I was reverse engineering a battery charger (Lidl brand Parkside 20V battery charger PLGS 2012 A1, with smart functions, among them charge current and termination voltage settings) and came across this unexplainable-to-me flyback transformer setup. -I stand corrected, as this is a forward converter-
As you can see in my below schematic the main SMPS transformer has 4 windings: a switched primary (between pins 1 and 2), an output secondary(which does the actual battery charging), a grounded shield winding that is only connected on one end (pin 4), and my "rogue" winding (between pins 1 and 3). You can see that the latter shares the same starting point at the rectified mains rail as the primary winding, but on the other end only connects to the cathode of a diode (D2), whose anode connects to ground. Measuring with a multimeter there is no connectivity between pin 3 and 4 (at first I thought this was simply an AUX winding but it isn't).
The switching is done via NPN transistor, driven by a CP1252, which has secondary side feedback via an optocoupler so doesn't need an AUX winding, and derives its VCC from a small second SMPS (driven by a KP2130), which provides around 10V to the primary side ICs via its AUX winding, and 5V to power ICs on the secondary side via its secondary winding.
In the schematic I have marked in red my two oscilloscope probe points, 1 and 2 (with the scope grounded) for the two waveforms you can see below.
My "rogue" winding practically has the inverse waveform of the primary. I imagine this has something to do with efficiency/PFC, but I have never seen such a setup and do not understand what function this "rogue" winding is preforming. It seems to me that D2 never conducts, it certainly isn't snubbing the "rogue" winding?
I'd really appreciate some insight into this unique setup. Thanks for the replies!
And the oscilloscope waveforms: