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LTC3784 (LTC3787)- 1200W 4 phase FCC design - Hot inductors while idling, no load connected

Hi Linear-Staff,

we designed a 4-phase layout with the LTC3874 in a 2/4 phase arrangement. Everything seems to perform like expected but the inductors getting really hot when using FCC with no load connected.

The design is made with the following specs:

fsw: 230kHz

FET/Sync: Toshiba TPH2R506PL

inductors: Coilcraft XAL1580 3uH

DCR sensing

VIN: 10-45V

VOUT: 51V

IOUTmax: 23A

POUTmax: 1200W

Using Burst-Mode or Pulse-Skip of course wont heat up the inductors. Any hint? I can't see any frequency-jitter, compensation seems to work fine as well (with rather low bandwidth of roughly 4kHz (70+dB PM) at 10V VIN). The loop compensation is design made with LTPowerCAD.

Iripple is about 13A per phase at VIN=10V, shouldn't be an issue for the inductors.

I haven't spoken to my local FAE as this is a proof of concept student project which may go into production in the future. (No time to market yet)

Any ideas?

Best regards, Christian

www.360customs.de - www.stromrichter.org

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  • Dear ,

    thanks for the reply. I suspect the gain of knowledge by just getting the EVM is zero to none as it doesn't address the actual problem. I studied the EVMs design and part choice while designing the actual implementation from the begining. Even our FAE had an initial look at it. Would you mind to explain why, in your opinion, the Coilcraft XAL series isn'ta good choice with this IC/implementation? The inductor series is listed as recommendation within LTPowerCAD as well. For sure, 3uH is a bit on the low end and can be increased in value. Even Coilcraft states that these are "optimal" for VRM/VRD application.

    I scoped the ITH pin for PS and FCC configuration at idle and 50W load. The AC-coupled results are shown below. (These measurements where taken with 4u7H inductors (I changed the XAL1580-3R0 -> XAL1510-4R7 to see the benefits on core and lowered Rcomp from 6k2 to 5k, everything else is unchanged)

    Beside this, is there any way to use Burst-Mode with more than 2 phases?

    Best regards, Christian

Reply
  • Dear ,

    thanks for the reply. I suspect the gain of knowledge by just getting the EVM is zero to none as it doesn't address the actual problem. I studied the EVMs design and part choice while designing the actual implementation from the begining. Even our FAE had an initial look at it. Would you mind to explain why, in your opinion, the Coilcraft XAL series isn'ta good choice with this IC/implementation? The inductor series is listed as recommendation within LTPowerCAD as well. For sure, 3uH is a bit on the low end and can be increased in value. Even Coilcraft states that these are "optimal" for VRM/VRD application.

    I scoped the ITH pin for PS and FCC configuration at idle and 50W load. The AC-coupled results are shown below. (These measurements where taken with 4u7H inductors (I changed the XAL1580-3R0 -> XAL1510-4R7 to see the benefits on core and lowered Rcomp from 6k2 to 5k, everything else is unchanged)

    Beside this, is there any way to use Burst-Mode with more than 2 phases?

    Best regards, Christian

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