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UCC28880: UCC28880 / UCC28881 Flyback Primary Current Oscillation

ALIREZA TAJFAR
Prodigy 150 points
Part Number: UCC28880
Other Parts Discussed in Thread: UCC28881,

Hi,

I am using these two chips in an isolated Flyback converter (Vin = 300V, Vo=700V) and with the same transformer (Lpri = 5mH; n=1:2) I get different results.

When I use UCC28881 everything is perfect and I can charge the output capacitor at 700V fast enough as needed by the application.

However, when I change the IC to UCC28880 (which has a lower current limit) the primary current is somehow disabled and I see only an spike for about 200ns (it is not the lower current limit of this chip or the transformer saturation).

Now, when I change the input voltage to 200V, I clearly see a ramp-shaped input current but with higher oscillation as compared with UCC28881.The oscillation frequency is  around 500KHz.

Cin = 100nF

L_primary = 5mH

Can one of you experts please help me to find out the reason for these oscillations and resolve the issue?

V_IN = 300V, CH4: Primary current of Flyback transformer, CH2: Drain-source voltage

V_IN = 200V, CH4: Primary current of Flyback transformer, CH2: Drain-source voltage

Please note the oscillation (around 500KHz) at the very start of the primary current ramp which is not damping fast either.

  • 0
    TI__Mastermind 26215 points

    Hi Alireza,

    Normally this type of ringing would be due to parasitics connected to the switch node (capacitance from drain to ground, leakage inductance of the transformer, parasitic capacitance on the transformer, layout parasitics, etc). The ringing comes very close to the minimum on time for UCC28880 (220ns typical) and may be the reason the controller is going into current limit. The UCC28881 has a longer minimum on time of 270ns typical.If you tested the UCC28880 on the same board as the UCC28881 I would suggest cleaning any flux off the board that may have been there from the soldering process. You could also try increasing the strength of your clamp to see if this reduces the ringing.

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Prodigy 150 points

    Thanks for your response. So you think is is going to current limit mode because of oscillations? 

    But how does this behavior depend on the input voltage? As when I change the input voltage from 300V to 200V I can see a ramp current waveform on the primary of transformer.

    Thanks again for your help and support.

  • 0 in reply to ALIREZA TAJFAR
    TI__Mastermind 26215 points

    Hi Alireza,

    My feeling is with the high input voltage, the drain dV/dt when the MOSFET turns on will be larger and results in larger oscillation amplitude. 

    Best Regards,

    Ben Lough