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UCC2895: Output voltage drops down when power up ( phase shift topology)

Part Number: UCC2895

Hi Experts;

I am currently facing a problem regarding the output voltage. Nearly all test conditions are normal but when I increased output power, up to 3300watt for instance, the output voltage drops approximately 10-20V. In other words, output is normal, 220V for example, for light load.  But operation is abnormal when I turn to heave load ( 10ohm). 
I am wondering that this type of controller has the current clamping mechanism or something goes wrong with the design parameter. 
I am also wondering that if the current sense transformer has the problem, for example, noise level is large. That may induce this issue?

Thanks for your reply and kindly regards 

Yishan 

  • Hi Yishan,

    Thank you for the query. We are looking into the issue and get back to you shortly.

    Regards,

    Harish

  • Hi Yishan,

    My first thought is that the controller is going into current limit at full load - have a look at the CS pin and see if it is getting to the current limit trip point. If so then you should change the scaling of the CS signal. Again noise on CS pin can also affect this. Can you please probe the signal on CS/ EAOUT pins

    The other thing that might be happening is that the controller is running into a Dmax limit. The duty cycle of the PSFB is not a strong function of load but it is possible. Can you probe the voltage across the primary of the transformer (using a differential probe) if this is not possible then you can probe from 0V out to the centre tap of the transformer secondary. This will allow you to observe the duty cycle directly. The duty cycle is normally designed to be about 60% to 80% during 'normal' running.

    Please let me know if you have any questions.

    Thank you

    Regards,

    Harish

  • Thanks for your reply, but of cause indeed we found that the noise level is higher at some cases. I am confused that if the sensed current is overlimited, the controller would be shut down rather than clamp the current right? I am curious about the mechanism of this and which signal may clamp the current.

    Thank you

    Regards

    Yishan

  • Hi Yishan,

    Thanks for the reply.

    If the current sense signal reaches the 2.5V, an output disable initiated by this overcurrent fault and results in a restart cycle, with soft stop, followed soft start. This can be checked by probing the SS pin which will discharge to 0.5V and restart. 

    There is also a 2V current sense comparator in peak current threshold performing cycle by cycle current limit and this could potentially limit any further increase in input current. Because of the limit on the input current, the output voltage starts to reduce and the duty cycle reduces too.

    You might want to check the CS/SS pin as one of the above might be causing this voltage reduction at full load.

    Regards,

    Harish