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LM5575: Two questions regarding this part

Colin Waterhouse
Prodigy 20 points
Part Number: LM5575

Hello,

I am a service technician and have had a number of products come in for repair which use the LM5575 converter. The fault is caused by the circulating diode going short circuit  (3A Schottky, 60 volts reverse voltage).

Product 1 has 3 x LM5575MH converters with outputs of 3.3V, 5V, and 12V from a 48V nominal input - failure rate of this has been moderate

Product 2 has 3 x LM5575MH converters with outputs of 3.3V, 5V, and 12V from a 56V nominal input - failure rate of this has been high

The rail which fails is random in each power supply, I have had one instance where two diodes have failed in one unit.

It has been the directive to replace the 60V diode with one which has a reverse voltage of 100V. The LM5575MH datasheet specifies allowing an overhead for this parameter without stating a percentage of overhead. The reference design uses a 100V diode for a 75V maximum input voltage - 25% overhead in that case.

The advise I have been given is to replace the diodes only (even if shorted) and then if the supplies measure OK, then to put them back into service. If the supply doesn't work, then to replace the whole supply. I have a had a few modules where the switch mosfet in the LM5575 has been damaged, and they obviously were rejected.

The assumption is that service technicians are unable to perform the rework required to replace the LM5575 reliably, but this is not a problem in my case - I have no problem with this operation.

Question 1 - should the LM5575MH be replaced if it has been run into a shorted circulating diode for an unknown length of time, and an unknown number of power cycles?

Question 2 - what is the recommended percentage overhead for the reverse voltage of the circulating diode for a given input voltage?

My goal is to ensure long term reliability of the product, and prevent future failures.

Thank you,

Colin Waterhouse.

  • 0
    TI__Guru 57555 points

    Hello

    For #1:  It would be my opinion that the LM5575 should be replaced when a shorted diode is found in the power supply.

    As a matter of fact, I am surprised that many of the LM5575s will survive with a shorted diode.

    For #2:  I think that 25% is a good overhead to stick with.  So, 60-V is good for a 48Vin.  However you must take 

    into account your highest Vin, and not just the nominal.

    There is one other consideration that can cause failure of the diode and perhaps the regulator.

    Some Schottky diodes can "snap-on" in the blocking direction if the dV/dt on the diode exceeds a certain limit.

    When the power FET in the regulator turns on, the diode is suppose to turn off.  However, the rising voltage on the

    cathode of the diode, during this time, is moving very quickly; high dV/dt.  Some diodes can try to turn back on

    when subjected to this high dV/dt.  This will cause a large current flow that can damage both the diode and the regulator.

    Unfortunately, not all diode manufacturers specify a maximum dV/dt rating on the diode data sheet; although some do so.

    The other difficulty is that the actual dV/dt seen on the SW pin will vary due to capacitance on the board and input voltage, etc.

    If you wish to pursue this particular possibility, then the best plan is to take a few good supplies and measure the voltage on the 

    SW pin with a fast 'scope and note the voltage rising slew rate; preferably at full load and max input voltage.

    You can then look for candidate diodes that meet this specification.  

    Also, you may want to contact the diode manufactures to determine if they have dV/dt information available.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 20 points

    Thank you for the fast reply, Frank.

    It has been my instinct to replace the LM5575 in cases where the circulating diode has failed, and not leave it to chance where it may potentially fail at a later date.

    The product(s) in question are fairly labor intensive to work on and it seemed prudent (to me) to minimize the risk of potential returns for this problem.

    I had a look at the datasheet for the 60V diode, Diodes Inc B360A, and could not find a specification for the dV/dt.

    The replacement diode has already been specified by the manufacturer.

    I appreciate the extra insight into potential other causes of this type of failure, however.