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UCC28950: Miss switching cycles occasionally from half load to full load

Eric Lai
Expert 4940 points
Part Number: UCC28950
Other Parts Discussed in Thread: UCC28951

Hi Expert,

We see the UCC28950 miss switching cycles occasionally from half load to full load.It cause the system level failure.

 

Pls see the major parameters and project information:

Applicant : PSU 

Output power :1000W

Vo:12V

Issue waveforms:Pls see attachment

Key parameter of design:

--Transformer ratio: 47T:2T:2T

--60KHz switching Frequency

 

Now the issue can be avoided from change the Tmin from 130K to 30k.The engineer want to know the root cause and concern if had.   

Pls help to clarify:

1)      Why change the Tmin from 130K to 30k can solve the switching cycles missing

2)      Can parallel a small cap on Tmin pin for noise immunity improvement

3)      To charge the Tmin from 130K to 30k can help the noise immunity

4)      Can TI share detail Block Diagram to know the Tmin behavior

5)      Any suggested solution to slove miss switching cycles occasionally if Tmin from 130K to 30k is not preferred.

 

Best regards,

Eric Lai

  

Field Application Engineer

Texas Instruments Taiwan Limited

O: +886-2-2175-2582

M: +886-909-262-582

E-Mail:Eric.Lai@ti.com        UCC28950 miss switching cycles Issue.docx

  • 0
    TI__Mastermind 25570 points

    Hello Eric

    I'd suggest that the customer try the UCC28951. This is a 100% compatible, drop-in replacement for the UCC28950 and no changes to external components are necessary. The reason I suggest this is that the UCC28951 works better at wide duty cycles while in current limit and I think that the customer system may be reaching this condition during the transient. If this prevents the problem then I'd recommend that they change their design to use the UCC28951 device.

    I don't know why lowering Tmin would affect the behaviour but it will affect the point at which the system enters burst mode so the customer should check that this is acceptable to them.

    It's not normally necessary to add a capacitor at the TMIN pin but the voltage at the pin is DC so adding a small capacitor is ok.

    I'm afraid that I cannot share details of the internal structure of the device.

    Regards
    Colin