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1/8/2018 10:56 PM
 
Hello,

We have used the MP4559DN in a DC-DC converter producing +15V from +24V to +55V input voltage. The nominal current load is equal to 0.8A.
The switching waveform on the chip pin 1 looks as expected with the nominal 0.8A load current.



With decreasing load current (using an electronic load) the waveform becomes more and more "jittery" on the oscilloscope screen. The picture below shows the switching waveform with 0.3A load current.



The switching frequency is expected to decrease with reduced load, but it also appears to be less stable. Is this normal or a sign of the DC-DC converter malfunction?

Also, when increasing the input voltage from 24V to 55V with constant 0.8A load current the switching waveform can become unstable on the oscilloscope screen when the input voltage is around 50V or higher. On a few occasions the waveform as on the following picture has been captured.

 

What could be reason for this?

We are greatly concerned whether the MP4559DN can work with input voltages higher than 50V as we had one failure where the MP4559DN switch short circuited when powered from 55V damaging other two DC-DC converters on the same board powered from its 15V output. The exact cause of this failure remains unknown.

Any suggestions what to check would be appreciated.

Regards,
Les
 
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1/9/2018 5:42 PM
 

Hi Les,

 

Could you please share your schematic with us?

 

Thanks,

Kevin

 
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1/9/2018 10:14 PM
 
Hi Kevin,


Here is the schematic:
Power Supplies Schematic.PDF

Regards,

Les
 
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1/10/2018 6:39 PM
 

Hi Les,

 

It seems like the circuit may be unstable. The regulator probably is operating in boundary mode in the third scope shot with instability. I ed with your circuit, but I couldn't duplicate the issue you are seeing. I would suggest you to try different values on the COMP pin to see if it improves. For example, C152=1.2nF, R158=210k, and C150=1pF. Also, I would suggest you to check the layout to see if there noise got coupled to the FB and Comp pin.

 

Regards,

Kevin

 
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1/10/2018 7:47 PM
 
Hi Kevin,

I have checked the suggested compensation components values and actually they make the instability worst. I have found that the wider the DC-DC converter bandwidth the more unstable the switching waveform looks. Yes, that would suggest some noise coupling into the FB and COMP pins, but I can't see anything obviously wrong in the board layout.
This is a pdf with the 15V DC-DC converter layout in case you would like to have a look: SIL-60-1035-00-A 15V DC-DC Layout.pdf

This layout was based on the EV4559-N00A evaluation board from MPS.

Regards,
Les
 
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