Flyback converter common mode filter impact on stability

[fchastel]

Hi all,

I recently to take over the design of a colleague who has left our company and and have difficulties with it. It is a current mode Flyback converter operated in DCM. The circuit is very similar to that described in Plexim "Design of a TL431-Based Controller for a Flyback Converter".
The input side consists of a common mode choke and a capacitor (Ccm-) between the return line and the case. The input also implements an LC filter (=> differential mode noise).
I have several questions:

- to mitigate common mode noise, I would expect a second capacitor (Ccm+) between the line and the case. Should one expect any positive effect when only one capacitor is installed?

- Ccm has a drastic effect on the flyback. Without the capacitor, I can see a lot of jitter on the PWM pulse width (when the flyback is loaded with constant dummy loads) whereas when Ccm is installed, the PWM pulse with is perfectly stable. Can someone explain me the positive benefit of Ccm on the converter?

- while trying to search for information on this issue, I've seen that many designs have a small capacitor between the primary and secondary ground. Should I add this capacitor also or is it only required in very specific cases?

Thanks in advance for your help ad best regards
Fred

 

[FlapJack]

Ccm is parasitic capacitance from any part of the flyback to earth ground/case.

I found "Design of a TL431-Based Controller for a Flyback Converter", and it does not have an input filter or any capacitor to the case.

 

[BradtheRad]

- Ccm has a drastic effect on the flyback. Without the capacitor, I can see a lot of jitter on the PWM pulse width (when the flyback is loaded with constant dummy loads) whereas when Ccm is installed, the PWM pulse with is perfectly stable. Can someone explain me the positive benefit of Ccm on the converter?

Compared to DCM, through the identical transformer, CCM has smaller Ampere swings, at a higher frequency.

As long as you keep Ampere level out of saturation, CCM can obtain more net power through the transformer.

- while trying to search for information on this issue, I've seen that many designs have a small capacitor between the primary and secondary ground. Should I add this capacitor also or is it only required in very specific cases?

Thanks in advance for your help ad best regards
Fred

It probably is not essential to operation, however the capacitor may be included for all-purpose reasons, same as with many appliances.