UCC28911: How to improve the effect of the magnet on the output

Hello,

Thank you for your interest in  UCC28911.

I believe your flyback transformer is saturating at the peak of the primary line, with the increasing magnetic field(H) the flux density(B) will move towards its saturation value. In order to prevent fast saturation, it is helpful to make the transformer with a high number of primary turns because it reduces the magnetic flux ϕ . The reason for this is that a core with a higher magnetic reluctance Rm (due to larger air gap) must be used to maintain the same primary inductance. The flux density is, simply, the magnetic flux ϕ divided by the effective core area Ae. Therefore, a high number of primary turns (limiting flux ϕ) combined with a large effective core area Ae, reduces the flux density in the core. The tradeoffs are a higher core and winding losses.  

In order to make the power supply robust against the external magnetic field (300mT), you will need a different transformer. A Sendust core transformer is normally a good compromise between the robustness and efficiency from the previous experience. There are some less efficient Iron power cores which can withstand higher ext magnetic field.

The important thing to note here is that the transformer provides an ideal path for the external magnetic field, therefore the core “amplifies” the external field, and if the magnetic field lines of the external magnet have the same direction as the transformer flux density, the field inside the core will be much higher than 300mT. For this reason, the orientation of the transformer plays a critical role.

I hope this helps.

Regards,

Sonal

Hi Sonal,

Thank you for your reply.

I found another issue above. With the same flyback transformer,the same flux density B and magnetic flux ϕ ,AC-DC power made of Primary-Side Regulation is more terrible than that of Secondary-Side feedback.So,the issue is whether the Primary-Side Regulation have a more effect on the magnet?

Best regards,

Tingting Cui

Hello,

This is expected behavior of a PSR feedback because in PSR the output voltage is sensed across the auxiliary winding of the transformer and when the external magnetic field is introduced it changes the flux density of the flyback transformer and thus the voltage on the aux winding changes.

In an SSR feedback, the output voltage is sensed directly from the output terminals and thus the introduction of a mag field will not affect the regulation.

I hope this answers your question.

Regards,

Sonal

Hi Sonal,

Thank you for your reply again.

Now, I would like a AC-DC power with ucc28911 to work well with a magnetic body(300mT), what can I do?
Changing the orientation of the transformer can reduce the impact,but not solve the problem. I think that a high number of primary turns and using the Sendust core will be the same .Are there other methods?
Or from your previous experience, can the design ,a AC-DC power with PSR, achieve against the magnetic field though some methods?

Best regards,

Tingting Cui