What topology is this PS AUDIO Powerpack UPS

[CaptainBucko]

I picked up a PS Audio Power Pack 1500MkII from eWaste. It had dead SLA batteries (dated 2013) and a blown fuse. Replacing both of those and the unit appears to work as normal. From what I have read through google search, this is a decent UPS in terms of performance, and is true sine wave. I don't have a UPS for my lab, and this might be good for the sensitive test equipment.

However, I don't understand the topology of this unit. I understand that true sine wave UPS generally rectify mains AC into high voltage DC and then digitally reconstruct the waveform back into AC. However, this unit has no large electrolytic high voltage capacitors or bridge rectifier justifying a 1500W rating. I cant find any schematic or service manual for this unit.

The transformer shown in the photos is truly massive - there are four banks of four (16 in total) of IRF3205 MOSFETs. During normal mains AC operation, I can see 13v AC coming from the large winding that is connected to the FETs. The backup battery is 24v.

I would appreciate any information on how the unit works as I am quite interested.

EDIT & UPDATE : This is the topology.  https://www.ti.com/lit/an/slaa602a/slaa602a.pdf