LM5168: Why the secondary output voltage will drop significantly during heavy load while the primary output voltage will rise a little bit, why the output ripple will have a pulse

Hi Change,

Flybuck designs need a minimum output current in order to create enough ripple for the FB pin. That's why there's some strange behavior especially when the current on the secondary side is 0mA.

This problem should go away if you add a minimum load at the output.

Thanks,
Andrew 

Hi Change,

Your welcome. Have a nice day!

Thanks,

Andrew

Hi Andrew,

Can you tell me why a pulse is produced in the ripple wave?

Vout2 ripple have a pulse, I want to understand why it happen.

Thanks,

Change

Hi Change,

I believe the pulse could be sudden decrease in current on the IOUT2. Does adding a 100nF capacitor at both the primary and secondary sides get rid of the pulse?

Thanks,

Andrew

Hi Andrew,

I want to know why this pulse is generated by secondary output ripple. Just like this one.

I guess this pulse noise is probably due to the reverse recovery of the diode.

How do you think?

Vo2 ripple.pptx

Hi Change,

I was thinking that it had to do with the sudden change in current as V = L(di/dt). A sharp change in current is going to cause a voltage swing.

I kind of implied that in my previous post as shown in the wave form (a sharp drop in Iout2), but it was a bit unclear.

Thanks,
Andrew

Hi Andrew,

I think you're right; I analyze this situation due to the sudden change in current caused by diodes.

Thanks,

Andrew