A3 Fet Board; Output Circuits - Agilent Technologies 6571A Service Manual

Fan Speed Control
A fan speed control circuit (U304, U311) provides the dc voltage to operate the cooling fan. A pulse-width modulated
signal (FAN_PWM) from the secondary microprocessor on the A5 Control Board varies this voltage to increase or decrease
the fan speed according to the ambient temperature. The secondary microprocessor determines the level of FAN_PWM by
monitoring a fan speed signal (FAN_DETECT) and the ambient temperature, as measured by the voltage drop across a
thermistor (A4RT300). Chassis-mounted fan BI is shown on A4 AC Input Board schematic.

A3 FET Board

The A3 FET Board has 8 power FET stages and 2 bridge driver IC's (see A3 FET Board schematic). The power FETs are
mounted on, but are electrically isolated from, a heat sink assembly. The FET stages are connected from the +RAIL and the
-RAIL to the power transformer (T900) in an H-bridge configuration.
DRIVE_A and DRIVE_B pulses, received from the A5 Control Board. are used by the bridge driver ICs (U201 and U202) to
derive control pulses (DRIVEl and DRIVE2) for the FET switches. The width of these pulses determines the ON time of the
FET switches, thereby determining the magnitude of the output voltage or current. DRIVEl pulses turn on one set of +RAIL
(Q201, Q211) and -Rail (Q203, Q233) FETS, causing current to flow through power transformer T900 in one direction.
DRIVE2 pulses turn on the other set of +RAIL (Q204, Q244) and -Rail (Q202, Q222) FETS, causing current to flow through
T900 in the opposite direction. The FET on/off periods are controlled by Duty Cycle Detect and the peak current detection
circuits. If the output attempts to change, regulation is accomplished by the CV/CC Control Circuits on the A5 Control
Board (previously described). These circuits vary the width of the drive pulses and the duration of the FET on/off periods.

Output Circuits

The output circuits consist of chassis mounted components (power transformer T900, power rectifier D900, and inductor
L900), the A6 Output Filter Board, and the A7 Snubber Circuit Board. These circuits are shown on the corresponding
model schematic (see Chapter 6). Power transformer T900 couples the output pulses from the A3 FET Board to the power
full-wave rectifier D900. Filtering is provided by L900 and the A7 Snubber Board. The A6 Output Board provides both
normal-mode and common-mode filtering as well as downprogramming circuits. When activated, the downprogramming
circuits draw current from the output terminals.
82 Principles Of Operation