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EMI performance data?

Martin55843
Intellectual 400 points
Other Parts Discussed in Thread: TPS54527

I am looking for a synchronous buck converter, 6-9V input to 3V (adjustable) output, 3-4 A. As EMC compliance is an issue in my application, I'd like to use a converter with low radiated noise. I was looking for a TPS54527, which has the FETs integrated - this means there is no way of influencing switching speed (like reducing speed in order to reduce noise). Is there any EMI data available from TI? Or recommendations which families are better suited for reduced EMI? The only way I see right now is to purchase an eval board and do measurements with the spectrum analyzer.

  • 0
    TI__Guru**** 191445 points

    In general we do not supply any EMI data.  EMI is typically measured at the end item, where you will have some type of encloseure to aid in reducing radiated emissions and probably filters at the line input to suppress conducted emissions.  We only supply the ICs and do not have any control over the many possible end item configurations.  I can tell you that radiated noise is mostly due to layout and inductor selection.  In ultra sensitive applications it is also possible to shield the entire converter assembly or that portion on the pcb board.  I have seen this often in radio receivers and GPS units.

    I know for a fact that we do not have any EMI data for the TPS54527.  Also you should knoe that the TPS54527 uses DCAP-2 control with psuedo-fixed frequency.  There is no internal clock, only adaptive on time, semi-hysteretic off time, so the observed operating frequency will vary with time even under static line and load conditions.

     

  • 0 in reply to JohnTucker
    Intellectual 400 points

    You write inductor selection influences radiated noise. Can you elaborate this? Assuming I am using a shielded inductor, I am not sure if there are huge differences between similarly specified (eg. 10 uH 5A) inductors. On different projects I have seen about 150 MHz ringing on the switch node. I am not sure how much this is due to layout or parasitic (inductor?) effects.

    From my experience with MOSFET half bridges, a nasty property is the possible shoot-through when the topside FET switches on. This is caused by the high du/dt on the drain of the lowside fet, leading to temporary switching on (coupled by the miller capacity). These spikes cause wideband noise and there is little do do about it. Do you know if the integrated FETs of the TPS54527 (or other switchers) are immune to this? Does it, in your opinion, make sense to add a snubber circuit to the switching node? Adding a small resistor in series to the bootstrap capacitor  (VBST pin) would also delay the switching on of the high side FET - would this be an option or would it mess with the nonoverlap circuitry?