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LM51551: Rectifier Diode Selection

Allan Fan
Expert 7080 points
Part Number: LM51551

Tool/software:

Hi expert,

Customer used LM51551 for the application 9-36Vin to 54Vo/0.6A. There were two diodes they have tried.

1. DM1 used 200V/3A Schottky diode SM3200B.pdf. Loading is good and can be used up to full load.

2. DM1 used 600V/5A Ultrafast diode MUR5U60(TO-277).pdf. DM1 would be very hot and smoking. We would also observe the input power supply current keep increasing. 

Could you please have a look and advise how to choose the rectifier diode properly and what might goes wrong with the schematic? 

Schematic:

xfmr: PL3254-001SR_SPEC_A.pdf

UM2 MOSFET:

MTB013N10BRQ8.pdf

Thanks,

Allan

  • 0
    TI__Genius 15545 points

    Hi Allan,

    Thanks for using E2E.

    From my calculations, the peak current through the diode is 6A in you case. The 600V diode has a significantly higher forward voltage drop and with that a higher power dissipation (more than 7W peak). This might be too much for the package size. I recommend using a diode that is made for higher currents. In this case i would choose one with at least 10A rating.

    Best regards,

    Moritz

  • 0 in reply to Moritz
    TI__Expert 7080 points

    Hi Moritz,

    Thanks for prompt reply. Customer had also tried with two 600V diode in parallel (so current rating should be ok?) but still can't get stable output at full load 30W. We thought something relates to stability. Also want to clarify that the peak current 6A through the diode should only happened at the lowest 9V input condition but the test customer did is using 12Vin. 

    p.s. originally Rsns used 10mohm but can only power up, would shutdown as load increasing. Rsns changed to 2mohm but can only power up to 15W load. After 15W load, system seems unstable.

    I updated some test waveforms. LM51551 Issue.pdf

    Please have a look and advise what we can do next. 

    Thanks,

    Allan

  • 0 in reply to Allan Fan
    TI__Genius 15545 points

    Hi Allan,

    Paralleling Diodes is not a good idea. One Diode will always take most of the current.

    Also at 12V input, the peak current will still be 5.5A.

    Currently the customer is using a transformer ratio of 1:1. This will result in a very high duty cycle. This might be the main problem.

    For a flyback, a max. dutycycle of 50% is recommended. Please have look here: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/546358/maximum-duty-cycle-of-a-flyback-converter

    I would recommend to use a ratio of 1:3.

    Best regards

    Moritz

  • 0 in reply to Moritz
    TI__Expert 7080 points

    Hi Mortiz,

    Customer referred a similar reference design PMP20588 and the transformer Pri: Sec is 1:5.

    The transformer customer used Pri:Sec PL3254-001SR_SPEC_A.pdf is also 1:5(not 1:1). 

    Could you please review again what might be wrong?

    Thanks,

    Allan

  • 0 in reply to Allan Fan
    TI__Genius 15545 points

    Hi Allan,

    you are right, i looked at the wrong column.

    Just to clarify, everything works normal with the other diode?

    I would recommend to place a snubber across the rectifier diode to supress the large ringing and spikes.

    From the schematic, the rest seems to be ok, can you please measure some more waveforms?

    I would be interested in

    - VDS and anode voltage of diode together in one scopeshot

    - Vin, Vout, COMP in one scopeshot.

    Thanks and best regards

    Moritz

  • 0 in reply to Moritz
    TI__Expert 7080 points

    Hi Moritz,

    Pleas see some waveforms.

    Moritz said:
    - VDS and anode voltage of diode together in one scopeshot

    @no load

    @full load 30W

    Moritz said:
    - Vin, Vout, COMP in one scopeshot.

    @no load

    @full load 30W

    Regards,

    Allan

  • 0 in reply to Allan Fan
    TI__Genius 15545 points

    Hi Allan,

    thanks for the scopeshots.

    Did you get answers for the other questions? :

    Moritz said:
    Just to clarify, everything works normal with the other diode?

    Did they already install a snubber ?

    Moritz said:
    I would recommend to place a snubber across the rectifier diode to supress the large ringing and spikes.

    Best regards

    Moritz

  • 0 in reply to Moritz
    TI__Expert 7080 points

    HI Moritz,

    Moritz said:
    Just to clarify, everything works normal with the other diode?

    using Schottky can normally operate but if not Schottky, the 54Vout would be unstable and very hot at full load. 

    Moritz said:
    I would recommend to place a snubber across the rectifier diode to supress the large ringing and spikes.

    Adding snubber with diode MUR5U60(TO-277).pdf. still see unstable 54Vout and very hot. 

    Regards,

    Allan

  • 0 in reply to Allan Fan
    TI__Genius 15545 points

    Hi Allan,

    So i would recommend using the schottky diode where everything is working.

    Can you send scopeshots again with snubber installed? did the high spikes improve?

    Best regards

    Moritz