Fluke NetDAQ 2640A, Service Manual

Page: 53 / 260

Theory of Operation
Detailed Circuit Description
2
2-11
Raw DC Supply 2-24.
The raw dc supply circuitry receives input from power transformer T401, which operates
from an ac source of 107V to 264V ac. The power transformer is energized whenever the
power cord is plugged into the ac line; there is no on/off switch on the primary side of
the transformer. The transformer has an internal 275V ac metal-oxide varistor (MOV) to
clamp line transients. The MOV normally acts as an open circuit. When the peak voltage
exceeds approximately 400V, the line impedance in series with the line fuse limits
transients to approximately 450V. All line voltages use a time-delay 0.15 A, 250V fuse.
On the secondary side of the transformer, rectifiers A1CR2, A1CR3, and capacitor A1C7
rectify and filter the output. When ON, switch A1S1 (the rear panel POWER switch)
connects the output of the rectifiers to the filter capacitor and the rest of the instrument.
Depending on line voltage, the output of the rectifiers is between 8.0 and 35V dc.
Capacitor A1C2 is used for electromagnetic interference (EMI) and electromagnetic
compatibility (EMC) requirements. Capacitor A1C1 helps supply the high frequency
ripple current drawn by the switching regulator (described below).
When external dc power is used, the power switch connects the external dc source to
power the instrument. The external dc input uses thermistor A1RT1 for overcurrent
protection and diode A1CR1 for reverse input voltage protection. Capacitor A1C59 is
used for EMI/EMC requirements. Resistor A1R48, and capacitors A1C102 and A1C39
are also used for EMI/EMC performance requirements. If both ac power and dc power
are connected to the instrument, the instrument uses ac power when it exceeds
approximately eight times the value of the dc voltage. Automatic switchover occurs
between ac and dc power without interrupting instrument operation.
Auxiliary 6V Supply 2-25.
Three-terminal regulator A1U19, voltage-setting resistors A1R44 and A1R46, and
capacitor A1C34, make up the auxiliary 6-volt supply. This supply is used to power the
inverter oscillator and inverter driver.
5V Switcher 2-26.
The 5V switcher supply uses a controller/switch device A1U9 and related circuitry to
produce the 4.9V dc (Vcc) output.
4.9V dc (Vcc) The 8V to 35V dc input is regulated to 4.9V dc (Vcc) through
pulse-width modulation at a nominal switching frequency of 100 kHz. The output
voltage of the switcher supply is controlled by varying the duty cycle (ON time) of the
switching transistor in the controller/switch device A1U9. A1U9 contains the supply
reference, oscillator, switch transistor, pulse-width modulator comparator, switch drive
circuit, current-limit comparator, current-limit reference, and thermal limit. Dual
inductor A1T2 regulates the current that flows from the raw supply to the load as the
switching transistor in A1U9 is turned on and off. Complementary switch A1CR10
conducts when switching is turned off.