LLC Design with multiplier output

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I'm working on an LLC design with the HyperLCS device. The application note and spreadsheet assume a standard rectified output. However, with a quadroupler output, the multiplier capacitors in the multiplier/rectifier bridge reflect into the primary side and have an effect on the resonant tank.

How do I account for the reflected multiplier capacitors? I'm looking for a quick answer so I don't have to knuckle down and re-derive all the fundamental gain and resonance equations from scratch...

-thx,
Dave

If your quadroupler output circuit is after the diode, the multiplier capacitor will not refect into the primary side and will not effect the resonant tank.
What you may need to modifiy is the soft start up speed. With larger capacitive load, you may need to slow down the start up speed to avoid current spikes.
Also, depending on your output voltage, if your secondary winding turn number is high, you need to carefully design your secondary windings to minimize winding capacitance. Served Litz wire is recommended.

I've always seen multipliers with the series capacitor attached directly to the secondary winding - otherwise, with a diode between the multiplier cap and the secondary, the charge pumping action is defeated. See the attached 'rough' schematic.

Regarding the capacitance of the secondary winding - how does it affect the LLC resonance? Does is simply cause a shift in the resonance, such that by modifying Cres it can be accounted for; or does it introduce a new resonance that is destructive?

The primary winding is a single layer of 20 turns, the secondary is composed of 40 turns bifilar - so there are four layers to the secondary. I began to come to the conclusion today that the layer capacitance of the secondary is coming into play. So, my question is still: can it be compensated for, or is it destructive?

It can be compensated and it is not destructive. As you said it is simply cause a shift in the resonance. Equivalent secondary capacitor is in series with Cres. So the compensation is to choose an accordingly bigger Cres.
Besides your question, if you end up with using PI chips, your posted schematic could be modified following our datasheet and app notes recommendations.