

One doubt is on the K factor (equation 33)... I do not understand how to evaluate it.

 

[eem2am]

Re: Flyback Equation

Christophe Basso's (smps) book goes on about the K factor for smps control.

Equation 32 gives the K factor...it is Iover/VFBsat
as it says there.

you are using a prescriptive application note and it looks to me that if you want to use their FPS device then you are just going to have to "suck their lolly" and like it.

goodness only knows how you will find out how they derived that stuff.

You are in the field of Feedback Control.......the biggest area of confusion and secrecy in the SMPS field.

Strictly speaking, there is so much stray components in SMPS that you cannot declare an smps stable EVEN if you have derived everything and done all your homework...because the strays may screw up your feedback loop.

strictly speaking, THE ONLY WAY you can proove your SMPS is stable is by using a gain/phase analyser when you have designed and built it.

What a lot ot people do, is they use huge amounts of secondary capacitance say, and/or make the Unity gain Xover frequency very low so that stability is almost gauranteed at the expense of a slow transient response.

they then do transient load tests to see if its ok.

-----even though load step tests cannot be used to gaurantee stability.

I have never met a switch mode power supply engineer who was capable of deriving control to output transfer functions.

generally speaking, its not worth doing as your going to check it and test it anyway, and its possible to find transfer functions in books etc

what about ericksons book "fundamentals of....."

i am told that is good on stability, but i dont know as i never saw it.

Added after 10 minutes:

now my major problem with this flyback is the poor stability.
It regulate the voltage, but the PWM is not continuous and the control goes in a burst mode.

...oh right, just saw this......do the bursts have a certain frequency?, so to speak....ie the envelope of the bursts has a certain (approx) frequency?.

if its bursting a few pulses and then going off then that suggests that your feedback loop is too fast.

so basically , get your geek hat on, and think how you can stop this.....

..what you need is to slow down the feedback signal by filtering it, so to speak.....using the feedback compoenents, -or filter it more than its already being filtered.

BUT


is this happening at very light load?

because if it is, then thats normal..................all smps's eventually go into burst mode at light enough load.
----------------------------------------------------------------------

If its a low power SMPS, then do you really care if its in burst mode?

Burst mode is bad with higher power smps's because the burst frequency tends to be so low that it is not sufficiently attenuated by the EMC filter and it can give you a conducted EMC problem.
---------------------------------------------------------------------
Radar power supplies should not be in burst mode as the bursting gets coupled to the RF signal and your enemy can get the "signature" of your radar and know that you are coming and blow you away b4 you have supped your rations of Army Tea and Biscuits.......as well as that nice tinned sponge pudding that they have.

 

[berni80]

Re: Flyback Equation

Thanks for your answer..

Is the Christophe Basso's (smps) book available in pdf?

I'm using this application just for reference, but I'm not using the

> Equation 32 gives the K factor...it is Iover/VFBsat

Understood, but I cant understand Iover and VFBsat..

Is Iover the MOS peak current at full load?
what about VFBsat? Is device dependent? Is dependent on shunt I use to sense the current?

> is this happening at very light load?
> [cut]...........all smps's eventually go into burst mode at light enough load.

Ok, this in normal

Bye

 

[Avizor]

Hi,

I have exactly the same problem. I've been looking for information for two weeks, but I can't find a solution.

berni80, did you solve this problem? Or does anyone know where are these control to output transfer functions derived?

**broken link removed**, there's a reference (appendix B of Unitrode's SEM700), but the appendix isn't available anywhere either.

 

[BuckySparks]

see the appendix of the following paper from the 2003 seminar, it shows the derivation of the flyback small signal model.

the 1990 appendix b is most likely a paper from of the previous seminars, the link below has many of the papers

check the 2003 seminar paper titled "Achieving High-Efficiency w/Multi-Output CCM Flyback Supply"
https://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM401001

 

[Avizor]

check the 2003 seminar paper titled "Achieving High-Efficiency w/Multi-Output CCM Flyback Supply"

This is EXACTLY what I was looking for. There's an explanation in the last 3-4 pages of the document.

Thank you very much BuckySparks!!!