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WEBENCH® Tools/LM3424: Main mosfet power dissipation variations with Inductor

Mario Jimenez
Prodigy 200 points
Part Number: LM3424
Other Parts Discussed in Thread: CSD19534Q5A

Tool/software: WEBENCH® Design Tools

Hello,

I'm getting a lot of difference on M pd, M1 PdCond and M1 PdSw values under Op Vals tab when I choose a custom inductor( I barely move the suggested values) This is my input:

LED Operating Current 0.8A

Vforward 2.9V

Rdynamic: 0.174ohm (Cree's  XPLBWT-00-0000-000BV650E)

#Series: 8 (23.2V)

#Paralel: 2 (1.6A)

Vin Min: 20V Max 36V Amb. Temp. 30°C

Switching Frequency :100kHz

When I look at the Op Vals tab with this set-up, under power values categorie I get this values:

0.11W+0.08W+0.03W=0.22W

Webench could not find a suitable inductor for the design, but the custom inductor has this values:

L=78.589uH, DCR=0.02Ohm and IDC=8.09

When I try to select an alternate part, with custom part values, like L=82uH and everything else stays the same, I get this values on the new design:

0.28W+0.19+0.09W=0.56W!. Thats more than twice the first calculation. Is this calculation mistaken/bugged?

I'm trying to figure out if 2300HT-820-V-RC is a suitable inductor for this design without affecting the N MOS' power dissipation and conduction/switching loses.

Thanks in advance.

  • 0
    TI__Mastermind 25500 points

    Hello Mario,

    I'm not sure of the exact differences between the two, but I can tell you webench is not perfect. I looked at the inductor you are talking about and it should be just fine. If you use a good FET that should be fine too. It looks like it may be making some assumptions that I'm not quite sure of, reducing the ripple should reduce the RMS current in the inductor and should help (due to the higher value). So like I said, webench isn't always perfect. It just looks like it might be flagging an efficiency concern. If you are using a good FET and diode you should be absolutely fine with that inductor. If you want to be safe use a common diode and FET footprint so you can try various components if needed for them.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 200 points
    Hello Clinton,

    Thanks for answering.

    Yes I'm picking that inductor. I'm also considering to use the recommended FET by WEBENCH (CSD19534Q5A). It's a pretty uncommon footprint but I think BSC117N08NS5ATMA1 has a compatible footprint, but not sure because dimensions change a little bit.
  • 0 in reply to Mario Jimenez
    TI__Mastermind 25500 points

    Hello Mario,

    Sounds good. I'm a big fan of toroids, in fact they lose less inductance versus current so the AC losses might actually be better. Both of those FETs are good as well. I'm sure I should say use the TI FET, it is a good one, but the other one is very good as well. It looks like the pin spacing is the same, so if you just use some solder mask cutout on your PCB you should be able to swap one for the other if you would like to see what works best for you.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 200 points
    Hello Winston,

    Thanks. I think i'll make a footprint that could match those FETs.
    Looking at the stock of that inductor, is pretty low. I was considering to use PM2120-820K-RC instead, but it has a lower IDC(6.4A) and is cheaper as well. Is there any reason WEBENCH suggest such high IDC on the inductor? Because under Op Vals tab, L1 rms is 3.5A estimated, 8A is more than twice that value. PM2120-820K-RC isn't that high but I guess it could work. Both have similar DCR.

    Thanks.
  • 0 in reply to Mario Jimenez
    TI__Mastermind 25500 points

    Hello Mario,

    I'm not sure exactly all of the parameters webench selected for your design, but it is probably rated that high because it either so that it doesn't lose more than 20% of its inductance during normal operation and/or if the circuit hits its current limit. You do not want the inductor to saturate if current limit (set by the IS resistor) is hit for some reason. You can always lower the current limit if it is set high and use an inductor with a lower current rating. But the current limit, with tolerances, needs to be higher than the maximum peak inductor current at your minimum input voltage and maximum output voltage.

    Regards,

    Clint