Agilent Technologies 6571A, Service Manual

Page: 81 / 161

Principles Of Operation 81
the secondary microprocessor. The width of the output pulses from the Summing Amplifier vary as the
OR_GATE
control
signal decreases or increases. These pulses are applied to the Pulse-Width Modulator IC via the On Latch. The Pulse-Width
Modulator generates the square-wave pulses that are applied to the A3 FET Board to turn the FET switches on and off. The
Deadtime Latch resets the On Latch to provide a minimum dead (off) time for the FET switches .
The OV circuit compares the output voltage level with a signal ( OVREF ) that represents the programmed overvoltage level.
When the output voltage exceeds the programmed value, the downprogramming circuits are activated and the FET switches
are turned off.
The downprogramming control circuit generates control signal
DP
when an OV or
INHIBIT
condition has been detected, or
when the output is higher than was programmed. DP causes the downprogramming MOSFETS on the A6 Output Filter
Board to conduct current away from the load.
Power Circuits
A4 AC Input Board
The A4 AC Input Board (see A4 AC Input Board schematic) includes the following circuits:
Input Filter and Rectifier
The single-phase ac input (220/240 vac) is applied through chassis-mounted line filter FL1, a 25-ampere line fuse (A4F400)
and filter components on the A4 board to the front panel switch (S1). When the switch is turned on, filtered ac input is
applied to a bias transformer (A4T300) via a line voltage select switch (A4SW300) and through turn-on relay (A4K401)
contacts and R400/R401 to a bridge rectifier (A4D400).
The ac line voltage is rectified and filtered to provide the
±
RAIL (290 volts) input lines to the A3 FET Board. Because filter
capacitors are connected directly across the rectified line (
±
RAILS), large inrush current surges at turn-on could blow fuses
or affect the operation of other equipment connected to the same ac line. This is prevented by the time-delay action of one
of the turn-on relays (AK400). During the turn-on period (approximately 10 seconds), relay A4K400 is deenergized and
presents a higher input resistance that limits inrush current. After the turn-on period, relay A4K400 energizes and decreases
R400/R401 input resistance.
Bias Supplies and Bias Detect
The secondary windings of the bias transformer (A4T300) provide ac inputs to the +15Vp primary bias supply (components
D313-D316, U302), the
±
15V
s
secondary bias supply (components D306-D309, U305, U306) and the fan voltage supply
(components D310, D317). In addition, the bias transformer applies ac power to the +5V
p
Primary Interface Bias Supply on
the A2 GPIB or Isolator Board.
The +15Vp bias voltage is used to operate the turn-on relays (K400/K401), an Isolation Optoisolator (U309), and by
circuits on the A3 FET Board. The
±
15V
s
bias voltage is used by the fan speed control circuits and also powers the +5V
s
and
±
14V bias voltages used by circuits on the A5 Control Board.
The purpose of the Bias Detect Circuit (U308) is to disable the power supply output until the bias voltages have stabilized.
When power is initially applied, the circuit generates a
RESET signal to disable the supply’s output and to initialize the
secondary interface microprocessor on the A5 Control Board. When the bias circuit has stabilized, the Bias Detect Circuit
detects a BIAS_OK signal from the A5 Control Board and removes the
RESET
signal to enable the power supply output.