Query on Flyback waveform

[juihau]

flyback waveform

Hi, I found a problem while monitoring a flyback design.

There is too much oscillation at the drive winding when the PWM turn on. The drive winding waveform should be similar to the VDS. Considering parasitc components, I didn't expect the oscillation to be so severe.

Currently I'm still trying to find the root cause. I would appreciate if someone could offer some advices.

Thank you very much.
Ric

 

[eem2am]

flyback leakage inductance

no this is normal..its ok.....no worries

go to powerint.com and watch their apps tv...one explains for you that wavefrom.

if you are clever , you are using that waveform to tool yourself up with some decent low voltage switching...in fact. yuo may even want to shove some Capacitance DRAIN_source in there to make it bigger and go to zero for you to get ZVS

The oscillation is an LC oscillation caused by the interwinding capacitance ringing with the transformer primary inductance.

at turn off, there is usually higher freq wave and this is LC oscillation of transformer leakage inductance and the transformer interwinding capacitance

 

[eem2am]

waveform on flyback

if you see SOME transition mode flybacks with a feedback DEMAG winding,,,,,they actually make USE of this effect that you have seen and been so worried by......................but you not worry..........that oscillation is normal, and can be harnessed if you buy nice controller for you.

 

[juihau]

waveform voltage on flyback

Thanks eem2am for yr advices & recommendations.

You gave me the direction to tackle this problem. Despite the oscillation is normal but I found the osc noise is coupled to the secondary outputs. It is annoying.

I will do some study soon to try minimize it. Now I can only think of to play around with the windings of transformer & primary inductance.

 

[eem2am]

current waveform in flyback

i think on switchingpowermagazine.com there is something for you there?

but you say it is coupled to the secoondary.......well.....yes.......but....ok there is coupling....but you have forgotten(?) that on the secondary you have the diode swithcing on and off and that is causing the oscillation there off itself.........because the secondary can see the transformers interwinding capacitance and also it obviously can see the secondary inductance....so it has its own LC oscillation situation there..........you cannot stop this, it is normal......you can use RC snubber to smash it over the head if you like a bit..............