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Original question:

UCC28950: PMP8740 pulse generation issue

UCC28950: PMP8740 heating issue

Nidheesh BR
Expert 1295 points
Part Number: UCC28950
Other Parts Discussed in Thread: PMP8740,

Hi,

While developing PMP8740 reference design,  It is noticed that Q8 & Q15 are getting more heated at the same time Q9 & Q16 seems to be so cool(It is a self-made board).

I have shorted the shim inductance T7 footprint as it is not using. The issue is the same with and without freewheeling diodes D7&D10.

What may be the cause of this?

(The sound of witches seems to be so loud)

  • 0
    TI__Mastermind 25570 points

    Hello Nidheesh

    Assuming that you are using the same reference designators as in the pMP8740 then what is most likely happening is that the PA leg of the bridge which is being switched by Q8 and Q15 has lost ZVS while the AP leg switched by Q9 and Q16 is still maintaining ZVS. 

    The easiest way to see this is to look at the gate/source voltages on the bottom switches. If the switch is achieving ZVS then there will be no 'Miller Plateau' on the gate to source waveform. You should be able to confirm my suspicion by comparing the Vgs on Q15 and Q16.

    Loss of ZVS is not catastrophic but as you have noted the device temperatures will increase due to the increased switching loss and you will probably find an increase in the high frequency EMI spectrum because the dv/dt at the switched node is increased.

    The solution is to increase the amount of energy available to drive the ZVS transition on the PA leg by adding in some leakage inductance or adding a shim inductor. If you do add a shim inductor - you should also use the clamp diodes D10 adn D7.

    The effect is explained in the attached.

    /cfs-file/__key/communityserver-discussions-components-files/196/8461.ZVS_5F00_SubSet.docx

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Expert 1295 points

    Hi Colin,

    Thank you for your information.

    Let me check the waveform and will get back to you soon.

    In the meantime can you please give me any datasheet or any other information regarding T7 transformer.

    Regards.

    Nidheesh

  • 0 in reply to Nidheesh BR
    TI__Mastermind 25570 points

    Hi Nidheesh

    T7 is actually an inductor not a transformer - it has only a single winding and an inductance of 300uH. I don't have any information about it other than what is in the BOM for the PMP8740 design and I have copied this below.

    This inductor is a 300uH part however you should calculate the amount of inductance you need in your design from the formulae in the word document in my previous post - or use the Excel calculator for the UCC28950. To be honest, I'm surprised that the value is as high as 300uH but I haven't done the calculations.

    There are example calculations for the shim inductor in the UCC28950 data sheet and (P21) in the document at https://www.ti.com/seclit/ml/slup348/slup348.pdf

    I'd suggest that you make a provision for a shim inductor in your prototype even if your calculations show that you don't need it. It's always possible to put in a wire link instead of an inductor !

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Expert 1295 points

    Thank you Colin

    I will proceed accordingly.

  • 0 in reply to Nidheesh BR
    TI__Mastermind 25570 points
    Hi Nidheesh

    I'm going to close this thread, please feel free to re-open it or to open a new thread if you have any further questions.

    Regards
    Colin