4
Power Supply VSNUB
Transformer
From
To 3.3 V
Regulator
VSNUB
VSNUB is formed by rectifying the transformer secondary potential via power rectifiers D10-D13. An additional
rectifier, D16 prevents discharge of the filter capacitor, C48 through the FET switch, Q10, whenever this switch is
on. For operating conditions other than extremely low line potential, this permits the peak potential at C48 to
track closely to the peak secondary potential. This is substantially higher than the peak potential of the quasi-
regulated Vraw supply discussed below.
VSNUB also serves as a clamping potential into which some of the energy stored in the leakage reactance of
the mains transformer is discharged when Q10 is switched off. This additionally energy can cause the peak
potential for V-UNREG to actually be greater in magnitude than the peak potential of the transformer secondary
when Vraw current drain is large. VSNUB can range from 20 to 40 volts, depending on input voltage and Vraw
loading.
