Oscillation in magnetising current of half bridge SMPS..how damp this?

[treez]

Hello,
The primary magnetising current in this half bridge converter has an envelope of oscillation at 1.39KHz.
The oscillation is obviously between the primary series DC blocking capacitor, (5uF) and the one half of the split secondary (the half that’s not conducting at the time), being referred to the primary [1.62mH * (NS/NP)^2 = 2.62mH ]

Since this oscillation is at such low frequency, how can we damp it out without dissipating too much in the process?

The Half bridge converter is provided here as a PDF schematic, and as an LTspice simulation, and the 1.39KHz oscillation in the magnetising current is also shown.

 

[Orson Cart]

Make C4, C5 = 2.2uF,

If the oscillation doesn't affect the o/p or the pri side - then possibly you can live with it?


Regards, Orson

 

[treez]

c4 and c5 are each in parallel with c11 and c18, so it wouldn't be worth making them 2u2's

 

[FvM]

The oscillation is essentially an initial transient response to charging of the DC bus. It doesn't occur in normal operation.

 

[treez]

if you run the simulation you can see that this oscillation goes on and on.

 

[FvM]

The oscillation has high Q and is stoked up by the bus voltage ripple pulses. The magnitude is too low to cause any problems, I guess about 50 mA (*) rms. Of course it can be reduced by placing a RC snubber parallel to C7.

*Actually 25 mA rms.

 

[dick_freebird]

Are you sure this is not just numerical error in the transient
solution? Got a lot of discrepant activity in the higher level
assembly, and some high gain points that can amplify any
residue to visible levels. I'd try one run with Euler method
just to see whether it's circuit or settings / software.