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LM5122-Q1: About Frequency Selection and Warming Problem

Part Number: LM5122-Q1
Other Parts Discussed in Thread: LM5122, PMP7936

Hi,

I am using LM5122 for start-stop application. I developed pcb and tested. Used PMP7936 LM5122 reference design. Its below.

Design is working . But i have a problem. When the frequency value is 250 KHz, fets are warming up to 100°C. I solved this problem by reducing the frequency. I couldn't find anything about the minimum frequency.

I want to learn;

* Is it correct to lower the frequency to reduce warming?

* What is the advantage-disadvantage of low / high frequency?

* What else can be done to prevent overheating at high frequency? (I change snubber components. Another else?)

Your help is very valuable to me. Thanks in advance.

  • HI Onur,

    Thank you for considering the LM5122 in your design.  The IC is characterized between 100kHz and 1000kHz.  In the datasheet EC table, the VSLOPE is tested at 100kHz.  

    * Is it correct to lower the frequency to reduce warming? 

    A: Yes.

    * What is the advantage-disadvantage of low / high frequency?

    A: Higher frequency can help reduce the size of the inductor and capacitors for the same power than the lower frequency.  However, a too high frequency may become prohibitive owing to the increased switching frequency.  It is always a trade-off between the solution size and efficiency.

    * What else can be done to prevent overheating at high frequency? (I change snubber components. Another else?)

    A: In addition to the snubber, you may need to re-select the FETs and inductor to get the best tradeoff between the switching and conduction losses in your circuit. You circuit layout may also need to be improved to reduce both the switching and conduction losses.  Please reference to our EVM layout and also following the layout guide lines presented in the datasheet.

    Hope this clarifies.


    Thanks,

    Youhao Xi, Applications Engineering