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LM3450AEV230V30: LM3450AEV230V30

Ingo Ambaum
Prodigy 10 points
Part Number: LM3450AEV230V30

Hi,

 

in the past, we got several field returns regarding overheated components in the start-up path. Investigations didn’t show any malfunction after changing defect parts.

The question is, if there is any circumstance that can lead to a malfunction of the BIAS-Function (switching internal Z-Diodes).

I got one part that starts only with 14V Z-Diode (never with 20V) at pin BIAS. The reason for this behavior was a low resistive transistor in parallel to the resistor between Pin FB and GND. In normal case the resistor has a value of 5k1. The defect FET in parallel had a resistance of approximately 11k so that the result of both resistances in parallel was round about 3k5.

This behavior is replicable. A 3k5 at Pin FB to GND leads to a start-up voltage of 14V.

So there must be a correlation between the functions FB and BIAS. Is there any condition possible that activates the 20V during „normal“ operation?

 

In the attachment you will find the corresponding schematic.

 

 

Best regards

 

Ingo

 

2746.Schematic Prints.pdf

  • 0
    TI__Mastermind 28655 points
    Hello Ingo,

    If the fb resistor is 3.5 Kohm it is right at or below the Vfb-SD (shut down), 3.5K * 50 uA = 175 mV which is less than (168 mV + 20 mV). Since I only have part of the schematic I assume OPTO1 is for OVP and there is another OPTO?

    "So there must be a correlation between the functions FB and BIAS. Is there any condition possible that activates the 20V during „normal“ operation", yes, if the VCC capacitor is allow to discharge to UVLO, other wise it should not, the above condition is Shutdown.

    Regards,