Agilent Technologies Agilent 4396B, Service Manual

Page: 227 / 324

Line P ower Module
The line power module includes the main fuse . The main fuse , which protects the input side of
the preregulator from drawing too much line current, is also accessible at the rear panel. See
P ower R equirements in appendix C for the fuse replacement and other power considerations .
A40 Preregulator
The A40 preregulator contains a rectier and a switching regulator , converts the line voltage to
+24 V and provides it to the A50 DC-DC converter .
A50 DC-DC Converter
The A50 DC-DC Converter consists of the two switching regulators (1 and 2). The DC-DC
convereter provides an LED (visible at the top) to indicate circuit status . See Figure 5-12 in
chapter 5. The shutdown LED is turned o when the overcurrent protection circuit activates .
The circuit activates when an overcurrent is sensed on the +5 VD power line , when an
overcurrent is sensed on the four power supplies (618 V and 67.8 V), or when the F AN LOCK
signal is sensed. It shuts down the ve power supplies of the switching regulators (1 and 2).
F or A50 to work properly , the +7.8 V must be loaded (approximately 680 ohms , more than
125mW). If it is not, the other preregulated voltages in the A50 DC-DC converter will not be
correct.
Switching Regulator 1
Switching regulator 1 converts the +24 V to the regulated +5 VD (digital supply). The +5 VD
goes directly to the A1 CPU .
Switching Regulator 2
Switching preregulator 2 converts the +24 V to four DC voltages , +7.8 V , 07.8 V , +18 V , 018
V . The voltages are routed to the A2 post-regulator for nal regulation.
Regulated +5V Digital Supply (+5 VD)
The +5VD power supply is fully regulated in the A50 DC-DC converter . It goes directly to the
A1 CPU and is supplied to all assemblies requiring a digital +5 V supply through A1, and the
A20 motherboard. See Figure 11-3 .
A50 Shutdown LED
The A50 shutdown LED is on during normal operation. It turns o when the A50 protective
circuits are activated and shut down some power lines . The shutdown LED turns o when one
of the following conditions is sensed:
Overcurrent on +5 VD P ower Line .
Overcurrent on the four power supplies (618 V and 67.8 V)
F an is not rotating (F AN LOCK signal is sensed).
The fan obtains its power +24 V from A40 preregulator through the A50 DC-DC converter and
the A2 post-regulator . When the power is missing, the F AN LOCK signal shuts the switching
regulators down and turns the A50 shutdown LED o.
Theory of Operation 11-7