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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

attachments.zip
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  • Out of curiosity i compared the XAL1580/1510-series inductors against the SER2918H-series used on the EVM.

    In general, Coilcraft states better loss performance for the XAL1580/1510-series as shown in the attached data.

    To see the losses in detail, reduced ripple current for higher inductance values must must taken into account. So i compared XAL1580-302 vs. SER2918H-332H in the 3u0H range, and XAL1510-103 vs. SER2918H-103. The results are shown in the following table:

    The results are based on Vin=10V, Vout=51V.

    While this is true for core-losses at idle, the XAL1510 will loose it advantage at full load current due to x3 the DCR resistance.

    Getting back to the 3uH class, how does the XAL1580-302 performs against the SER2918H-332? Well, they do pretty well:

    So again, why do you think the XAL1580/1510 series is a "bad" choice? Using Pulse-Skip this XAL1580 inductors should really do the job, no?

    Best regards,

    Christian

    attachments.zip
Reply
  • Out of curiosity i compared the XAL1580/1510-series inductors against the SER2918H-series used on the EVM.

    In general, Coilcraft states better loss performance for the XAL1580/1510-series as shown in the attached data.

    To see the losses in detail, reduced ripple current for higher inductance values must must taken into account. So i compared XAL1580-302 vs. SER2918H-332H in the 3u0H range, and XAL1510-103 vs. SER2918H-103. The results are shown in the following table:

    The results are based on Vin=10V, Vout=51V.

    While this is true for core-losses at idle, the XAL1510 will loose it advantage at full load current due to x3 the DCR resistance.

    Getting back to the 3uH class, how does the XAL1580-302 performs against the SER2918H-332? Well, they do pretty well:

    So again, why do you think the XAL1580/1510 series is a "bad" choice? Using Pulse-Skip this XAL1580 inductors should really do the job, no?

    Best regards,

    Christian

    attachments.zip
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