Sepic snubber sizing?

[Flyback]

Hello,
The attached are schematics of two like-for-like SEPIC converters, however, one has an RC snubber across the sepic capacitor and the other doesn't.
The one without the snubber suffers terrible ringing of the sepic inductors and the sepic capacitor.
The (unwanted) L,L,C ringing frequency is 6850Hz.
The snubber 3dB frequency is 1590Hz.
The snubber obviously dissipates virtually nothing when the sepic is in steady state.
Why is this snubber frequency of 1590Hz so effective in totally snubbing out the unwanted resonant ringing of the SEPIC?
The SEPIC spec's are as follows:
F(sw) = 104KHz
Vin = 5V
Vout = 15V
Pout = 7.5W
Continuous Current mode (with slope compensation)

 

[ronsimpson]

I did not run the spice.
Are the inductors coupled?
In the circuit there is a L,C,L that wants to ring. Most spice models are too perfect. Inductors with out resistance and capacitors with out R and L. So very high Q (Ls and Cs) will ring. In you case 6850hz.
By adding 10 ohms to 1/2 of the capacitor (20uf) causes loss that probably stops the ring.

 

[Flyback]

The inductors aren't coupled. I am interested in non coupled inductor sepics because there are few off-the-shelf coupled inductors for sepic, but, as you know, there are huge amounts of off-the-shelf 27uH inductors of the requisite current rating.
It does ring until the RC snubber is added across the cap (simulation is attached of LTspice)
The 'R' of the RC snubber barely dissipates anything at all when the sepic is in steady state.
Interestingly, if the converter is run open loop in the simulation, with the same duty cycle and frequency, there is no ringing even without the snubber.