The drive transistors turn on and off at high speed to chop up the DC. You can think of these pulses as AC; they provide the changing magnetic field the transfo

The drive transistors turn on and off at high speed to chop up the DC. You can think of these pulses as AC; they provide the changing magnetic field the transformer needs. Since this charger is a resonant converter, it's a bit more complex than most; the inductor-capacitor circuit resonates and the pulses turn into sinusoids.