UCC28780: Understanding the UCC28780

Mo Mo

Part Number: UCC28780
Other Parts Discussed in Thread: LM25069

Hello,

I'm designing an AC/DC power supply (115Vac/400Hz - 32Vdc 30W) and i'm trying to use the UCC28780 active clamp flyback using Si MOSFETs.

Why in the electrical specifications in page 38 in the datasheet, the maximum frequency is 63Hz ?

What is the conduction mode of this controller ? Is it DCM or Critical conduction Mode ? i can't really understand it because apparently the switching frequency change with the load, what about the duty cycle ?
How the switching frequency is changing vs load ? same question for duty cycle.
Knowing that the AC line voltage is 115Vac, what is the DC voltage+/- seen by the transformer ?

Thank you in advance.

Have a nice day.

over 4 years ago

0 Mike O' over 4 years ago

TI__Guru 66135 points

Please see attached,

Hello Mohamed UCC28780 Frequency Variation Prediction.pdf

0 Mike O' over 4 years ago in reply to Mike O'

TI__Guru 66135 points

Also Mahamed,

If you have not done so you probably should order one of the EVMs to evaluate.

The excel design at the following link will also help you with your design.

Regards,

0 Mo Mo over 4 years ago in reply to Mike O'

Prodigy 130 points

Hello Mike,

Thank you for your answer.

I have another question concerning the hold up time and how the inrush current is managed by the controller.

In my application, a high hold up time is needed >200ms, which result in a high output capacitor >50mF and thus a high inrush current and as i should limit this current to 10 times the nominal output current, i'm planing to design with lower Cout, and include an active circuit between the low Cout and the 50mF which delays charge up of the 50mF until Vout is regulated.

Do you recommand any reference for this type of active circuit ? I thought about the TPS259827L or the LM25069, what do you think about it ?

Thank you.

Regards,

Mohamed

0 Mike O' over 4 years ago in reply to Mo Mo

TI__Guru 66135 points

Hello Mahamed,

The UCC28780 does no limit inrush current. So you are correct and external current limiting circuit is required. The TPS259827L or LM25069 may work for your design.

In regards to holdup time I would suggest increasing the input bulk capacitor to the design instead of the output capacitor. This is lower current and will require less capacitance.

Regards,

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UCC28780: Understanding the UCC28780