Envision Peripherals NV3128 - Manual (Page 73)

NV3128 RS-422A Machine-Control Data Routing Switch 4-11
Metal oxide varistors DV1 and DV2 clamp voltage spikes on the AC
input lines to safe levels.
Further, the PS2001 automatically bleeds all input capacitors when the
unit is removed from the frame. Normally, a diode is connected to
ground through the backplane, biasing FET Q4 off. Upon removal of
the module from the frame, the FET conducts, quickly discharging the
capacitors, thus minimizing the chance of electrical shock.
Switching FETs Q1 and Q2 chop the high voltage DC at 40 kHz into
a square wave. The resulting square wave is subsequently fed into a
step-downtransformer. The secondary side of the transformer has two
identical sets of windings which feed full-wave rectifiers. The use of
Schottky diodes for the rectifiers minimizes voltage losses in the
outputs.
Inductors and capacitors form two-stage filters for the secondary
rectifiers. They produce low voltage DC outputs at approximately
+/-16 to +/-18 Volts and +/- 6 to +/-8 Volts. Schottky diodes assure
proper load sharing among redundant supplies.
A pulse width modulation submodule tightly regulates the secondary
outputs. A primary-side auxiliary winding on transformer T1 monitors
the flux and feeds back a relative voltage to U1. This feedback
governs the pulse width of the submodule's outputs, thereby regulating
the drive signal applied to the switching FETs.
A boot-strap multivibrator circuit on the submodule initializes the
power-supply circuitry.
Although each of the NV3128 modules has its own on-board voltage
regulators and current limiters, the power supply itself is self limiting.
Transformer T2 monitors the primary current and steps it down to a
level where it can safely drive the PWM submodule, shutting it down
completely in case of excessive load. Operation resumes immediately
when the overload is corrected.